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Improved Use of Green Manure as a Nitrogen Source for Sweet Corn


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IMPROVED USE OF GREE N MANURE AS A NITROGEN SOURCE FOR SWEET CORN By COREY CHERR A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Corey Cherr

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To my new family and my old, may you help me to find balance in my life and make me a better person than I could ever be alone.

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iv ACKNOWLEDGMENTS I would like to acknowledge the wonderful help of Johan Scholberg, Andy Schreffler, Brian Jackson, Sam Willingham, Vony Petit-Frere, Lily Chang-Chien, Holly Nelson, Amy Van Scoik, John McQueen, Dipen Patel, Robert Wanvestraut, Alicia Lusiardo, and Huazhi Liu, as well as the sta ff of the UF-IFAS Plant Science Research and Education Unit in Citra. This research was funded by grants from the Sustainable Agriculture Research and Education program of the United States Department of Agriculture (grant number LS02-140, “A Syst em Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems”) and the Center for Cooperative Agricultural Programs (grant also titled “A System Approach for Improved Integration of Green Manure in Commer cial Vegetable Production Systems”).

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES.........................................................................................................xiv ABSTRACT....................................................................................................................... xv CHAPTER 1 INTRODUCTION........................................................................................................1 Overview....................................................................................................................... 1 Introduction................................................................................................................... 1 Rationale................................................................................................................1 Green Manure Management..................................................................................4 Approach....................................................................................................................... 7 Hypotheses....................................................................................................................8 Objectives..................................................................................................................... 9 General Set-Up and Design..........................................................................................9 Measurements.............................................................................................................10 2 GREEN MANURE GROWTH AND DECOMPOSITION.......................................20 Introduction and Literature Review............................................................................20 Materials and Methods...............................................................................................27 Set-up and Design................................................................................................27 Timeline of Operations........................................................................................27 2001-02.........................................................................................................27 2002-03.........................................................................................................28 Measurements......................................................................................................28 2001-02.........................................................................................................28 2002-03.........................................................................................................29 Analysis...............................................................................................................30 Results........................................................................................................................ .30 Sunn Hemp 2001.................................................................................................30 Growth..........................................................................................................30 Decomposition.............................................................................................32

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vi Sunn Hemp 2002.................................................................................................34 Growth..........................................................................................................34 Decomposition.............................................................................................36 Lupin 2001-2002.................................................................................................38 Vetch 2002-2003.................................................................................................40 Discussion...................................................................................................................42 Sunn Hemp..........................................................................................................42 Growth..........................................................................................................42 Decomposition.............................................................................................45 Lupin and Vetch..................................................................................................47 Conclusions.................................................................................................................51 3 GROWTH, YIELD, AND N-UPTAKE E FFICIENCY RESPONSE OF CORN TO AMENDMENT WITH GREEN MANURES......................................................57 Introduction and Literature Review............................................................................57 Materials and Methods...............................................................................................63 Set-Up and Design...............................................................................................63 Timeline of Operations........................................................................................63 2001-02.........................................................................................................63 2002-03.........................................................................................................64 Procedures and Measurements............................................................................64 Analysis of Data..................................................................................................66 Results........................................................................................................................ .67 N Applied to Corn...............................................................................................67 Ear Yields, 2002..................................................................................................67 Ear Yields, 2003..................................................................................................68 Growth Analysis, 2002........................................................................................69 Leaf indicators..............................................................................................69 Tissue characteristics...................................................................................70 Growth Analysis, 2003........................................................................................72 Leaf indicators..............................................................................................72 Tissue characteristics...................................................................................73 Nitrogen Uptake Efficiency a nd Unaccounted Applied Nitrogen.......................75 Discussion...................................................................................................................76 Conclusions.................................................................................................................82 4 EFFECTS OF GREEN MANURE AM ENDMENT ON SWEET CORN ROOT LENGTH DENSITY AND DISTRIBUTION............................................................94 Introduction and Literature Review............................................................................94 Materials and Methods...............................................................................................98 Set-up and Design................................................................................................98 Field and Lab Procedures....................................................................................98 Data Analysis.......................................................................................................99 Results.......................................................................................................................1 02 Overall Root Length Density.............................................................................102

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vii Root Length Density by Location.....................................................................103 Relative Root Length by Location....................................................................105 Root Length Density by Proximity....................................................................106 Relative Root Length by Proximity...................................................................107 Effective Rooting Depth....................................................................................107 Soil Water Potential...........................................................................................108 Discussion.................................................................................................................109 Conclusions...............................................................................................................114 5 EFFECTS OF A GREEN MANURE APPROACH TO SWEET CORN FERTILIZATION ON SOIL PROPERTIES...........................................................122 Introduction...............................................................................................................122 Materials and Methods.............................................................................................128 Set-Up and Design.............................................................................................128 Procedures and Measurements..........................................................................128 Data Analysis.....................................................................................................130 Results.......................................................................................................................1 31 Dry Matter Additions........................................................................................131 Microbial Biomass Carbon................................................................................132 Total and Particulate Carbon and Nitrogen pools.............................................132 Soil pH...............................................................................................................134 Discussion.................................................................................................................135 Conclusions...............................................................................................................140 6 EFFECTS OF GREEN MANURE APPROACHES ON CROP PESTS: PARASITIC NEMATODES AND WEEDS............................................................146 Introduction and Literature Review..........................................................................146 Materials and Methods.............................................................................................150 Set-Up and Design.............................................................................................150 Procedures and Measurements..........................................................................151 Results.......................................................................................................................1 52 Nematodes.........................................................................................................152 October 2001..............................................................................................152 March 2002................................................................................................153 April 2002..................................................................................................153 July 2002....................................................................................................153 March 2003................................................................................................154 June 2003....................................................................................................154 Weeds................................................................................................................155 Sunn hemp, October 2001..........................................................................155 Sunn hemp, October 2002..........................................................................156 Vetch, April 2003.......................................................................................157 Discussion.................................................................................................................157 Conclusions...............................................................................................................161

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viii 7 CONCLUSIONS......................................................................................................165 Review and Synthesis of Findings............................................................................165 Future Work..............................................................................................................172 APPENDIX A CHARACTERIZATION OF DOMINANT SOIL TYPES PRESENT IN FIELD..174 B CONTINUOUS MEASUREMENTS.......................................................................176 C SELECTED TISSUE FACTORS AND LEAF INDICATORS FOR SWEET CORN, 2002 AND 2003...........................................................................................177 D TABLES OF INTERACTIONS FOR ROOT LENGTH DENSITY BY LOCATION, 8 WEEKS AFTER EM ERGENCE, SWEET CORN 2003................195 LIST OF REFERENCES.................................................................................................199 BIOGRAPHICAL SKETCH...........................................................................................210

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ix LIST OF TABLES Table page 1.1 Review of green manure studies..............................................................................11 1.2 Overview of experimental treatments......................................................................19 2.1 Sunn hemp nitrogen concen tration by tissue type, 2001..........................................54 2.2 Selected sunn hemp growth indicators, 2001...........................................................54 2.3 Sunn hemp nitrogen concentration by tissue type after death, 2001-02..................54 2.4 Sunn hemp nitrogen concen tration by tissue type, 2002..........................................55 2.5 Selected sunn hemp growth indicators, 2002...........................................................55 2.6 Sunn hemp nitrogen concentration by tissue type after death, 2002-03..................55 2.7 Lupin nitrogen concentration by tissue type, 2001-02.............................................56 2.8 Selected lupin growth indicators, 2001-02...............................................................56 2.9 Vetch tissue nitrogen concentration, 2002-03..........................................................56 2.10 Selected vetch growth indicators, 2002-03..............................................................56 3.1 Pairwise contrasts of selected n itrogen factors an d ear yields, 2002.......................84 3.2 Pairwise contrasts of selected n itrogen factors an d ear yields, 2003.......................84 3.3 Ear yields at final harvest, 2002 and 2003...............................................................85 3.4 Leaf area index, 2002...............................................................................................85 3.5 Pairwise contrasts of leaf area i ndex and specific leaf nitrogen, 2002....................86 3.6 Leaf dry weight........................................................................................................87 3.7 Total dry weight, 2002.............................................................................................87 3.8 Leaf nitrogen content, 2002.....................................................................................88

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x 3.9 Total nitrogen content, 2002....................................................................................88 3.10 Pairwise contrasts of leaf dry weight and nitrogen content, 2002...........................89 3.11 Pairwise contrasts of total dry weight and nitrogen content, 2002..........................89 3.12 Leaf area index, 2002...............................................................................................90 3.13 Pairwise contrasts of leaf area i ndex and specific leaf nitrogen, 2003....................90 3.14 Leaf dry weight, 2003..............................................................................................91 3.15 Total dry weight, 2003.............................................................................................91 3.16 Leaf nitrogen content, 2003.....................................................................................92 3.17 Total nitrogen content, 2003....................................................................................92 3.18 Pairwise contrasts of leaf dry weight and nitrogen content, 2003...........................93 3.19 Pairwise contrasts of total dry weight and nitrogen content, 2003..........................93 4.1 Pairwise contrasts against Conv 267N for overall sampled root length density, 0-60 cm...................................................................................................................118 4.2 Significance of green manure, nitrogen rate position and depth and sub-effects when constituting linear model fo r sampled root length density...........................118 4.3 Various interactions with depth for root length density at 3 and 5 weeks after emergence...............................................................................................................119 4.4 Significance of treatment, position and de pth when constituting linear model for sampled root length density....................................................................................119 4.5 Pairwise root length density comparis ons against Conv 267N by depth and position at 5 weeks after emergence......................................................................119 4.6 Pairwise root length density comparis ons against Conv 267N by depth and position at 8 weeks after emergence......................................................................120 4.7 Significance of green manure, nitrogen ra te, and proximity to plant when constituting linear model for sampled root length density.....................................120 4.8 Significance of green manure, nitrogen rate, and proximity to plant when constituting linear model for sampled root length density.....................................120 4.9 Interactions between nitrogen rate an d proximity for root length density.............121

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xi 4.10 Pairwise root length density compar isons against Conv 267N by proximity at 8 weeks after emergence...........................................................................................121 5.1 Significance of green manure, nitrogen rate and year in balanc ed analysis of variance for soil carbon and nitrogen pools, July 2002 and June 2003.................142 5.2 Significance of treatment and year in full analysis of variance and pairwise contrasts of selected treatments for so il carbon and nitrogen pools, July 2002 and June 2003.........................................................................................................143 5.3 Analysis of variance for all treatments and pairwise contrasts of selected treatments within years for partic ulate organic carbon and nitrogen.....................144 5.4 Significance of date, green manure and n itrogen rate for pH of sampled soil.......145 6.1 Nematode soil population counts from sel ected treatments at selected dates........163 6.2 Nematode soil population counts at selected dates................................................164 A.1 Selected characteristics from a La ke Fine Sand; Typic Quarzipsamments, hyperthermic, coated; Ci trus County, FL...............................................................174 A.2 Selected characteristics from a Candl er Fine Sand; Typic Quarzipsamments, hyperthermic, uncoated; Alachua County, FL.......................................................175 B.1 Continuously measured environmental factors......................................................176 C.1 Corn applied nitrogen, unaccounted fo r applied nitrogen and chlorophyll meter readings by green manure and nitr ogen rate, sweet corn, 2002.............................178 C.2 Specific leaf area and sp ecific leaf nitrogen by green manure and nitrogen rate, sweet corn, 2002.....................................................................................................179 C.3 Stem dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2002...............................................................................................................179 C.4 Root dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2002...............................................................................................................180 C.5 Ear dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2002...............................................................................................................180 C.6 Stem and root nitrogen concentrations by green manure and nitr ogen rate, sweet corn, 2002...............................................................................................................181 C.7 Ear and total nitrogen concentrations by green manure and nitrogen rate, sweet corn, 2002...............................................................................................................181

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xii C.8 Pairwise contrasts of chlorophyll meter read ings and specific leaf area, sweet corn, 2002...............................................................................................................182 C.10 Pairwise contrasts of root dry weight and nitrogen content, sweet corn, 2002......183 C.11 Pairwise contrasts of ear dry weight and nitrogen content, sweet corn, 2002.......183 C.12 Pairwise contrasts of stem and root nitrogen concentrati ons, sweet corn, 2002....184 C.13 Pairwise contrasts of ear and total nitrogen concentrations sweet corn, 2002......184 C.14 Corn applied nitrogen, unaccounted fo r applied nitrogen and chlorophyll meter readings by green manure and nitr ogen rate, sweet corn, 2003.............................185 C.15 Specific leaf area and sp ecific leaf nitrogen by green manure and nitrogen rate, sweet corn, 2003.....................................................................................................186 C.16 Stem dry weight and nitrogen conten t green manure and nitrogen rate, sweet corn, 2003...............................................................................................................186 C.17 Root dry weight and nitrogen conten t by green manure and nitrogen rate, sweet corn, 2003...............................................................................................................187 C.18 Ear dry weight and nitrogen conten t by green manure and nitrogen rate, sweet corn, 2003...............................................................................................................187 C.19 Stem nitrogen concentration and root nitrogen concentr ation by green manure and nitrogen rate, sweet corn, 2003........................................................................188 C.20 Ear nitrogen concentration and total nitrogen concentration by green manure and nitrogen rate, sweet corn, 2003........................................................................188 C.21 Pairwise contrasts of chlorophyll mete r readings and specific leaf area, sweet corn, 2003...............................................................................................................189 C.22 Pairwise contrasts of stem dry weight and nitrogen content, sweet corn, 2003.....189 C.23 Pairwise contrasts of root dry weight and nitrogen content, sweet corn, 2003......190 C.24 Pairwise contrasts of ear dry weight and nitrogen content, sweet corn, 2003.......190 C.25 Pairwise contrasts of stem nitr ogen concentration and root nitrogen concentration, sweet corn, 2003.............................................................................191 C.26 Pairwise contrasts of ear nitr ogen concentration and total nitrogen concentration, sweet corn, 2003.............................................................................191 C.27 Leaf nitrogen concentration by green manure and nitrogen rate, 2002..................192

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xiii C.28 Pairwise contrasts of leaf n itrogen concentration, sweet corn, 2002.....................193 C.29 Leaf nitrogen concentration by green manure and nitrogen rate, 2003..................193 C.30 Pairwise contrasts of leaf n itrogen concentration, sweet corn, 2003.....................194 D.1 Root length density interaction betw een depth and position with green manure and chemical nitrogen rate held constant 8 weeks after emergence, sweet corn, 2003........................................................................................................................196 D.2 Root length density interaction betwee n depth and chemical nitrogen rate with green manure and position held constant, 8 weeks after emergence, sweet corn, 2003........................................................................................................................197 D.3 Root length density interaction betw een depth and green manure with position and chemical nitrogen rate held constant 8 weeks after emergence, sweet corn, 2003........................................................................................................................197 D.4 Root length density interaction between position and chemical nitrogen rate with green manure and depth held constant, 8 weeks after emergence, sweet corn, 2003........................................................................................................................198 D.5 Root length density interaction be tween position and green manure with chemical nitrogen rate and depth held constant, 8 weeks after emergence, sweet corn, 2003.....................................................................................................198

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xiv LIST OF FIGURES Figure page 2.1 Sunn hemp dry weight and nitrogen c ontent during growth and decomposition, 2001-02.....................................................................................................................51 2.2 Leaf area index and dry weight of each green manure during growth.....................52 2.3 Ratio of S:R-N to S:R-B of sunn hemp, lupin and vetch.........................................52 2.4 Sunn hemp dry weight and nitrogen c ontent during growth and decomposition, 2002-03.....................................................................................................................53 2.5 Lupin dry weight accumulation and n itrogen content during growth, 2001-02.......53 2.6 Vetch dry weight accumulation and n itrogen content during growth, 2002-03.......53 3.1 Marketable ear yields as fres h weight by treatment, 2002 and 2003.......................83 4.1 Name, location and relative volume of root core samples.....................................115 4.2 Effect of amendment with SH+L on sa mpled sweet corn root length density.......116 4.3 Effect of amendment with SH+L on samp led sweet corn root length density by depth at 5 weeks after emergnce............................................................................116 4.4 Effect of amendment with SH+L on sa mpled sweet corn root length density by proximity class at 8 weeks after emergence...........................................................117 4.5 Soil water potential at 15 cm and 60 cm during sweet corn growth......................117 5.1 Dry matter additions by treatment (20 01-2002, A; 2002-2003, B) and average soil pH by GM over two years (C).........................................................................141 6.1 Final weed dry weights and N concentrations.......................................................162

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xv Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science IMPROVED USE OF GREE N MANURE AS A NITROGEN SOURCE FOR SWEET CORN By Corey Cherr August 2004 Chair: Johannes Scholberg Major Department: Agronomy A green manure (GM) is a crop used primar ily as a soil amendment and a nutrient source for future crops. Leguminous GMs may represent a substantia l source of on-farm nitrogen (N) also capable of increasing soil organic ma tter and suppressing weeds and parasitic nematodes. In temperate and tropi cal environments, GMs such as hairy vetch ( Vicia villosa ) and sunn hemp ( Crotalaria juncea ) have been found to accumulate 150250 kg N ha-1 while fully satisfying N-requirement s of subsequent crops. However, spring crop production with GMs in Florida re mains particularly challenging because N accumulation and subsequent GM benefits of temperate winter legumes are reduced while tropical summer legumes cannot survive freezes and may experience unacceptable levels of N-loss during winter fallow. Establ ishing a winter GM after the summer GM may significantly reduce N leaching losses during winter and retain N benefits for spring crops, but this has not been studied.

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xvi We conducted a 2-year field study to eval uate yield response of sweet corn ( Zea mays var. Rugosa) to GMs of sunn hemp and/or lupin ( Lupinus angustifolius ; winter 2001-02) and cahaba white vetch ( Vicia sativa ; winter 2002-03) and supplementation with 0, 67, or 133 kg inorganic N ha-1. Unamended (non-GM) treatments receiving 0, 67, 133, 200 or 267 kg inorganic N ha-1 were used for comparison. Growth and N analyses were conducted for all crops and decompos ition monitored for ove rwintering sunn hemp. These analyses revealed substantial grow th and N-accumulation for sunn hemp (up to 12.2 Mt ha-1 and 135 kg N ha-1), but rapid N-loss (60-66% 2-4 weeks after death) occurred when senesced leaves and upright stems decomposed separately in our reduced tillage and reduced mowing system. Winter GM growth (2-4 Mt ha-1 and 35-40 kg N ha-1) was and not enhanced by following sunn he mp. Green manures resulted in N benefit for sweet corn of ~50-70 kg N ha-1, and ear yields for corn with sunn hemp plus winter GM and 133 kg N ha-1 only were similar to unamended corn with recommended N-rate (200 kg N ha-1) in either year. Amendment with G Ms significantly increased corn root length density, but did so in the upper 15 cm soil layer and close to the plant, possibly interfering with late -season ear-fill by exposure to water and N-stress. Apparent Nrecovery was not affected by use of GMs in this system. Green manure approaches significantly increased particulate orga nic C and N pools over two years, though it remains unclear if these change s can create effective differenc es in soil organic matter. Living sunn hemp reduced end-of-season weed biomass up to 80% and suppressed lesion and stubby-root nematodes while wint er legumes had mixed affects.

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1 CHAPTER 1 INTRODUCTION Overview This chapter serves as a brief overview ty ing together the individual components of this study. Detailed introductions, literature reviews, materials and methods, results, discussions and conclusions are provided in relevant chapters. Green manure (GM) growth, nitrogen (N) accumulation, decompositi on and N-release are treated in Chapter 2. Chapter 3 evaluates effects of GMs on sweet corn growth, N-accumulation, N-status indicators, and ear yields. A sweet corn root study, conducted to complement information from growth and yield analysis is described in Chapter 4. Effects of GM approaches on soil carbon (C) and N pools are treated in Ch apter 5, with effects on weeds and plant parasitic nematodes assessed in Chapter 6. Chap ter 7 provides a review and synthesis of findings, followed by Appendices (of se lected tables) and References. Introduction Rationale The last century of American agricultur e has been characterized by a shift from highly diversified low-input systems to highl y specialized operati ons greatly depending on external non-renewable resources. Dramatic increases in farm size, along with an erosion of farm and crop diversity, have resu lted in agroecosystems more vulnerable to pressures of urbanization, climate change, a nd volatile global market s. Need exists to provide farmers with economically viable alternatives that harness ecological processes,

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2 farm and biological diversity, and on-farm resources (see Gold 1999, Dinnes et al. 2002 for discussions). With a production area of 2.3 million acres and a crop value of 3.8 billion dollars, vegetable, fruit and field crop production compri se major agricultural activities in Florida (Florida Agricultural Statistics Service 2004). Many soils in this region possess little organic matter (<1%) and exhi bit poor water and nutrient re tention (for examples, see Appendix A or Carlisle et al. 1988), especi ally those experienci ng regular disturbance (through tillage) and low input rates for organic matter. Conventional cropping systems on such soils therefore require continuous application of large amounts of external nutrients and irrigation water, yet remain vulnerable to la rge losses of these inputs. Producers, consumers, government agencies, and researchers have therefore expressed increasing interest in alternative and/or organic production systems (Gold 1999, Dinnes et al. 2002). A green manure (GM) is a crop used primar ily as a soil amendment and a nutrient source for subsequent crops. In most produc tion environments, lack of N limits plant growth more than any other nutrient. Crop pl ants effectively satisfy their N requirement only by acquisition of mineralized N (NH4 + and NO3 -). Atmospheric N (N2), though abundant, cannot be utilized by plants. Legumes, however, possess a symbiotic relationship with rhizobial bacteria capable of transforming atmospheric N2 into plantusable form and may accumulate large amounts of N via this pathway. Legumes utilized as GMs therefore represent a potentially renewable source of on-farm, biologically fixed N. Unlike chemical N fertilizers, legumes ma y also fix and add large amounts of C to a cropping system (Hargrove 1986, Sharma and Mittra 1988, Goyal et al. 1992). Legumes

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3 can also correct phosphorus (P) imbalances typi cally associated with excess applications of animal waste products because legume P leve ls are similar to those of other plants. The slow release of N from decomposing GM resi dues may be better timed with plant uptake, possibly increasing N-uptake efficiency and crop yield while reduc ing N leaching losses (Bath 2000, Wivstad 1997). Green manure appr oaches may also drive long-term increases of soil organic matter and micr obial biomass, further improving nutrient retention and N-uptake efficiency (for exampl e, see Agustin et al. 1999). When used in place of fallow, well-chosen GMs may reduce erosion and suppress weeds and specific crop pests (Ross et al. 2001, McSorley 1999). Green manures may also offer habitat or resources for beneficial organisms (Altieri and Letourneau 1982, Yeates et al. 1999, Bugg et al. 1991a,b). Presently, GM management is difficult rela tive to chemical fertilizer approaches. Nitrogen release from plant residues depends on a large number of interactive factors including chemical composition and N concentr ation, temperature, and water availability (Schomberg et al. 1994, Andren 1992). Practic al information about the composition and N concentration of GMs as they change ove r a growing season is often lacking. Most existing information on GM performance come s from studies conducted in temperate or tropical environments on fine-textured soils, the results of which may not hold in north Florida with its sub-tropical/sub-temperate cl imate and sandy soil. Especially for high Ndemanding crops, GMs may not supply adequate N if amount and timing of N-release do not match crop demand. At times, GM-amended crops may require supplementary inorganic N fertilizer to prev ent yield reductions, but little or no information exists on optimal supplementation levels. It also remain s unclear if long-term improvements in soil

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4 fertility can be achieved under Florida condi tions. Current guidelines to GM use and previous GM research have not yet devel oped management techniques necessary to produce economic yields comparable to chemi cally fertilized, high-N demanding crops in north Florida. Improved integration of GMs in such cropping systems will require more precise and detailed information about GM growth and decomposition, subsequent crop yield responses, and effects on soil a nd pests over time in specific production environments. Green Manure Management Performance of GMs varies by species, grow th environment (climate, soil, weather, pests,etc.), and management (e.g., planting da te, length of growing season,etc.). Table 1.1 summarizes the dry matter and N accumulati on of about 50 GM species from 40 studies and includes reported information about study location, soil type, and length of growing season. Green manures generally fall into two categories: tropical (“warm weather”) and temperate (“cool weather”). Few, if any, tr opical legumes can survive hard freezes (when temperature drops below –2 C for several hours), though they can usually tolerate temperatures in excess of 40 C. Temperate legumes, on the other hand, often decline at temperatures over 25 C but may persist without injury at –10 C or lower. The most widely used tropical GM legumes probably include those in genera Crotalaria (sunn hemp), Glycine (soybean), Indigofera (indigos), Mucuna (velvetbean), Vigna (cowpea), Cajanus (pigeonpea), and Sesbania while the temperate GM legumes often include Trifolium (clovers), Vicia (vetches), Medicago (alfalfa, trefoils, a nd other medics), and Lupinus (lupins). Typical non-legume temper ate GMs consist of cereal rye ( Secale cereale ), mustards ( Brassica spp), radishes ( Raphanus spp), buckwheat ( Fagopyrum esculentum ), millet ( Echinocloa spp), oats ( Avena spp), and wheat ( Triticum spp).

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5 Legume GMs are often preferable to non-legum es because they supply their own N, but in production scenarios where N is less limiting, where a specific GM service other than high N supply (such as allelopathy) is sought and/or where legumes do not perform well, non-legumes or mixtures of legumes and non-legumes may be more desirable (for example, see Karpenstein-Machan and Stue lpnagel 2000). Because they do not derive direct sales profit, GMs are often chosen that require some acceptably low level of nutrients, irrigation, and pest control and fit into otherwis e unplanted fallow periods. When biological N-fixation is not water or temperature limited, legumes are often selected as GMs due to their N-fixation cap acity. Probably because they are adapted to and grown in warmer climates with higher li ght levels, tropical legumes often accumulate biomass and N faster than wint er legumes. Genetic differences (species and variety) also may dictate that some legumes grow larg er and accumulate more N than others. Environment (temperature, soil type, nutrien t and water availability) and management (planting density and timing, mowing, pest cont rol,etc.) may further alter performance of individual GM species. For example, sunn hemp ( Crotalaria juncea ; a tropical legume) generally grows more rapidly than temperate legumes, accumulates greater biomass and N than most tropical legumes (likely becau se it is capable of growing upright and becoming quite stemmy), with reduced pe rformance on low-fertility soils and under water stress (Seneratne and Ratnasinghe 1995, Ladha et al. 1996, Mansoer et al. 1997, Jeranyama et al. 2000, Ramos et al 2001, Steinmaier and Ngoliya 2001) Climate probably limits GM selection more than any other single factor. In very cold climates, temperate legumes survive during the spring, summer, and fall. As one moves to warmer climates, increasing winter temperatures permit temperate legumes to

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6 persist during winter months while tropical legumes become better suited during warmer months. Where lowest “winter” temperatur es remain above freezing, tropical legumes may survive year-round, and high temperatures may begin to exclude use of temperate legumes. However, precipitation, soil type, and pest pressures al so interact with temperature to determine how specific GMs w ill perform in a given location. Potential N accumulation and growing season of a GM must fit a particular crop rotation. Desirability of GMs may also include or exclude ability to reseed, growth habit (upright, prostrate, viney,etc.), aggressiveness, a nd presence of toxic or allelopathic chemicals affecting livestock, crops, and/or plant pests. For exam ple, sunn hemp may be desirable preceding a fall vegetable crop because it accumulates much N, thrives in high summer temperatures, is killed easily by stem br eakage, does not become weedy by reseeding itself (at least in Florida), and may suppress parasitic nematodes. However, sunn hemp may be ill-suited to grow near trees because its height and mass make it competitive for light, water, and nutrients and it would require replanting on an annual basis. Both tropical and temperate GMs may be used in north Florida, but their production level and/or growing time is often restricted by variable temperatures and low-fertility soils. Compared to temperate environments, effective N-accumulation from temperate GM legumes in Florida may occur slowly and/or have limited potential due to high temperatures and poor adaptation to sandy soils. More producti ve summer legumes do not survive winter freezes in north Flor ida, and decomposition during this period can result in heavy losses of residue N. If GMs do not supply adequate N to meet requirements of subsequent crops, then suppl ementary inorganic N may be required to

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7 prevent yield reductions. Crops with high N-de mand planted just afte r winter therefore pose a particularly acute challenge for GM use in north Florida. During the winter, a temperate GM may ta ke up significant amounts of N from a decomposing tropical legume, possibly boosting the growth and N accumulation of the temperate legume and reducing N leaching lo sses. Overwintering residue with low N content and/or high C:N ratio may also re duce N leaching losses (Stopes et al. 1996, Wyland et al. 1996). In some systems, it ma y therefore be advantageous to follow a stemmy, vigorous summer GM with a well-adap ted winter GM, and to preserve as much recalcitrant litter as possi ble by reducing tillage. However, excessive build-up of crop residues may interfere with grow th of a number of crops; sele ction of less sensitive crops and/or periodic tillage may become important. Approach This project focused on a GM approach to production of a spring planted, high-N demanding vegetable crop – sweet corn ( Zea mays var Rugosa) – in north Florida. Based on information from the University of Florida-Institute of Food and Agricultural Science (UF-IFAS) Electronic Data Information S ource (EDIS), extension recommendations for sweet corn on sandy Florida soil s include at least 180-200 kg N ha-1 (Hochmuth and Cordasco 2000). Florida farmers generally us e chemical fertilizers to satisfy the N requirements of sweet corn. We conducted our study at the Plant Science Research and Education Unit in Citra, Florida, primarily on Candler and Lake sands (see Appendix A for soil characterization data). To help develop improved GM manage ment techniques appropriate for spring crops in north Florida, a novel approach to a GM/sweet corn cropping system was investigated. Sunn hemp was planted in late summer, grown for 12-14 weeks to optimize

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8 N content, biomass and overall recal citrance. Afterwards, blue lupin ( Lupinus angustifolius ; winter 2001-02) or cahaba white vetch ( Vicia sativa ; winter 2002-03) was planted into the standing sunn hemp residue s to capture mineralized N and/or fix additional N. Sunn hemp and lupin/vetch were also evaluated alone After mowing of all residues, sweet corn was then planted in th e spring. All crops were planted using reduced or zero-tillage. Several rates of supplemen tary inorganic N were applied to both GMamended and unamended (conventional) swee t corn. We believed the combined GM approach would supply significant amounts of N to sweet corn while also providing longterm benefits by increasing soil organic ma tter and microbial biomass and suppressing parasitic nematodes and weed production. The overall project is therefore planned to continue for at least 5 years, dependi ng on availability of external funding. Hypotheses 1. Sunn hemp stem residues would immob ilize a significant amount of N during winter decomposition (Chapter 2). 2. Growth of winter legumes following s unn hemp would be enhanced, reaching levels similar to those reported for te mperate environments (Chapter 2). 3. The double-GM approach would significan tly reduce chemical N required by sweet corn to achieve ear yields similar to an optimal level identified in the conventional approach (Chapter 3). 4. Green manures would increase N recovery rates of sweet corn (Chapter 3). 5. Amendment with GMs would increase sweet corn root length density and redistribute it nearer to th e GM residue (Chapter 4). 6. GMs would increase total soil C and N as well as specific soil C and N pools often indicative of recent organic additions including microbial biomass C and particulate organic C and N (Chapter 5). 7. Green Manures would significantly suppre ss weed biomass and plant parasitic nematode population (Chapter 6).

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9 Objectives The objectives of this research were as follows: 1. To generate detailed information about GM biomass and N accumulation by tissue fraction during growth, and subsequent decomposition and N-release by the summer GM during winter months (Chapter 2). 2. To gauge impacts of GMs on sweet corn growth, N-status in dicators, and root distribution patterns throughout the season (Chapters 3 and 4). 3. To estimate chemical N-supplementation needed to achieve acceptable sweet corn ear yields for GM approaches (Chapter 3). 4. To determine if any GM approach can pr oduce corn ear yields equivalent to the conventional approach (Chapter 3). 5. To estimate N recoveries and losses for GM and conventional approaches (Chapter 3). 6. To evaluate effects of GMs on soil prope rties, weeds, and parasitic nematodes having long-term implications for the effi cacy of the system (Chapters 5 and 6). General Set-Up and Design Table 1.2 lists the 15 overall treatments of the study, which began in August 2001 and was conducted at the Plant Science Res earch and Education Unit near Citra, FL. Candler fine sand (Typic Quarzipsamments, hyperthermic, uncoat ed; 98% sand in the upper 15 cm) and Lake fine sand (Typic Qu arzipsamments, hyperthermic, coated; 97% sand in the upper 15 cm) were the domina nt soil types (see Appendix A for detailed characterization). Study design c onsisted of four randomized complete blocks with plots 7.6 m x 8.8 m (25 ft x 30 ft). Tr eatments were composed of tw o main effects: GM level and chemical N-rate level (chemical N app lied to sweet corn only). Green manure level consisted of: a summer leguminous GM (s unn hemp) followed by a winter legume (blue lupin, winter 2001-02; cahaba white vetch, year 2002-03) denoted as SH+L; summer legume only (SH); winter legume only (L); a nd a conventional leve l with no GM (Conv). Following summer and winter, a spring crop of sweet corn wa s planted in all plots and

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10 supplemented with 0, 67, and 133 kg NH4NO3-N ha-1 for each GM level (denoted as 0N, 67N, and 133N). Conventional GM level also possessed fertilizati on rates of 200 and 267 kg NH4NO3-N ha-1 (Conv 200N and Conv 267N) representing 3/3 and 4/3 the chemical N-rate recommended for sweet co rn in Florida by UF-IFAS ex tension. A final treatment of complete fallow (Fal) receiving chemical w eed control (identical to other treatments) but no GM, sweet corn, chemical-N, or ti llage was included for comparison purposes. More detailed materials and methods are found in relevant chapters (see Overview, above). Measurements Measurements taken for all crops were: pl ant numbers at beginning of season; leaf area and leaf, stem, root, and reproduc tive (flowers/pods/ear s) fresh and dry weights taken every 2-4 weeks and at fina l samplings, as well as N concentration and content of all tissues. Measurements taken for sweet corn were: leaf chlorophyll read ings every 2 weeks and at final samplings, ear number and ear gr ade at final samplings, and root length density for selected plots at selected dates in 2003. Measurements taken for soil (0-25 cm) were: Total Soil C and N, Particulate Organic C and N, Microbial Biomass C, and pH at selected dates. Continuously taken measurements were: Pr ecipitation, air temperature at 1m, soil temperature at a depth of 5 cm, and re lative humidity using Watchdog Dataloggers (Spectrum Technologies; Plainf ield, IL; see Appendix B). Measurements of nematodes were: soil counts for selected plots at selected dates. Measurements taken for weeds were: to tal dry weight, N concentration and N content in all plots at en d of sunn hemp and vetch.

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11Table 1.1. Review of green manure studies. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Oats A vena sativa Dyck & Liebman 1995 3.3-4.3 80-82 Sandy loam, Maine 3 months Azolla A zolla microphylla Ladha et al. 2000 2.1-2.3 61-75 Silty Clay, Philippines 6-9 weeks; flooded Colza B rassica campestris N 'Dayegamiye & Tran 2001 2.1-4.6 59-99 Silt loam, Canada 4 months; 30 kg N ha-1 applied Mustard B rassica hirta N 'Dayegamiye & Tran 2001 2.3-3.8 62-72 Silt loam, Canada 4 months; 30 kg N ha-1 applied Pigeonpea Cajanus cajan Ladha et al. 1996 6.5-9.0 154-235 Clay or loam?, Philippines ~6 months; clipped to 20-30 cm 5 times Canavalia Canavalia ensiformis Ramos et al. 2001 4.4 58 Sandy loam, Cuba 8-9 weeks Centro Centrosema pubescens Steinmaier & Ngoliya 2001 1.2 27 Sandy loam, Zambia 14 weeks (?) Rhodes Grass Chloris gayana Steinmaier & Ngoliya 2001 14 167 Sandy loam, Zambia 14 weeks (?) Clitoria Clitoria ternatea Ladha et al. 1996 6.9-7.7 256-306 Clay or loam?, Philippines ~6 months; clipped to 20-30 cm 2-3 times Sunn Hemp Crotalaria juncea Jeranyama et al. 2000 0.9-2.9 23-82 Loamy sand, Zimbabwe 6-7 weeks Ladha et al. 1996 7.6-7.8 277-279 Clay or loam?, Philippines ~6 months; clipped to 20-30 cm 2-3 times Mansoer et al. 1997 4.8-7.3 120-138 Sandy loam, Alabama 9-12 weeks Ramos et al. 2001 11.1 195 Sandy loam, Cuba 8-9 weeks Seneratne & Ratnasinghe 1995 6.1-9.6 161-252 N R, Sri Lanka 8-9 weeks Steinmaier & Ngoliya 2001 12.1 227 Sandy loam, Zambia 14 weeks (?) Crotalaria Crotalaria ochroleuca Carsky et al. 1999 5.0 (13 wks), 8.0 (19 wks) 114 (13 wks), 137 (19 wks) Loamy sand, Nigeria 13 and 19 weeks; AAR = 1350 mm Carsky et al. 1999 2.0 (13 wks), 3.3 (19 wks) 52 (13 wks), 63 (19 wks) Clay loam, Nigeria 13 and 19 weeks; AAR = 900 mm AAR = average annual rainfall.

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12Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Sunn Hemp Marejea Crotalaria zanzibarica cv Marejea Steinmaier & Ngoliya 2001 14.6 328 Sandy loam, Zambia 14 weeks (?) Desmanthus D esmanthus virgatus Ladha et al. 1996 8.0-9.1 251-283 Clay or loam?, Philippines ~6 months Millet E chinochloa crus galli N 'Dayegamiye & Tran 2001 2.3-11.2 65-139 Silt loam, Canada 4 months; 30 kg N ha-1 applied Buckwheat F agopyrum esculentum N 'Dayegamiye & Tran 2001 2.1-3.7 52-65 Silt loam, Canada 4 months; 30 kg N ha-1 applied Soybean Glycine max Thonnissen et al. 2000a 2.8-5.8 106-141 N R, Taiwan & Philippines 2-2.5 months Indigo I ndigofera tinctoria Agustin et al. 1999 2.3-2.9 56-57 Clay loam, Philippines 5-6 months after death of other intercrops Thonnissen et al. 2000a 0.2-2.0 5-44 N R, Taiwan & Philippines 2-2.5 months Lablab L ablab purpureus / D olichos lablab Carsky et al. 1999 1.9 (13 wks), 2.0 (19 wks) 71 (13wks), 47 (19wks) Loamy sand, Nigeria 13 and 19 weeks; AAR = 1350 mm Carsky et al. 1999 0.6 (13 wks), 1.8 (19 wks) 23 (13 wks), 49 (19wks) Clay loam, Nigeria 13 and 19 weeks; AAR = 900 mm Kouyate et al. 2000 0.7-1.7 N R Loamy sand, Mali N R; AAR = 619 mm Kouyate et al. 2000 0.6-2.0 N R Loam, Mali N R; AAR = 619 m m Steinmaier & Ngoliya 2001 5.8 115 Sandy loam, Zambia 14 weeks (?) Black Lentil L ens culinaris Brandt 1999 2.3-2.7 53-64 Loam, Saskatchewan N R; AAR = 359 m m Guldan et al. 1996 1.0-2.2 34-58 Sandy loam, New Mexico ~22 weeks; interseeded in sweet corn after 2 weeks Guldan et al. 1996 0.9-1.1 34-35 Sandy loam, New Mexico ~17 weels; interseeded in sweet corn after 7 weeks Rye Grass L olium multiflorum Dapaah & Vyn 1998 1.3-2.5 N R Loam, Ontario 7 months; intercropped with barley Stopes et al. 1996 0.7-17.5* 15-346* Clay loam, England *6-25 months of growth; periodic mowing AAR = average annual rainfall; NR = not reported.

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13Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Blue Lupin L upinus angustifolius Forbes 1970 5.3-6.7 N R N R (Sandy loam?), Tifton, GA N R Gallaher 1991 ~1.0 ~20 Sand, Gainesville 24 weeks; 25 plants m-2 Gallaher 1991 ~1.8 ~30-35 Sand, Gainesville 24 weeks; 50 plants m-2 Gallaher 1991 2.1 36 Sand, Gainesville 24 weeks; 100 plants m-2 Suman (in Forbes 1970) 2.8-3.1 N R N R, South Carolina N R Siratro M acroptilium atropurpureum Ladha et al. 1996 4.9-5.5 132-178 Clay or loam(?), Philippines ~6 months Steinmaier & Ngoliya 2001 2.4 62 Sandy loam, Zambia 14 weeks (?) Trefoil M edicago lupulina Stopes et al. 1996 0.6-20.4* 15-459* Clay loam, England *6-25 months of growth; periodic mowing Burr Medic M edicago polymorpha Shresthra et al. 1999 1.1 (C), 1.6 (N) N R Loam, Michigan 90 days; cut for forage at 60 days (C) or not (N) Burr&Snail Medics M edicago polymorpha M. scutellata Jeranyama et al. 1998 0.6-3.1 17-75 Loam, Michigan 9-11 weeks; 5 planting dates Jeranyama et al. 1998 0.1-1.3 2-32 Loam, Michigan 9-11 weeks; 5 planting dates, intercropped with corn Gamma Medic M edicago rugosa Shresthra et al. 1999 1.4 N R Loam, Michigan 13 weeks Alfalfa M edicago sativa Griffin, et al. 2000 3.7-5.7 105-174 Silt loam, Maine 1 year Guldan et al. 1996 1.1-1.5 41-53 Sandy loam, New Mexico ~22 weeks; interseeded in sweet corn after 2 weeks Guldan et al. 1996 0.5-1.2 21-43 Sandy loam, New Mexico ~17 weeks; interseeded in sweet corn after 7 weeks Shresthra et al. 1999 1.6 N R Loam, Michigan 13 weeks Singogo et al. 1996 2.8-5.7 107-138 Sandy loam, Kansas 7-8 months NR = not reported.

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14Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Barrel Medic M edicago truncatula Guldan et al. 1996 2.4-4.5 72-131 Sandy loam, New Mexico ~22 weeks; interseeded in sweet corn after 2 weeks Guldan et al. 1996 1.0-2.3 37-69 Sandy loam, New Mexico ~17 weeks; interseeded in sweet corn after 7 weeks Shresthra et al. 1999 1.4 (C), 3.2 (N) N R Loam, Michigan 13 weeks; cut for forage at 60 days (C) or not (N) Yellow Sweet Clover M elilotus officianalis Blackshaw et al. 2001b 3.1-5.4 N R Sandy clay loam, Alberta N R; AAR = 387 mm; multiple intercrops Mucuna M ucuna aterrima Ramos et al. 2001 2.1 64 Sandy loam, Cuba 8-9 weeks Velvet Bean M ucuna pruriens / M. atropurpuriem / M deeringiana Carsky et al. 1999 4.0 (13 wks), 6.2 (19 wks) 131(13 wks), 154 (19 wks) Loamy sand, Nigeria 13 and 19 weeks; AAR = 1350 mm Carsky et al. 1999 1.7 (13 wks), 3.4 (19 wks) 53 (13 wks), 85 (19 wks) Clay loam, Nigeria 13 and 19 weeks; AAR = 900 mm Steinmaier & Ngoliya 2001 9.3 183 Sandy loam, Zambia 14 weeks (?) Glycine N eonotonia wightii Steinmaier & Ngoliya 2001 0.9 21 Sandy loam, Zambia 14 weeks (?) Winter/Field Pea P isum sativum Karpenstein-Machan and Stuelpnagel 2000 ~4.8 ~200 Silty clay, Germany ~ 4 months Soon et al. 2001 1.1-3.5 (stover) 8.2-28.3 (stover) Sandy loam, Alberta N R; pea-wheat-canola-wheat rotation Winter Pea + Rye P isum sativum + Secale cereale Karpenstein-Machan and Stuelpnagel 2000 ~6-12 ~200 Silty clay, Germany ~ 4 months; 3 different seeding mixtures Winter Pea (Austrian) P isum sativum subsp arvense Singogo et al. 1996 3.2-7.6 107-230 Sandy loam, Kansas 7-8 months Oilseed radish R aphanus sativus Dapaah & Vyn 1998 2.5-3.5 N R Loam, Ontario 3 months; intercropped with barley Rye Secale cereale Cline & Silvernail 2001, 2002 3.5-4.0 (0N), 4.0-9.0 (140N) 28-43 (0N), 43-64 (140N) Silt loam, Kentucky 8 months; 0 or 140 kg N ha-1 for preceeding corn crop Griffin, et al. 2000 4.1-6.6 52-66 Silt loam, Maine 9 months AAR = average annual rainfall; NR = not reported.

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15Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Rye Secale cereale Karpenstein-Machan and Stuelpnagel 2000 ~9-13.5 N R Silty clay, Germany ~4 months Ranells and Wagger 1996 1.5-5.7 17-64 Loamy sand, Georgia 6 months Ross et al. 2001 2.7-3.4 (n), 6.4 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 0.5-0.6 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Tollenaar et al. 1993 1.0-6.1 N R Loam, Ontario 8 months; 4 rye cultivars Sesbania Sesbania macrantha Steinmaier & Ngoliya 2001 7.1 124 Sandy loam, Zambia 14 weeks (?) Sesbania Sesbania rostrata Kouyate et al. 2000 0.7-1.4 N R Loamy sand, Mali N R; AAR = 619 m m Kouyate et al. 2000 2.3-4.6 N R Loam, Mali N R; AAR = 619 m m Sesbania Sesbania rostrata Ladha et al. 2000 3.2-4.6 71-88 Silty clay, Philippines 6-9 weeks; flooded Stylo Stylosanthes guianensis Steinmaier & Ngoliya 2001 4.3 88 Sandy loam, Zambia 14 weeks (?) Teramnus Teramnus uncinatus Steinmaier & Ngoliya 2001 3.8 80 Sandy loam, Zambia 14 weeks (?) Berseem Clover Trifolium alexandrinum Ross et al. 2001 6.7-10.2 (n), 9.2 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 4.0-6.0 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Shresthra et al. 1999 1.9 (C), 4.1 (N) N R Loam, Michigan 13 weeks; cut for forage at 60 days (C) or not (N) Kura Clover Trifolium ambiguum Zemenchik et al. 2000 6.2-10.7 N R Silt loam, Wisconsin N R; intercropped with corn, then grown alone Alsike Clover Trifolium hybridum Ross et al. 2001 3.0-4.6 (n), 6.1 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 2.5-2.7 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) AAR = average annual rainfall; NR = not reported.

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16Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Crimson Clover Trifolium incarnatum Abdul-Baki et al. 1996 4.2-5.7 151 Sandy Loam, Maryland 8 months Dyck & Liebman 1995 5.8-7.3 130-143 Sandy loam, Maine 3.5 months Dyck et al. 1995 4.8-5.1 117-123 Sandy loam and silt loam, Maine 2-2.5 months Karpenstein-Machan and Stuelpnagel 2000 ~4-10.5 ~200 Silty clay, Germany ~4 months Ranells and Wagger 1996 1.4-5.0 35-134 Loamy sand, Georgia 6 months Crimson Clover Trifolium incarnatum Ross et al. 2001 2.1-4.0 (n), 5.7 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 3.7-5.1 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Crimson Clover + Rye Trifolium incarnatum + S. cereale Karpenstein-Machan and Stuelpnagel 2000 ~6-12 ~200 Silty clay, Germany ~4 months; 3 different seeding mixtures Ranells and Wagger 1996 2.3-5.2 42-111 Loamy sand, Georgia 6 months Balansa Clover Trifolium michelianum var balansae Ross et al. 2001 2.5-4.5 (n), 7.2 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 2.3-3.5 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Red Clover Trifolium pratense Dapaah & Vyn 1998 2.4-3.7 N R Loam, Ontario 10 months; intercropped with barley Davis & Liebman 2001 1.5-3.0 72-115 Sandy loam, Maine N R; intercropped with whea t Guldan et al. 1996 0.8-1.9 29-49 Sandy loam, New Mexico 22 weeks; interseeded in sweet corn after 2 weeks Guldan et al. 1996 0.3-0.6 13-16 Sandy loam, New Mexico 17 weeks; interseeded in sweet corn after 7 weeks N ’Dayegamiye & Tran 2001 0.6-0.7 13 Silt loam, Canada 4 months; 30 kg N ha-1 applied Ross et al. 2001 1.7-2.9 (n), 5.2 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) NR = not reported.

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17Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Ross et al. 2001 2.1-2.2 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Soon et al. 2001 1.7-3.6 51-94 Sandy loam, Alberta N R; red clove r -wheat-canola-wheat rotation Stopes et al. 1996 0.8-25.4* 21-741* Clay loam, England *6-25 months of growth; periodic mowing White Clover Trifolium repens Ross et al. 2001 0.8-2.1 (n), 4.0 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 2.7-3.0(n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Stopes et al. 1996 0.6-25.0* 17-592* Clay loam, England *6-25 months of growth; periodic mowing Persian Clover Trifolium resupinatum Ross et al. 2001 1.7-3.4 (n), 7.2 (m) N R Silty clay loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Ross et al. 2001 3.7-4.6 (n) N R Loam, Alberta 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n) Wheat Triticum aestivum Singogo et al. 1996 4.9-9.8 81-87 Sandy loam, Kansas 7-8 months; Hairy Vetch Vicia villosa Abdul-Baki et al. 1996 4.4-5.2 167-197 Sandy Loam, Maryland 8 months Cline & Silvernail 2001, 2002 3.5-4.0 (0N 140N) 115-164 (0N 140N) Silt loam, Kentucky 8 months; 0 or 140 kg N ha-1 for preceeding corn crop Guldan et al. 1996 1.8-3.8 70-124 Sandy loam, New Mexico 22 weeks; interseeded in sweet corn after 2 weeks Guldan et al. 1996 1.5-2.8 58-88 Sandy loam, New Mexico 17 weeks; interseeded in sweet corn after 7 weeks Puget & Drinkwater 2001 4.4 N R Silt loam, Pennsylvania ~ 8 months Ranells and Wagger 1996 2.9-4.8 125-182 Loamy sand, Georgia 6 months Sainju & Singh 2001 3.0-6.7 104-257 Sandy loam, Georgia ~ 6 months; 3 tillage types, 2 kill dates Singogo et al. 1996 5.6-8.9 233-247 Sandy loam, Kansas 7-8 months NR = not reported.

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18Table 1.1. Continued. Green Manure Study Dry Weight (Mt ha-1) N Content (kg ha-1) Environment Growth Time; Management Notes Hairy Vetch + Rye Vicia villosa + Secale cereale Abdul-Baki et al. 1996 5.9 120-162 Sandy Loam, Maryland 8 months Cline & Silvernail 2001,2002 4.0 (0N), 4.0-10.0 (0N) 104-152 (0N), 141-149 (140N) Silt loam, Kentucky 8 months; 0 or 140 kg N ha-1 for preceeding corn crop Griffin, et al. 2000 3.6-6.9 57-209 Silt loam, Maine 9 months Ranells and Wagger 1996 3.0-5.4 82-200 Loamy sand, Georgia 6 months Black Gram Vigna mungo Seneratne & Ratnasinghe 1995 7.1-8.8 (stover) 104-155 (stover) N R, Sri Lanka 11-12 weeks Mung bean Vigna radiata Seneratne & Ratnasinghe 1995 3.1-5.5 (stover) 30-88 (stover) N R, Sri Lanka 11-12 weeks; 2 cultivars Thonnissen et al. 2000a 1.1 26 NR, Taiwan & Philippines 9-11 weeks Cowpea Vigna unguiculata Carsky et al. 1999 0.6 (13 and 19 wks) 16 (13 wks), 21 (19 wks) Loamy sand, Nigeria 13 and 19 weeks; AAR = 1350 mm Carsky et al. 1999 1.4 (13 wks), 2.3 (19 wks) 45 (13 wks), 58 (19wks) Clay loam, Nigeria 13 and 19 weeks; AAR = 900 mm Jeranyama et al. 2000 0.6-4.6 15-154 Loamy sand, Zimbabwe 11 weeks Kouyate et al. 2000 1.5-2.5 N R Loamy sand, Mali N R; AAR = 619 mm Kouyate et al. 2000 1.1-2.4 N R Loam, Mali N R; AAR = 619 m m Seneratne & Ratnasinghe 1995 3.7-8.5 42-155 N R, Sri Lanka 11-12 weeks; 3 cultivars AAR = average annual rainfall; NR = not reported.

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19 Table 1.2. Overview of experimental treatments. Treatment Crop 1 Crop 2 Crop 3 July-November N ovembe r -April April-July (1) SH+L 0N Sunn hemp Lupin(Year1) or Vetch(Year2) Sweet Corn + 0 N (2) SH+L 67N Sunn hemp Lupin(Year1) or Vetch(Year2) Sweet Corn + 67 N (3) SH+L 133N Sunn hemp Lupin(Year1) or Vetch(Year2) Sweet Corn + 133 N (4) SH 0N Sunn hemp Fallow Sweet Corn + 0 N (5) SH 67N Sunn hemp Fallow Sweet Corn + 67 N (6) SH 133N Sunn hemp Fallow Sweet Corn + 133 N (7) L 0N Fallow Lupin(Year1) or Vetch(Year2) Sweet Corn + 0 N (8) L 67N Fallow Lupin(Year1) or Vetch(Year2) Sweet Corn + 67 N (9) L 133N Fallow Lupin(Year1) or Vetch(Year2) Sweet Corn + 133 N (10) Conv 0N Fallow Fallow Sweet Corn + 0 N (11) Conv 67N Fallow Fallow Sweet Corn + 67 N (12) Conv 133N Fallow Fallow Sweet Corn + 133 N (13) Conv 200N Fallow Fallow Sweet Corn + 200 N (14) Conv 267N Fallow Fallow Sweet Corn + 267 N (15) Fal Fallow Fallow Fallow SH = sunn hemp; L = winter legume; Conv = conventional; Fal = Complete fallow; N = kg NH4NO3-N ha-1.

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20 CHAPTER 2 GREEN MANURE GROWTH AND DECOMPOSITION Introduction and Literature Review Due to coarse texture, high temperatures and high rainfall, many Florida soils contain little organic matte r (less than 1-2%) and posse ss poor water and nutrient retention. This is especially true for agricu ltural soils that experi ence regular tillage and low carbon (C) input rates. Legumes utilized as green manures may be useful as a component of sustainability in such producti on environments. A green manure (GM) is a crop used primarily as a soil amendment and a nutrient source for future crops. Legumes may add nitrogen (N) to the system th rough biological fixation and can correct phosphorus (P) imbalances typically associat ed with excess applications of animal manures. The slow release of N from deco mposing GM residues may be better timed with plant uptake (Bath 2000, Wivstad 1997). Un like chemical fertilizers, legumes may fix and add large amounts of C to a cropping system (Hargrove 1986, Sharma and Mittra 1988, Goyal et al. 1992) and may drive long-te rm increases of soil organic matter and microbial biomas (Goyal et al. 1992, 1999). Gr een manures may provide other benefits such as reduction of soil er osion, conservation of soil water, improved retention of other crop nutrients, and control of plant pests, pa thogens and weeds with less reliance on offfarm chemical inputs (Bugg et al. 1991a McSorley 1999, Ross et al. 2001). Effective use of GMs is often hampered by lack of precise information about N availability for future crops. Nitrogen accumulation and subsequent release from decomposing GMs depends largely on re sidue composition a nd N concentration,

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21 temperature, water availabili ty, and residue management (S chomberg et al. 1994, Andren 1992), which in turn depend on GM species, si te environment (climate, soil, weather, etc.), and cropping system. Table 1.1 outlines the dry matter and N accumulation of about 50 GM species from 40 studies. While not exha ustive, this review probably represents a major cross section of recent GM studies, most of which took place in temperate (and high latitude) or tropical (equatorial) regions on relatively fine-textured soils. Results of such studies may not extend to intermediate regions such as Florida. In temperate regions, the cooler and often le ss variable temperature regime s with longer daylight hours may be more conducive to temperate legumes. Compared to tropical regions, much of Florida experiences winter fr eezes that kill warm weather an nuals used as both GMs and many of the crops that follow them. Sandy so ils and unique pest problems also make establishment more challenging, especially for temperate GMs such as clovers and medics. Additionally, pertinent information about composition and N concentration of GMs as they change over a growing season is of ten lacking. Almost all studies report end-ofseason GM biomass and N content/concentratio n only. For example, of the peer-reviewed literature sampled in Table 1.1, only Karp enstein-Machan and Stuelpnagel (2000) provide growth analysis of investigated G Ms. This poses an obstacle to GM adoption because growing time in an on-farm producti on system differs from that studied in research, creating yet another way GM biom ass and composition in an on-farm setting may differ from reported findings. Biological N-fixation and overall N-accu mulation rates are primary factors governing the adequacy of a GM as an N-source. Estimates of N accumulation for

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22 leguminous GMs and the relativ e contribution of biological N fixation in this process ranges broadly depending on soil fertility, wate r availability, and GM species. Generally speaking, legumes will take up what N is avai lable from the soil, but thereafter will accumulate N from biological fixation to meet demand. For example, sunn hemp ( Crotalaria juncea ) has been estimated to fix 27-39% (Ramos et al. 2001), 72-81% (Ladha et al. 1996), and 91% (Sen aratne and Ratnasinghe 1995) of its total N in different study locations and conditions. Water stress and deficiency of nutrients other than N may significantly reduce fixation, either directly or through reduced availability of assimilates from photosynthesis. Although they did not re port rainfall or temperature conditions, Ladha et al. (1996) attributed a 9% differentia l in relative contributi on of fixed N in sunn hemp over two years to differences in weat her patterns that dir ectly affected plant growth. Reduction of soil N through compe tition may increase rates of biological Nfixation. Karpenstein-Machan and Stuelpnagel (2000) found that relati ve contribution of N-fixation to hairy vetch ( Vicia villosa ) and crimson clover ( Trifolium incarnatum ) N increased when intercropped with an increa singly larger propor tion of cereal rye ( Secale cereale ). Nitrogen contributions from below-ground tis sues of GMs (roots, root nodules) is difficult to determine due to rapid turnover of these tissues and possible root exudation of N. For example, Ramos et al. (2001) determ ined that 39-49% of all N accumulated by Canavalia ensiformis and Mucuna aterrima GMs was belowground, and 10-12% of all accumulated N transferred to the soil by root and nodule turnover and root exudation. In a 3-year study, Griffin et al (2000) reported 56%, 46%, a nd 38% of biomass and 32%, 28%, and 19% of total N in roots at final sampling for alfalfa ( Medicago sativa ), cereal

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23 rye, and hairy vetch plus ry e intercrop. However, both of these studies occurred on fine textured and/or high fertility soils. Soil-based residue decomposition and N-rele ase generally occur faster for residues with lower C:N ratios and li gnin and polyphenol contents, w ith optimum temperature and water availability usually around 35 C and field capacity, re spectively (Andren et al. 1992, Lomander et al. 1998, Vigil and Kissel 19 95). Mathematically, these investigators often characterize decomposition as a negative exponential dec line in residue biomass or C over time, with the rate affected by a “decay rate constant” that may depend on temperature, water availability, N availability, and chemical quality of the residue. Investigators may gain greater accuracy by st atistically resolving decomposition into two “pools” with faster and slower decay rate constants. For example, Somda et al. (date unknown) used a litterbag study of a numb er of legumes and non-legumes; C:N and lignin:N ratios were generally lower for le gumes (8:1-27:1, and 2:1-9:1, respectively) than for non-legumes (27:1-186:1, and 4:1-44: 1, respectively) as were decay rate constants of both fast and slow pools. Kuo and Sainju (1997) showed mixing hairy vetch residue with increasingly large propor tions of cereal rye and rye grass ( Lolium multiflorum ) residues slowed the relative rate of N release. Working in Georgia, Ranells and Wagger (1996) also found faster decom position and N-release for hairy vetch and crimson clover grown alone than when grown with cereal rye, but still found no net Nimmobilization in any treatment (including rye alone). For materials with low lignin:N, C:N may control decomposition, while lignin:N ratio may become more important as it becomes higher. Decomposition of mixed materials over time may ther efore involve control by C:N initially, then lignin:N as

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24 recalcitrant material makes up more of the remainder (Mueller et al. 1998). Palm and Sanchez (1991) also found that polyphenol concentration may exert more control over breakdown rates than lignin and N concentra tions for residues high in polyphenols. In a review of previously published data from 11 studies, Seneviratne (2000) found that: when N availability is limiting (typically, plant resi dues with N concentra tions less than 2%) a positive linear relationship existed between N-release and N concentration (r2 = 0.63); when N concentration was non-limiting (typical ly, residues with N concentration greater than 2%) N concentration does not affect N -release; C:N ratio was a good predictor of N release over a wide range of N contents; and polyphenol content better predicted Nmineralization than lignin:N in low N residues in tropical environments. Leaf C:N ratio and lignin content is generally much lower than stems or roots of the same plant, and in most studies leaf d ecomposition and N release occurs significantly faster than for other tissues. Prolong ed periods of N-immobilization (when decomposition results in a net accumulation of N) are often recorded for recalcitrant stems and roots (Collins et al. 1990, Cobo et al. 2002). Cobo et al. (2002) characterized decomposition, N, P, K, Ca, and Mg release, as well as C, lignin, polyphenol, and cell wall contents of leaves and stems of about one dozen different tropical legumes. On average leaves decomposed five times fast er than stems, decomposition was closely related to cell wall content, and N release most dependent on lignin:N ratio. Cobo et al. (2002) found that decomposition and N-release was faster for stems mixed with leaves than for stems alone, and slower for leaves mixed with stems than leaves alone. Both Cobo et al. (2002) and Collins et al. (1990) s howed the decomposition rate of different tissue types decomposing together was fast er than predicted by summing individual

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25 decomposition rates. These studies suggest f ungal decomposers may redistribute N from leaves to more recalcitrant tissues during decomposition. Puget and Drinkwater (2001) used 13C to compare the decomposition of root and shoot derived C in hairy vetch residues on a silt loam in Pennsylvania. In their study, shoot and root biomass for vetch was 3.71 and 0.89 Mt ha-1 respectively, lignin:N six times higher in roots, and non-structural car bohydrates (eg, sugars a nd starches) was four times higher in shoots. Twenty-two weeks af ter soil-incorporation, 13% of shoot C and 49% of root C remained in the soil, ma king the overall mass of shoot and root contributions to soil organic C relatively equal at this time. Soil incorporation of plant residues ma y speed decomposition and N release by buffering temperature and water regimes relativ e to the surface. Hargrove et al. (date unkown), Schomberg et al. ( 1994), and Thonissen et al. ( 2000) showed more rapid decomposition of soil incorporated residues vs surface residues in no-till systems. Schomberg et al. (1994) also found greater N-immobilization potential for surface sorghum ( Sorghum bicolor ) and wheat ( Triticum aestivum ) residue, although initial Nimmobilization was more rapid when the resi dues were buried. At peak immobilization (5 months to 1 year or more), highly reca lcitrant (sorghum and wh eat) residues tied up 150-170% of their initial N content. For thes e low-N residues net N immobilization lasted longer than one year on soil surface (study e nded after 1 year) and only 1/3 year for buried residues. Nitrogen immobilization ende d and release began only when 45-55% of the residue mass had decomposed. Bowen et al. (1993) found that 60-80% of N applied within 10 legume GMs was released as i norganic-N within 120-150 days after soil incorporation, while Thonnissen et al. (2000b) found similar levels of N-release to take

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26 place faster (within 2-6 weeks) for soybean ( Glycine max ) and vetch. In Alabama, however, Mansoer et al. (1997) found less than 50% of N remaining in surface and soilincorporated sunn hemp, respectivel y, at 16 weeks after plant death.. Most studies reviewed here found best correlation with tw o-pool exponential models for decomposition and N-release (see Katterer et al. 1998 for a review). More complex decomposition/N-release models exist that make use of residue quality, soil, and weather data to predict decomposition, including the CERES and CENTURY models used by DSSAT (Decision Support System for Agrotechnology Transfer, see Jones et al. 2003). These and similar models can often be adjusted to more accurately reflect actual decomposition and N-release data obtained in field experiments (Bowen et al. 1993, Hadas et al. 1993, Quemada et al. 1997). As part of a larger study on improved use of GMs in vegetable cropping systems in Florida, we investigated a GM sequence of sunn hemp (SH) followed by a winter legume (L) of blue lupin ( Lupinus angustifolius winter 2001-02) and cahaba white vetch ( Vicia sativa winter 2002-03) as an N-source for sweet corn ( Zea mays var Rugosa). We hypothesized that sunn hemp stem residues wo uld immobilize a significant amount of N during winter decomposition, and that growth of winter legumes following sunn hemp would be enhanced, reaching levels more typi cally seen in temperate environments. The objectives of this particular study component were to generate deta iled information about GM biomass and N accumulation by tissue fraction during growt h, and subsequent decomposition and N-release by th e summer GM over the winter.

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27 Materials and Methods Set-up and Design This study consisted of nine of the 15 overa ll treatments related in Chapter 1 (Table 1.2). Only treatments with GM components in the rotation were investigated here: sunn hemp followed by lupin (winter 2001-02) or ve tch (winter 2002-03; treatment denoted as SH+L), sunn hemp followed by fallow (SH), fa llow followed by lupin or vetch (L). Only methods relevant to GM growth, N-accu mulation, decomposition and N-release are presented in this chapter. Me thods regarding GM effects on sweet corn growth analysis, root dynamics, and yield, and effects on soil pr operties and plant pests are discussed in relevant chapters. Timeline of Operations 2001-02 On 7 August 2001, sunn hemp was planted following complete disking and plowing of the field. Seed was inoculated w ith cowpea-type rhizobium and planted at 2-4 cm depth. In-row spacing was 3.12 cm (1.25 in), between-row spacing was 76 cm (30 in). Sunn hemp emerged 11 August 2001 and grew until 31 October 2001 when it was killed with an application of Gramoxone (Syngenta; Basel, Switzerland). Lupin was inoculated with lupin-type rhizobium and planted on 19 November 2001 using a rip-strip planter and with spacing identical to sunn hemp. Lupi n emerged 22 November 2001 and grew until 12 April 2002. All plots were then mown and field treated with RoundUp (RoundUp; Columbus, OH). Sweet corn (variety GS 0966, Syngenta) was planted 26 April 2002 using a rip-strip planter, with in-row spacing of 18 cm and between-row spacing of 75 cm.

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28 2002-03 On 19 July 2002, inoculated sunn hemp was pl anted with a rip-strip planter at the same spacing and depth as year 1, emergi ng 21 July 2002 and growing until 30 October 2002 when it was killed with Diuron/Touchst one. Cahaba white vetch was inoculated with vetch-type rhizobium and planted 15 N ovember 2002 with a zero-till grain-drill at a rate of ~35 kg ha-1 (30 lbs acre-1). Sweet corn (variety GS 0966, Syngenta) was directly planted into vetch on 7 April 2003. Measurements 2001-02 Sunn hemp was sampled from 8 of the 24 plots every two weeks after emergence (WAE), and at final sampling all plots were sampled. Sunn hemp was also sampled at 4, 6, 10, 12, and 16 weeks after death (WAD). D ecomposition was therefore quantified for undisturbed material (not drie d). Due to poor stand establishment, lupin was sampled from 6 of 24 plots at 4, 8, 12, and 16 WAE, and at final sampling (20 WAE) all plots were sampled. In each sampled plot, 61 cm (2 ft) of row length representative of the entire plot and with uniform emergence wa s removed at each sampling, brought to the UF Environmental Agronomy Lab (University of Florida, Gainesville, FL), refrigerated before processing (no longer than one week). Entire plants were removed including roots. In Gainesville, heights for all sampled plants were recorded, with plants subsequently separated into leaves, stems, roots, and re productive tissues (flowers and pods, where existing). Roots were washed clean of soil and debris. Total sample leaf number and area and leaf, stem, root, and reproductive (flowers /pods) fresh weights were taken for each sample. Leaf area was determined with an LI-3000 (Li-cor; Linc oln, NE). Dry weights were recorded for subsamples after oven-drying at 65 C for 72 hours. Afterwards, all

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29 subsamples were ground in a Wiley mill to pass through a 2 mm screen, and a thoroughly mixed 5 g portion of each grinding was subs equently stored. Grindings were then subjected to a wet-acid Kjeldahl digesti on, diluted, filtered, and analyzed for total Kjeldahl N at the UF-IFAS Analytical Resear ch Lab (University of Florida, Gainesville, FL; EPA Method 351.2; Jones and Case 1991). For each sample, shoot to root ratio of biomass (S:R-B) was calculated as the sum of above ground dry matter divided by root dry matter (kg kg-1), and shoot to root ratio of N (S:R-N) was calculated as shoot N c ontent divided by root N content (kg kg-1). Specific leaf area (SLA) was calculated as cm2 leaf g-1 leaf dry weight, and specific leaf N (SLN) was calculated as g N cm-2 leaf. Leaf area index (LAI) wa s determined by sample leaf area divided by sampled area (sampled row length x between row space; m2 leaf m-2 ground). 2002-03 Sunn hemp was sampled from all 24 plot s at 2, 6, 10, and 14 WAE, and also from treatments SH 0N and SH 133N at 4, 8, and 12 WAE. Sunn hemp residue was sampled at 2, 4, 6, 8, 11, 14, and 18 weeks after death WAD. Vetch was sampled from all 24 plots every 3 weeks after emergence. Row length sampled remained 61 cm. When plants became large, all but 1-3 plants were clippe d at ground level, weighed, and returned to the plot. The subsample of 1-3 representati ve plants was excavated and taken to the Environmental Agronomy Lab for measuremen t of the same growth parameters as described for the previous year, with iden tical grinding and N analysis. Throughout both years, continuous measurements of solar radi ation, air temperature and relative humidity at 1 m, rainfall/irrigation, and soil temperature at 12.5 cm were made using a Watchdog datalogger (Spectrum Tec hnologies; Plainfield, IL).

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30 Analysis Numerical trends and figures were developed using MS Excel. Using SAS statistical software package (Statistical Anal ysis Systems; Cary, NC), a general linear model was developed for final sampling data to assess the possibility of significant differences due to GM combinations, sweet corn chemical N-fertilization rate, interaction between GM type and N-rate, and replicati on effects for all measurements. Green manure type, chemical N-rate, and the interacti on of the two were insignificant at the = 0.05 level of significance in either year. Results are therefore presented as averages of all sampled treatments. Results Sunn Hemp 2001 Growth As the initial crop of the experiment, all pl ots had identical histories (same previous crops, same fertilization levels), and th roughout the season no si gnificant differences existed for any factor of sunn hemp growth due to main or sub-effects. Figure 2.1(a) illustrates the accumulation and subsequent decomposition of sunn hemp biomass by tissue type for 2001. Figure 2.1(b) shows accumu lation and subsequent loss of sunn hemp N by tissue type. Sunn hemp produced a total of 8.00 0.40 Mt ha-1 and 76 4 kg N ha-1 by final sampling at 12 WAE. Of this, 6.95 0.37 Mt ha-1 (87%) and 72 4 kg N ha-1 (94%) was above ground. Maximum LAI of 3.59 0.25 occurring 10 weeks after emergence (WAE) (Figure 2.2a). Average da ily maximum temperatures remained around 36 C during August and September of 2001 and around 33 C during October of 2001. In terms of biomass, leaves accounted fo r the largest tissue fraction in samplings during the first 4 WAE (58% and 51% of to tal biomass at 2 and 4 WAE). By 6 WAE

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31 stems became the largest single tissue fr action, accounting for over half of total dry weight by 8 WAE. Stem and r oot dry weight increased th roughout the entire growth season reaching final values of 4.76 0.26 and 1.05 0.06 Mt ha-1 respectively; leaf biomass increased for 10 weeks to a maximum value of 1.50 0.07 Mt ha-1. Flowers did not appear until 8 WAE and increased to 0.76 0.05 Mt ha-1 by 12 WAE (Figure 2.1a). Leaves and flowers possessed relatively hi gh N concentrations that changed little throughout the season (20.1-21.8 g N kg-1 and 24.1-29.0 g N kg-1 respectively). Stems and roots had much lower N concentrations that tended to decrease as a negative exponential throughout the s eason (from 12.0 to 5.0 g N kg-1 and from 8.5 to 4.6 g N kg-1 respectively, with r2 = 0.93 and 0.97). Total N concen tration showed an exponential decay over time from 16.2 to 10.0 g N kg-1, reflecting increasing c ontribution from stems and roots (Table 2.1). Leaves formed the largest N pool of any tissue throughout the growing season, reaching a maximum at 33 2 kg N ha-1 at 8 WAE. Leaves and flowers (when flowers existed) together accounted for most sunn hemp N throughout the season, beginning at 76% (2WAE) and decreasing down to 62% at final sampling (12 WAE). Flower N was maximum at final sampling (18 1 kg N ha-1). In terms of N cont ent and as proportion of total plant N content, stem N increased thr oughout the season reaching final values of 25 1 kg N ha-1 and 32%, respectively. Except at first sampling, roots typically formed the smallest N pool (about 6-8% of total N), reaching a final N content of only 5 <1 kg N ha-1 (Figure 2.1b). Shoot to root biomass ratio (S:R-B) increas ed linearly (r2 = 0.92) from 2.7 0.3 kg kg-1 to 6.8 0.3 kg kg-1 over the 12 week growth season. Shoot to root N ratio (S:R-N)

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32 also increased linearly (r2 = 0.95) from 6.2 0.6 kg kg-1 to 17.5 2.3 kg kg-1 (Table 2.2). Both changes reflected the increasing amounts of biomass and N sent to shoots relative to roots. The ratio of S:R-N to S:R-B remained almost unchanged throughout the season at an average of 2.36 0.13 (Figure 2.3a), show ing that partitioning of N and biomass to roots and shoots remained consistent relati ve to each other. Sp ecific leaf area (SLA) varied between 209 and 297 cm2 g-1 over the season and was somewhat described by a polynomial behavior (r2 = 0.60), showing a maximum at 4-6 WAE. Because leaf N concentration changed very little over the se ason, specific leaf N (SLN) was basically a mirror image of SLA, varying from 74 to 97 g N cm-2 leaf, showing a minimum at 4-6 WAE and also being somewhat described by a polynomial (r2 = 0.50; Table 2.2). Decomposition Residue decomposition (loss of dry matte r) and N-loss were greatest during the first two weeks after death, after which re ductions were much less rapid or even nondetectable (Figure 2.1). Total plant decompos ition and N-loss were 40% 9% and 61% 7%, respectively, at 2 weeks after death (2 WAD). Final residue dry weight at 16 WAD was 52% 14% of the original 8.00 Mt ha-1, but after the initia l sharp drop at 2 WAD there was no little change in dry weight. Final residue N content at 16 WAD was only 20% 6% of the original 61 kg N ha-1, and although residue N-loss was also slow and not always resolvable between sample dates fi nal, N content was significantly lower than N content at 2 WAD (Figure 2.1b). Most rapid decomposition and N-loss occurr ed for leaves and flowers (which were pooled together as they were too difficult to separate). Leaf and flower (combined) dry weight and N content at 2 WAD were only 25% 3% and 15% 2%, respectively, of the original amount before death. Rate of loss ma y have been inflated because two herbicide

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33 applications were required to kill sunn hemp over a two-week period, during which leaves generally died before stems and root s. However, no significant leaf and flower material persisted by 12 WAD to sample. R oots also showed an initial flush of decomposition, with only 50% 8% and 25% 8% of dry weight and N content, respectively, remaining at 2 WAD (Figures 2.1 and 2.2). Root N concentration declined from 3.7 0.2 g N kg-1 at death to 2.2 0.1 g N kg-1 at 2WAD and 2.0 0.3 g N kg-1 at 4 WAD (Table 2.3). Afterwards, roots showed little dry weight decomposition until final sampling at 16 WAD when only 0.28 0.06 Mt ha-1 (26% 6% of original) remained (Figure 2.1). At the same time, roots showed a consistent trend towards Nimmobilization, with N concentration rebounding steadily to 3.0 g N kg-1 by final sampling (16 WAD; Table 2.3) and root N c ontent increasing back up to 40% 11% of the original at 12 WAD, though this trend am ounted to immobilization of no more than 2 kg N ha-1 at that time (with a total ro ot N content of 2 < 1 kg N ha-1; Figure 2.2). By final sampling at 16 WAD, root N c ontent decreased to 1 < 1 kg N ha-1 (28% 6% of the original; Figure 2.1b). Stem decomposition and N-loss occurred more slowly than all other tissue types, maintaining 77% 14% and 89% 19% of original dry weight and N content, respectively, at 2 WAD. Stem dry weight remained stable after 2 WAD with little changes, with 3.86 1.02 Mt ha-1 (81% 22% of original ) remaining at 16 WAD. However, sample variability was quite high throughout the decomposition period (Figure 2.1). Average stem N content also showed lit tle change after 2 WAD, though there was a general decrease and by 12 WAD stem N content (14 3 kg N ha-1) was significantly less than original. Stem N content at final sampling (11 3 kg N ha-1) was 56% 15% of

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34 original (Figure 2.1b). Stem N concentra tion after 2 WAD also declined steadily, reaching 3.0 0.1 g N kg-1 at 16 WAD (see Table 2.3). Sunn hemp 2002 Growth Figure 2.4a illustrates the accumulation and subsequent decomposition of sunn hemp biomass by tissue type for 2002, and Figure 2.4b shows accumulation and subsequent loss of sunn hemp N by tissue type. Sunn hemp produced a total of 12.26 0.38 Mt ha-1 and 134 5 kg N ha-1 by final sampling at 14 WAE, exceeding 2001 final production by 53% and 106% respective ly (Figure 2.2b). Of 2002 production, 11.12 0.35 Mt ha-1 (91% of total) and 127 5 kg N ha-1 (95% of total) was above ground. Maximum LAI of 6.07 0.28 occurred at 10 WAE (Figure 2.2a). Average daily maximum temperatures remained around 36 C from planting until final sampling throughout the 2002 season. Although production was increased, dry ma tter partitioning among tissue fractions was almost identical to 2001. Leaf and stem production in 2002 exceeded 2001 by 6 WAE. Leaves accounted for the largest tissu e fraction in samplings during the first 4 WAE (59% and 53% of total biomass at 2 and 4 WAE). By 6 WAE stems became the largest single tissue fraction, accounting for ove r half of total biom ass by 8 WAE. Stem and root biomass increased throughout the enti re growth season reaching final values of 8.76 0.30 and 1.14 0.05 Mt ha-1 respectively; leaf biomass increased for 12 weeks to a maximum value of 1.94 0.15 Mt ha-1, although changes in leaf biomass after 10 WAE were not significant. Flowers did not a ppear until 10 WAE and increased to 0.61 0.05 Mt ha-1 by 12 WAE (Figure 2.4a).

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35 Total N concentration and content was much higher in 2002, perhaps reflecting increased water availability (Figure 2.4b, Ta ble 2.4). However, partitioning patterns remained similar. Leaves and flowers ag ain had relatively hi gh N concentration compared to stems and roots, although drops oc curred in leaf N concentration just before flower appearance (10 WAE) a nd in flower N concentration at final sampling (probably due to a contribution from pods, which were pooled with flowers). Stems and roots again had lower N concentrations which tended to decrease as a ne gative exponential throughout the season (from 15 to 6 g N kg-1 and from 23 to 4 g N kg-1, respectively, with r2 = 0.90 and 0.94, respectively). Total N c oncentration also showed a negative exponential trend decreasing ove r time from 30 to 12 g N kg-1 (Table 2.4). Leaves again formed the largest N pool of any tissue throughout the growing season, reaching a maximum at 63 3 kg N ha-1 at 10 WAE. Leaves and flowers (when flowers existed) together accounted fo r most sunn hemp N throughout the season, beginning at 75% (2 WAE) and decreasing down to 57% at final sampling (14 WAE). Flower N-content was maximum at final sampling (15 1 kg N ha-1). In terms of N content and as proportion of total plant N content, stem N increased throughout the season reaching final values of 50 2 kg N ha-1 and 37%, respectively. Except at first sampling, roots again formed the smallest N pool (about 4-9% of total N), reaching a final N content of only 7 1 kg N ha-1 (Figure 2.4). Biomass based shoot to root ra tio (S:R-B) increased linearly (r2 = 0.99) from 3.2 0.2 kg kg-1 to 10.2 0.4 kg kg-1 over the 14 week season. Except at final harvest (when a drop occurred), shoot to root N ra tio S:R-N also increased linearly (r2 = 0.96) from 4.53 0.31 kg kg-1 to 36.0 1.9 kg kg-1 (Table 2.5). The ratio of S: R-N to S:R-B did not remain

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36 constant as in 2001, but increased linearly (r2 = 0.89) except at final harvest when a drop occurred. This ratio increased from 1.43 0.04 to 3.90 0.15 from 2 to 12 WAE (Figure 2.3a), showing that partitioni ng to shoot biomass did not keep pace with partitioning to shoot N (relative to root N). Specific leaf area and specifi c leaf N behaved similarly in 2002 as in 2001, but were greater than in 2001 by an averag e 26% and 69% over the season, respectively. Specific leaf area varied between 256 and 355 cm2 g-1 over the season and was somewhat described by a polynomial behavior (r2 = 0.62), showing a maximum at 6 WAE (Table 2.5). As in 2001, leaf N concentration changed very little over the season, and specific leaf N (SLN) was again a mirror image of SLA, varying from 105 to 135 g N cm-2 leaf, showing a minimum at 10 WAE and also being somewhat described by a polynomial (r2 = 0.72; Table 2.5). Decomposition Although residue decomposition and N-loss we re again greatest during the first 2 WAD, residue decomposition proceeded more slowly and was less dramatic during this time. Total plant decomposition and Nloss were 24% 10% and 66% 3%, respectively, at 2 weeks afte r death (2 WAD; Figure 2.4). Fi nal residue dry weight at 16 WAD was 6.91 1.10 Mt ha-1 (56% 9% of the original 12.26 Mt ha-1), but there was almost no significant change after 4 WAD (Fi gure 2.4a). Final residue N content at 16 WAD was only 21 kg N ha-1 (16% 3% of the original 134 kg N ha-1), and, like decomposition, overall residue N-loss was vi rtually complete after only 4 WAD (Figure 2.4b). Most rapid decomposition and N-loss again o ccurred for leaves and flowers, but the dynamics differed from 2001, with initial decomposition occurring more slowly (64%

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37 8% and 46% 9% of original remaining at 2 and 4 WAD) and complete decomposition occurring rapidly between 4 and 6 WAD (4-5 weeks earlier than in 2001; Figure 2.4a). Nitrogen loss occurred more quickly than did decomposition, with only 28% 3% and 15% 3% of original leaf and flower N remaining at 2 and 4 WAD (Figure 2.4b). Initial root N loss also proceeded more quickly than decomposition, with 34% 3% and 86% 12% of N content and dry we ight, respectively, remaining at 2 WAD (Figure 2.4). Root N concentra tion declined from 6.1 0.3 g N kg-1 at death to 3.3 0.2 g N kg-1 at 2WAD and 1.6 0.2 g N kg-1 at 6WAD (Table 2.6). After 6 WAD, roots showed little dry weight decomposition th rough final sampling at 16 WAD (61% 16% remaining). As in 2001, roots eventually showed a consistent trend towards Nimmobilization, with N concentration rebounding at 8 WAD (3.3 0.3 g N kg-1) and remaining at 2.2-2.5 g N kg-1 through final sampling (16 WAD). However, the increase was generally not large enough to increase root N content in the face of decomposition (Table 2.6). Stem decomposition and N-loss again occurr ed more slowly than all other tissue types, maintaining 82% 12% and 54% 7% of dry weight and N content, respectively, at 2 WAD (Figure 2.4). Beginni ng at 6 WAD, stem N concen tration remained relatively unchanged at 0.25-0.30%, with only one sample da te falling out of th is range (Table 2.6). Stem dry weight remained stable after 6 WAD, with 6.21 1.06 Mt ha-1 (71% 12% of original) remaining at 16 WAD, although sa mple variability was high throughout the decomposition period (Figure 2.4a). Except fo r a single outlying date, stem N content also remained relatively stable af ter 4 WAD, between 16 and 20 kg N ha-1 (Figure 2.4b).

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38 Stems made up roughly 90% of total sunn hemp residue dry weight and N content after 8 WAD, with roots making up the remainder. Lupin 2001-2002 Due to poor establishment, only treatments with lupin alone (L) were sampled until the final sampling date (20 WAE) when all lupin plots (L and SH+L) were sampled. At this sample date, a general linear model was developed to assess sign ificance of previous sunn hemp presence on growth factors. Duncan comparisons were made at the = 0.05 level of significance, and effect of sunn he mp presence was non-significant. Lupin results are therefore presented as average of all treatments. Figure 2.5 illustrates the accumulation of lupin biomass and N, respectively, by tissue type for 2001-02. Averaged over all tr eatments, lupin produced a total of 4.03 0.18 Mt ha-1 and 53 6 kg N ha-1 by final sampling at 20 WAE. Of this, 3.52 Mt ha-1 and 47 kg N ha-1 (87% and 90%) was above ground. Maximum LAI of 1.50 0.09 also occurred at the final sampli ng at 20 WAE (Figure 2.2a). Roots accounted for the largest tissue fracti on at 4 WAE (44% of total), thereafter leaves (40% and 47% of total dry matter at 8 and 12 WAE) followe d by stems (46% and 56% of total dry matter at 16 and 21 WAE) became dominant (Figure 2.5a). Stem and leaf dry matter increased thr oughout the entire growth season reaching final values of 2.25 0.12 and 1.21 0.03 Mt ha-1 respectively, although the incr ease in leaf dry matter from 16 to 20 WAE was small. Root dr y matter reached a maximum 0.60 0.13 Mt ha-1 at 16 WAE. Pods did not appear until fina l sampling at 20 WAE when they accounted for only 0.06 0.01 Mt ha-1. Large increases in biomass occurred between 8-12 and 12-16 WAE, with total dry weight more than qua drupling between each sampling and occurring

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39 simultaneously with heavy root nodulation. Ho wever, biomass and N accumulation up to 12 WAE was relatively low (0.78 Mt ha-1 and 10 kg N ha-1; Figure 2.5). Leaves possessed relatively high N concentr ation, increasing logistically from 15.2 to 22.6 g N kg-1 by 16 WAE (Table 2.7). And although lupin stems and roots generally had lower N concentrations, stem N concentration increased linearly (r2 = 0.91) from 5.5 to 7.5 g N kg-1 over the season, while root N concen tration increased exponentially from 3.8 g N kg-1 at 4 WAE to a peak of 17.1 g N kg-1 at 16 WAE, then dropping to 11.0 g N kg-1 at 20 WAE. Total N concentr ation increased from 7.7 g N kg-1 (4 WAE) to 14.2 v (16 WAE), with a small drop at final sampli ng (20 WAE; Table 2.7). These trends in N concentration probably reflected the large in creases in root nodulation seen around midseason, followed by a general die-off of root nodules by final sampling. Leaves formed the largest N pool of any tissue throughout the growing season, reaching a maximum at 27 1 kg N ha-1 by 16 WAE and representing 52-68% of total plant N throughout the season (Fig ure 2.5b). Root N content as a fraction of total plant N showed an initial decrease from 4 to 8 WA E (22% to 11%) followed by an increase from 8 to 16 WAE (11% to 22%) and a final drop back to 11% from 16-20 WAE (Table 2.9); root biomass also decreased from 16 to 20 WAE (Figure 2.5b). Again, root trends were probably related to increased root nodulation at mid-seas on, with late-season nodule die off and pod production responsible for the drop in N concentration of other tissues. As fraction of total plant N, stems showed th e opposite trend (increase from 20% to 32% from beginning to end of season, with a dr op to 17% at 12 WAE). Maximum root and stem N contents were 11 3 and 17 3 kg N ha-1 at 16 and 20 WAE respectively. Roots

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40 generally formed the smallest N pool (about 11-22% of total N), although this was greater than the relative N pool of sunn hemp roots (Figure 2.5b). As in sunn hemp, S:R-B increased linearly (r2 = 0.93) from 1.6 to 7.2 kg kg-1 over the 20 week growing season (Table 2.8). Howe ver, S:R-N saw periods of early and late increase (4-8 WAE and 16-20WAE) around a period of linear d ecrease (8-16 WAE), probably reflecting nodulation patterns. The overall range for lupin S:R-N was 3.7 to 10.8 kg kg-1. As a result, ratio of S:RN to S:R-B shows a linear (r2 = 0.96) decrease for the first 16 weeks, followed by an increase from 16-20WAE. This ratio showed a range of 0.8 to 2.8 (Figure 2.3b), which was similar to that seen for sunn hemp in 2002 but not 2001 (Figure 2.3a). Specific leaf area (SLA) va ried between 154 and 103 cm2 g-1 over the season, the pattern of change being well describe d by a negative polynomial function (r2 = 0.99) with the lowest measurement made at 16 WAE. Specific leaf N (SLN) increased linearly (r2 = 0.99) for the first 16 WAE, followed by a small drop at 20WAE (probably due to pod formation), varying from 98 to 219 g N cm-2 throughout the season (Table 2.8). Vetch 2002-2003 Due to variable stand performance, results for vetch are presented for the best 10 plots (of 24 total) beginning at 12 WAE unl ess otherwise noted. These 10 plots showed dry weight production at or above 1.00 Mt ha-1 by final sampling. Because these 10 plots were evenly distributed across all GM and N-rate levels without apparent trend, and because vetch growth was so variable, previ ous GM plantings and sweet corn chemical N applications produced no signifi cant differences by final sampling. Figure 2.6 shows the accumulation of vetch dr y weight and N cont ent, respectively, by tissue type for 2002-03. Averaged over the be st 10 plots, vetch produced a total of

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41 1.95 0.25 Mt ha-1 and 37 6 kg N ha-1 by final sampling at 18 WAE. Of this, 1.52 Mt ha-1 and 34 kg N ha-1 (78% and 93%) was above ground. Maximum LAI of 1.02 0.13 also occurred at the final sampling at 18 WA E (Figure 2.2a). Two plots with excellent performance produced over 3.0 Mt ha-1 and 60 kg N ha-1 at final sampling. Roots accounted for the largest tissue fraction for the first 9 WAE (41-52% of total), thereafter leaves (44% at 12 WAE) followed by stem s (45% and 43% at 15 and 18 WAE) became the largest tissue fraction. St em, leaf and root biomass in creased throughout the entire growth season reaching final values of 0.85 0.13, 0.68 0.11 and 0.43 0.05 Mt ha-1, respectively at final sampling (Figure 2.6a). L eaf N concentration remained low (22-27 g N kg-1) until a linear (r2 = 0.97) increase began afte r 9 WAE, bringing leaf N concentration to 36 g N kg-1 at 18 WAE. Stem N concentration (11-17 g N kg-1) remained relatively constant, while root N concentration decr eased linearly (r2 = 0.96). Total N concentration remained constant between 16-20 g N kg-1 (Table 2.9). Except at first sampling, leaves formed the largest N pool of any tissue throughout the growing season, reaching a maximum at 24 3 kg N ha-1 at 18 WAE and representing 37-65% of total plant N thr oughout the season (Figure 2.6b). Stem N content increased throughout the season reaching a ma ximum at final sampling of 12 2 kg N ha-1. Stems accounted for an increasing fracti on of total plant N over time (17% at 3 WAE to 34% at 18 WAE). Root N content as a fraction of total plant N decreased throughout the season, but was marked by an initi al period of relative importance for the first 9 WAE (30-46% of total plant N) followed by a large drop (6-8% of total plant N) as shoot growth increased. Maximum root N content reached only 3 1 kg N ha-1 (18 WAE; Figure 2.6b).

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42 Neither S:R-B nor S:R-N experienced much change during the first 6-9 weeks of the season (values ranging between 0.9-1.4 kg kg-1 and 1.2-2.3 kg kg-1, respectively; Table 2.10). However, from 6-9 WAE until final sampling both indices increased logarithmically (r2 = 0.92 and 0.99, respectively) to 4.8 kg kg-1 (S:R-B) and 19.9 kg kg-1 (S:R-N). As a result, ratio of S:R-N to S:R-B showed a linear (r2 = 0.98) increase at during this period, going from 1.1 to 4.1 by final sampling (Figure 2.3b). Vetch SLA varied between 141 and 314 cm2 g-1 over the season, the pattern of change being well described by a negative logarithmic function (r2 = 0.94) beginning at 9 WAE. Vetch SLA was generally higher than lu pin SLA and comparable to that of sunn hemp. Vetch SLN was highly variable, show ing no overall trend and ranging from 87.4 to 326 g N cm-2 over the season (Table 2.10), likely re lated to variable performance. Discussion Sunn Hemp Growth Sunn hemp appeared quite well adap ted to the sandy soils and hot summer temperatures of north Florida with rapid nodulation and little damage from pests or disease until the end of the second year Biomass in both years (8.00 and 12.26 Mt ha-1; Figues 2.1 and 2.5) and N accumulation in 2002 (134 kg ha-1; Figure 2.6) was higher than that achieved by Mansoer et al. (1997; 5-6 Mt ha-1 and up to 120 kg N ha-1) in Alabama, but similar to findings by Seneratne and Ratn asinghe (1995) and Steinmaier and Ngoliya (2001) under tropical conditions. Ni trogen accumulation in 2001 (76 kg N ha-1; Figure 2.6) appears similar to that found by Jera nyama et al. (2000) under low precipitation conditions. Sunn hemp also produced greater dry matter than that of other summer legumes including cowpea evaluated in anot her on-going study at the same site

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43 (including cowpea ( Vigna unguiculata ), hairy indigo ( Indigofera hirsuta ), and velvet bean ( Mucuna atropurpureum ); Linares and Scholberg, unpu blished), although its size and stemmy nature (reaching 2.6 m in 2002) may make it inappropriate for some forms of intercropping, agroforestry, or plastic mulch systems if sunn hemp is allowed to grow past 4-8 weeks. Increased biomass and N accumulation in 2002 compared to 2001 may have resulted from both longer growing season as well as deeper root systems and improved water availability following mechanical “r ipping” of a plow-p an after 2001. Although production remained excellent, water stress during the 2001 appeared to reduce sunn hemp biomass, LAI (through reductions in bot h leaf dry matter and SLA) as well as N concentration and SLN while decreasing bot h S:R-B and S:R-N (Tables 2.1, 2.2, 2.4, and 2.5). Sunn hemp dry weight in 2002 exceeded th at in 2001 at similar sample dates by 6 WAE. Sunn hemp is reportedly capable of becoming extremely large (up to 20 Mt ha-1), and it does appear that sunn he mp was able to take advant age of the extra two weeks from earlier planting in 2002. Final sunn hemp dry weight and N accumulation was 53% and 76% greater, respectively, in 200 2 than in 2001 (Figures 2.1 and 2.4). In both years, leaf material dominated early growth of sunn hemp (>50% of total plant dry weight for the first 4 WAE; Figures 2.1a and 2.4a) and stems retained relatively high-N through 4-6 WAE (Tables 2.1 and 2.4). L eaves and flowers made up the largest N pool throughout the growing season (57-76% of total plant N) and decomposed most quickly after death (6-12 WAD; Figures 2.1b a nd 2.4b). However, large decreases in total plant N concentration occurre d after 6-8 WAE when stems began to dominate biomass (up to 71% in 2002) and stem N concentrati on became relatively low. Root contributions

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44 to biomass and especially total plant N were relatively low and decreased with growth – sometimes to less than 10% and 3% of tota l biomass and N, respectively. Such root contributions were lower than that found by Gr iffin et al. (2000) working with temperate GMs in a cooler climate (Maine) with fine textured soil (silt loam). Relative root contributions were higher (S:R-B and S:R-N were lower) during 2001, when water availability was apparently diminished. A lthough root turnover or exudation was not accounted for, the extremely consistent linear increases seen in S: R-B and S:R-N over the season and the small root biomass and N-cont ent appear to be in line with findings by Thonnissen et al. (2000a) in a tr opical environment. Because it occurred so consistently in different years, this shoot-dominated growth behavior appe ars genetic, although differences in water availability show capab ility of exerting some changes (Tables 2.2 and 2.5). Differences between years also occurred in slope of the S:R-N to S:R-B ratio over time, with slope being nearly zero in 2001 but distinctively positive in 2002 (Figure 2.3). This suggests that water stress in 2001 also created a situation where shoot growth was more N-limited. In 2001, increases in shoot N partitioning were associated with a consistent biomass partitioning response to shoots. In 2002, with higher N concentration and higher S:R-N, increases in shoot N part itioning were not “kept up with” by similar increases in biomass partitioning to shoots, s uggesting biomass partitioning to shoots was decreasingly N-limited in 2002. It therefore appears that both longer growth time and gr eater water av ailability strongly increased N accumulation and dry we ight accumulation, but that the relative sizes of tissue pools were mo re strongly affected by growing time while water

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45 availability may have exerted more contro l over tissue N concen trations and leaf characteristics (SLA and SLN). Decomposition Previous studies on surface applied resi due and mixed residues of different recalcitrance show more rapid N-mineraliza tion when overall plan t N-concentration is high (C:N < 20 or %N > 2 %), with low-N residue (C :N > 30-40) exhibiting Nimmobilization and even immobilizing N fr om nearby, high-N sources (Kuo and Sainju 1997, Ranells and Wagger 1996, Schomberg et al. 1994, Collins et al. 1990, Mansoer et al. 1997) However, in our study we found that leaves and flowers decomposed rapidly while low-N stems showed net N-release at all times, rather than N-immobilization as expected (Figures 2.1b and 2.4b). The spatial separation between stems (which remained somewhat upright or raised above the soil surface during decompos ition) and leaves and flowers (which decomposed primarily on the ground) in our reduced tillage and reduced mowing approach may have prevented movement of N from areas of high availability to low availability. The initial N flush exhibited by stems may reflect decomposition of the relatively succulent stem-tips which others (Marshall 2002) have shown to possess high N-concentration. Lack of homogenization ma y have also prevented movement of N between these stem fractions. On the other hand, root dry weight stabili zed and slight N-imm obilization occurred beginning at about 8 WAD in both years. The initial flush in root decomposition and Nrelease was probably due to decompos ition of finer roots and nodules. Net Nimmobilization probably occurred when only th e more recalcitrant large roots remained, and also because availability of N within the soil was likely much greater than on the surface. However, the relatively small pool of root biomass could not immobilize more

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46 than 2 kg N ha-1. Residue N losses from sunn hemp totaled 60% and 66% of initial N in the first 2-4 WAD in 2001-02 and 2002-03, resp ectively. Final residue N-losses were 49 and 123 kg N ha-1 (80% and 84%) in 16 weeks of 2001-02 and in 18 weeks of 2002-03, respectively (Figures 2.2 and 2.6). Dry wei ght decomposition losses (44-48% in 16-18 WAD) were much less than those of N, but al so demonstrated most losses during the first 2-4 WAD. While leaves and flowers decompos ed rapidly, stem dry weight loss at all dates after 2-4 WAD remained within one standard error of each other, making decomposition unresolvable. Root dry matter loss was almost as slow as that of stems (Figures 2.1a and 2.4a). Mansoer et al. (1997) hom ogenized sunn hemp residue by mowing but experienced similar levels of N-loss over the winter. Because it may help buffer water and temperature in a decomposing litter layer, it is unclear if mowing as a means of homogenization would lead to net N-immobili zation in our environment. Had sunn hemp had much greater root production, one could al so speculate greater immobilization might occur based on our results. Given the findings of numerous other investigators (for example, Schomberg et al. 1994 and Thonnisse n et al. 2000b), it appears that soil incorporation would unacceptably intensify long-term N-loss of overwintering residue; Mansoer et al. (1997) found mu ch greater N-loss from so il-incorporating sunn hemp residue in Alabama during the winter. A lthough the high N-losses from overwintering sunn hemp residue were contrary to our ma nagement goals, our findings suggest this reduced-tillage and reduced -mowing system may provide a “double” benefit if sunn hemp (or another legume with similar growth habit) is followed immediately by one or more economic crops. In this way, sunn hemp could provide quickly available N from

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47 decomposing leaves and flowers; at a later da te, the recalcitrant natu re of left-over sunn hemp stems (once mowed or pushed dow n to the ground) may immobilize surface applied N, improving synchrony of N-release from chemical or animal manure sources (see Chapter 3). On the other hand, sunn hemp accumulated more than half of its N between 6 and 8 WAE in both study years, wh ich was also around the time when leaves and stems were equally dominant in terms of dry weight. If moderate N supply with less residue is desired, our results s uggest that killing sunn hemp b ack at this time would yield 30-70 kg N ha-1 and 3-7 Mt dry matter ha-1 (Figures 2.1 and 2.4) depending on growing conditions. Sunn hemp is a crop which may be easily ki lled without pesticide by using a roller, which kills the plants by breaking their stem s. Our experience and the experience of others suggests sunn hemp may be planted in to directly (“live mulc hed”) prior to cold weather. Tractor tires tend to “roll over” many of the rows and open up the canopy for a new crop. At the onset of freezing temperatures, the rest of the sunn hemp will die. This method of planting may make more effici ent use of sunn hemp N by delaying much decomposition until another crop is alrea dy established beneath the sunn hemp, eliminating “gap” time that occurs betw een “wholesale” death of sunn hemp (from herbicide or mowing) and subsequent planting and growth of another crop. Lupin and Vetch As cool-weather legumes, blue lupin and cahaba white vetch behaved quite differently than sunn hemp. Their growth appeared controlled by time required for effective nodulation to begin (when gr owth and N-accumulation increased) and subsequent time until reproduction and rising te mperatures (when total plant growth and N-accumulation slowed and root nodules died off). These legumes may require longer

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48 periods of cool temperature combined with longer daylength duri ng fall and spring than found in north Florida. Unseasonably warm weather (lupin, 2001-02) and poor adaptation to sandy soil (vetch, 2002-03) may also have reduced performance of these legumes, but as these conditions are typical in north Florid a this may also point out general weaknesses of cool-weather legumes in the region. L upin and vetch still accumulated 4.03 and 1.95 Mt ha-1 dry weight and 53 and 39 kg N ha-1, respectively, similar to other results from Florida (Gallaher 1991) and Ne w Mexico (Guldan et al. 199 6; see Figures 2.5 and 2.6). However, results were highly variable and mu ch lower than findings in temperate regions with finer soils and longer growing times (Forbes et al. 1970, Abdul-Baki et al. 1996, Cline and Silvernail 2001 and 2002, Puget a nd Drinkwater 2001, Ranells and Wagger 1996, Sainju and Singh 2001, Singogo et al. 1996). That neither lupin nor vetch were significantly affected by presence of sunn hemp residue probably reflec ts the heavy initial N-loss from sunn hemp, but also indicates that sunn hemp ha d no apparent allelopathic effect on either crop as well. In our study, linear growth phase of these sp ecies probably initiated far too late (8-9 WAE) for them to significantly reduce N losses from sunn hemp residue, and any N benefit from sunn hemp residue early in the season became insignificant by final samplings. Even in lupin, biomass and N pr oduction up to 12 WAE was relatively low (0.78 Mt ha-1 and 10 kg N ha-1). However, although these species are not always as productive as some summer legumes, they may still provide a significant source of N. As a monocrop, lupin appeared better adapted to our environment than vetch. Because its performance was generally poor, cahaba white vetch was terminated at 18 weeks. Only 13 of 24 plots produced greater than 1 Mt ha-1, and at final sampling plants exhibited

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49 severe root decay (likely caused by nematode s) and nutrient deficiency. However, some plots of cahaba white vetch produced over 3.0 Mt ha-1 and 60 kg N ha-1, closer to reported production of 3-9 Mt ha-1 elsewhere but still quite lower than potential N accumulation (100-250 kg N ha-1; see Table 1.1). Performance of lupin and vetch were quite variable. Nearby trials of other wint er legumes including l upin, vetch, and clovers indicate lupin may be the most productiv e as a monocrop (Linares and Scholberg, unpublished), and the seemingly low growth of lupin and (the better plots of) vetch may simply be near potential for winter legumes in north Florida. Results from continuation of this study with a mixture of hairy vetch a nd cereal rye, and anecdotal evidence from 4way mixtures of vetch, rye, crimson clover and radish ( Raphanus sativus ) in the same field suggest combinations of legumes, gr asses, and/or non-leguminous dicots may provide for more uniform and productive wint er GMs in our area (Lavila and Scholberg, unpublished). Compared to sunn hemp, lupin and vetch ro ots accounted for a greater fraction of total biomass in the first 49 weeks (41-52%), but thereafte r the emphasis of growth on leaves followed by stems was similar (Figur es 2.1 and 2.4-2.6). Unlike sunn hemp, total N concentration increased (lupin) or remained relatively cons tant (vetch) over the season despite increases in stem tissue fraction, prim arily because stem production was relatively low (never more than 56% for lupin a nd 45% for vetch) and exhibited higher N concentration than sunn hemp. Allowing these winter legumes to become more “stemmy” did not lead to an apparent increase in their recalcitrance, although lupin and vetch differed in their N concentration. Individual ti ssue and total N concentration of lupin was relatively low and was comparable to that found in sunn hemp duri ng the dry summer of

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50 2001. Vetch leaf and whole plant N concentr ations were higher than lupin and comparable to sunn hemp in the rainier summer of 2002 (Tables 2.1, 2.4, 2.7 and 2.9). Total plant and root biomass, N-concentrati on and N-content for vetch and lupin showed exponential increases followed by a leveling o ff or decline near the end of the season, apparently following patterns in nodule in itiation and nodule death around the onset of reproduction (Figures 2.6 and 2.7, Tables 2.7 and 2.9). Because they die back at the onset of warm weather and/or reproduction, and because they are not extremely large, cool-w eather legumes in north Florida may be good candidates for live mulch during spring. Mowing, st rip tillage, strip herb icide, or tractor traffic may be used to create openings for a spring crop planted into a cool-weather legume. As mentioned earlier, this may re duce N losses from decomposition by reduction of “gap time” between the two crops. In our experience, sweet corn strip tilled into vetch suffered no adverse effects, and others (Pha tak et al. 1999) have shown good results for cotton no-till planted into live clover. Mi xtures of leguminous GMs with non-legumes capable of earlier growth and better “N-scav enging” may be desirable. Rye, oats, or mustards may be well suited for such mixt ures (for example, Abdul-Baki et al. 1996, Cline & Silvernail 2001 and 2002, Griffi n et al. 2000, Ranells and Wagger 1996, Karpenstein-Machan & Stuelpna gel 2000). Our preliminary expe rience with mixtures of multiple winter GMs shows great promise and should be investigated more, as performance of any one GM (especially le gumes) may be variable in north Florida winters. These mixtures may also be more appropriate for systems requiring lower growing, less stemmy, and/or less aggressive GMs than sunn hemp.

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51 Conclusions Results of all crops from both years high light the dynamic nature of legume cover crops. Patterns of biomass accumulation, N c ontent and N concentration change over the course of a season, but these pa tterns are quite different be tween cool and warm season legumes, between different species growing at th e same time of year and within the same species growing in different years. 0 2 4 6 8 10 0481216202428 Weeks After EmergenceDry Weight (Mt ha-1) Flower Leaf Stem Root A 0 20 40 60 80 0481216202428 Weeks After EmergenceN Content (kg N ha-1) Flower Leaf Stem Root B Figure 2.1. Sunn hemp dry weight (A) and n itrogen content (B) during growth and decomposition, 2001-02. Error bars reflect standard errors.

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52 0 1 2 3 4 5 6 704812162024 Weeks After EmergenceLAI (m2 m-2) SH 2001 SH 2002 Lupin 2001-02 Vetch 2002-03 A 0 2 4 6 8 10 12 1404812162024 Weeks After EmergenceDry Weight (Mt ha-1) SH 2001 SH 2002 Lupin 2001-02 Vetch 2002-03 B Figure 2.2. Leaf area index (A) and dry weight (B) of each GM during growth. Error bars reflect standard errors. 1 2 3 4 5 0481216 Weeks After EmergenceS:R-N / S:R-B SH 2001 SH 2002 A 0 1 2 3 4 5 6036912151821 Weeks After EmergenceS:R-N / S:R-B Lupin 2001-02 Vetch 2002-03 B Figure 2.3. Ratio of S:R-N to S:R-B of sunn hemp (A) and lupin and ve tch (B). Error bars reflect standard errors.

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53 0 2 4 6 8 10 12 14 048121620242832 Weeks After EmergenceDry Weight (Mt ha-1) Flower Leaf Stem Root A 0 30 60 90 120 150 048121620242832 Weeks After EmergenceN Content (kg N ha-1) Flower Leaf Stem Root B Figure 2.4. Sunn hemp dry weight (A) and n itrogen content (B) during growth and decomposition, 2002-03. Error bars reflect standard errors. 0 1 2 3 4 5048121620Weeks After EmergenceDry Weight (Mt ha-1) Pods Leaf Stem Root A 0 15 30 45 60 048121620 Weeks After EmergenceN Content (kg N ha-1) Pods Leaf Stem Root B Figure 2.5. Lupin dry weight accumulation (A) and N content (B) during growth, 200102. Error bars reflect standard errors. 0 1 1 2 2 3 0369121518 Weeks After EmergenceDry Weight (Mt ha-1) Leaf Stem Root A 0 15 30 45 0369121518 Weeks After EmergenceN Content (kg N ha-1) Leaf Stem Root B Figure 2.6. Vetch dry weight accumulation (A) and nitrogen content (B) during growth, 2002-03. Error bars reflect standard errors.

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54 Table 2.1. Sunn hemp nitrogen co ncentration by tissue type, 2001. WAE N Concentration Leaf Stem Root Flower Total g N kg-1 2 21.1 0.7 12.0 0.6 8.5 0.6 16.2 0.6 4 21.8 0.5 8.2 0.4 7.2 0.4 14.9 0.4 6 21.3 0.5 6.2 0.3 6.1 0.3 12.1 0.3 8 21.8 0.4 5.3 0.2 4.9 0.2 27.3 0.4 10.3 0.2 10 21.7 0.3 5.0 0.2 4.7 0.2 24.1 0.5 10.0 0.1 12 20.1 0.4 5.2 0.2 4.6 0.2 29.0 4.3 10.1 0.5 WAE = weeks after emergence. Table 2.2. Selected sunn hemp growth indicators, 2001. WAE SLA SLN S:R-B S:R-N cm2 g-1 g cm-2 kg kg-1 kg kg-1 2 222 8 96 4 2.7 0.33 6.1 0.6 4 297 8 74 2 3.8 0.24 9.0 0.7 6 287 6 75 3 5.0 0.20 10.9 0.6 8 235 6 93 3 4.5 0.19 11.0 0.8 10 238 7 92 3 6.3 0.49 14.6 1.1 12 209 5 97 3 6.8 0.33 17.5 2.3 WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B = biomass-based shoot to root ratio; S: R-N = N-based shoot to root ratio. Table 2.3. Sunn hemp nitrogen concentra tion by tissue type after death, 2001-02. WAD N Concentration Leaf Stem Root Total g N kg-1 0 16.1 0.3 4.2 0.1 3.7 0.1 7.7 0.1 2 10.8 0.2 4.7 0.3 2.2 0.2 3.5 0.2 4 9.8 1.0 3.6 0.2 2.0 0.3 3.8 0.1 8 9.0 0.5 3.2 0.1 2.4 0.3 3.5 0.1 12 3.2 0.1 2.9 0.3 3.2 < 0.1 16 3.0 0.1 3.0 < 0.1 3.0 0.1 WAD = weeks after death.

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55 Table 2.4. Sunn hemp nitrogen co ncentration by tissue type, 2002. WAE N Concentration Leaf Stem Root Flower Total g N kg-1 2 37.7 1.0 15.3 0.3 23.2 0.7 30.4 0.8 4 39.0 0.4 14.7 0.1 14.7 1.7 27.6 0.4 6 40.3 0.5 12.0 < 0.1 10.1 0.4 22.4 0.2 8 34.3 0.6 9.0 < 0.1 6.6 0.8 16.9 0.2 10 32.9 0.5 6.0 < 0.1 5.4 0.4 40.2 0.9 13.1 0.2 12 31.8 0.6 6.0 < 0.1 4.0 0.6 42.7 4.8 12.5 0.5 14 31.3 0.6 5.7 < 0.1 6.1 0.3 21.1 < 0.1 11.7 0.7 WAE = weeks after emergence. Table 2.5. Selected sunn hemp growth indicators, 2002. WAE SLA SLN S:R-B S:R-N cm2 g-1 g cm-2 kg kg-1 kg kg-1 2 281 8 135 3 3.2 0.2 4.5 0.3 4 331 8 118 3 5.0 0.2 12.3 0.3 6 355 4 114 2 6.1 0.2 15.6 1.0 8 288 7 121 7 7.3 0.3 22.9 3.5 10 318 7 105 3 8.5 0.3 25.8 2.9 12 284 7 113 4 9.2 1.0 36.0 6.8 14 256 7 124 4 10.2 0.4 21.0 1.9 WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B = biomass-based shoot to root ratio; S: R-N = N-based shoot to root ratio. Table 2.6. Sunn hemp nitrogen concentra tion by tissue type after death, 2002-03. WAD N Concentration Leaf Stem Root Total g N kg-1 0 31.3 0.0.6 5.7 < 0.1 6.1 0.3 11.7 0.7 2 11.2 0.0.6 3.4 0.2 3.3 0.2 4.4 0.2 4 7.3 0.1.2 2.8 0.2 1.8 0.1 3.1 0.3 6 2.7 0.3 1.6 0.2 2.6 0.2 8 1.7 0.2 3.3 0.3 1.9 0.2 11 2.6 0.3 2.2 0.5 2.5 0.3 14 2.5 0.1 2.5 0.3 2.5 0.1 18 3.0 0.3 2.5 0.4 2.9 0.3 WAD = weeks after death.

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56 Table 2.7. Lupin nitrogen concentration by tissue type, 2001-02 WAE N Concentration Leaf Stem Root Pod Total g N kg-1 4 15.2 0.2 5.5 0.2 3.8 < 0.1 7.7 0.3 8 15.6 < 0.1 6.2 0.5 4.7 0.7 9.6 0.4 12 19.1 < 0.1 6.8 0.4 10.4 1.4 13.1 0.6 16 22.5 0.4 6.7 0.2 17.1 0.8 14.2 0.4 20 22.6 0.7 7.5 0.4 11.0 0.7 35.0 13.0 0.4 WAE = weeks after emergence. Table 2.8. Selected lupin growth indicators, 2001-02 WAE SLA SLN S:R-B S:R-N cm2 g-1 g cm-2 kg kg-1 kg kg-1 4 154 4 98 3 1.6 0.3 4.5 0.8 8 118 8 135 7 3.4 0.1 8.3 0.8 12 107 8.9 185 17 4.2 0.6 5.9 1.0 16 103 3 219 6 4.6 0.2 3.7 0.2 20 124 3 183 7 7.2 0.5 10.8 2.0 WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B = biomass-based shoot to root ratio; S: R-N = N-based shoot to root ratio. Table 2.9. Vetch tissue nitrog en concentration, 2002-03. WAE N Concentration Leaf Stem Root Total g N kg-1 3 27.3 1.3 16.6 0.5 17.8 0.5 20.2 0.7 6 22.0 1.0 11.3 0.3 15.5 0.5 16.5 0.6 9 21.8 0.9 13.0 0.5 12.9 0.8 16.1 0.4 12 27.5 2.5 15.0 1.1 7.8 0.5 18.8 1.8 15 29.7 2.4 13.3 1.0 5.8 0.3 17.0 1.5 18 35.6 1.5 14.5 0.6 6.4 0.8 20.3 1.3 WAE = weeks after emergence. Table 2.10. Selected vetch gr owth indicators, 2002-03. WAE SLA SLN S:R-B S:R-N cm2 g-1 g cm-2 kg kg-1 kg kg-1 3 259 8 105 3 0.9 <0.1 1.2 0.1 6 252 5 87 4 1.3 0.1 1.4 0.1 9 314 30 326 29 1.4 0.1 2.3 0.3 12 210 5 130 10 3.5 0.5 10.0 1.5 15 141 14 250 45 4.8 1.0 16.9 3.7 18 141 15 239 10 7.2 1.6 19.9 3.7 WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B = biomass-based shoot to root ratio; S: R-N = N-based shoot to root ratio.

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57 CHAPTER 3 GROWTH, YIELD, AND N-UPTAKE EFFI CIENCY RESPONSE OF CORN TO AMENDMENT WITH GREEN MANURES Introduction and Literature Review Utilized as green manures (GMs), legumes may represent a substantial source of on-farm nitrogen (N) for subsequent crops. In temperate environments on fine textured soils, winter legumes such as vetch ( Vicia spp.), clover ( Trifolium spp.), and medics ( Medicago spp.) are capable of accumulating large amounts of biomass (7-10 Mt ha-1) and N (150-250 kg N ha-1) and delivering substantial N bene fit to subsequent crops. On a silt loam soil in Maine, Griffi n et al. (2000) found alfalfa ( M. sativa ) and winter rye ( Secale cereale ) plus hairy vetch ( Vicia villosa ) combinations as GMs capable of satisfying the N requireme nts of sweet corn ( Zea mays var Rugosa) in two of three study years. In Kentucky, sweet corn N requirements were fully met when vetch N was equal to or greater than 166 kg N ha-1 (Cline and Silvernail 2002). In tropical environments, warm weat her legumes such as sunn hemp ( Crotalaria juncea ), cowpea ( Vigna unguiculata ), and mungbean ( V. radiata ) may also accumulate large amounts of biomass and N. Because no freezes occur in tropical environments, these legumes may be followed immediatel y by frost-sensitive crops. For example, studies in Asia have shown such GMs capab le of supplying the N-requirements of rice ( Oryza sativa ; Ladha et al. 2000, Agustin et al. 1 999, Aulakh et al. 2000). However, few GM studies have been conducted with high-N demanding spring crops under north Florida conditions (sandy soils, sub-temperat e climate). In this environment, the two

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58 greatest challenges to leguminous GM appro aches for spring cropping systems remain accumulation of adequate N by GMs and delay of N-release during the winter to better match timing of spring crop uptake. As discussed in more detail in Chapte r 2, temperate legumes often do not perform as well in north Florida, while the rotati on from tropical legume to spring crop is interrupted by feezing temperat ures over the winter. Based on ear leaf N concentration, Gallaher (1993) reported N-substi tution values by blue lupin ( Lupinus angustifolius ), hairy vetch and crimson clover ( Trifolium incarnatum ) of only 67 kg N ha-1 compared to chemical N for a variety of different residue management systems. In another study near Gainesville, Florida, Gallaher and Eylands (1985) reported N substitution value for blue lupin near 56 kg N ha-1 based on sorghum ( Sorghum bicolor ) grain yields. In a low-input system on a loamy sand in Zimbabwe, Jera nyama et al. (2000) found relatively low fertilizer N equivalency for sunn hemp (36 kg N ha-1). Need exists to develop GM management techniques for north Florida and like environments that deliver N benefits similar to those achievable elsewhere. The slow release of N from decomposing GM residues may be better timed with plant uptake (Bath 2000, Wivstad 1997). Indeed, so me researchers have found Nsubstitution values for GMs in excess of th eir actual N accumulation, suggesting that GM N is either used more efficiently than chemi cal fertilizer N, that GMs modify the soil environment and/or crop growth such that gr eater crop N uptake is possible, or that GMs also supply some other nutrient which is lim iting crop growth (such as phosphorus). In a low-land rice system, Agustin et al. (1999) found 58 kg N ha-1 from indigo ( Indigofera tinctoria ) comparable to 120 kg N ha-1 from urea and speculated that all of the above

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59 mentioned factors may have been involve d. Also studying rice, Aulakh et al. (2000) found application of 84 kg N ha-1 in the form of cowpea and sesbania ( Sesbania rostrata ) equivalent to 104 kg N ha-1 applied as chemical-N. Green manures may provide other benefits such as reduction of soil erosion, recy cling of other crop nutrients, and control of plant pests, pathogens and weeds with less reliance on off-farm chemical inputs (see Chapters 5 and 6). In a 3-year study using several differe nt GMs on a silt loam in Canada, N’Dayegamiye and Tran (2001) f ound yield benefits for wheat ( Triticum aestivum ) of 3090 kg N ha-1 and an increase in fertilizer N recovery with GM use. Recovery rates for GM derived N in the same study ranged fr om 19-36%, which were lower than many of the fertilizer N recovery rates (25-52%). Lo wer recovery rates for GM-derived N may have been due to stabilization of N in organic forms rather than loss through leaching, volatilization, and denitrificat ion. In another study by N’Da yegamiye (1990), 15% of red clover ( T. repens ) N applied to maize was taken up, while 19% and 28% were recovered in microbial biomass and soil organic frac tions respectively. Steinmaier and Ngoliya (2001) evaluated the use of 11 GMs as N s ources for maize on a sandy loam in Zambia. Although a formal control was not used, co mparison to low producing GMs suggests a N benefit of around 50 kg N ha-1 or more from sunn hemp and velvet bean ( Mucuna pruriens ). On a sandy clay loam in India, Sharma et al. (2000) repor ted N-replacement of 60 kg N ha-1 for rice when a mungbean GM was plowed in prior to planting. Studying fertigated, mulched tomatoes ( Lycopersicon esculentum ), Abdul-Baki et al. (1996) found that plastic-mulch with 112 kg N ha-1 (recommended rate) produced lower yields than hairy vetch, crimson clover, and hairy vetch plus rye live mulches with only 56 kg N ha-1.

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60 Synchrony between GM-N availability and subsequent crop N-demand remains difficult to achieve. Many investigators ha ve shown more rapid decomposition for plant residues when soil incorporated (see Chap ter 2). Rapidly grow ing crops immediately following GMs may benefit from soil-incorporat ion of GM residues, especially in cool climates and/or on fine-textured soils with hi gh N-retention. For example, Shrestha et al. (1998), finding no N-replacement benefit for winter canola ( Brassica napus ) in Michigan even when a spring GM produced over 100 kg N ha-1, hypothesized that GM-N release occurred as the canola crop went into dorma ncy. Griffin and Hester man (1991) found that legume GMs increased biomass, N uptak e and N concentration of potato ( Solanum tuberosum ) but did not benefit tuber yields. Th ey concluded that N from GMs became available too late to benefit tuber growth. However, in warm environments GM-N release more often occurs so rapidly that peak av ailability takes place well before peak N demand from a subsequent crop. Potential Nleaching losses under these circumstances may eliminate advantages of GMs. In Georgi a, Sainju and Singh (2001) showed greater corn N-uptake and ear yield following hairy ve tch for no-till compared to conventional tillage, although the opp osite trend occurred for corn following (highly recalcitrant) winter wheat. In these case s reduced or zero-tillage may better synchronize leguminous GM-N release with subsequent crop demand. The N loss incurred by overwintering of decaying residues may negate any N benefit to a subsequent spring crop. For exampl e, on a loam soil in Saskatchewan, Brandt (1999) saw no N benefit from pr oduction of less than 3 t ha-1 of black lentil ( Lens culinaris ) on a subsequent crop of wheat. In Alab ama, Mansoer et al. (1997) found close to 2/3 N loss in mowed, overwintering sunn hemp. Reduced tillage as means of slowing

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61 GM decomposition during winter months ma y increase GM-N availability during the spring. Modification of residue quality (especially C:N ratio or N concentration) may also control timing of residue N-re lease. Some field grown gra ss-legume mixtures have shown potential to increase GM C:N and total GM N content relative to legumes alone, improving both the amount and synchrony of GM N-release (Ranells and Wagger 1996). Nitrogen accumulation of such mixtures may be reduced, however, if legume seed rate is too low (Karpenstein-Machan and Stuelpna gel 2000, Cline and Silvernail 2002). Based on subsequent ear yields, small grain GMs do not appear capable of satisfying corn N demand (Griffin et al. 2000, KarpensteinMachan and Stuelpnagel 2000, Cline and Silvernail 2002, Gallaher and Eylands 1985). Al ternatively, overwintering residue with low N concentration and/or high C:N rati o may also highly reduce N leaching losses (Stopes et al. 1996, Wyland et al. 1996). Selecting leguminous GMs capable of accumulating large fractions of stemmy, lowN biomass may create opportunity for both high GM-N accumulation and improved N-retention. Green and Blackmer (1995) found N-immobilization (followed by N-release) by soybean ( Glycine max ) residue helped explained N benefits to subsequent corn. Establishing a winter GM after the summer GM may si gnificantly reduce N leaching losses and enhance performance of th e winter GM, but the effective N benefit to a subsequent spring crop from such a double-GM approach has not been studied. In some systems, it may therefore be advantageous to follow a vigorous and stemmy summer GM with a well established winter GM and to preserve as much r ecalcitrant litter as possible by reducing tillage.

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62 If GMs do not supply adequate N to meet requirements of subsequent crops, then supplementary inorganic N may be required to prevent yield reductions. Many studies have compared use of GMs alone against synthe tic fertilizers (for example, see Carsky et al. 2000), and others have also investigated GMs used in combination with synthetics, (for example, see Ladha et al. 2000). Howe ver, these studies usually do not establish optimums for chemical N rate whether used alone or with GMs, making it difficult to assess how much (if any) chemical N is re quired “on top of” GMs for optimal production. A number of studies (such as Prasad et al. 2002) do so fo r “cut and carry” systems where GMs are not grown in place, but this does not reflect comm on agricultural practice in developed countries. As part of a larger study on improved use of GMs in vegetable cropping systems in the southeast US, we inves tigated a GM sequence of sunn hemp followed by a winter legume (blue lupin, winter 2001-02; cahaba white vetch, Vicia sativa winter 2002-03) as an N-source for sweet corn. Details of GM growth and decomposition patterns can be found in Chapter 2. In these studies, sunn hemp followed by winter legume produced a cumulative 12-15 Mt dry matter ha-1 and up to 170 kg N ha-1. We evaluated sweet corn growth and leaf characteri stics throughout the season for GM amended and unamended corn supplemented with multiple chemical N-rates. We hypothesized the double-GM approach would significantly reduce chemical N required by sweet corn to achieve ear yields similar to an optimal level identified in the convent ional approach, and that GMs would increase N-uptake efficiency of sweet co rn. Objectives of the study were to gain greater understanding of the im pacts of GMs on sweet corn growth throughout the season

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63 and to estimate chemical N-supplementation ne eded to achieve acceptable sweet corn ear yields. Materials and Methods Set-Up and Design This study consisted of 14 of the 15 overall treatments related in Chapter 1 (Table 1.2). Treatments consisted of sweet corn fo llowing rotations of sunn hemp (summer) and lupin (winter 2001-02) and vetch (winter 2002-03 ), denoted as SH+L; sunn hemp alone, denoted as SH; winter legu me (lupin 2001-02, vetch 2002-03) alone, denoted as L; and unamended corn denoted as Conv (for c onventional). Each GM level received supplementation with 0, 67, or 133 kg inorganic N ha-1 (0N, 67N, and 133N). Other unamended (Conv) treatments also received 200 or 267 kg inorganic N ha-1 (Conv 200N and Conv 267N). Only methods relevant to co rn growth and N-accumulation analysis are considered here. Methods regarding GM gr owth and accumulation, root dynamics, and effects on soil properties and plant pests are discussed in relevant chapters. Please see Chapter 1 for overview. Timeline of Operations 2001-02 On 7 August 2001, sunn hemp was planted following complete disking and plowing of the field. Seed was inoculated w ith cowpea-type rhizobium and planted at 2-4 cm depth. In-row spacing was 3.12 cm (1.25 in), between-row spacing was 76 cm (30 in). Sunn hemp emerged 11 August 2001 and grew until 31 October 2001 when it was killed with an application of Gramoxone (Syngenta; Basel, Switzerland). Lupin was inoculated with lupin-type rhizobium and planted on 19 November 2001 using a rip-strip planter and with spacing identical to sunn hemp. Lupi n emerged 22 November 2001 and grew until

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64 12 April 2002. All plots were then mowed and field treated with Round-Up (RoundUp; Columbus, OH). Sweet corn (variety GS 0966 Syngenta) was planted 26 April 2002 using a rip-strip planter, with in-row spacing of 18 cm and between-row spacing of 76 cm. Corn emerged 1 May 2002. For each treatment, chemical N was applied as NH4NO3 in three equal applications: at emergence and 3 and 5 weeks after emergence (WAE). 2002-03 On 19 July 2002, inoculated sunn hemp was pl anted with a rip-strip planter at the same spacing and depth as in 2001, emerging 21 July 2002 and growing until 30 October 2002 when it was killed with Diuron/Touchst one. Cahaba white vetch was inoculated with vetch-type rhizobium and planted 15 N ovember 2002 with a zero-till grain-drill at a rate of roughly 40 kg ha-1 (35 lbs acre-1). Sweet corn (variety GS 0966) was directly planted into vetch on 7 April 2003 with the same planter, spacing, and depth as 2002. Corn emerged 15 April 2003. For each treatment, chemical N was again applied as NH4NO3 in equal applications at emergence a nd 3 and 5 weeks after emergence (WAE). Procedures and Measurements At emergence a plant count was made to determine an average plant population. In both years, sweet corn biomass was samp led five times (2, 4, 6, 8, and 9 WAE) and ears were harvested at maturity (9 WAE). The final biomass samplings were taken the day before harvesting ears. Ear harvest was cond ucted in an inner area of the plot kept free from destructive biomass and soil samp ling. This inner area was roughly 4.6 m (15 feet) by 4.6 m, allowing harvest of the central 4.6 m of row length from each of the six inner rows of corn (out of a total of 10 rows in each plot). Representative subsamples of ears from harvest were graded using USDA standards (United States Department of Agriculture 1997).

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65 Biomass sampling was conducted outside this inner area but away from plot edges using three feet of row length representative of the entire plot in plant number, size, spacing, and appearance. Within each sample, one representative subsample plant was dug from the ground with a shovel to include roots (except at 2 WAE when all plants were dug out). All other plants were cli pped at ground level. Clipped plants were weighed and counted. Roots were cut from th e subsample plant and stored separately, and the subsample plant top was then weighed and refrigerated until further processing in Gainesville. The clipped plants were return ed to the plots when possible. Relative humidity, air temperature at 1 m, soil temper ature at 5 cm depth, and precipitation were recorded continuously with a Watchdog data logger (Spectrum Technologies; Plainfield, IL). At the UF Environmental Agronomy Lab (U niversity of Florida, Gainesville, FL), height and total plant leaf numbers were taken for each subsample plant. Chlorophyll meter readings (CMR), taken with a Minolta SPAD-502 (Spectrum Technologies; Plainfield, IL), were made on the two most recently matured leaves (before tasseling) or the third and fourth leaves from the top (after tasseling). Plants were then separated into tissue type: leaves, stem, dead leaves, and ear s (where applicable). Roots were washed clean of soil and debris and fresh weights were taken for all tissues. Leaf area was determined for each subsample plant using an LI-3100 (Li-cor; Lincoln, NE). All tissues were then bagged and dried for 72 hours at 65 C and then reweighed. Afterwards, all tissues were ground in a Wiley mill to pa ss through a 2 mm screen, and a thoroughly mixed 5 g portion of each grinding was subs equently stored. Grindings were then subjected to a wet-acid Kjeldahl digestion, diluted and filtered. The diluted samples were

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66 then analyzed for total Kjeldahl N (TKN) at the UF Analytical Research Laboratory (University of Florida, Gainesville, FL; EPA Method 351.2; Jones and Case 1991). Nitrogen applied to corn (NAC) for each plot was calculated as: NACx = Chemical-Nx + Residue-Nx; where Chemical-Nx = N applied as NH4NO3 to corn in plot “x” and Residue-Nx = TKN present in any winter GM, winter weeds, and sunn hemp residue in plot “x” at the final sampling prior to corn planting. Nitrogen-uptake efficiency (NUE) was calculated as: NUEx = (Total N Contentx – Total N ContentConv 0N) / NACx; where Total N Contentx = TKN present in total corn biomass in plot “x” and Total N ContentConv 0N = average TKN present in total co rn biomass of Conv 0N treatment. Unaccounted applied N (UAN) was calculated as: UANx = NACx Total N Contentx. Analysis of Data A balanced analysis of variance (ANOVA) was conducted to assess the effect of GM application on corn growth, yield, and Nuptake efficiency responses, as well as the effect of chemical N-rate and possible interactions of chemi cal N-rate with GMs. For all measured values, this ANOVA was conducted on SAS software (Statistical Analysis Systems; Cary, NC) using data from all trea tments receiving N rates of 0, 67 or 133 kg N ha-1 (4 GM levels x 3 N-rates = 12 treatments; = 0.05). Measured values were regressed with a linear model (PROC GLM) based on GM level, N-rate, GM x N-rate interaction, and block. A randomization term for block was included. Significance of main effects and the interaction term (GM x N-rate) are shown. Where interacti on of the two main effects is non-significant, Duncan multiple range test ( = 0.05) of pooled averages are shown for main effects. Interaction between GM level and N-rate was never significant. Pairwise contrasts were conducted to assess the pari ty of GM treatments supplemented with 1/3 (67N) or 2/3 (133N) th e recommended N-rate for sweet corn with

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67 conventional treatments receiving 3/ 3 (Conv 200N) or 4/3 (Conv 267N) of the recommended N-rate (6 GM treatments compared with 2 Conv treatments = 12 contrasts). Using SAS software, contrasts we re made with an ANOVA based on a linear model (PROC GLM) of treatment and block. All 14 treatments were included in the ANOVA. Possible error from the high number of contrasts was mitigated as much as possible by evaluating relevance of contrast results within the context of the overall statistical, numerical, and graphical trends. Results N Applied to Corn In 2002, average amounts of N applied to corn (NAC) derived from GM residues of SH+L (56 kg N ha-1) and L (57 kg N ha-1) were statistically similar to each other and greater than SH (11 kg N ha-1; see Appendix C, Table C.1). In 2003, SH+L (51 kg N ha-1) applied significantly more N to corn than both SH (30 kg N ha-1) and L (21 kg N ha-1; see Appendix C, Table C.14). As a result, NAC in both years was nume rically greater for Conv 200N and Conv 267N compared to any GM with 67N or 133N, with differences significant everywhere except SH+L 133N si milar to Conv 200N in 2002 (Tables 3.1 and 3.2). Ear Yields, 2002 For all treatments, marketable ear yiel ds (fresh weight) for 2002 are shown in Figure 3.1(A). Amendment with GM increase d end-season marketable, fancy, and total ear yields by 30-45%, 46-68%, and 15-24% respectively, with no differences between GM types (Table 3.3). Yields for grades N o.1 and No.2 as well as non-marketable ears were not affected by GM application (data not shown). Reduction in N-rate from 133N resulted in much lower yields for mark etable ears (5% and 60% for 0N and 67N,

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68 respectively, compared to 133N), fancy ears (2% and 50% for 0N and 67N, respectively), and total ears (17% and 73% for 0N and 67N respectively). Interactions between GM and N-rates were non-significant in all cases (Table 3.3). For all residue levels, fraction of ear yield as fancy and marketable increased as chemical N-rate went up. Optimal marketable ear yields were ac hieved with Conv 200N (within 4% of maximum yielding Conv 267N). Amendment with SH+L 133N produced statistically similar marketable and fancy ear yield to amendment with Conv 200N or 267N, and similar total ear yields to 200N, though yields with SH+L 133N were numerically less in all cases. Corn with SH 133N also produ ced similar fancy ear yields to Conv 200N. Otherwise, marketable, fancy, and total ear yi elds were significantly greater with Conv 200N or 267N than with any GM plus 67N or 133N (Table 3.1). Ear Yields, 2003 For all treatments, fresh weight of ma rketable ears for 2003 are shown in Figure 3.1B. Due to earlier planting date and near ly 50% higher plant population, overall ear yields compared to 2002 increased for treatm ents receiving 133N or more and decreased for treatments receiving 0N or 67N. Unlike 2002, optimal ear yield for the conventional treatment was not reached at Conv 200N (Conv 267N was greater than Conv 200N by 15%). Only amendment with SH+L significantly increased end-season marketable, fancy, and total ear yields relative to Conv (Table 3.3). Yields for grades No.1 and No.2 as well as non-marketable ears were unaffected (dat a not shown). Increase in N-rate again resulted in much higher yields for marketab le, fancy, and total ears. Interaction between GM and N-rates was non-significant in all ca ses (Table 3.3). Pair-wise contrasts showed that SH+L 133N produced similar marketab le, fancy, and total ear yield to Conv 200N,

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69 but lower yields in all cases when comp ared to Conv 267N. Otherwise, marketable, fancy, and total ear yields were greater with Conv 200N or 267N than with any GM plus 67N or 133N (Table 3.2). In 2003 compared to 2002, fraction of ear yield as USDA fancy grade decreased while fraction as USDA No.1 and No.2 increa sed for all treatments (except Conv 0N which had no fancy ears in either year). Decr ease in fraction as fa ncy was particularly high for nearly all GM treatments (12.3-28.7 pe rcentage points lowe r in 2003 than in 2002), but less for all Conv treatments as well as SH+L 0N and SH 0N (2.1-9.7 percentage points lower in 2003 than in 2002). For highest yielding treatments (Conv 267N, Conv 200N, SH+L 133N, SH 133N, a nd L 133N) fraction of ear yield as marketable ears decreased slightly, from 0.6-7.6% percentage points lower in 2003 than in 2002, depending on the treatment (Tables 3.1 and 3.2). Growth Analysis, 2002 Leaf indicators For nearly all treatments, leaf area index (LAI), chlor ophyll meter readings (CMR), and specific leaf N (SLN) showed lin ear or logarithmic increases up through the time of ear appearance (6 WAE) or the follo wing sample date (8 WAE; see Tables 3.4 and 3.5 and Appendix C, Table C.2). Green ma nures affected these indicators weakly with statistical differences a ppearing primarily at the begi nning (2-4 WAE) or end (9 WAE) of the growing season. However, LAI, CMR, and SLN for GM-amended corn usually showed numerical advantage comp ared to Conv throughout the entire season. Advantages were strongest for SH+L and SH and most pronounced in LAI, often ranging from 30-45% for all the leaf indicators (exc ept SLA) during the first 2-4 WAE and up to 16% afterwards. Specific leaf area showed no consistent response to GMs. Chemical N-

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70 rate strongly affected LAI, CMR, and SL N at almost every sample date, always producing increases from 0N to 67N (significant at all da tes) and from 67N to 133N (significant in about half of sample dates) with most pronounced benefits in the middle 4 weeks of the season (4-8 WAE) Greatest response to increas ing N-rate occurred for LAI (46-126% and 70-145% increases for 67N and 133N respectively, compared to 0N), with response from CMR and SLN on the order of 15-50% (67N compared to 0N) and 2560% (133N compared to 0N; Tables 3.4, 3.5 a nd Appendix C, Table C.2). Specific leaf area (SLA) decreased in response to chemical N, but effects were less strongly significant than for other leaf indicators. In terms of LAI, SH+L 133N and SH 133N showed numerical advantage over Conv 200N and Conv 267N at 2 and 4 WAE (T able 3.5). Otherwise, Conv 200N and Conv 267N generally showed nu merical advantage over all GMs with 67N and 133N for LAI, SLN and CMR, though these differences did not become significant until 8-9 WAE. In terms of LAI, CMR, and SLN, Conv 267N showed more frequent and more significant advantages over GM treatments than C onv 200N, and SH+L 133N remained the only GM treatment statistically similar to Conv 267N and Conv 200N throughout the season (Table 3.5 and Appendix C, Table C.8). Tissue characteristics Dry weights and N contents fo r leaf, stem, and total plant increased logarithmically or exponentially during the first 6 WAE (tim e of ear appearance; Tables 3.6-3.9 and Appendix C, Tables C.3-C.5). During this ti me, benefit from GM application generally ranged from 5-45%. Consistently significant benefit from GMs occurred for dry weights and N-contents of leaf and stem tissue and (t o a lesser extent) for the total plant. At or after ear appearance, GM benefit for dry we ight and N content of vegetative factors

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71 became somewhat reduced (less than 20%), but more pronounced for ears themselves (15-30%). Compared to leaf, stem, and ear dry weights, GM benefits were somewhat lower and less consistently si gnificant for root dry weight as well as tissue N contents (generally, 5-30%) and had little effect on tissue N concentrations (see Tables 3.6, 3.8 and Appendix C, Tables C.6-C.7 and C.27). Adva ntages were typically greatest for SH+L and SH, although SH+L generally showed greatest tissue dry weights by late season and greatest N content th roughout the season. In terms of all tissue N contents and dry weights, SH+L 133N and SH 133N showed numerical advantage over Conv 200N and Conv 267N during the first 2-4 WAE. Otherwise, Conv 200N and C onv 267N maintained greater tissue dry weights and N contents than all GM treatments, though diffe rences did not become significant until late season (6-8 WAE and 8-9 WAE relative to GMs with 67N and GMs with 133N, respectively). Compared to other GM treatm ents, late-season differences against Conv 200N and Conv 267N treatments were genera lly less dramatic for SH+L 133N (see Tables 3.10, 3.11 and Appendix C, Tables C.9-C.11). In regards to tissue N concentration, pairwise contra sts showed consistent statis tical advantage for Conv 200N against GMs with 67N throughout the season, but not until the end of the season when compared to GMs with 133N. As with other growth factors, tissue N concentration for SH+L 133N remained closer to Conv 200N and Conv 267N than any other contrasted GM treatment (see Appendix C, Tables C.12-C.13 and C.28). Chemical N-rate strongly affected al l tissue characteristic s (dry weights, N contents, and N concentrations) on all sample dates, with tissue N contents showing strongest response. Application of 0N a nd 67N (compared to 133N ) reduced vegetative

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72 tissue N content by roughly 60-80% and 25-40%, respectivel y, while reductions for vegetative dry weights were about 40-70% and 10-20%, re spectively, with greatest differences occurring just be fore or at ear appearance (4-6 WAE; Tables 3.6-3.9 and Appendix C, Tables C.3-C.5). At final bi omass sampling, application of 0N and 67N resulted in ear N content of 14% and 62%, respectively and ear dry weight by 15% and 72%, respectively, compared to 133N (Appendi x C, Table C.5). Vegetative tissue N concentrations showed similar patterns to dry weight and N content before ear appearance, though reductions du e to lower chemical N-rate were generally less (not more than 50%). After ear appearance, root stem and ear N concentrations typically remained lowest for corn with 67N even compared to 0N primarily due to dilution effect (biomass increases outpaced increa ses in N accumulation) with stronger N remobilization to ears from vegetative tissues possibly playing a ro le as well (Appendix C, Tables C.6-C.7 and C.27). Growth Analysis, 2003 Leaf indicators Leaf indicators showed similar behavi or in 2003 compared to 2002, although GM effects were weaker. In term s of LAI, CMR, and SLN, benefit from GM amendment typically remained within 2030%, with significant differenc es less consistent than in 2002. However, as in 2002 greatest GM benef its occurred for SH +L and (to a lesser extent) SH (Table 3.12 and Appendix C, Tabl e C.15). Neither GM nor chemical N-rate significantly affected SLA. Chemical N-rate again strongly affected LAI, CMR and SLN throughout the season with significant increases from 0 to 67N and from 67N to 133N at all sample dates, and smallest relative bene fits at 2 WAE. As in 2002, LAI response to increased chemical N-rate (26-107% a nd 44-134% for 67N and 133N, respectively,

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73 compared to 0N) was greater than for CM R and SLN (generally, 30-70% and 50-100% for 67N and 133N, respectively, compared to 0N; see Table 3.12 and Appendix C, Table C.15). Relative to 2002, LAI values were gr eater by 30-60% and SLN values lower by 30-45% within treatments at similar samp les date in 2003. Values for SLA and CMR changed little from 2002, especially for treatments with 133N or more. Both SH+L 133N and SH 133N maintained similar LAI compared to Conv 200N and Conv 267N, although LAI for SH 133N dr opped in comparison at final sampling (Table 3.13). Values of CMR for SH+L 133N and SH 133N were statistically similar, though numerically less, than those of Conv 200N and Conv 267N. Contrasted GM treatments did not demonstrate consistent early-season numerical advantage against Conv 200N and Conv 267N in terms of CMR and SLN (see Appendix C, Table C.21). Tissue characteristics Amendment with SH+L and SH consiste ntly increased tissue dry weights and N contents by 10-45% throughout the season (Tables 3.14-3.17; see also Appendix C, Tables C.16-C.18). Like 2002, rela tive advantages from GMs ge nerally peaked at or just prior to ear appearance (4-6 WAE) and ther eafter declined. However, more dramatic declines in some characteris tics for Conv at final sampling (9 WAE) created apparent benefits for GMs similar to those seen at 46 WAE. Relative advantages within each GM level remained qualitatively similar across all tissue dry weights and N contents. Advantages were again stronger for SH+L and SH compared to L, and tissue N concentration again showed almost no eff ect from GM amendment (see Tables 3.14-3.17 and Appendix C, Tables C.16-C.18). Changes in chemical N-rate in the 0N to 133N range also produced effects qualitatively similar to 2002. Chemical N-rate strongly affected most tissue

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74 characteristics at all sample dates, with tissue N contents showi ng strongest response. Application of 0N and 67N (compared to 133N) lowered vegetative tissue N content by roughly 50-80% and 15-40%, respectively, while reductions of vegetative dry weights were about 40-70% and 10-15%, respectively, with greatest di fferences again occurring just before or at ear appearance (4-6 WAE; for examples, see Tables 3.14-3.17 and Appendix C, Tables C.16-C.18). At final bi omass sampling, application of 0N and 67N decreased ear N content to 8% and 42%, respec tively, and ear dry wei ght to 9% and 48%, respectively, compared to 133N (Appendi x C, Table C.18). Vegetative tissue N concentrations showed similar patterns to dry weight and N content before ear appearance, though decreases due to lower chem ical N-rate were generally less (as in 2002, not more than 50%). After ear appearan ce, root, stem and ear N concentrations again remained lowest for corn with 67N (see Appendix C, Tables C.19-C.20). Tissue and total dry weights generally remained numerically superior throughout the season for SH+L 133N compared to Conv 200N and Conv 267N, although differences were often non-si gnificant. Tissue and total N c ontents for SH+L 133N also remained similar to Conv 200N and C onv 267N throughout most of the season. Additionally, tissue dry weights and N contents of SH 133N we re rarely less than Conv 200N and Conv 267N until a decline at final sampling (Tables 3.18 and 3.19 and Appendix C, Tables C.22-C.24). Tissue N concentrations for GMs with 133N again stayed lower than those of Conv 200N a nd Conv 267N, becoming significantly lower at or after 6 WAE but remaining closest for SH+L 133N. Stem dry weights for all GMs with 67N were often numerica lly (sometimes significantly) greater than for Conv 200N and Conv 267N (Appendix C, Table C.22), lik ely as a result of much lower ear

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75 production by GMs with 67N. Root, leaf, and total dry weights for GMs with 67N also remained statistically similar (though nu merically less) to Conv 200N and Conv 267N until final harvest. Statistical advantag e of Conv 200N and Conv 267N over GMs with 67N were detected for the first 6 WAE fo r stem N content and N concentration and afterward for leaf and ear N contents a nd N concentrations (Tables 3.18-3.19, Appendix C, Tables C.24, C.26 and C.30). Values for tissue dry weights of all treat ments in 2003 (compared to values from similar sample dates in 2002) increased 20-80% while values for total N content in 2003 fell by 40-50% for most treatments beginning at 4 WAE. Values from TKN may have been reduced by a percentage consistent for all samples, which are being resubmitted for analysis. New N data will likely be proportiona l to that listed here which will therefore not change statistical trends for tissue N c ontents and concentrations. Also, unlike 2002, when little or no net N uptake occurred af ter 6 WAE (Table 3.11), tissue samples in 2003 revealed 40-45% of total plant N uptake occu rred between 6 WAE and final harvest (9 WAE) for highest yielding treatments (Conv 267N, Conv 200N, and SH+L 133N; Table 3.19). Nitrogen Uptake Efficiency and Unaccounted Applied Nitrogen With few exceptions, Nuptake efficiency (NUE) was not significantly affected by GMs or chemical N-rate in either year, nor did any statistical differences for NUE exist between Conv 200N and Conv 267N compar ed to GMs with 67N or 133N (Tables 3.1-3.2 and Appendix C, Tables C.1, C.14). In 2002 and 2003 NUE showed a decreasing trend as chemical N-rate increased beyond 67N with decreases also occurring from Conv 133N to Conv 200N and from Conv 200N to Conv 267N. Nevertheless, none of these trends were significant.

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76 Corn amended with SH+L 133N and L 133N showed unaccounted applied N (UAN; defined as N applied at or after corn planting not accounted for in corn tissues) similar to Conv 200N in both years. Otherw ise, UAN was significan tly greater for Conv 200N and Conv 267N compared to all GMs with 67N or 133N. However, when one includes the N accumulated by SH and weeds duri ng the fall of each year but lost before corn planting, UAN from SH+L 133N and SH 133N become similar to or greater than Conv 200N and Conv 267N while L 133N, SH+L 67N and SH 67N become similar to Conv 200N (Tables 3.1-3.2). However, it mu st be remembered that this UAN pool includes any N still present in non-corn residue s or in the soil and therefore does not necessarily indicate lo ss from the system. Discussion Amendment with GMs resulted in ear yield, growth and N accumulation benefit for sweet corn. However, GM approaches in th is particular management system delivered only 13-51 kg N ha-1, with SH+L supplying highest N in both years (Tables 3.1-3.2). Benefits from GMs were usually greatest ear ly in the season (2-4 WAE), strongest from the combination of SH+L and weakest for L alone (Tables 3.3-3.4, 3.6-3.9, 3.12, 3.143.17), and required chemical N supplementa tion at least two-thirds (133 kg N ha-1 or more) of the recommended N-rate (200 kg N ha-1) to achieve ear yields similar to the conventional approach with recommended inor ganic N (Figure 3.1[A,B]). Results were similar to other experiments where wint er-decomposed residues from tropical GMs (Brandt et al. 1999) or low-performing temperate GMs (Gallaher 1993, Gallaher and Eyelands 1985) could not satisfy N demand for spring crops. As suggested by our GM growth and decomposition study (Chapter 2) as well as other studies of sunn hemp decomposition (Mansoer et al. 1997) and poten tial use of hairy ve tch GMs (Sainju and

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77 Singh 2001) in the southeast US, our GM approaches were limited by low N accumulation and/or rapid N loss duri ng winter-time GM decomposition. In 2002, growth of sweet corn amended with any GM plus two-thirds the recommended N-rate (133N) generally fell behi nd that of unamended corn with the full or high N-rate (Conv 200N and Conv 267N) af ter showing initial advantage during the first 2-4 WAE (Tables 3.5, 3.10-3.11). In 2003, Conv 200N and Conv 267N showed advantage over SH+L 133N for final ear harvest only, with almost no differences occurring throughout the season in terms of tissue dry wei ghts, N contents and leaf indicators (Tables 3.13, 3.18-3.19) Significant reduction in tissu e and ear yields for SH 133N relative to Conv 200N and Conv 267N during 2003 also occurred only at 8-9 WAE. Corn with SH+L 133N produced ear yields statistically similar to, though numerically lower than, Conv 200N (2002 and 2003) and Conv 267N (2002 only). All other corn amended with GMs plus 67N or 133N produced ear yields significantly lower than Conv 200N or Conv 267N (Tables 3.1 a nd 3.2). Griffin and Hesterman (1991) showed similar results for potato, with great er GM benefit for vegetative growth than reproductive yields. We were unable to detect in teresting differences in NUE based on GM or N-rate N, and direct measures of N-loss (via suction lysimeter sampling; see Chapter 4) failed to produce data. In terms of all N (plant and/or chemically-derived) applied to corn in each treatment SH+L 133N remain ed similar to Conv 200N and less than Conv 267N. However, when one includes N accumulated by SH and weed tissues but lost by decomposition before corn planting, unacc ounted applied N (UAN) for SH+L 133N becomes similar to Conv 267N (Tables 3.1 and 3.2). Generally, NUE ranged from 25-

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78 35% in 2002 and 15-25% in 2003. These results do not differ radically from those of N’Dayegamiye and Tran (2001) and N’Dayegamiye (1999). Results for growth factors between y ears were similar qua litatively, although generally not quant itatively. Dry weights, N concentrati ons and N contents for all tissues (leaf, stem, root, ears) and tota l plant as well as leaf indicat ors such as LAI, SLN, and CMR revealed GM benefits up to 45% in the first 4 to 6 WAE in both years. Among leaf indicators, LAI responded most strongly to GM application and N-ra te, as did leaf and stem dry weights and N contents am ong tissue factors (Tables 3.4, 3.6, 3.8, 3.12, 3.14, 3.16). Tissue N concentrations were somewhat variable, often showing lower values for GM amended corn especially at mid-season when ears appeared. These lower values for GM amended corn probably reflected greater N-stress and N remobilization to ears from vegetative tissues. Specific leaf area (SLA) never displayed any consistent response to GMs (see Appendix C, Tables C.2 and C.15). In terms of pairwise comparisons at ear ly and mid-season, no leaf or tissue characteristic showed statistical differences predictive of the “finer” but significant differences in final ear yield patterns among highest producing treatments (Conv 200N and Conv 267N produced greater ear yields than all GMs with 67N or 133N except SH+L 133N; Tables 3.1-3.2, 3.5, 3.10-3.11, and 3.18-3.19). Vegetative tissue and leaf characteristics often did not display differences reflective of ear final yields until 8-9 WAE, far too late for a grower to generate a yield response by adjusting management. Most net GM N release probably occurred with in the first 2-4 weeks after emergence, and total N delivered to corn via GMs was significantly less than the extra 67 and 133 kg N ha-1 received by Conv 200N and Conv 267N – likely resulting in a “running out” effect

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79 after the first 4 WAE (Tables 3.1 and 3.2). Early season advantages and late-season declines for GMs with 133N (esp ecially in terms of ear-fill) may also have been the result of rapidly changing root growth and proliferation patterns, ex plored in detail in Chapter 4. Taken together, this suggests N content of GM residues at planting may better indicate required levels of N supplementation. Manageme nt of GM approaches to fertility may thus need to be “preventative” rather th an “therapeutic” because plant and root characteristics will not provide adequate warning time to adjust management. Despite 50% higher plant population, lower N recovery for all tr eatments occurred during 2003 compared to 2002 (Tables 3.11 and 3.19). While values for tissue dry weights of all treatments in 2003 (compared to values from similar sample dates in 2002) increased 20-80%, values for total N conten t in 2003 decreased. As mentioned above, Nvalues from all corn tissue samples in 2003 ma y have been underestimated by a constant fraction (which would not change statistical findings). However, the decreased N may be partly explained by rainfall distribution – less than 25 mm (less than one inch) fell during the first six weeks of corn gr owth in 2002 but greater than 100 mm (greater than four inches) fell during the same period in 2003. Chem ical N, applied at 0, 3, and 5 WAE, as well as mineralized N from GM residues, may have suffered far more leaching in 2003, especially if topsoil was alre ady near field capacity from irrigation. Suction lysimeters were installed to quantify such N-leaching losse s in selected treatments, but due to coarse soil texture sample extraction was far too inc onsistent to yield results. Nonetheless, unlike 2002, when little or no net N uptake o ccurred after 6 WAE, tissue samples in 2003 revealed 40-45% of total plant N uptake occu rred between 6 WAE and final harvest (9 WAE) for highest yielding treatments (C onv 267N, Conv 200N, and SH+L 133N; Tables

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80 3.11 and 3.19). Considering the possibly low N c ontent of corn plants in 2003 and the lack of yield plateau at 200N, such late -season N-uptake does not seem unreasonable. With higher plant density and lower appa rent N-recovery in 2003, differences in size and location of the available N pool a ppear to have become more important. Treatments supplemented with less chemical N showed lower total ea r yield gains and/or greater reductions in market able ear yield as a fracti on of total ear yield in 2003 compared to 2002 (Tables 3.1 and 3.2). The lower N-content of vetch residues in 2003 may have also become a greater liability. Ev en in the best 10 (of 24) plots where it was planted, vetch N accumulation in 2003 was 10 kg N ha-1 less than lupin in 2002, with 3040 kg N ha-1 reductions in some of the worst plots. As a result, average N content of SH+L residue at the time of corn planting was little more than 50 kg N ha-1, and N released from decomposing SH+L residue dur ing the corn growing season may have only been some fraction of this total. Nitrogen from vetch residues – which possessed highest N concentrations of all GMs studied – may ha ve mineralized rapidly and been especially vulnerable to leaching loss during early-season rains of 2003. Increased root distribution near the soil surface and near the plant may have ameliorated potential N and water stresses for SH+L 133N during early to mid season, but may also have exacerbated them during la te-season ear fill, especially as late-season N-uptake appears to have been a factor (s ee Chapter 4). Taken together, lower GM N content, greater release of N early in the corn growing season during higher rainfall, higher plant population, and root patterns combined with continued N demand through late season may have reduced the relative ea r yield benefit of GMs in 2003 compared to 2002.

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81 Decline in ear quality fo r GM treatments during 2003 was evidenced by greater reductions in fancy ears as fraction of total yields compared to conventional (Tables 3.1 and 3.2). At final harvest in 2003, ears from GM treatments with 133N also appeared to suffer more from overmaturity, which was not quantified but may have reduced apparent yields and grade quality. Data from a colla borative study in Tifton (Phatak et al. unpublished) suggests corn ears may indeed have matured earlier with GMs than without. These changes in ear yield timing and/or quality may also be related to some combination of rooting patterns and GM N release potential. Given our particular management strate gies, it appears insufficient N from our summer leguminous GM was immobilized over the winter until spring corn planting, nor did our winter leguminous GM perform well en ough to accumulate N at levels similar to those seen in temperate environments. Our re sults were therefore similar to those of previous investigators in north Florida (Gallaher 1993, Ga llaher and Eyelands 1985) or similar environments (Mansoer et al. 1997, Jera nyama et al. 2000), with less benefit from GMs than found in temperate (Griffin et al. 2000, Cline and Silvernail 2002) and tropical (Ladha et al. 1996, Seneratne and Ratnasi nghe 1995, Aulakh et al. 2000, Agustin et al. 1999) environments. Scheduling of chemical N supplementation, which delivered twothirds of applied NH4NO3 during the first 4 weeks of gr owth, may have conflicted with simultaneous release of N from GM residues. In both years, conventi onal treatments with 200N and 267N may have gained advantage by r eceiving more N at final application date (5 WAE) than GM treatments, especially dur ing a year with high ear ly season rainfall. Notwithstanding possible long-term benefits for weed and pest control (see Chapter 6) or changes in soil properties conducive to crop production (see Chapter 5), we should

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82 consider the following options: altering manage ment of sunn hemp (or similar GMs) in our reduced-tillage/reduced -mowing system to better immobilize N during winter decomposition (see Chapter 2) and improve corn root growth patterns (see Chapter 4); following sunn hemp with a fall/winter economic crop that will make better use of sunn hemp N; moving sunn hemp to the spring and following it with a summer or fall economic crop; and/or substituting sunn hemp with another legume for which seeds may be harvested, thereby generating an econom ic benefit while removing “excess” N from the system. Substitution of our winter GM monocrop with mixtures of winter legumes, grasses, small grains and/or non-leguminous dicots in the continuation of this project appears to have dramatically improved winter GM poten tial (Lavila and Scholberg, unpublished; see also Karpenstein-Machan and Stuelpnage l 2000, Cline and Silver nail 2002). We also recommend better exploitation of direct pl anting winter GMs or economic crops into living sunn hemp (or another easily broken GM) so as to eliminate gap time between rapid sunn hemp decomposition and crop N uptak e (see also Chapters 1 and 2). Chemical (or animal manure) N supplementation in GM approaches in the north Florida environment should probably deliver more N at mid season to avoid unnecessary coincidence with early season GM N re lease and reduced crop N demand. Finally, organic approaches to crop production relyi ng heavily on GM N may be less risky with lower crop plant populations and with use of crops having lower N demand and without price premiums for large fruit size. Conclusions Green manure approaches to N fertilization of spring sweet corn in a north Florida reduced tillage system significantly increased vegetative and reproductive tissue growth,

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83 N accumulation and most leaf indicators. Great est benefits often came during the first 2-4 weeks of growth, although performance of corn amended with SH+L and 133 kg N ha-1 and (to a lesser extent) SH and 133 kg N ha-1 rivaled that of corn with 200 or 267 kg N ha-1 but fell behind in terms of fi nal ear yields. Final ear yield trends were best predicted by statistical differences in total N applied to corn at planting in th e form of residue and subsequent NH4NO3 supplementation, but may also be re lated to timing of N availability and dynamic changes in root gr owth patterns (explored in Chapter 4). Improvement of GM benefits may require selection of differe nt GMs or GM mixtures and modification of management techniques including residue ma nagement, selected crop rotation, planting procedure, and scheduling of N supplementation. 0 2 4 6 8 10 12 14 16 SH+LSHLConvMarketable Ear FW (Mt ha-1) 0 67 133 200 267 kg NH4NO3-N ha-1 A 0 4 8 12 16 20 24 SH+LSHLConvMarketable Ear FW (Mt ha-1) 0 67 133 200 267 kg NH4NO3-N ha-1 B Figure 3.1. Marketable ear yields as fres h weight by treatment, 2002 (A) and 2003 (B). Error bars reflect standard errors.

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84Table 3.1. Pairwise contrasts of select ed nitrogen factors and ear yields, 2002. Treatment NUE NAC UAN UAN-Total# Fancy Ears Marketable Ears Total Ears kg kg-1 kg N ha-1 kg N ha-1 kg N ha-1 Mt ha-1 Mt ha-1 Mt ha-1 Conv 200N 0.33 200† 127† 129† 12.3 13.9 14.8 Conv 267N 0.28 267* 178* 187* 12.9 14.4 15.4 SH+L 67N 0.34 116*† 61*† 138† 6.5*† 8.1*† 9.9*† SH 67N 0.40*† 79*† 32*† 123† 5.2*† 7.3*† 9.5*† L 67N 0.24 127*† 81*† 96*† 5.3*† 7.3*† 9.3*† SH+L 133N 0.28 211† 139† 218* 11.8 12.8 13.8† SH 133N 0.34 144*† 79*† 163* 10.7† 11.8*† 12.9*† L 133N 0.19 209† 151† 168† 10.6*† 12.1*† 12.9*† SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; NUE = N uptake efficiency; NAC = N applied at or after corn planting; UAN = applied N not recovered by corn; # includes N fr om sunn hemp residue and weeds prior to sunn hemp death; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively. Table 3.2. Pairwise contrasts of select ed nitrogen factors and ear yields, 2003. Treatment NUE NAC UAN UAN-Total# Fancy Ears Marketable Ears Total Ears kg kg-1 kg N ha-1 kg N ha-1 kg N ha-1 Mt ha-1 Mt ha-1 Mt ha-1 Conv 200N 0.24 203 149 184 14.2† 17.0† 18.9† Conv 267N 0.19 271 213 252 17.7* 20.1* 21.7* SH+L 67N 0.15 113*† 86*† 214† 2.8*† 6.2*† 7.6*† SH 67N 0.21 99*† 70*† 200† 2.4*† 5.7*† 7.0*† L 67N 0.20 81*† 58*† 81*† 2.7*† 5.3*† 6.7*† SH+L 133N 0.24 184*† 139† 261* 11.9† 14.8† 16.3† SH 133N 0.20 163*† 128*† 258* 10.4*† 13.3*† 14.9*† L 133N 0.22 168*† 140† 170† 8.3*† 11.9*† 13.6*† SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; NUE = N uptake efficiency; NAC = N applied at or after corn planting; UAN = applied N not recovered by corn; # includes N fr om sunn hemp residue and weeds prior to sunn hemp death; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively.

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85 Table 3.3. Ear yields at final harvest, (fresh weight, kg ha-1), 2002 and 2003. 2002 2003 Fancy Marketable Total Fancy Marketable Total GM x N-Rate NS NS NS NS NS NS GM ** ** *** * SH+L 6.1 a 7.2 a 8.7 a 4.9 a 7.2 a 8.3 a SH 5.3 a 6.5 a 8.1 a 4.3 ab 6.4 ab 7.6 ab L 5.5 a 6.9 a 8.4 a 3.7 ab 5.9 ab 7.1 ab Conv 3.6 b 5.0 b 7.0 b 3.3 b 5.5 b 6.7 b N-Rate *** *** *** *** *** *** 0N 0.2 c 0.5 c 2.1 c < 0.1 c 0.3 c 0.9 c 67N 5.1 b 7.0 b 9.2 b 2.4 b 5.4 b 6.7 b 133N 10.2 a 11.6 a 12.7 a 9.7 a 13.0 a 14.7 a NS: means within columns for GM a nd N-rate not different at the p 0.05 level; *, **, ***: means within columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within ve rtical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test. Table 3.4. Leaf area index, 2002 (m2 leaf m-2 ground). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM * NS NS NS SH+L 0.18 a 1.06 a 1.98 1.88 1.13 a SH 0.20 a 1.17 a 2.16 1.78 1.08 ab L 0.17 a 0.85 b 1.58 1.82 0.90 b Conv 0.14 b 0.81 b 1.86 1.67 0.99 ab N-Rate *** *** *** *** *** 0N 0.12 c 0.51 b 1.30 b 1.05 c 0.68 c 67N 0.18 b 1.15 a 2.18 a 1.96 b 1.05 b 133N 0.21 a 1.25 a 2.21 a 2.36 a 1.34 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test.

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86Table 3.5. Pairwise contrasts of leaf ar ea index and specific leaf nitrogen, 2002. Treatment Leaf Area Index (m2 m-2) Specific Leaf Nitrogen (g N cm-2) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 0.21 1.29 2.78 2.45 1.52 54.2 57.4 76.8 73.5 71.3 Conv 267N 0.20 1.40 2.83 2.76 1.67 48.2 63.5 72.8 74.4 76.3 SH+L 67N 0.18 1.18 2.31 2.09† 1.03*† 42.9* 50.7† 57.0 62.6 48.4*† SH 67N 0.20 1.43 2.73 1.93*† 1.23*† 44.2* 47.2*† 44.9*† 47.0*† 61.4† L 67N 0.16 1.00 1.57*† 1.97*† 0.90*† 30.8* 56.0 81.6 55.6*† 51.1*† SH+L 133N 0.22 1.39 2.12 2.49 1.42 49.2 56.9 79.9 63.1 66.5 SH 133N 0.25 1.51 2.41 2.33† 1.29† 51.7 56.2 64.3 63.2 61.6† L 133N 0.22 1.09 2.16 2.38† 1.34*† 49.4 58.4 71.0 61.1 65.2 WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively.

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87 Table 3.6. Leaf dry weight, 2002 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM * NS SH+L 49 a 420 a 926 a 1051 a 702 a SH 54 a 462 a 935 a 982 ab 679 a L 45 a 357 b 951 a 953 ab 539 ab Conv 36 b 336 b 740 b 893 b 617 b N-Rate *** *** *** *** *** 0N 32 c 195 b 499 c 543 c 401 c 67N 48 b 462 a 1002 b 1080 b 647 b 133N 58 a 523 a 1163 a 1286 a 854 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test. Table 3.7. Total dry weight, 2002 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM * NS NS SH+L 89 a 757 ab 3734 5712 5385 a SH 98 a 825 a 3920 5492 5042 ab L 85 ab 657 ab 3872 5063 4661 b Conv 68 b 598 b 3375 4869 4705 b N-Rate *** *** *** *** *** 0N 61 c 347 c 1469 b 2555 c 2373 c 67N 89 b 819 b 4551 a 6225 b 5334 b 133N 105 a 962 a 5155 a 7072 a 7138 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test.

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88 Table 3.8. Leaf nitrogen content, 2002 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM * NS NS SH+L 0.8 a 5.3 ab 10.7 a 11.0 a 6.1 SH 0.9 a 5.7 a 10.0 ab 9.4 ab 6.2 L 0.7 b 4.6 ab 10.8 a 9.9 ab 5.1 Conv 0.6 b 4.3 b 8.7 b 9.0 b 5.1 N-Rate *** *** *** *** *** 0N 0.5 c 1.8 c 4.3 c 4.4 c 3.0 c 67N 0.8 b 6.0 b 11.7 b 10.4 b 5.4 b 133N 1.1 a 7.1 a 14.1 a 14.7 a 8.5 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test. Table 3.9. Total nitrogen content, 2002 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS GM NS NS NS ** SH+L 1.5 ab 12.1 a 41.5 47.8 41.8 a SH 1.7 a 12.6 a 39.7 42.9 36.3 a L 1.4 ab 10.7 ab 42.8 40.8 35.7 a Conv 1.1 b 9.3 b 37.9 39.9 32.0 b N-Rate *** *** *** *** *** 0N 0.9 c 3.7 c 13.0 c 16.8 c 14.8 c 67N 1.4 b 13.0 b 45.9 b 47.0 b 35.7 b 133N 2.0 a 16.9 a 62.7 a 64.6 a 58.7 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test.

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89Table 3.10. Pairwise contrasts of leaf dry weight and nitrogen content, 2002. Treatment Leaf Dry Weight (kg ha-1) Leaf N Content (kg N ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 60 541 1314 1404 1020 1.1 7.3 17.8 17.9 10.8 Conv 267N 54 591 1338 1560 1055 1.0 8.9 19.3 20.7 12.6 SH+L 67N 49 448 984*† 1221† 635*† 0.8 5.9† 11.8*† 13.1*† 4.9*† SH 67N 54 563 1064*† 1059*† 793*† 0.9 6.9 12.1*† 9.0*† 7.3*† L 67N 41* 409† 1073† 1048*† 514*† 0.5* 5.6† 12.8*† 10.6*† 4.5*† SH+L 133N 60 590 1224 1383 905 1.1 7.9 16.1 15.7† 9.4† SH 133N 73† 600 1304 1303† 840† 1.3 8.4 14.3*† 14.8† 8.0*† L 133N 61 471 1203 1233† 823*† 1.1 6.4† 14.1*† 14.4† 8.8† WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively. Table 3.11. Pairwise contrasts of total dry weight and nitrogen content, 2002. Treatment Total Dry Weight (kg ha-1) Total N Content (kg N ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 107 994 5950 7886 7730† 2.2 18.4 96.9 81.5 72.5† Conv 267N 98 1122 5800 7984 8486* 2.0 22.0 82.4 90.3 89.2* SH+L 67N 90 800† 4345 6896 5498*† 1.4* 12.9*† 46.9*† 54.8*† 38.3*† SH 67N 99 994 4742 6138 5675*† 1.6 14.9† 45.2*† 46.9*† 40.7*† L 67N 78 739† 4509 6048† 4956*† 1.1* 12.6*† 46.4*† 45.8*† 33.3*† SH+L 133N 108 1082 4856 7674 7651† 2.1 19.1 63.4* 71.8 68.9† SH 133N 130 1093 5370 7453 6978† 2.5 19.4 62.3* 65.3† 53.3*† L 133N 109 884 5042 6459 6834*† 2.1 15.6† 66.7* 57.8*† 57.9*† WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively.

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90Table 3.12. Leaf area index, 2002 (m2 leaf m-2 ground). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM NS NS SH+L 0.29 1.96 a 2.24 ab 1.95 1.64 ab SH 0.30 1.78 ab 2.51 a 2.15 1.66 a L 0.29 1.53 ab 1.99 ab 1.71 1.38 b Conv 0.27 1.56 b 1.94 b 1.78 1.34 b N-Rate *** *** *** *** *** 0N 0.23 c 0.99 b 1.33 c 1.05 b 0.96 c 67N 0.29 b 1.99 a 2.24 b 2.18 a 1.49 b 133N 0.34 a 2.14 a 2.94 a 2.46 a 2.07 a WAE: weeks after emergence. NS: means within co lumns for GM and N-rate not different at the p 0.05 level; *, **, ***: means within columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means w ithin vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Dun can’s Multiple Range Test. Table 3.13. Pairwise contrasts of leaf area index (m leaf m-2 ground) and specific leaf nitrogen (g N cm-2), 2003. Treatments Leaf Area Index (cm2 cm-2) Specific Leaf N (g N cm-2) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 0.36 2.05 2.89 2.42 2.28 57.8 29.5 40.5† 49.1 45.6 Conv 267N 0.35 2.42 2.79 2.74 2.71 55.4 28.2 48.6* 51.6 45.6 SH+L 67N 0.31 2.13 2.32 2.25 1.42*† 43.2*† 21.9 29.7*† 36.0*† 31.1*† SH 67N 0.30 1.94† 2.25* 2.45 1.82† 46.0*† 26.6 30.4*† 36.4*† 26.9*† L 67N 0.31 2.01 2.28* 1.99† 1.35*† 47.7* 21.7 34.7† 32.9*† 26.9*† SH+L 133N 0.32 2.43 2.84 2.68 2.42 54.1 27.5 40.8† 48.3 35.1*† SH 133N 0.35 2.17 3.67*† 2.50 2.00† 55.5 33.3 38.0† 44.2 33.1*† L 133N 0.35 1.83† 2.62 2.18 1.89† 47.6* 28.7 35.2† 45.8 32.0*† WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively.

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91 Table 3.14. Leaf dry weight, 2003 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM NS NS NS SH+L 91 420 ab 1122 ab 1064 ab 953 a SH 98 462 a 1206 a 1170 a 925 ab L 93 357 b 1041 bc 943 ab 792 ab Conv 84 336 b 949 c 962 b 760 b N-Rate *** *** *** *** *** 0N 68 c 195 b 627 c 567 b 531 c 67N 95 b 462 a 1140 b 1204 a 888 b 133N 111 a 523 a 1471 a 1333 a 1155 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test. Table 3.15. Total dry weight, 2003 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM NS * NS SH+L 163 757 ab 5080 a 6847 ab 8071 a SH 174 825 a 5238 a 7297 a 7586 a L 168 657 ab 5009 a 6032 b 7254 ab Conv 153 598 b 4155 b 5971 b 6380 b N-Rate *** *** *** *** *** 0N 125 c 347 c 2395 c 3289 b 3766 c 67N 171 b 819 b 5601 b 7917 a 7956 b 133N 198 a 962 a 6617 a 8404 a 10247 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test.

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92 Table 3.16. Leaf nitrogen content, 2003 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM NS NS NS NS SH+L 1.3 4.3 ab 7.4 a 7.5 5.3 SH 1.4 4.5 a 7.8 a 7.5 4.7 L 1.3 3.6 ab 6.4 b 6.0 3.8 Conv 1.2 3.4 b 5.3 b 6.3 4.0 N-Rate *** *** *** *** *** 0N 0.8 c 1.4 c 2.4 c 2.1 c 2.1 c 67N 1.4 b 4.6 b 6.8 b 7.1 b 4.2 b 133N 1.7 a 5.9 a 10.9 a 11.2 a 7.1 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test. Table 3.17. Total nitrogen content, 2003 (kg ha-1). 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE GM x N-Rate NS NS NS NS NS GM NS NS NS NS SH+L 2.0 6.7 17.8 a 26.1 a 28.0 a SH 2.2 6.8 17.9 a 25.1 ab 24.2 ab L 2.0 5.4 16.3 ab 19.2 c 23.4 ab Conv 1.9 5.2 12.8 b 20.0 bc 21.9 b N-Rate *** *** *** *** *** 0N 1.3 c 2.0 c 5.8 c 7.2 c 8.5 c 67N 2.1 b 6.9 b 17.3 b 24.7 b 22.2 b 133N 2.7 a 9.2 a 25.5 a 35.8 a 42.5 a WAE: weeks after emergence. NS: means within columns for GM and N-rate not different at the p 0.05 level; *, **, ***: means with in columns different at the p 0.05, 0.001, and 0.0001 level, respectively; means within vertical columns for GM and N-rate followed by the same letter do not differ at the p 0.05 level according to Duncan’s Multiple Range Test.

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93Table 3.18. Pairwise contrasts of leaf dry weight and nitrogen content, 2003. Treatment Leaf Dry Weight (kg ha-1) Leaf N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 123 541 1400 1352 1250 2.0 6.1 11.9 12.2 10.4 Conv 267N 119 591 1406 1497 1527 2.0 6.8 13.5 13.9 12.3 SH+L 67N 95 448 1134*† 1239 920*† 1.4*† 4.7† 6.9*† 7.8*† 4.4*† SH 67N 98 563 1151 1352 1064† 1.4*† 5.1 6.9*† 8.6*† 4.9*† L 67N 102 409† 1206 1139† 773*† 1.5*† 4.2† 8.0*† 6.7*† 3.6*† SH+L 133N 110 590 1467 1463 1312 1.7 6.6 11.7 12.8 8.6† SH 133N 119 600 1752*† 1364 1085 1.9 7.0 13.7 11.1 6.8*† L 133N 111 471 1380 1152† 1113 1.7 5.2 9.2 9.2† 6.0*† WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively. Table 3.19. Pairwise contrasts of Tota l Dry Weight and N Content, 2003. Treatment Total Dry Weight (kg ha-1) Total N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 217 994 6328 8853 10251 3.1 9.8 30.3 40.9 54.7 Conv 267N 221 1122 5762 8857 10720 3.2 11.5 34.2 46.0 57.8 SH+L 67N 173 800† 5484 7984 8233*† 2.1*† 7.1† 17.2*† 27.2† 21.8*† SH 67N 171 994 5740 8448 9037 2.1*† 7.4† 17.4*† 29.0† 26.1*† L 67N 184 739† 6348 8132 7706*† 2.3*† 6.4*† 21.7*† 23.4† 21.4*† SH+L 133N 196 1082 6560 9544 11091 2.7 10.6 27.9 44.1* 50.1 SH 133N 211 1093 7201† 8485 9424 3.0 10.8 30.2 35.3† 37.9*† L 133N 200 884 6609 7141 10587 2.6 8.0† 22.0*† 27.9† 40.6*† WAE: weeks after emergence. SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; mean different from Conv 200N and Conv 267N at the p 0.05 level, respectively.

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94 CHAPTER 4 EFFECTS OF GREEN MANURE AM ENDMENT ON SWEET CORN ROOT LENGTH DENSITY AND DISTRIBUTION Introduction and Literature Review Information on root distribution patt erns may improve understanding of crop responses to inputs. Root length density (RLD) – defined as length of roots per unit volume of soil – may give some indication of plant response to environmental factors. Increased RLD in response to increased nut rient and water availability, a phenomenon known as root proliferation may reflect greater water a nd nutrient uptake potential (Paolillo et al. 1999). However, root water and nutrient uptake may be dictated by a complex interplay of RLD, soil water and nutrien t availability, root ag e, plant stress, and soil aeration status. Additionally, soil water and nutrient status may change more rapidly (hours to days) than RLD can respond (day s to weeks). For example, Coelho and Or (1999) found that RLD for a corn ( Zea mays ) row crop was most associated with root water uptake when water source was in-row and on the soil surface, but less so for buried in-row and surface between-row water sources. On a loamy sand in North Carolina, Durieux et al. (1994) fou nd that application of NH4NO3 at 0 and 4 weeks after emergence (WAE) increased RLD in field corn at silki ng and at 20 days before silking, but reduced RLD at physiological maturity. Working with co nventional tillage on a silty clay loam in Nebraska, Eghball and Maranville (1993) found th at a deeply-rooted (to 0.9 m) field corn genotype had greater yield response than shallo w-rooted varieties when irrigation led to

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95 deeper water infiltration, and that moderate N and water stress increased RLD uniformly throughout the soil profile while severe water and N stress reduced RLD. Effective rooting depth may li mit plant access to water and nutrients as they move down the soil profile. In a greenhouse study w ith a potting soil and sand mixture, Eghball et al. (1993) removed entire corn root sy stems and found 52.7%, 37.6%, and 9.7% of root length in the 0-0.3 m, 0.3-0.6 m, and 0.6-0.9 m de pths. Root age may also play a role in potential uptake; Gao et al. (1998) showed that short-term nutrient uptake in corn and wheat may be affected by root system age, with newer roots taking up more nutrients. However, as plants themselves became olde r (after 48 days), the investigators found less correlation of nutrient uptake with new RLD. Due to different nutrient release ch aracteristics and effects on soil water, temperature, and biota, root growth pattern s may be markedly different following green manures (GMs) compared to chemical fertili zer. Because their N-release is driven by decomposition, GMs may represent a source of slow-release N. Spa tial distribution of GM residue may be heterogeneous, creating lo calized areas of N-release and other GMmediated impacts (for example, see Mahm oudjafari et al. 1997). Green manures may have effects on soil moisture transfers, temp erature, and populations of root-parasitizing organisms such as nematodes. To help explain ear yield patterns, Goldst ein (2000) studied field corn roots on a fine textured soil in Wisconsin under three conventionally tilled management systems: corn monoculture with chemical N (CS1), corn-soybean-winter whea t-red clover rotation (CS3), and corn-oat-alfalfa with dairy co w manure (CS5; highest yielding treatment). According to Goldstein, previous research based on N application da ta could not account

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96 for significantly higher corn yields respons es for CS5 compared to CS1 and CS3. In the top 15 cm of soil Goldstein found RLD explai ned 68-79% of unexplai ned variability of ear yield within each system, while corn amended with organic N-sources maintained healthier roots (76%, 63%, and 59% roots without visible damage for CS5, CS3, and CS1, respectively) and required lower RLD for maximum ear yields (1.63, 1.74, and 2.12 cm cm-3 for CS5, CS3, and CS1, respectively) a ccording to regression or actual data. Results apparently contradict ed Pallant et al. (1997) w ho found greater RLD for GMamended corn in the upper 30 cm. Goldstei n (2000) may have underestimated RLD for GM treatments by not accounting for roots below 15 cm, especially as GM and animal manure material had been soil-incorporated. Further, the higher RLD for conventional monocropped corn in the upper 15 cm may have rendered it more susceptible to water stress. According to Nickel et al. (1995), RLD for corn grown in rotation with soybean ( Glycine max ) tends to be higher than for corn grown in monoculture even under high inputs. In a conventional tillage system, Nickel et al. (1995) found greater RLD for monocropped corn in the upper 12.5 cm at 4 WA E and greater RLD for corn in rotation (with soybean) at deeper soil depths of 12.5-25cm (early season), 37.5-50cm (mid-late season), and 12.5-37.5cm (mid-late season), a lthough soybean RLD tended to be higher under monoculture. Pallant et al. (1997) also found that incr eases in soil organic matter significantly increased corn RLD on two of four sample dates. For potato ( Solanum tuberosum ), Opena and Porter (1999) report that organic amendments (compost plus beef cattle manure) significantly increased RLD in the 0-30 cm plow layer and did not change relative distribution of roots by depth (~85% of RLD in 0-30 cm layer). However, Thorup-Kriste nsen and van der Boog aard (1999) found that

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97 surface applying increased amounts of high-N GM residue reduced carrot ( Daucus carota ) root proliferation in the upper 1 m of soil and shifted r oot proliferation closer to the plant. As part of a larger study on improved use of GMs in vegetable cropping systems in the southeast US and north Fl orida, we investigated a GM sequence of summer planted sunn hemp (SH) followed by a winter legume (L) of blue lupin ( Lupinus angustifolius winter 2001-02) and cahaba white vetch ( Vicia sativa winter 2002-03) as an N-source for sweet corn under redu ced tillage. Details of GM grow th and decomposition patterns can be found in Chapter 2, and yield responses of sweet corn to the GM sequence and to the component GMs alone is discussed in Chapte r 3. In these studies, SH+L produced a cumulative 12-15 Mt dry matter ha-1 and up to 170 kg N ha-1 annually. Nitrogen benefit from SH+L to a subsequent crop of sweet corn was highest during the first 2-4 weeks after corn emergence, and although growth re mained largely equivalent to conventional corn with 200-267 kg NH4NO3-N ha-1 throughout the season, ear yields and some tissue factors at maturity were not as high with SH or SH+L supplemented with 133 kg NH4NO3-N ha-1. To better explain early season advant age and late season decline of GM amended corn, we initiated a root study of selected treatments. We hypothesized that amendment with SH+L would increase overall sweet corn RLD in the sampled area, that sweet corn RLD would be redist ributed nearer to the GM resi due (in this case, near the surface as we used reduced-tillage), but that corn with a high chemical N-rate (267 kg NH4NO3-N ha-1) would show greater RLD than co rn at lower N-rates (0 or 133 kg NH4NO3-N ha-1) with or without SH+L.

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98 Materials and Methods Set-up and Design Root growth analysis utilized five trea tments that were part of the larger GM study described in Chapter 1. Timeline of ope rations and corn planting patterns are described in Chapter 3. Related experimental design and set up only are described here. The five treatments used for this analysis consisted of sweet corn following: sunn hemp and lupin/vetch rotation with 0 kg N ha-1 applied to corn (SH+L 0N); sunn hemp and lupin/vetch rotation with SH+L 133 kg N ha-1 (SH+L 133N); and “conventional” sweet corn with 0, 133, or 267 kg N ha-1 applied (Conv 0N, Conv 1 33N, Conv 267N; See Table 1.2). After initial field plow all crops were pl anted with zero or re duced tillage and all chemical N was band applied to corn rows by hand. Samplings of these treatments for RLD analysis took place in their second year (2003). All treatments were repeated four times within the larger randomized co mplete block design (see Chapter 1). Field and Lab Procedures Using a 5 cm soil auger (Forestry Suppliers, Inc; Jackson, MS) of known volume, soil cores were extracted at 3, 5 and 8 week s after emergence (WAE ) of sweet corn. In each plot, soil was extracted from three different depths: 0-15 cm; 15-30 cm; and 30-60 cm; and in three different surface positions: in-row and immediatel y adjacent to a corn plant (IR0); in-row and halfway between two corn plants (IR0.5); and between-row (halfway between two corn rows; BR), givi ng nine unique locations (Figure 4.1). Soil extraction was conducted away from border area s and near plants representative of the plot in both size and spacing. Soil cores were placed into pl astic bags and refrigerated until root extraction (not more than 2 weeks). At that time, soil cores were washed in a grain sieve with pores 4.5 mm in diameter Although fine roots could not be accounted

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99 for, this grain sieve satisfactorily retained visible roots which were then separated from debris. Using software and hardware from Regent Instruments (Quebec City, Canada), roots were scanned and subse quent image analyzed with th e Winrhizo program for root length. The results were transferred to MS Ex cel (Microsoft Corporation, Los Angeles, CA) for graphical analysis and preparation fo r SAS (Statistical Analysis Systems; Cary, NC). Data Analysis To test the effects of SH+L amendment and chemical N-rate on root length and distribution at each sample date, five different balan ced ANOVAs were run for each sample date for results from SH+L 0N, SH+L 133N, Conv 0N, and Conv 133N using SAS software package. Sample data wa s organized in two primary ways: on a density basis expressing root length per cubic cm of soil (cm cm-3), and on a relative basis with root length data expressed as a fraction of the total from that plot (unitless). Use of RLD allowed straightforward comparisons of root proliferation, while root length expression on a relative basis permitted comparisons of root distribution between treatments regardless of absolute size. Within each of these two techniques, data was expressed in two more ways: data sets w ith individual entries for all 9 unique locations based on the three depth levels and three positions ( location ), and data sets with pooled values based arbitrarily on whether samples we re near or far from the plant ( proximity ). The proximity groupings were established to de velop clearer trends based on a natural spatial pattern in root growth, with 62% or more of sample d RLD in all treatment s being found in four locations “near” the plant (IR0 and IR0.5, 0-15 and 0-30cm) and the remainder of RLD (generally less than one third) found in the ot her five locations “far” from the plant. A

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100 fifth ANOVA was run for average RLD of all sampled volume in each treatment to compare overall differences in total sampled root length. Root length densities were log transformed by log10(x+1) before ANOVA to maintain homoscedasticity. Model statements re gressed RLD or relative root length as a function of N-rate (0 or 133 kg N ha-1), GM application (SH+L or Conv), depth (0-15cm, 15-30cm, 30-60cm) and position (IR0, IR0.5, and BR) or proximity (near or far) where applicable, all possible first-degree interactions of these variables, and block (four blocks; see Chapter 1). Where interaction terms were significant, separate ANOVAs were run to compare levels of one interacting variable within specific levels of the other interacting variable(s). Non-interacting va riables (except block) were no t included in the interaction model statements. Because of the inhere nt variability in root measurements, = 0.10 was considered significant. Comparisons of means were always made with Duncan’s multiple range test. To compare root length and distribution patterns of these treatments to highfertilized, high-producing corn, pairwise contrasts of Conv 267N were made with each of the other treatments. These ANOVAs used the same independent variables as the balanced design except that GM level and N-rate level were substituted with the appropriate overall treatment title (C onv 0N, Conv 133N, Conv 267N, SH+L 0N, and SH+L 133N). Where interaction of treatment with depth and/or location or proximity were significant to = 0.10, separate ANOVAs were run to compare treatments within specific levels of the interacting variable(s ). Non-interacting vari ables (except block) were not included in the inte raction model statements.

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101 Actual data, whether expressed as RLD or relative root length, is for sampled volumes only. Because three positions (IR0, IR0.5, BR) were sampled in a triangular fashion, no more than two positions can be connected with any line within a given depth, and therefore any attempt to interpolate RLD between the two position points lacks information needed to fit any non-linear behavior. However, the three depths sampled with in each position allo wed for fitting of nonlinear trendlines (where appropriate) reflecti ng RLD as a function of depth. An effective rooting depth was therefore defined as depth at which 90% of calculated RLD within the top 100 cm of soil occurred. Using Maple 8 ma thematical software (Maplesoft; Waterloo, Canada), the following equation was solved for x* : ( 0cm x* y dx) / ( 0cm 100cm y dx) = 0.9; where y = RLD (cm cm-3) as a function of x, x = depth (cm), and x* = effective rooting depth (cm). Three slightly different gra phs could be developed to ge nerate an equation for RLD as a function of depth. Average RLD for any gi ven layer could be graphed: (1) once, in the center-point of the layer; (2) at both the upper and lower bounds of the soil layer; and (3) in the center-point and the upper and lowe r bounds of the layer. All three trendlines resulted in similar effective rooting dept hs (see below). Given the variability of measurements and the extrapolation of RL D beyond measured depth, this calculated effective rooting depth is a general indicator only, based on the assumption that sweet corn root growth beyond 60 cm follows the same pattern with respect to depth established within the upper 60 cm. We th erefore do not make strict statistical comparisons of effective rooting depth.

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102 Suction lysimeters (constructed with materials from Soil Moisture; Goleta, CA) and transducing tensiometers (model “R” irrome ters; Spectrum Technologies; Plainfield, IL) were installed in selected pl ots to monitor N-leaching. Unfortunately, highly inconsistent lysimeter samples (probably due to low soil wa ter content of our sa ndy soil) could not be meaningfully analyzed. Data from transduc ing tensiometers buried at 15, 60, and 90 cm in SH+L 133N and Conv 200N, intended to co mplement the lysimeter study, is, however, reported here. Data from tensiometers was recorded continuously by Watchdog dataloggers (Spectrum Technologies; Plainf ield, IL). Treatment Conv 200N was not sampled for root cores. Results Overall RLD Over the season, RLD for the sampled volume in the upper 0-60 cm soil layer remained within one order of magnitude in all treatments, from 0.15-1.50 cm cm-3, consistent with other studies (Goldstein 2000, Pallant et al. 1997, Nickel et al. 1995). Across both N-rates, amendment with SH+L increased total corn RLD in the sampled 060 cm profile by 32%, 54%, and 27% at 3, 5, and 8 WAE respectively, although increases at 3WAE were not statistically si gnificant unless sample spatial location was included in the regression model (Figure 4.2; also, see below). Increase of N-rate from 0 to 133 kg N ha-1 increased total RLD by 52%, 119% and 93% at 3, 5, and 8 WAE respectively. There was no inte raction between GM amendment and chemical N-rate for overall sampled corn RLD. Corn with SH+L 133N maintained highest overall RLD throughout the season, even compared to corn with 267N (SH+L > Conv 267N by 45%, 29%, and 6%, at 3, 5, and 8 WAE, respectivel y). However, differences in pairwise

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103 contrasts became significant only when the mo del statement included spatial information (Table 4.1; also, see below). RLD by Location As can be seen in Table 4.2 and Figur e 4.2, at 3 WAE amendment with SH+L significantly increased overall RLD when de pth and position were included in the regression model ( = 0.10). In general RLD decreased from in-row (IR0, IR0.5) to between-row (BR) and decreased with incr easing depth. However, increased root proliferation towards the soil surface was most resolvable within IR0 with significant changes in RLD values occurring at both depth transitions. At the IR0.5 position, RLD dropped significantly only at the deeper transition between 15-30 cm and 30-60 cm, while at the BR position a signi ficant drop occurred only at the first (0-15 cm to 15-30 cm) depth transition. In both 0-15 cm and 15-30cm depths a significant drop in RLD values occurred only as one moved from the IR positions to the BR position (no significant difference between IR0 and IR0.5; see Table 4.3). Corn with SH+L 133N showed greater overall RLD than Conv 267N (45% advantage), becoming significant when the model statement included depth a nd position (Tables 4.1 and 4.4). Otherwise no other treatment showed signi ficantly different RLD from Conv 267N nor did treatment level interact with depth and/or position at 3 WAE. At 5 WAE, position, GM amendment and N -rate all interacted individually with depth (Table 4.2). Application of SH+L increased RLD within all depths, but increase was greatest and significant only in the upper soil depth (64% for 0-15 cm compared to 36% for 15-30 cm and 55% for 30-60 cm; Figure 4.3). Application of 133N also increased RLD at all depths (44%, 58%, and 27% for 0-15, 15-30, 30-60 cm, respectively) but the increase was non-significan t at the 15-30 cm level due to variability

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104 (Table 4.3). Interactions be tween position and depth at 5 WAE remained identical to those seen at 3 WAE except that a significan t decrease in RLD occurred at all soil depths (including 30-60 cm) when moving from either of the IR positions to the BR position (Table 4.3). The full ANOVA used for pairwise contrasts showed tr eatment interacted individually with depth and position (Table 4.4). Root length densities for Conv 267N were statistically greater than RLDs for SH +L 0N and Conv 0N in the 0-15 cm and 15-30 cm layers and the IR0.5 position but statistica lly less than SH+L 133N in the 0-15 cm and 30-60 cm layers (Table 4.5). However, high va lues for SH+L 133N in the 30-60 cm layer may have been affected by sample variability (Table 4.5). At 8 WAE, a significant 4-way interac tion occurred between position, depth, GM, and N-rate ( = 0.06; Table 4.4; see Appendix D fo r sub-effects). Amendment with SH+L benefited corn RLD more and more uni formly at the zero N-rate (0N). In all locations but two (IR0 and IR0.5 15-30 cm) both percent-wise and abso lute increases in RLD from amendment with SH+L was greater for non-fertilized corn (relative to corn with 133N). At all locations but one (IR0.5 15-30 cm), RLD for non-fertilized corn showed increase (23%-105%) when amended with SH+L, with the benefit significant in four of eight locations. For corn at 13 3N, increase in RLD from SH+L amendment occurred only at positions IR0 and IR0.5 fo r depths 0-15 cm and 15-30 cm only (9%-53% increases) while at all other locations (all BR and all 30-60 cm) SH+L amendment on top of 133N actually decreased RLD (2-43% re ductions). However, only one of these differences (decrease of RLD at BR 1530 cm) was statistically significant. Application of chemical N (133N) also benefited RLD for unamended (Conv) corn more and more uniformly than for SH+L amended corn. Root length density for

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105 unamended corn increased at a ll locations with a pplication of 133N (compared to 0N) by 74%-204%, and these increases we re statistically significant everywhere except at IR0.5 15-30 cm and 30-60 cm only. For SH+L amende d corn, increased RLD (by 6%-122%) in response to chemical N fertiliz ation took place at all depths for both IR positions and at all positions across the 0-15 cm soil layer, although these increases were smaller than seen for Conv and significant only at IR 0 15-30 cm and IR0.5 15-30 cm and 30-60 cm. At the 133N level, amendment with SH+L pr oduced non-significant reduction in RLD at BR 15-30 cm (53%) and 30-60 cm (17%), and in fact the weakest increase for SH+L 133N compared to Conv 133N was also seen at BR 0-15 cm (6%; See Appendix D). Compared to any Conv treatment (even with 267N), corn with SH+L 133N maintained numerically higher RLD in 7 of 9 sampling locations at both 3 and 5 WAE (data not shown). At 8 WAE, however, RLD for SH+L 133N showed numerical advantage only in the four “near” loca tions (IR0 and IR0.5 at 0-15 cm and 15-30 cm) while RLD for Conv 133N and/or Conv 267N wa s numerically higher in all five “far” locations (all locations within BR and within 30-60 cm depth). Despite these numerical trends, data analysis using depth and position resolved few effects of SH+L and few differences between SH+L 133N and Conv 267N at 8 WAE (Table 4.6; Appendix D). Greater analytical resolution at this date came with data grouped on a proximity basis (see below). Relative Root Length by Location Statistical patterns of relative root le ngth by depth and position were similar to those of absolute RLD, although relative root length distributions we re less affected by GM amendment and N-rate (data not shown). Corn with SH+L exhibited significantly greater root length distribution towards th e soil surface at 3 WAE, and the trend

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106 continued (though non-significantly) at 5 and 8 WAE. Few statis tical differences existed in pairwise comparisons of relative root length between Conv 267N and all other treatments throughout the season. Throughout th e season, relative root length sampled remained greatest in the IR0 0-15 cm location (23-27%) compared to all other locations, roughly half or more (up to 55%) of sampled r oot length was always in the 0-15 cm layer, with both the upper 30 cm as well as the tw o IR positions together each containing roughly 85% of sampled root length (data not shown). RLD by Proximity Effects of SH+L on RLD by proximity became increasingly resolved over time. At 3 WAE, amendment with SH+L increased near RLD by 22% and far RLD by 68%. However, ANOVA based on proximity at 3 WAE was not as sensitive as that based on depth and position. No significan ce was detected for GMs, pr oximity, or the interaction of the two when making up a linear model (T able 4.7), nor did ANOV A show significant differences between RLD for SH+L 133N a nd Conv 267N based on proximity analysis (Table 4.8). At 5 WAE, the increase in RL D from amendment with SH+L (41% and 98% in near and far RLD, respectively) was significant with proxim ity included in the regression, but the interaction between GM level and proxim ity was again non-significant (Table 4.7). At 8 WAE, the interactio n between amendment and proximity became significant (Table 4.7 and 4.9), w ith SH+L preferentially increas ing near RLD. Corn with SH+L 133N maintained (non-sign ificantly) greatest RLD in both near and far categories throughout the sample period except at 8 WAE, when far RLD for Conv 267N was statistically greater (Tables 4.10). Although the three-way intera ction was not detected as significant at 8 WAE (Table 4.7), effects of SH+L on far RLD may have differed

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107 depending on chemical N-rate; application of SH+L at 0N appeared to increase far RLD, while SH+L at 133N may have led to decrease of far RLD (Table 4.10). At 3 WAE, application of chemical N (133N) significantly increased RLD by 65% near the plant (0.62 vs 0.37 cm cm-3 for 0N and 133N, respectively), but increase was lower (19%) and non-significant far from the plant (0.07 vs 0.08 cm cm-3 for 0N and 133N, respectively; Table 4.9). For both N-ra tes, however, sampled RLD near the plant was more than 5 times greater far RL D. Although ANOVA indicated significant interaction of N-rate with proximity at 5 a nd 8 WAE, Duncan comparisons of sub-effects revealed no differences (Tab le 4.9). Compared to Conv 0N, both SH+L 0N and Conv 133N showed numerical increases in near and far RLD with far RLD increasing more dramatically than near RLD and 133N giving greater numerical bene fit than SH+L. Near RLD continued to be 3-3.5 times greater than far RLD in all treatments at both 5 WAE and 8 WAE (Table 4.10). Relative Root Length by Proximity Trends for relative root length by proxim ity differed little from those of RLD. A three-way interaction between N-rate, GM amendment, and proximity occurred at all three sample dates for relative RLD (data not shown). Generally, amending corn with SH+L only (SH+L 0N) increased root distri bution far from the plant compared to amendment with both SH+L and 133N (SH+L 133N) or neither (Conv 0N). At 8 WAE, amendment with both SH+L 133N shifted root distribution towards the plant (76% near) compared to amendment with SH+L 0N ( 64% near) and Conv 133N (62% near). Effective Rooting Depth Root length density always decreased exponentially with increasing depth from the surface. In general, effective rooting dept h (depth at which 90% of calculated RLD of

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108 the top 100 cm occurred) remained within 40-60 cm of the soil surfac e for all treatments and dates. No apparent differences existed in effective rooting depth between the surface positions, or between any GM or chemical N-rate, reflecting the general lack of differences in measures of relative root length by depth, es pecially below 30 cm (discussed above). The method used to genera te the RLD trendline as a function of depth also created no apparent differences. Soil Water Potential Soil water potential for SH+L 133N and Conv 200N at 15 cm showed two distinct phases. From 17 April until 20 May (~0-5 WA E) soil water potential under SH+L 133N remained significantly higher than Conv 200N by an average of 2.6 0.2 kPa (See Table X). Average soil water potentials at 15 cm during this period were -16.5 0.4 kPa for SH+L 133N and -19.1 0.5 kPa for Conv 200N. Greatest differences occurred during the 12-day period from 19 April to 1 May when soil water potential under SH+L 133N was 3.9 0.1 kPa higher than under Conv 200N. From 21 May until final harvest on 19 June (~5-9 WAE) this trend reversed, with so il water potential under Conv 200N (-13.7 0.2 kPa) becoming significantly higher than in SH+L 133N (-14.9 0.1 kPa) by an average of 1.2 0.2 kPa. Overall, average daily soil wa ter potential for both treatments increased logarithmically over the seas on, probably reflecting greater water potential near the surface after canopy closure a nd shading (Figure 4.5A). Soil water potential at 60 cm also unde rwent two distinct phases but with an intermediate “buffer” period during which wa ter potential in both SH+L 133N and Conv 200N were relatively equal. From 17 April to 5 May (~0-3 WAE) soil water potential at 60 cm remained lower in SH+L 133N (-15.1 0.5 kPa) compared to Conv 200N (-13.9 0.3 kPa) by an average of 1.2 0.3 kPa. Soil water potential for the two treatments at 60

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109 cm were not significantly different from 6 May to 25 May (~3-6 WAE; Figure 4.5B), but from 26 May to final harvest (~6-9 WAE) te nsiometer readings s howed higher soil water potential for SH+L 133N (-11.8 0.4 kPa) compared to Conv 200N (-13.8 0.5 kPa) by an average of 2.0 0.2 kPa. Tensiometer read ings from 90 cm showed lower soil water potential throughout the season for SH+L 133N (-10.2 0.4 kPa) compared to Conv 200N (-8.8 0.3 kPa) by an average of 1.4 0.3 kPa (data not shown). Discussion Despite variability inherent in root core work, data analysis from several different aspects resolve definite pattern s of corn root distribution within the study environment. These patterns help explain corn growth a nd yield performance deta iled in the previous chapter. Generally, amendment with SH+L increased RLD throughout the season, but as the season progressed overall advantages declined and became more concentrated at the upper soil depths and near the plant, esp ecially when chemical N was band applied. In terms of rooting patterns, RLD decreas ed from in-row (IR0, IR0.5) to betweenrow (BR) and decreased with increasing de pth. For all treatments throughout the season relative RLD sampled remained greatest in the IR0 0-15 cm location nearest the plant (23-27% of sampled root length) compared to all other locations; roughly half or more (up to 55%) of sampled RLD was always in the 0-15cm layer, with roughly 85% of sampled RLD occurring in the IR positions (c ombined) as well as in the upper 30 cm, similar to findings reported by Eghball et al. (1993). However, interactions between depth and position altered these distribution patterns on all sample dates. Early in the season (3 WAE), RLD showed more pronounced distribution towards the soil surface as one moved to positions closer to the plant. At the BR position, early-season root exploration below 15 cm remain ed so little that RLD in these two layers were not

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110 significantly different from each other but were significantly less than RLD in the top soil layer (BR 0-15cm). Root exploration of the 30-60 cm soil layer was so sparse at 3 WAE that position relative to the plant had no eff ect at this depth. By mid-season (5 WAE), RLD values were close to maximum recorded during the study, but rooting patterns by depth and position were similar to those at 3 WAE. However, by this time RLD in both IR positions showed similar tendency towards proliferation in the upper soil layer. Root length densities in the 30-60 cm layer becam e much greater in the IR positions, but RLD values in BR 30-60 cm remained low (Table 4.3). By late-season (8 WAE) RLD patterns significantly interacted with GM amendm ent and/or N-rate (discussed below). Amendment of corn with a substantial SH+L green manure (15 Mt ha-1 year-1) over two years significantly increased total corn RLD in the top 60 cm of soil in two of three sample dates (Figure 4.2), and when the st atistical model included terms for sample depth and position the increase became significant at all da tes notwithstanding interactions. Overall RLD with SH+L 133N showed advantage even over Conv 267N, though this diminished over time (Table 4.2). At no time could increased biomass (total or root dry weights) by SH+L amended corn explain benefi ts in RLD. Root and total dry weight benefits from SH+L varied between 7%-31% over the seas on, and were always 530 percentage points less than the benefit associated with RLD. At 2 and 4 WAE, root and total plant dry weights for SH+L 133N we re less than those of Conv 267N despite significantly greater overall RLD for SH+L 133N at 3 WAE (Table 4.2; see also Chapter 3). However, GM effects on RLD interacted with sample location, chemical N supplementation, and sample date. Analysis of variance with a regression model

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111 containing arbitrarily chosen depth and positio n of samples was most useful early and mid-season, whereas regression model based more “natural” categories of sample proximity (near or far) to plant provided mo re resolution of trends during late-season. In general, corn amended with SH+L alone (SH+L 0N) exhibited greater distribution of sampled root length far from the plant compared to corn with both SH+L and 133N (SH+L 133N) or neither (Conv 0N). Early during the season (3WAE) GM amendment increased RLD throughout the soil prof ile with little intera ction with specific location or proximity (Tables 4.2 and 4.7), though an alysis of relative root length at this time indicated greater prolifer ation in the upper 15 cm of soil for SH+L amended corn. Application of chemical N also tended to increase near RLD (by 65%) more than far RLD (increase of 19%) at this time. At mi d-season (5WAE) SH+L application increased RLD most at 0-15 cm depth (Figure 4.3) and RLD for SH+L 133N demonstrated significant advantage against C onv 267N in this soil layer (Table 4.5). A large jump in RLD far from the plant also occurred for SH+L 133N (Table 4.10), but this may have been due to sample variability. At final sampling (8 WAE; one week before maturity) GM effects interacted with chemical N-rate, soil depth, and position. These patterns were most succinctly described when analyzing data by natural grouping based on sample proximity to the plant (near or far). At this time, amendmen t with SH+L significantly incr eased RLD near the plant but had almost no effect on RLD far from th e plant (Figure 4.4). Although analysis of variance detected no signifcance for the intera ction term (GM x N-rate), the effect may have been particularly pronounced for SH+L amended corn supplemented with 133N (which was band applied to rows) and not fo r corn with SH+L only (Tables 4.7 and 4.10).

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112 Such root proliferation near the plant during time of intens e ear-fill may have increased vulnerability to water and N stress for corn with both surface applied GM and banded chemical N. Similar root di stribution effects were reporte d by Thorup-Kristensen and van der Boogard (1999) for carrot amended w ith large amounts of surface applied GM. Additionally, at 8 WAE Conv 267N showed significantly greater RLD far from the plant compared to SH+L 133N (Table 4.10). These results complement and contrast findings under conventional tillage, wh ere GM amendment also appe ars to increase overall RLD but at lower soil depths (Pallant et al. 1997, Nickel et al. 1995). Coelho and Or (1999) showed that for a crop of corn a relatively minor fraction of total RLD located between-rows can make disp roportionately large contributions to root water uptake when water becomes more available there than closer to the plant. These far roots may therefore have benefited conventiona lly fertilized corn in terms of water and N-uptake potential during la te season. In the 2003 season, when N may have been particularly limiting, between 40-50% of total plant N uptake occurred from 6-9 WAE for many treatments, including Conv 267N and SH+L 133N (see Chapter 3, Table 3.19). Differences may have been exacerbated as effective N application for Conv 267N was nearly 90 kg N ha-1 greater than SH+L 133N (Chapter 3, Table 3.2). Tensiometer data reveals surface residue in combination with band-applied chemical N may have created an environment of greater water and N availability near the surface, especially early in the season befo re canopy closure. Coupled with possible net N-release from decomposition, this promoted root prolifera tion and may have benefited plants during early season. By late season mo isture level near the soil surface in SH+L 133N treatments dropped below that of conventi onally fertilized corn. Higher late season

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113 water potential at 60cm in SH+L 133N plots (compared to conventional) also suggests that SH+L 133N roots were unable to adjust gr owth further away from plants at this time (Figure 4.4). Because RLD always dropped exponentially with depth, and because sampled RLD in the 30-60 cm layer was relatively small (~10-15% of total), a total RLD was calculated for the upper 100cm of soil. The depth at which 90% of this calculated RLD occurred was defined as the effective rooting depth. E ffective rooting depth generally existed no deeper than 40-60 cm regardless of treatme nt, positions, GM levels, and N-rates at all sample dates, and regardless of the manner in which the sampled RLD was graphed so as to generate trendlines for RLD as functions of depth (see Materials and Methods). More detailed statistical analysis that could reveal greater diffe rences would not have been appropriate without more root samples take n from more finely divided soil layers. However, these results generally agree with those from Eghball et al. (1993) who excavated intact root system s from corn grown under contro lled conditions and found an effective rooting depth of roughly 60cm. Thes e data suggest water and nutrients leached well below 40-60 cm become less available to sweet corn regardless of GM application, chemical N-rate, and stage of growth, unle ss a more deeply rooted variety is used (Eghball and Maranville 1993). Irrigation management of sweet corn should probably use 50 cm as an effective rooting depth in our Florida environment. Additionally, although sweet corn at 3 WAE showed an effective rooting depth of 40-60 cm, actual RLD values and N-demand are so small that water a nd N uptake around such a depth are probably insignificant, and an effective rooting dept h of 30 cm may be more appropriate.

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114 Conclusions To summarize, amendment with SH+L in a reduced tillage system with band applied chemical N increased RLD throughout the season, but as the season progressed overall advantage declined a nd became more concentrated in the upper 15-30cm of soil nearest to the plant, especially for plants receiving chemical N. Corn with high amounts of chemical N (Conv 267N) sustained greater root growth far from the plant in the between row area and at 30-60cm depth during late season. This may help explain the advantages of GM-amended corn during the early season and in terms of vegetative growth as well as late-season gains in ear yield by chemically fertilized corn. Soil incorporation of GM resi due to help encourage deeper root growth under these circumstances would be favorab le if subsequent nutrient lo ss from decomposition did not negate the benefits. However, in warm humi d areas with coarse textured soil, reduced tillage is often desired to slow organi c matter decomposition and nutrient loss and improve low soil water retention. In such redu ced tillage systems, improved use of GMs may necessitate different irrigation manage ment, including drip lines buried below surface residue to increase infiltration. Us e of GMs or GM mixtures with more substantial below ground production may be an even less expensive and laborious way to create a better rooting environment at deeper depths. Using early se ason deficit irrigation to encourage root exploration or developmen t/use of varieties of economic crops with deeper root systems may hel p, but providing GMs with N-content closer to that applied with chemical fertilizer must remain a priority.

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115 Figure 4.1. Name, location and relative volume of root core samples.

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116 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 358 Weeks after EmergenceRoot Length Density (cm cm-3) SH+L Conva a b* bab Figure 4.2. Effect of amendment with SH+L on sampled sweet corn root length density. Error bars reflect standard erro rs; lower case letters reflect ANOVA differences within sample date, p 0.10; means significa ntly different when depth and position included in regression model. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0-1515-3030-60 Depth (cm)Root Length Density (cm cm-3) SH+L Convab a a a a Figure 4.3. Effect of amendment with SH+L on sampled sweet corn root length density by depth at 5 weeks after emergence. Erro r bars reflect standard errors; lower case letters reflect ANOVA differen ces within sample depth, p 0.10.

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117 0.00 0.50 1.00 1.50 2.00 2.50 3.00 NearFar Proximity ClassRoot Length Density (cm cm-3) SH+L Conva a a b Figure 4.4. Effect of amendment with SH+L on sampled sweet corn root length density by proximity class at 8 weeks after emer gence. Error bars reflect standard errors; lower case letters reflect ANOVA di fferences within proximity class, p 0.10. -25 -20 -15 -10 -517Apr 01May 15May 29May 12JunSoil Water Potential (kPa) SH+L 133N Conv 200N A -20 -15 -10 -5 017Apr 01May 15May 29May 12JunSoil Water Potential (kPa) SH+L 133N Conv 200N B Figure 4.5. Soil water potential at 15 cm (A ) and 60 cm (B) during sweet corn growth.

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118 Table 4.1. Pairwise contrasts against Conv 267N for overall sampled root length density (cm cm-3), 0-60 cm. Treatment 3 WAE 5 WAE 8 WAE Conv 267N 0.22 1.11 1.32 SH+L 0N 0.17 0.62† 0.86† Conv 0N 0.17 0.44† 0.51† SH+L133N 0.32X 1.42 1.39 Conv 133N 0.20 0.88 1.26 WAE = weeks after emergence; † means within column for date different from Conv 267N at the p 0.05 level. X SH+L 133N different from Conv 267N at the p 0.10 level when sample depth and position are in cluded in the regression model. Table 4.2. Significance of green manure, nitroge n rate, position and depth and sub-effects when constituting linear model for sampled root length density. Model Term Probability (p) 3 WAE 5 WAE 8 WAE N-rate x GM x Pos x Depth NS NS 0.0602& N-rate x Pos x Depth NS NS GM x Pos x Depth NS NS Pos x Depth 0.0006* 0.0036* N-rate x GM x Depth NS NS N-rate x Depth NS 0.0021* GM x Depth NS 0.0448† N-rate x GM x Pos NS NS N-rate x Pos NS NS GM x Pos NS NS N-rate x GM NS NS Pos Depth GM 0.091# N-rate 0.011 0N 0.17 cm cm-3 133N 0.26 cm cm-3 WAE = weeks after emergence; Pos = Po sition; NS not significant at the p 0.10 level; See Table 4.3; # See Figure 4.2; † See Figure 4.3; & See Appendix D; N = kg NH4NO3-N ha-1.

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119 Table 4.3. Various interactions with de pth for root length density (cm cm-3) at 3 and 5 weeks after emergence. Depth 3 WAE 5 WAE Position Position N-rate (cm) IR 0 IR 0.5 BR IR 0 IR 0.5 BR 0N 133N 0-15 0.61Aa 0.51Aa 0.21Ab 2.61A a 2.05Aa 1.11Ab 1.15Ab 2.69Aa 15-30 0.40Ba 0.47Aa 0.08Bb 1.35Ba 1.34Ba 0.25Bb 0.71Ba 1.25Ba 30-60 0.06Ca 0.06Ba 0.05Ba 0.36Ca 0.27Ba 0.06Bb 0.13Cb 0.33Ca WAE = weeks after emergence; = 0.10; means within rows for depth having same lower case letter do not differ at the p 0.10 level according to Duncan’s MRT; means within columns for position or N-rate having sa me capitalized letter do not differ at the p 0.10 level according to Dun can’s Multiple Range Test. Table 4.4. Significance of treatment, position an d depth when constituting linear model for sampled root length density. Model Term Probability (p) 3 WAE 5 WAE 8 WAE Treatment x Pos x Depth NS NS NS Treatment x Pos NS 0.0354† 0.0008& Treatment x Depth NS 0.0084† 0.0040& Pos x Depth <0.0001 0.0004 <.0001 Pos NS Depth NS Treatment 0.0169* WAE = weeks after emergence; Pos = pos ition; NS not significant at the p 0.10 level; See Table 4.1; † See Table 4.5; & See Table 4.6. Table 4.5. Pairwise root length density (cm cm-3) comparisons against Conv 267N by depth and position at 5 weeks after emergnce. Treatment Depth Position 0-15cm 15-30cm 30-60cm IR 0 IR 0.5 BR Conv 267N 2.42 1.62 0.20 1.18 1.69 0.46 SH+L 0N 1.41X 0.80X 0.13 0.82 0.74X 0.30 Conv 0N 0.89† 0.62X 0.12 0.68 0.45† 0.19 SH+L 133N 3.37X 1.46 0.42X 1.92 1.59 0.76 Conv 133N 2.02 1.04 0.24 1.26 1.16 0.23 WAE = weeks after emergence; X, † means within column for depth or position different from Conv 267N at the p 0.10 and 0.05 levels respectively.

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120 Table 4.6. Pairwise root length density (cm cm-3) comparisons against Conv 267N by depth and position at 8 weeks after emergence. Treatment Depth Position 0-15cm 15-30cm 30-60cm IR 0 IR 0.5 BR Conv 267N 2.96 1.39 0.46 2.04 1.08 0.83 SH+L 0N 1.98X 0.88 0.29X 1.20 0.85 0.53 Conv 0N 1.01† 0.70X 0.17† 0.66X 0.61 0.27X SH+L 133N 3.22 1.63 0.36 2.13 1.56 0.49 Conv 133N 2.83 1.31 0.45 1.62 1.49 0.67 WAE = weeks after emergence; X, † means within column for depth or position different from Conv 267N at the p 0.10 and 0.05 levels respectively. Table 4.7. Significance of green manure, nitroge n rate, and proximity to plant (near vs far) when constituting linear model for sampled root length density. Model Term Probability (p) 3 WAE 5 WAE 8 WAE GM x N-rate x Proximity NS NS NS N-rate x Proximity 0.0716* 0.0324 0.0032 GM x Proximity NS NS 0.0914 GM x N-rate NS NS NS Proximity N-rate GM NS 0.005 WAE = weeks after emergence; NS not significant at the p 0.10 level; see Table 4.8. Table 4.8. Significance of green manure, nitroge n rate, and proximity to plant (near vs far) when constituting linear model for sampled root length density. Model Term Probability (p) 3 WAE 5 WAE 8 WAE Treatment x Proximity NS 0.0740 0.0106 Proximity 0.0708 Treatment <0.0001* WAE = weeks after emergence; NS not significant at the p 0.10 level. Mean of Conv 267N not significantly different from mean of any other contrasted treatment at the p 0.10 level.

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121 Table 4.9. Interactions between nitrogen rate and proximity for r oot length density (cm cm-3). Proximity 3 WAE 5 WAE 8 WAE N-rate N-rate N-rate 0N 133N 0N 133N 0N 133N Near 0.37 Ab 0.62 Aa 1.18 Ab 2.50 Aa 1.37 Ab 2.86 Aa Far 0.07 Ba 0.08 Ba 0.21 Bb 0.48 Ba 0.35 Bb 0.56 Ba WAE = weeks after emergence; means within rows for proximity class having same lower case letter do not differ at the p 0.10 level; means within columns for N-rate having same capitalized letter do not differ at the p 0.10 level according to Duncan’s Multiple Range Test. Table 4.10. Pairwise root length density co mparisons against Conv 267N by proximity at 8 weeks after emergnce. Treatment 5 WAE 8 WAE Proximity Proximity Near Far Near Far Conv 267N 2.60 0.37 2.57 0.70 SH+L 0N 1.39X 0.24 1.69X 0.44X Conv 0N 0.96X 0.18X 1.05† 0.25† SH+L 133N 2.91 0.68† 3.21 0.48X Conv 133N 2.09 0.28 2.52 0.63 WAE = weeks after emergence; X, † means within column for proximity class different from Conv 267N at the p 0.10 and 0.05 levels respectively.

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122 CHAPTER 5 EFFECTS OF A GREEN MANURE APPROACH TO SWEET CORN FERTILIZATION ON SOIL PROPERTIES Introduction Crop requirements and climate aside, soil fe rtility largely determines nutrient and water supplementation required in agricult ural production – thereby having major economic and ecological implications. Textur e, organic matter cont ent, and geological composition usually exert greatest infl uence on soil fertility (Brady and Weil 1999, Tinker and Nye 2000). Soil texture and geologi cal composition are interrelated and cannot be altered in any practic al sense. In contrast, changes in soil organic matter (SOM) including content, spatial di stribution, chemical propertie s (such as carbon-to-nitrogen ratio, C:N), and related soil bi ological properties (such as mi crobial biomass and activity) may be more readily driven by agricultura l practices. Current ma nagement practices utilizing regular soil disturbance (tillage ) and exclusively dependent on chemical fertilization may limit equilibrium SOM to the detriment of potential production and input use efficiency, es pecially in subtropical, sandy areas such as Florida. In such environments, leguminous green manure (GM) and reduced tillage a pproaches to soil fertility may provide significantly greater orga nic matter inputs and slow rates of organic matter decomposition compared to chemical fe rtilization, but without the often inhibitive costs and phosphorous imbalances associat ed with animal manure application. Nevertheless, modification of SOM is typi cally a long-term process. For example, for every 1% of dry weight as SOM in the upper 15 cm, a soil contains roughly 25 Mt

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123 SOM ha-1 (assuming a soil bulk density of about 1.65 g cm-3). Many economic crops contribute no more than 5 Mt ha-1 post harvest organic matter (dry weight). Effective leguminous GMs, grown during fallow peri ods and without need for chemical N fertilization, may provide 5-10 Mt ha-1 or more. However, decomposition occurring after death of crops and GMs reduces the fraction of such residues transforming into stable SOM. In two similar experiments after ~10 years of a pearl millet and wheat rotation on a low organic matter (~0.40-0.50% organic C) sand y loam (65-69% sand) in India, Goyal et al. (1992, 1999) found combinations of inor ganic fertilizer and organic amendments (wheat straw, manure, or se sbania green manure) genera lly increased soil organic C (SOC), total N (TN), microbial biomass C (MBC), and enzyme activity more than inorganic fertilizer alone in the top 15 cm of soil (plow-layer). Still, with manure/sesbania and crop stover additions in these studies varying between 8-15 Mt ha-1 annually, these increases in SOC amounted to only about 5-15%. Realization of greater SOC/SOM increases in hot, humid, sandy enviro nments may require greater and/or more consistent residue additions, esp ecially under conventional tillage. Generally, organic matter associated with th e finer (smaller) sized soil fractions – silt and clay – may experience more phys ical and chemical protection from decomposition than that associated with more coarse (larger) sized soil fractions. For example, even on a sandy loam soil having about 35% sand and 1.6% SOC, Kandeler et al. (1999) found most SOC associated with the clay-sized soil fraction (<2m) and roughly 90-95% of total SOC accounted for within silt and clay-sized fractions together (<63 m). Extreme sand content of Florida mineral soils (in many cases 95-97% sand) has therefore provoked doubt that organic matter can be meaningfully increased in such

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124 soils. Data from native upland and anthropogeni c silvicultural systems appears somewhat limited. Daubenmire (1990) determined SOM at five sites representing three widespread Florida tree communities on sandy soil (with an d without fire protection), finding average SOM levels less than 1.50% in the upper 10 cm in all co mmunities. Gholz and Fisher (1982) quantified SOM in sandy soils under sl ash pine management of seven ages between 2 and 34 years. In pine 5 to 34 year s of age, organic matter in the A1 horizon (~12-14 cm) averaged 2.18%. Despite large standing plant biomass in these systems, actual leaf litter-fall a dditions may be only on the order of 1-4 Mt ha-1 annually, depending on age (Nemeth 1972). Early indicators of long-term change s in SOM are desirable given the time limitations of agricultural research. Pool size of coarse particles of SOM – known as the particulate organic matter (POM) pool – often reflects the most recent additions of plant residues that have yet to undergo major deco mposition. Results from Magid et al. (1997), Magid and Kjaergaard (2001), a nd Mueller et al. (1998) show recent additions of plant residues primarily contributed to lo w-density (“light”; density < 1.4 g cm-3) fractions of POM, with C-loss during the fi rst 2-4 months occurring prim arily from these light POM fractions. Carbon to nitrogen ratios for all POM fractions in these studies also tend to decrease with decomposition over time. On sandy, loamy and clayey soils, Hassink (1995) studied decomposition rate constants of SOM, separating “macroorganic” matter (>150 m; heavy, intermediate and light densi ties) from “microorganic” matter (150-20 m and < 20 m), finding decomposition rate constants fastest for macroorganic matter and for lighter fractions – i ndependent of soil type. These results complement those of Kandeler et al. (1999), sugges ting that larger-s ized POM tend to be less physically

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125 protected within the soil matrix and that lighter POM facilitates enzymatic action (see also Wander and Bidart (2000), who use an alternative POM separa tion into physically “loose” and “occluded” fractions). Consequent ly, changes in POM levels may provide early indication of ongoing change s of in overall SOM as well as potential soil nutrient release via decomposition. Because decomposition and nutrient release from plant residues is a microbially mediated process, levels of microbial biom ass reflect instantaneous decomposition rates and may also indicate potent ial soil nutrient release or im mobilization as well as gross differences in SOM (for example, see Fran zluebbers et al. 1999) Investigators use combinations of periodic field sampling of soil, laboratory soil incubation techniques, and/or 13C and 15N radioisotope techniques to qu antify decompositi on and inter-pool movements of soil organic matter (Gonzalez-P rieto et al. 1995, Hadas et al. 1993). Soil microbial biomass can be determined di rectly by chloroform fumigation (BletCharaudeau et al. 1990, Wardle et al. 1999, Franzluebbers 1999a, Franzluebbers and Arshad 1996, Franzluebbers et al. 1995 and 1996, Goyal et al. 1992 and 1999) and indirectly by bioluminescence (Blet-Charaud eau et al. 1990), near -infrared reflectance spectroscopy (Palmborg and Nordgren 1993) and plate-count techniques (BletCharaudeau et al. 1990). Resp iration procedures used to estimate potential microbial activity also provide an indire ct estimation of microbial biom ass (Blet-Charaudeau et al. 1990, Neely et al. 1991, O’Connell 1990, Palmbo rg and Nordgren 1993, Wardle et al. 1999, Franzluebbers 1999a,b, Franzluebbers a nd Arshad 1996, Franzluebbers et al. 1995 and 1996), as do relatively simple arginine a mmonification protocols (Franzluebbers et

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126 al. 1996). Most of these investigations, conduc ted on soils with significant clay and silt fractions, find that microbial biomass res ponds positively to plant residue additions. Tillage reduction may increase equilibrium soil organic matter compared to conventional tillage, but may also shift orga nic matter accumulation closer to the soil surface. Investigating long-term (~10 years) ti llage effects within different soil depths and particle size classes in a sandy loam in Austria, Kandeler et al. (1999) found reduced and minimum tillage increased overall SOC, TN, and microbial biomass N of the bulk soil (top 30 cm), doing so mainly within the largest particle-size fraction (>200 m) and the top 10 cm of soil. Microbial biomass N and enzyme activity per unit SOC remained relatively uniform in the top 30cm under conve ntional tillage, but increased both toward the soil surface and in larger particle si ze fractions with reduced and minimum tillage. Franzluebbers et al. (1995) found potential C and N mineralization and MBC in the upper 30 cm of a silty clay loam generally greater under zero tilla ge compared to conventional tillage. However, incorporat ion of crop residues in conve ntional tillag e resulted in temporary increases in soil MBC, potential C mineralization and immobilization of inorganic N, indicating imme diate and rapid decompositi on. Many other workers have shown slower decomposition and greater N-i mmobilization for surfa ce applied residues compared to soil-incorporated residues (see Chapter 2 for discussion). These results suggest long-te rm tillage reduction potentially increases soil organic matter and nutrient cycling by creating a slow ly decomposing litter layer where plant residues accumulate followed by delayed transf er to the uppermost soil layers. Over the long-term, potential SOM levels may therefor e increase under reduced tillage, with POM and microbial biomass fractions indicati ng changes earlier than the total SOM pool.

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127 Because the soil environment speeds decomposition by buffering water and temperature regimes (see Chapter 2), and because soil disturbance promotes microbially-based decomposition by destroying macroaggregat es (Franzluebbers 1999b), long-term conventional tillage may reduce overall reside nce times for organic matter and promote more rapid nutrient release after tillage even ts. Tillage reduction may therefore be more important to increase SOM and soil/residue nu trient retention in hot, humid environments where decomposition already takes place rapi dly, and on coarse-textured soils with negligible small-sized particle fractions (see also Franzluebbers and Arshad 1996 and Franzluebbers et al. 1995). To test if significant increases in soil C and N pools can be achieved in coarsetextured soils under Florida conditions, we investigated a GM sequence of summer planted sunn hemp (SH) followed by a winter legume (L) of blue lupin ( Lupinus angustifolius winter 2001-02) and cahaba white vetch ( Vicia sativa winter 2002-03) as N-source for sweet corn ( Zea mays var Rugosa) on a sandy so il using reduced tillage. Sweet corn treatments with one GM or no GMs (conventional), as well as complete fallow (no corn or GMs, zero-tillage and pe riodic weed control) were also included. Effects of treatments on dry matter additions total soil C (TC), total soil N (TN), particulate organic C (POC), particulate organic N (PON), microb ial biomass C (MBC) and soil pH were assessed. We hypothesized that the double GM approach would add significantly more dry matter to the system than other approaches, driving greater increases in the soil C and N pools than all ot her treatments, and that increases in soil C and N pools would generally be greater w ith any GM approach compared to the conventional. We expected to see greate st differences in POC and MBC pools.

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128 Materials and Methods Set-Up and Design The 15 overall treatments as well as site and experimental design are more fully described in Chapter 1. Treatments consisted of sweet corn preceded by: a summer GM of sunn hemp and a winter GM of blue lupin (winter 2001-02) a nd cahaba white vetch (winter 2002-03) denoted as SH+L; sunn he mp only (SH); lupi n (winter 2001-02) and vetch (winter 2002-03) only (L); and unamende d “conventional” corn (Conv). Each GM level was supplementated with 0, 67, or 133 kg inorganic N ha-1 (0N, 67N, and 133N). Unamended (Conv) treatments also received 200 or 267 kg inorganic N ha-1 (Conv 200N and Conv 267N). A complete fallow (Fal) rece iving only weed control (no tillage and no planting) was also used for comparison. The study was conducted at the Plant Scie nce Research and Education Unit near Citra, Florida. Candler and Lake fine sa nds (~95-97% sand in the upper 15 cm) were dominant soil types (see Appendi x A) with soil survey indicating organic matter between 1.1% and 2.1% depending on location with in the field (data not shown). Procedures and Measurements Dry matter additions to the field from crop residues over both study years were calculated for each plot from data presente d in Chapters 2 and 3. These included all residues from sunn hemp, winter legumes, weeds, and corn stover (corn vegetative tissue) at respective final samplings. Due to resource limitations, only pH was determined for all treatments at all dates. Evaluation of soil MBC was conducted at all da tes for all treatments receiving 0N and 133N as well as Conv 267N and Fal. Total soil C and N, POC and PON were determined for all treatments after the end of each year’s sweet corn crop (July 2002 and June 2003).

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129 Soil samples of no less than 200 g were removed from each plot on four sample dates: November 2001 (at the end of sunn hemp ), April 2002 (at the end of lupin), July 2002 (at the end of sweet corn) and June 2003 (at the end of sweet corn). Samples were taken from the top 15 cm from different areas wi thin each plot, stored in plastic bags and refrigerated immediately. Samples were hom ogenized and subsamples of 30-40 g were weighed, dried at 100 C for 24 hours, and re weighed to determine gravimetric water content. Microbial bioma ss C was determined via chloroform fumigation method (procedure provided by Dr. Yu Wang, UF Wetlands Biogeochemistry Lab, Gainesville) for all GM levels (SH+L, SH, L, Conv) at 0N and 133N N-rates, as well as Conv 267N and Fal treatments. Fumigated samples were exposed to chloroform in an evacuated vacuum chamber for 24 hours, with as much chloroform as possible removed by repeated air entry and evacuation. Carbon from fumi gated samples and non-fumigated controls was extracted using 25 ml of 0.5 M K2SO4 solution. Samples and extractant were shaken for 1 hour and then centrifuged for 10 minut es at 6000 rpm. Resulting supernatant was seperated, vacuum filtered, acidified using 37 N sulfuric acid, and refrigerated. Solutions were then analyzed for dissolved carbon usi ng a TC analyzer and original soil samples frozen until further analysis. Microbial bi omass C for each plot was calculated as (Cf – Cuf)/0.41, where Cf and Cuf were C contents of fumigated and unfumigated samples, respectively, and 0.41 was a cal ibration constant as determined by Voroney and Paul (1984). Particulate organic matter was separate d using a procedure adapted from one provided by Dr. Alan Franzluebbe rs (USDA Agricultural Research Service, Watkinsville, GA). Subsamples of no less than 50 g so il were mixed with 100 ml of 0.1 M Na4P2O7 and

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130 shaken overnight for 12-16 hours to disperse soil coatings and other inorganic C. Subsamples were then rinsed and filtered us ing a 0.053 mm sieve with debris greater than 0.5 cm removed. Separated, sieved subsamples were then dried at 100 C for 24 hours and weights recorded. After thorough homogenizatio n, roughly 10 g of this material was ground using a ball mill and analyzed for C and N using a Carlo Erba CN analyzer (Carlo Erba Reagenti; Milan, Italy). Ground subsamples of unseparated soil were also analyzed for C and N using the same equipment. This total C and N and particulate organic C and N were ascertained for all treatments after sweet corn crops in both July 2002 and June 2003. However, to further resolve differences, samples in June 2003 were taken and analyzed for both 0-7.5 cm and 7.5-15 cm soil la yers (samples were later recombined for MBC and pH analysis). Soil pH was determined for all treatments at all sample dates using procedure of the UF-IFAS Analytical Research La b (University of Florida, Gain esville, FL). A mixture of 20 g soil was stirred with 40 g of pH-neutral DDI water and allowed to equilibrate for 20 minutes, with pH measured afterwards using a pH probe. Data Analysis Data were analyzed using SAS (Stati stical Analysis Systems; Cary, NC). Balanced analysis of variance (ANOVA) wa s conducted with results for observations from all GM levels (SH+L, SH, L and Conv) and N-rates of 0N, 67N, and 133N (where applicable) for all sample dates. Results were regressed linearly on sample date, GM level, chemical N-rate, all interaction terms of these three variables, and block. A randomization term was included for block. Where interaction terms became significant, separate ANOVAs were run to compare treatm ents within specific levels of the interacting variables. Non-interacting variab les (except block) were not included in the

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131 interaction model statements. Comparisons of means were always made with Duncan’s multiple range test. To compare results of treatments with GMs plus 133N against high-fertilized, highproducing conventional corn, pairwise c onstrasts against Conv 267N and Conv 200N were made (where available). These tr eatments (GMs with 133N, Conv 200N and Conv 267N) were also constrasted against Fal, w ith Conv 200N and Conv 267N also contrasted against each other. For these contrasts, necessary ANOVA used the same independent variables as the balanced design except that GM level and N-rate level were substituted with the appropriate overall treatment ti tle (SH+L 67N, SH+L 133N, SH 67N, SH 133N, L 67N, L 133N, Conv 200N, and Conv 267N where applicable). The same protocol was followed when interaction between sample date and treatment became significant. All significant differences discussed occurred at p 0.05. Results Dry Matter Additions End-of-year dry matter additions from all plant residues (GMs, sweet corn, and weeds) ranged from 4.1 to 23.3 Mt ha-1 depending on treatment and year (Figure 5.1A,B). Additions remained significantly lower for Conv compared to all GM levels in both years. In 2001-02, when winter legume pr oduction was higher, SH+L added 144% more residue than Conv, while additions with SH and L alone were 96% and 59% greater, respectively, than Conv; residue contributions significantly increased from Conv to L to SH to SH+L in that order. Dry matter pr oduction for SH in 2002 was about 50% greater than in 2001 (see Chapter 2), leading to gr eater year-long residue additions for SH and SH+L in 2002 (about 20 Mt ha-1 in 2002, which was about 230% more than produced by Conv). The low yield of cahaba white vetc h in 2002-03 resulted in no statistical

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132 difference (p > 0.05) for dry matter additions between SH and SH+L in that year. Residue contributions for L and Conv did not differ st atistically between y ears despite 50% higher corn plant population in 2003 compared to 2002 (Figure 5.1A,B; see Chapter 3). By increasing corn vegetativ e growth, increase from 0N to 67N or 133N resulted in a significant increase in dry matter addi tion of 19-32% depending on N-rate and year. Increase from 67N to 133N generally increase ear growth more than vegetative growth (Chapter 3), therefore no difference in dr y matter additions occurred between 67N and 133N (Figure 5.1A,B). Interaction between ye ar and N-rate was not significant (not shown). In both years, treatments with any GM plus 133N added significantly more dry matter to the system than Conv 267N and Conv 200N. All GMs with 133N as well as Conv 200N and Conv 267N produced greater dry matter additions than Fal. However, dry matter additions from weeds in Fal amount ed to 45-50% of the total from Conv 200N and Conv 267N, and numerically these two high -N treatments contributed little more biomass than did Conv 67N and Conv 133N (Figure 5.1A,B). Microbial Biomass C Microbial biomass C (MBC) showed no response to date, GM, or chemical N-rate over the course of the study (data not shown) Pairwise treatment contrasts showed no significant differences for GMs plus 133N re lative to Conv 200N, Conv 267N or Fal, nor did pairwise contrasts show differences among Conv 200N, Conv 267N and Fal (data not shown). The overall study average for soil MBC was 105 4 mg C kg-1 dry soil. Total and Particulate C and N pools Values for TC and TN ranged from 7.1-8.9 g C kg-1 (0.71-0.89%) and 0.35-0.57 g N kg-1 (0.035-0.057%) depending on treatment a nd year. These results corresponded

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133 roughly to pre-existing soil survey data indi cating soil organic matter varying from 1.12.1% across the field (not s hown). Particulate organic C and N generally made up 3040% and 20-30%, respectively, of the total soil C and N pools, while TC:TN and POC:PON values remained near 19:1 a nd 26:1, respectively (Table 5.1). Across both years, amendment with SH+L and SH incr eased TN (by 15% and 18%, respectively), POC (by 17% and 18%, respectively) a nd PON (by 27% and 24%, respectively) compared to Conv while lowering TC:TN (by 7% for both SH+L and SH; Table 5.1). Amendment with L showed similar effects only for TC:TN and PON, but otherwise was not significantly different from Conv, SH+L or SH (Table 5.1). Over the period of one year, values for POC, PON, POC:TC and PON:TN all increased significantly (by 12%, 20%, 18% and 25%, respectively) when ANOVA was conducted for the balanced desi gn (12 treatments including a ll four GM levels at three chemical N-rates; Table 5.1). However, ANO VA for the full study (balanced design plus Conv 200N, Conv 267N, and Fal; Table 5.2) reve aled significant interaction between treatment and year for POC and PON. The apparent discrepancy occurred because POC and PON decreased for Conv 200N (by 36% and 30%, respectively), Conv 267N (26% and 27%, respectively) and Fal (12% and 21%, respectively) from 2002 to 2003, although only the decrease in PON for Fal was signifi cant. Subsequent pair wise contrasts of treatments within years showed greater PO C and PON for treatments with any GM plus 133N compared to Conv 200N, Conv 267N and Fa l at the end of th e second year (2003) only. Relative increases for GMs plus 133N co mpared to these treatments amounted to roughly 30-50% for POC and 30-100% for PON (Table 5.3). Otherwise, pairwise

PAGE 150

134 contrasts between GMs with 133N against Conv 200N, Conv 267N, a nd Fal resolved few differences (Table 5.2). Similarly, full ANOVA revealed a signi ficant decrease for TC across all treatments from 2002 to 2003 – a trend weakly significant ( p 0.06) in the balanced ANOVA as well (see Tables 5.1 and 5.2). The decrease may reflect conversion of the field from pasture to tilled row-crop system about 2 years prior to the start of our experiment. Assuming a soil bulk density of 1.65 g cm-3, the decrease of about 0.5 g C kg-1 soil was roughly equivalent to a loss of 2.5 Mt SOM ha-1. Increases in POC, especially for GM treatments appear of similar size. In 2003, analyzing samples from the upper 7.5 cm of the 0-15 cm showed similar trends regarding the soil C and N pools (data not shown). Especially for SH and SH+L treatments, POC and PON values were 3040% greater in the upper 7.5 cm of soil compared to the upper 15 cm of soil as a whol e. However, due to variability, p-values for these differences between GM levels and be tween contrasted treatments became larger with ANOVA for the upper 7.5 cm (compared to the upper 15 cm). Soil C and N pools displayed more homogenized values from 7.5-15 cm, with no significant differences between GM levels, N-rates, years, or treatment (dat a not shown). By reducing variability, averaging the 0-7.5 cm and 7.5-15 cm layers together (f or the 0-15 cm layer) increased statistical resoluti on. Nevertheless, these results show changes in soil C and N pools as result of GM use under reduced tillage is occurring primarily in the upper 7.5 cm. Soil pH Soil pH was close to neutral (Table 5.4) higher than indicated by pre-existing soil survey data (not shown) by about 0.5, although this may have occurred as a result of

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135 fertility management during the interveni ng time or due to use of different pHdetermination procedure. Compared to Conv, amendment with SH+L and SH significantly decreased soil pH in the upper 15 cm, though differences were small (7.18 for Conv, 7.09 for SH, and 7.10 for SH+L). Sample date also significantly affected soil pH, with average higher values in April 2002 (7. 31; at the end of lupin) and lower values in October 2001 (7.03; at the end of sunn he mp) and July 2003 (7.06; after sweet corn). Differences by sample date may have been rela ted to temperature; the 5-6 months prior to April 2002 would have been colder, probably slowing soil organic matter decomposition and other biological activity. Chemical N-ra te showed no significant effect on soil pH, nor were any interactions between N-rate, samp le date, and/or GM (p > 0.05; Table 5.4). Pairwise contrasts of GMs with 133N ag ainst Conv 200N, Conv 267N and Fal showed no interesting trends, nor did contrasts am ong Conv 200N, Conv 267N and Fal (data not shown). Discussion Soils in this study are highly sandy, alt hough typical of those found in Florida. Dominant soil types for the experimental fi eld (Lake Fine Sand and Candler Fine Sand) are characterized as having 95-97% sand in the upper 15 cm (Carlisle et al. 1988; see Appendix A). Amendment with GMs, especia lly SH+L and SH, signi ficantly increased annual field residue additions (Figure 5.1A,B), POC, PON, and TN in both years, as well as decreased TC:TN (Table 5.1), compared to Conv in the upper 15 cm of soil, with changes probably taking place mostly in the upper 7.5 cm. Furthermore, analysis of variance for all treatments (including complete fallow and high-N, chemically fertilized treatments not present in the balanced GM /N-rate/year ANOVA) not only confirmed that TC among all treatments declined from 2002 to 2003, but indicated a year by treatment

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136 interaction suggesting that POC and PON for treatments with combined GM and chemical approach may be growing while that for conventionally fertilized treatments (Conv 200N and Conv 267N) and complete fallo w (Fal) may be declining (Tables 5.25.3). These patterns may reveal important tr ends “superimposed” on each other. The decline in TC likely reflects conversion from pasture in the medium-term past (2-3 years prior to the study) and the changes in POC and PON signa ling shorter-term changes in the current regime of SOM additions. Across al l treatments, the decline in TC (about 0.5 g kg-1 soil) appears roughly matched by increase s in POC for treatments with SH and SH+L. As the current POC and PON pools turnov er into smaller-fraction organic matter, declines in TC for conv entionally fertilized corn (Conv 200N and Conv 267N) and complete fallow (Fal) treatments may become increasingly rapid while TC declines in GM treatments may slow or even reverse. However, changes in the size of future additions to POC as well as its potential decomposition rate will determine what divergence, if any, we see betw een these cropping approaches. Higher POC and PON of soil under GMs plus 133N compared to soil under Conv 200N, Conv 267N, and Fal in 2002 became highl y significant after two years. Even though it is considered an early indicator, changes in the soil POM pool may still require several years to reach equilibrium with steady inputs, especially afte r recent adoption of reduced tillage; large pi eces of sunn hemp stem residue visibly remain on the soil surface for 2 or more years in our system. Additionally, because many of these patterns became significantly resolved on ly after data from the second year became available for analysis, we expect differences between treatments to become larger and more significant in the

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137 future. However, relative increases genera ted in the POC (30-50%) and PON (30-100%) pools by GM use after two years are still small in an absolute sense, indicating additions of only about 0.5-1.0 g C kg-1 and 0.03-0.07 g N kg-1, respectively. Without greater POM increases in the future, SOM increase due to GM addition may never exceed a fraction of a percent – especia lly on sandy soils. In our system, PON may be the earliest a nd most sensitive indicator to changes in other SOM pools. After th e initial year of data, only P ON showed a significant trend (all GMs > Conv), and after two years of data the significant tren ds in PON always occurred at the highest levels of significance (Table s 5.1 and 5.3). At the same time Fal (complete fallow treatment; receiving identical herbicide application as all other treatments, but otherwise undisturbed) received signifi cantly less dry matter inputs than all other treatments (only 4.1-4.6 Mt ha-1 annually; Figure 5.1A,B). Therefore, the significant decrease in PON for Fal after two years is likely one of the earlie st significant year-toyear responses we would expect to generate due to a specific experimental treatment (Table 5.3). Work by Robles and Burke on a sandy lo am (~65% sand) in Colorado showed POM fraction (53-2000 m) accounted for 30-40% of SOM, similar to our results (POC and PON equaled 30-40% TC and 20-30% TN, respectively; Table 5.1). Hassink (1995) showed decomposition rate of POM to be closel y related to particle density irrespective of source soil texture. Results from Ma gid and Kjaergaard (2001) suggest that POM density may correspond directly to particle si ze, which would facilitate inexpensive and readily simple manual separation, allowi ng more detailed analysis of POM decomposition potential. It would be of inte rest to examine relationships of size and

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138 density to occlusion as well, as this ma y further explain POM decomposition potential (Wander and Bidart 2000). Because deco mposition is known to decrease POC:PON values (Magid et al. 1997, Magid and Kjaergaard 2001, and Mueller et al. 1998), decreased values for POC:PON from 2002 to 2003 may reflect partial decomposition of the 2002 POM additions. Unlike studies on soils with greater clay a nd silt fractions, MBC values exhibited in this study remained low (105 4 mg kg-1) and exhibited no differences based on amendment regime. Using similar methodology (chloroform-fumigati on), Goyal et al. (1992, 1999) and Franzluebbers (1999a) found MBC values of 180-355 mg kg-1, 147-423 mg kg-1, and roughly 200-600 mg kg-1, respectively, on soils in India and Texas with 6580% sand. Expressed as a fraction of TC (~13.5 mg-1 g) or on a land area basis (~25.5 g m-2), MBC values recovered in this study also amounted to about 25-30% of those reported by Goyal et al. (1992, 1999) and Franzluebbers at el (1995). However, results from Kandeler et al. (1999) for chloroform -extracted microbial biomass N associated with soil particle-size fractions (coarse sand, fine sand, silt, and clay) suggest our values may be expected for sand. Because such low levels of microbial biomass appear associated with sand fractions, MBC may be a poor indicator of potential microbial action in our soils. Significantly decreased soil pH following SH+L and SH compared to Conv is expected because soil pH is known to fall as a consequence of organic matter decomposition (for example, Goyal et al. 1992, 1999, Simek et al. 1999). The small change (7.09 and 7.10 for SH and SH+L, respectively, compared to 7.20 for Conv) probably creates no practical difference but doe s indirectly indicate greater (microbial)

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139 soil decomposition activity following GM add itions – even if microbial biomass was unaltered. From results presented in Chapter 2, we certainly know that large GM residue additions are being decomposed (~65% of s unn hemp decomposes in 14-16 weeks, with winter legumes likely faster). Comparison of POC and TC suggests differences in soil C of about 1-2 Mt ha-1 between GM and Conv treatments, whereas SH and SH+L residue addition probably amounted to an additional 6-7 Mt ha-1 C annually. Kandeler et al. (1999) show ed microbially-based xylan ase and protease enzymes were more highly associated with sand frac tions; activities of th ese enzymes and/or measures of substrate-induced respiration would probably bett er gauge potential activity of the microbial community in our sandy soil. It is possible that increased microbial enzyme activity and respiration (not in creased microbial biomass) account for decomposition in regions such as Florida. Therefore, soil microbes conceivably function with reduced N-limitation in our environmen t. If so, they cannot be expected to immobilize significant amounts of N here, and this would have major implications for residue management intended to immobilize N via increased tissue recalcitrance (as discussed in Chapter 2). The relatively high TC:TN (~19:1) and PO C:PON (~26:1) ratios seen in our study (Tables 5.1 and 5.2) may support this theor y. Most studies (conducted in temperate environments and/or on fine textured soils ) report equilibrium soil organic C:N around 12:1. Residue with higher C:N values do not result in net-N release upon decomposition because the potential microbial biomass increase associated with the potential C respiration is N-limited until C:N ratio approa ches 12:1. Our equilibrium value of soil TC:TN (which did not change from 2002 to 2003) suggests microbial respiration

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140 (decomposition) in our system is N-limited only beyond 19:1. Even higher values for POC:PON are expected because of greater physical and/or enzymatic accessibility to POM (Hassink 1995, Kandeler et al. 2000, Wa nder and Bidart 2000). The greater sensitivity (to residue additi ons) of the PON and TN pools co mpared to POC and TC, and the significantly lower TC:TN values f ound with GM amendment, may result from increased microbial respiration or enzyme activity per unit biomass N. Soil microbes may in sandy soils may “burn off” more GM-deriv ed C additions without attacking N, making POC and TC levels more homogenized than PON and TN between different treatments. If, like microbial biomass C, microbial biomass N did not change with GM decomposition, but substrate induced respirat ion and/or enzyme activity increased, we would have further support for this hypothesis. Conclusions High residue additions (15-23 Mt ha-1 year-1) under reduced tillage increased soil C and N pools and reduced C:N ratios of partic ulate and total C and N pools – even on a sandy Florida soil after only 2 years. Given a historical conversion fr om pasture into rowcrop system, use of GMs may create important SOM differences in the future. However, for differences to become of practical si gnificance, greater increases in POC and PON pools may be required. Also, microbially-based decomposition in Florida sandy soils may be less characterized by biomass increase a nd less N-dependent than that found in temperate environments and/or on fine-texture s soils. For residue management systems in our region, this may reduce the effectiven ess of attempts to immobilize N through microbial biomass growth via in creased C and/or lignin inputs.

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141 0 5 10 15 20 25 SH+LSHLConvFalDry Matter Addition (Mt ha-1) 0 67 133 200 267 kg NH4NO3-N A 0 5 10 15 20 25 30 SH+LSHLConvFalDry Matter Addition (Mt ha-1) 0 67 133 200 267 kg NH4NO3-N B 7.00 7.05 7.10 7.15 7.20 7.25SH+LSH LConvFalSoil pH (0-15 cm)ba b ab C Figure 5.1. Dry matter additions by treatme nt (2001-2002, A; 2002-2003, B) and average soil pH by GM over two years (C). Error bars reflect standard error; means with the same lower case letter ar e not significantly different at the = 0.05 level according to Duncan’s Multiple Range Test.

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142Table 5.1. Significance of green manure, nitrogen rate, and year in balanced analysis of variance for soil TC, TN, TC:TN, POC, PON, POC:PON, POC:TC, PON:TN, for sample s taken in July 2002 and June 2003. Probability (p) TC TN TC:TN POC PON POC:PON POC:TC PON:TN Year x GM x N-rate NS NS NS NS NS NS NS NS Year x GM NS NS NS NS NS NS NS NS Year x N-rate NS NS NS NS NS NS NS NS N-rate x GM NS NS NS NS NS NS NS NS N-rate NS NS NS NS NS NS NS NS Year NS NS NS ** *** *** *** (g kg-1) (g kg-1) (g g-1) (g kg-1) (g kg-1) (g g-1) (g g-1) (g g-1) 2002 8.3 0.45 19.1 2.7 0.10 26.1 0.33 0.24 2003 7.9 0.42 19.2 3.0 0.13 24.6 0.38 0.30 GM NS * ** *** NS NS NS (g kg-1) (g kg-1) (g g-1) (g kg-1) (g kg-1) (g g-1) (g g-1) (g g-1) SH+L 8.3 ab 0.45 a 18.7 b 3.0 a 0.13 a 24.5 0.37 0.29 SH 8.4 a 0.46 a 18.7 b 3.0 a 0.12 a 25.1 0.36 0.27 L 8.0 ab 0.44 ab 19.0 b 2.8 ab 0.11 a 25.2 0.36 0.27 Conv 7.7 b 0.39 b 20.1 a 2.6 b 0.10 b 26.4 0.34 0.26 GM = green manure; TC = total soil C; TN = total soil N; POC = particulate organic C; PON = part iculate organic N; NS model ter m not significant at the p 0.05 level. *,**,*** model te rm significant at the p 0.05, 0.01, and 0.001 levels, respectively; means within columns for measured quantities and within year or GM group having identical lett ers not significantly different at the p 0.05 according to Duncan’s Multiple Range Test.

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143 143Table 5.2. Significance of treatment and year in full analysis of variance and pairwise contrasts of selected treatments for so il TC, TN, TC:TN, POC, PON, POC:PON, POC:TC, PON:TN, for samples taken in July 2002 and June 2003. Probability (p) TC TN TC:TN POC PON POC:PON POC:TC PON:TN Year x Treatment NS NS NS (see Ta ble 5.3) (see Table 5.3) NS NS NS Year NS NS NS * (g kg-1) (g kg-1) (g g-1) (g kg-1) (g kg-1) (g g-1) (g g-1) (g g-1) 2002 8.3 0.44 19.3 26.5 0.33 0.25 2003 7.8 0.41 19.3 25.5 0.37 0.28 Treatment NS NS * NS NS (g kg-1) (g kg-1) (g g-1) (g kg-1) (g kg-1) (g g-1) (g g-1) (g g-1) Conv 267N 8.0 0.40 20.0 32.7† 0.35 0.22 Conv 200N 7.7 0.40 19.2 26.4 0.35 0.26 SH+L 133N 8.5 0.45 19.0 25.0 0.36 0.27 SH 133N 8.3 0.49 17.3† 24.0 0.38 0.27 L 133N 7.9 0.41 19.4 26.0 0.38 0.28 Fal 8.3 0.41 20.4 27.0 0.31 0.24 TC = total soil C; TN = total soil N; POC = particulate organic C; PON = particulate organic N; NS model term not significant a t the p 0.05 level. *,**,*** model term significant at the p 0.05, 0.01, and 0.001 levels, respectively; † mean different from Fal at the p 0.05 level.

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144 144Table 5.3. Analysis of variance for all treatm ents and pairwise contrasts of selected treatments within years for POC and PON. Probability (p) POC 2002 POC 2003 PON 2002 PON 2003 Treatment NS *** NS *** (g kg-1) (g kg-1) (g kg-1) (g kg-1) Conv 200N 3.5 2.2 0.10 0.07 Conv 267N 3.0 2.5 0.11 0.09 SH+L 133N 2.9 3.2 †‡# 0.11 0.14 †‡# SH 133N 2.9 3.3 †‡# 0.12 0.14 †‡# L 133N 2.6 † 3.3 †‡# 0.11 0.12 †‡# Fal 2.8 2.4 0.11& 0.09& POC = particulate organic C; PON = particulate organic N; NS mode l term not significant at the p 0.05 level. *,**,*** treatment significant within year at the p 0.05, 0.01, and 0.001 leve ls, respectively; † mean different from Fal at the p 0.05 level; †,‡,# mean different from Conv 200N, C onv 267N, and Fal at the p 0.05 level, respectively; & years significant within treatment at the p 0.05 level.

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145 Table 5.4. Significance of date, green manure and n itrogen rate in analys is of variance for pH of sampled soil. Model Term Probability (p) Date x GM x N-rate NS Date x GM NS Date x N-rate NS GM x N-rate NS GM (See Figure 5.1) Date *** October 2001 7.03 b April 2002 7.31 a July 2003 7.06 b N-rate 0N 7.12 ab 67N 7.19 a 133N 7.09 b GM = green manure; NS model te rm not significant at the p 0.05 level. *,**,*** model term significant at the p 0.05, 0.01, and 0.001 levels, respectively; means within columns for date or N-rate having identical letters not significantly different at the p 0.05 according to Duncan’s MRT.

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146 CHAPTER 6 EFFECTS OF GREEN MANURE APPROACH ES ON CROP PESTS: PARASITIC NEMATODES AND WEEDS Introduction and Literature Review Concerns about environmental and econom ic costs have created interest in alternative methods of weed and pest control in agricu ltural systems that make use of ecological processes. When weeds occur in cropping systems they may compete with crops for resources, possibly reducing econom ic profits beyond th e potential costs of their control. Nematodes, a group of unsegmented roundworms, represent major actors in the soil food web including micro-floral graze rs, predators, and plant parasites. Like weeds, some plant-parasitic nematodes may cause an unacceptable level of economic loss in cropping systems. Some weeds may also act as hosts for parasitic nematodes. With proper selection and management green manures (GMs) may provide multiple services by supplying biologically fi xed nitrogen (N) to crops and adding carbon (C) to soils while also suppr essing weeds and parasitic nema todes that might otherwise require chemical or cultural (physical) intervention. Green manures may control weeds and nematodes through physical, biotic, a llelopathic, and adap tive interactions. Physically, GMs may outcompete weed speci es for light, nutrients, and water at crucial stages and may otherwise disrupt th e life-cycle of nematodes by acting as nonhosts. Blackshaw et al. (2001) found yellow sweet clover ( Melilotus officinalis ) suppressed weed biomass by 77%, 96%, and 99% in each of th ree years by direct competition. Ross et al. (2001) investigated seven clover species, finding them to have

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147 greatest weed suppression on a low-fertility si te when unmowed, esp ecially with tallergrowing annual clovers such as berseem clover ( Trifolium alexandrinum ). In the same study, weed suppression by clovers on high-fertility sites wa s enhanced by mowing and did not differ among species. Selection of resistant crops or crop varie ties represents one of the most important aspects of nematode damage control. Resist ance to particular nematodes may differ among varieties of the same species, and resist ance to different nematodes may also vary within a crop species or variety. Plants may also show different levels of susceptibility to regional races and local isolates of nematode species. Overuse of resistant crop varieties may also select for resistance “breaking” nematodes (see McSorley 2001 for discussion). Crop rotation with a non-host or nematode suppressant GM may help reduce such selection pressures by providing an alternative opportunity to disrupt nematode life cycles. Meloidogyne spp. (root-knot nematodes), one of the most problematic nematodes in Florida for corn ( Zea mays ) and tomato ( Lycopersicon esculentum ), have a wide host range. Greenhouse and field studi es have shown a number of GMs act as non-hosts or suppressors of one or more specie s of root-knot nematodes: castor ( Ricinus communis ), iron-clay cowpea ( Vigna unguicalata cv. Iron Clay), showy crotalaria ( Crotolaria spectabilis ), jointvetch ( Aeschynomene americana ), marigolds ( Tagetes minuta and T. erecta ), sesame ( Sesamum indicum cv. Paloma), sunn hemp ( Crotalaria juncea ), barley ( Hordeum vulgare ), green panic ( Panicum maximum ), glycine ( Neonotonia wightii ), horsebean ( Canavalia ensiformis ), velvetbean ( Mucuna spp.), and Sudex ( Sorghum bicolor x S. sudanese ) (McSorley 1999, Sipes and Arak aki 1997, Al-Rehiayani and Hafez 1998; also see McSorley 2001 for discussion).

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148 On the other hand, use of potential GMs may be limited if they exacerbate infestations of plant parasitic nematodes by acting as hosts. In Hawaii, Sipes and Arakaki (1997) found populations of Meloidogyne spp. on taro ( Colocasia esculenta ) increased significantly following alfalfa ( Medicago sativa ), sweet corn ( Zea mays var Rugosa), cowpea ( Vigna unguicalata variety unrepor ted), lablab ( Lablab purpureus ), hairy vetch ( Vicia villosa ), mustard ( Brassica napus ), oat ( Avena sativa cv Coker), okra ( Hibiscus esculentus ), rhodes grass ( Chloris gayana ), cereal rye ( Secale cereale cv Danka), grain rye ( Lolium multiflorum cv Alamo), siratro ( Macroptileum atropurpureum cv Siratro) and wheat ( Triticum aestivum multiple cultivars). In Florida, McSorley (1999) found significantly increased root-knot nemat ode populations on roots of pearl millet ( Pennisetum typhoides syn P. glaucum ) and Japanese millet ( Echinochloa frumentacea ). Some GMs known as non-hosts or direct suppressors of Meloidogyne spp. may have undesirable characteristics, or may vary in th eir adaptability to a particular environment and management system. As discussed above, some GMs well-suited for control of one type of nematode may show susceptibility to others; sunn hemp has been found to be a poor host of reniform nematodes ( Rotylenchulus reniformis ) but may support a slow population increase over time (Caswell et al. 1991; Wang et al 2001). Al-Rehiayani and Hafez (1998), working in Idaho, found varieties of buckwheat ( Fagopyrum esculenta ), mustard ( Brassica napus ) and corn to be nonor poor hosts for a Meloidogyne chitwoodi race, while Sipes and Arakaki ( 1997) found opposite results for with Meloidogyne javanica in Hawaii. Green manures may control pests indirectly by providing habitat for organisms that feed on or parasitize weeds and nematodes. Ye ates et al. (1999) conducted a 7-year study

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149 on impacts of different weed management stra tegies (two types of cultural control, two types of herbicides, and application of sawdust mulch) on nematode population and diversity in annual and perenni al cropping systems. Over the long-term, saw-dust mulch raised populations of predatory nematodes more than other treatments, possibly by increasing availability of fungal and bacteria l grazing nematodes. Greenhouse studies in Florida using sandy soil have shown sunn hemp can increase omnivorous and predatory nematodes on soils with low organic matter (< 2%), though perhaps not enough to control parasitic nematodes such as Meloidogyne spp. (Wang et al 2003a). Wang et al. (2001) found application of sunn hemp residues to a si lty clay at a somewhat high rate (1 g dry residue 100g-1 dry soil) enhanced nematode-trapping fungi. Release of allelopathic chemicals by GMs may directly inhibit weed growth and nematodes, although this is difficult to pr ove formally. Leachate collected from sunn hemp residues have shown allelopathic properties against Rotylenchulus reniformis (Wang et al. 2001). Blackshaw et al. (2001) found up to 97% lower weed density 10 months after yellow sweet clover had been terminated. Because no difference in weed suppression existed when yellow sweet clover residues remained in the field or were removed, the authors speculate d that isoflavanoid and pheno lics (identified in other studies) released during growth and/or root decomposition ma y have explained some of the suppression. Small-seeded weeds may be more suscep tible to growth-redu cing stresses than larger seeded crops (see Davis and Lieb man, 2001, for discussion). Allelopathic chemicals and slower release of N from d ecomposing GMs may therefore reduce smallseeded weed growth more than that of large seeded crops. Davis and Liebman (2001)

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150 found red clover ( Trifolium pratense ) residues significantly reduced and delayed the maximal relative growth rate for wild mustard ( Brassica kaber ) but not for corn. Based on other studies, investigators speculated al lelopathic were responsible. Dyck et al. (1995) suggest more slowly available N fr om decomposing GMs favors large-seeded crops over small-seeded weeds. These i nvestigators found clov er residue reduced lambsquarters ( Chenopodium album ) biomass more than that of corn at 2 weeks after emergence (72% and 31% reductions, respectiv ely). By final harvest, corn biomass following crimson clover recovered to levels achieved by chemical fertilizer, while lambsquarters remained 39% lower relative to conventionally fertilized treatments. We initiated a study to evaluate use of combined summer and winter GMs in a reduced tillage system as an N source for sweet corn in Florida. Weed and nematode pressure may have long-term implications for the profitability of such a system. Therefore we also assessed effects of GM and conventional approaches on weed growth and nematode populations. We hypothesized th at GMs would significantly outcompete (reduce biomass) of weeds and lower damage potential and population counts of parasitic nematodes, particularly Meloidogyne spp. (root-knot nematodes), Pratylenchus spp. (lesion nematodes), Paratrichodorus spp. (stubby-root nematodes), and Criconemella spp. (ring nematodes). Study objectives were to quantify weed dry matter production in each cropping approach and assess impacts of GMs on parasitic nematode populations and damage potential in general. Materials and Methods Set-Up and Design The 15 overall treatments as well as site and experimental design are more fully described in Chapter 1 (Table 1.2). Treatment s consisted of sweet corn preceded by: a

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151 summer GM of sunn hemp and a winter GM of blue lupin ( Lupinus angustifolius ; winter 2001-02) and cahaba white vetch ( Vicia sativa ; winter 2002-03; rotation denoted as SH+L); sunn hemp only (SH); lupin (winte r 2001-02) and vetch (winter 2002-03) only (L); and unamended “conventional” corn (C onv). Each GM level was supplementated with 0, 67, or 133 kg inorganic N ha-1 (0N, 67N, and 133N). Other unamended (Conv) treatments also received 200 or 267 kg inorganic N ha-1 (Conv 200N and Conv 267N). A complete fallow (Fal) receivi ng only weed control was also used for comparison. Procedures and Measurements Weed samples consisted of roots and s hoots taken from a representative, 0.23 m2 (2.5 ft2) area in each plot at the end of sunn hemp (2001 and 2002) and vetch (2003). All samples were bagged and dried for 72 hours at 65 C, then weighed for total dry weight after removal of soil from roots. Afterwards all samples were ground in a Wiley mill to pass through a 2-mm screen, and a thoroughl y mixed portion of each grinding was then subjected to a wet-acid Kjeldahl digestion, diluted and filtered. The diluted samples were analyzed for total Kjeldahl N (TKN) at the University of Florida Analytical Research Laboratory (EPA Method 351.2; Jones and Case 1991). Soil samples for nematode analysis were collected on six occasions during the twoyear study. Each sample consisted of six soil cores (2.5 cm diameter x 20 cm deep) from a plot. After thorough mixing of the aggregate sample, a 100 cm3 subsample was removed for nematode extraction using a si eving and centrifugation procedure (Jenkins, 1964). Extracted nematodes were identified a nd counted under an i nverted microscope. Data was analyzed using log-transformation: y = log(x+1) where y = log-transf ormed data point

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152 x = nematode population count in a single sample Initially, limited plantings of cucumber in all plots (summer 2002) were used to assess potential for root galling. However, after results showed extreme variability this procedure was terminated. Therefore, plots used for nematode soil samples did not constitute all treatments until the final tw o samplings (when nematode numbers had increased and when it became known that cucu mber root data could not be used). Previous to that, soil samples were collected from selected plots only to gauge effects of GM, N-rate, and/or fallow on nematodes. In those cases, balanced ANOVAs were developed to compare appropriate effects. Da ta from weed samplings and full nematode samplings were analyzed similarly to corn biomass data (Chapter 3) except that the complete fallow treatment (Fal) was also co ntrasted against GM and Conv treatments. Results Nematodes Root-knot, lesion, stubby-root and ring nematodes remained present for counting throughout the study. Xiphinema spp. (dagger nematodes), Helicotylenchus spp. (spiral nematodes), and Belonolaimus spp. (sting nematodes) appeared only periodically; their populations are discussed but not shown in tables. October 2001 Plots planted to sunn hemp only were comp ared to conventional (Conv) plots that had not been planted with any GM. Soil coun ts of ring nematodes were weakly higher ( = 0.10) in plots with sunn hemp (42 11 individuals 100 cm-3) compared to plots without (19 5 individuals 100 cm-3). Samples taken from Conv plots showed weakly higher (p 0.10) stubby-root counts compared to sunn he mp plots, but counts of these nematodes were very low (2 or less individuals 100 cm-3). No significant differences existed between

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153 plots with and without sunn hemp for lesion, root-knot, and spir al nematodes at this time, with plot counts for these nematodes also ve ry low (never more th an 7 individuals 100 cm-3; Table 6.1). March 2002 Near the end of lupin growth, plots plan ted to lupin only were compared to Conv plots not planted with any GM. No significan t differences existed for stubby-root, lesion, root-knot, sting, and dagger nematodes at this time. Average counts were never more than 11 individuals 100 cm-3 for any nematode within eith er treatment and no significant differences occurred (Table 6.1). April 2002 Plots were sampled about six weeks afte r the March 2002 sampling, just after corn planting. Plots previously plan ted to lupin only were again compared to Conv plots that were previously fallow (before corn planting) Soil counts made for root-knot nematodes were much higher (p 0.05) in plots that had lupi n (199 94 individuals 100 cm-3) compared to plots previously fallow (5 2 individuals 100 cm-3). Soil counts for stubbyroot, lesion and ring nematodes were of the sa me order of magnitude as in the previous sampling and again showed no statistical di fferences between lupin and fallow (Conv) treatments (Table 6.1). July 2002 Plots were sampled just after corn harv est. Nine different treatments were compared: SH+L 133N, SH 133N, L 133N, Conv 0N, Conv 67N, Conv 133N, Conv 200N, Conv 267N, and Fal. Soil counts for r oot-knot were highest (generally, 30-160 individuals 100 cm-3), but counts for lesion and ring nema todes were also higher than in earlier samplings (generall y, 10-20 individuals 100 cm-3). Stubby-root nematode counts

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154 remained low (not more than 11 individuals 100 cm-3). Balanced ANOVA for GM level (comparing SH+L 133N, SH 133N, L 133N, Conv 133N) showed that, by the end of the corn season, lesion nematodes were significan tly lower in all plots previously planted with GMs compared to Conv treatments (T able 6.2). Root-knot, stubby-root, and ring nematodes were not significantly affected by GM history at this time. Average counts for all four nematode types remained lower (p 0.05) in Fal treatments compared to L and Conv treatments. Balanced ANOVA for stubby-root nematode as affected by N-rate level (Conv 0N, Conv 67N, Conv 133N, Conv 200N, Conv 267N) showed somewhat of a polynomial response to N-rate p eaking at 200N, but soil counts of stubby root nematode were rather low across all N-rates (Table 6.2). March 2003 All plots were sampled at the end of ve tch growth (prior to corn planting). Nematode populations were generally less than just after corn in July 2002 (Table 6.2) but greater than one year earlier in Ma rch and April of 2002 (Table 6.1). Root-knot nematode soil counts were not statistical ly affected by GM history, but stubby-root nematodes were significantly increased after presence of winter legumes and decreased by sunn hemp (L > SH+L > SH Conv). Ring nematode populations increased weakly in response to SH relative to SH+L or Conv. Although significant only for stubby-root nematodes, Fal plots showed population counts nu merically as low or lower than all other treatments. Chemical N-rate from the previ ous year’s corn produced no trends, nor did any interaction exist between N-ra te and GM type (Table 6.2). June 2003 All plots were sampled at the end of corn growth. Soil counts for root-knot, lesion and ring nematodes showed populations higher th an those seen at any other time (Table

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155 6.2). However, none of these nematodes showed clear responses to GM or chemical Nrate. Complete fallow plots showed nematode levels lower (p 0.05) than all GM or Conv groups for lesion and root-knot, but sign ificant against SH only for ring. Root-knot nematodes remained the dominant plant parasi te present with averag e counts typically > 100 individuals 100 cm-3. Treatment averages for lesion and ring generally remained between 35-100 individuals 100 cm-3, with treatment averages for stubby-root nematodes around 5-15 individuals 100 cm-3 (Table 6.2). Weeds Sunn hemp, October 2001 At the end of the 2001 growing season, weed dry weight under sunn hemp totaled 2.60 0.17 Mt ha-1, a reduction of 38% compared to 4.19 0.18 Mt ha–1 weed dry weight in plots without sunn hemp (Figure 6.1A). Although relatively low, weed plant N concentration was significantly higher (by 42%) under sunn hemp compared to fallow (0.48% 0.01% and 0.34% 0.11% for sunn hemp and fallow plot weeds, respectively; Figure 6.1B). As a result, no significant di fference existed for weed N content under either sunn hemp (12.4 0.8 kg N ha-1) or in fallow plots (14.2 0.9 kg N ha-1) at this time. Although not quantified in any way, crow’s foot grass ( Dactyloctenium sp.) was the dominant weed, with other grasses such as Digitaria sp., and non-leguminous dicots such as Florida pusley ( Richardia scabra ) and purslane ( Portulaca sp.), making moderate contributions. Legumes such as alyce clover ( Alysicarpus vaginalis ), hairy indigo ( Indigofera hirsuta ), and volunteer peanuts ( Arachis glabrata ) contributed a small amount of weed biomass.

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156 Sunn hemp, October 2002 Weed dry weight under sunn hemp (SH and SH+L treatments) at the end of the 2002 growing season totaled only 0.76 0.11 Mt ha-1, amounting to 21.5% of the 3.52 0.17 Mt ha-1 found in plots without sunn hemp (Conv and L treatments; Figure 6.1C). Weed dry weight in 2002 was 29% and 84% of 2001 values in SH and non-SH plots, respectively. Lower weed biomass may have been due in part to reduced tillage, possibly reducing germination of weed seeds through decreased soil disturbance and increased light absorption by the litter layer. Anecdot ally, crow’s foot grass became much less prevalent while pusley and other non-legumi nous dicots made up a majority or nearmajority of weed biomass. Alyce clover and hairy indigo again made small contributions. Reduced biomass may have also been due to a change in weed species composition (though this may have been re lated to reduced-tillage). Weed N concentration was again signifi cantly greater under sunn hemp, this time by 114% (1.46% 0.06% versus 0.68% 0.02 % for weeds in sunn hemp and fallow plots, respectively; Figure 6.1D). Compared to 2001, weed N concentration was 2-3 times as high, possibly as a result of the shift in weed species composition from grass to nonleguminous dicots and/or from greater ava ilable N derived from d ecomposing residues. Also unlike 2001, weed N content under sunn hemp (10.3 1.3 kg N ha-1) was significantly less than in fallow plots (24.3 1.6 kg N ha-1). Competition for N from sunn hemp may have produced more effect in 2002 because sunn hemp was 50% larger than in 2001. Weed N content for SH plots in 2002 wa s 27% lower compared to 2001, but was 71% greater in non-SH plots during 2002 compared to 2001.

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157 Vetch, April 2003 Greatest weed dry weight occurred for SH (1.81 0.11 Mt ha-1) lowest weed dry weight for L (0.66 0.05 Mt ha-1), and mid-level weed dry weights for Conv (0.97 0.07 Mt ha-1) and SH+L (1.04 0.13 Mt ha-1; Figure 6.1C). Similar grouping occurred for weed N content, except that Conv was not significantly different from L. Nitrogen content in weeds was quite low, ranging from 2.2 to 8.1 kg N ha-1 depending on treatment. Nitrogen concen trations were unaffected by GM and ranged from 0.37% to 0.46%. Both Chemical N-rate and N-rate/GM interaction were again insignificant for weed dry weight, N concentration, and N cont ent. Weeds consisted almost entirely of small non-leguminous dicots including Richardia spp ., Gnaphalium pennsylvanicum Lepidium spp., and Geranium spp. Discussion Although patterns in nematode population ma y require some time to equilibrate to new conditions, data from the first two year s of the study already indicate changes based on cropping system and GM presence. Rootknot, lesion, stubby-root, and ring nematode all showed greatest increase following corn with strongest responses coming from rootknot and lesion. Populations of these pest nema todes also appeared to climb over time in all treatments with sweet corn compared to complete fallow, with root-knot nematode maintaining highest population c ounts (over 100 individuals 100cm-3 by the end of the sample period; Tables 6.1 and 6.2). Many invest igators have demonstrated host status of corn to these parasitic nematodes (AlRehiayani and Hafez 1998, Sipes and Arakaki 1997). In the first year of the study, lesion nematode populati ons showed a significant increase under lupin from March to April (2002) coinciding with warming weather and

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158 longer availability of increas ed lupin biomass (Table 6.1). However, by the end of the subsequent corn crop (July 2002), lesion nematode population counts were significantly lower in all plots previously planted with GMs (includi ng lupin) compared to Conv treatments. Root-knot nematode counts were numerically higher for corn following L and lower following SH (Table 6.2). Reduction of parasitic nematodes due to specific nonhost or suppressive interacti on, especially with sunn hemp, may therefore have occurred in some instances. Sunn hemp has demonstrat ed allelopathic and antagonistic effects on root-knot and reniform nematodes under gr eenhouse conditions (Wang et al. 2001, 2003), and non-host or poor-host status of sunn hemp w ith respect to root-knot nematode is well documented by previous studies (McSorley 1999, Sipes and Arakaki 1997, Al-Rehiayani and Hafez 1998). In March 2003 (prior to sweet corn pl anting), stubby-root nematodes showed a significant increase following vetch and decr ease following sunn hemp, but by the end of the 2003 corn crop no clear trends existed base d on GM level or chemical N-rate. It is possible that stubby-root may have demonstr ated a slight incr ease with increased chemical N-rate, and that ring nematodes may have increased following sunn hemp, but the low populations and count variability prevent any firm conclusion without further data. Over time, the favorable host status of corn to many of the nematodes present affected their population dynamics more th an GM crops. The increase of parasitic nematodes under corn may have possibly masked repressive effects from GMs. Trends based on GMs may therefore become more det ectable as nematode populations increase toward their potential over time and become more spatially unifor m within plots, but these trends may remain short-liv ed if preferred hosts such as corn continue to be used.

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159 Given an initial plow-down or herbicide ki ll of weeds, sunn hemp showed excellent potential for reducing weed gr owth. Greatest reduction was seen in 2003 when a 14 week growth season and maximum sunn hemp l eaf area index approaching around 6 m2 leaf m2 ground limited weed production to less than 1 Mt ha-1, a 78.5% reduction compared to plots without sunn hemp (Figure 6.1B). Data from an ongoing study at the same research facility suggests close to 100% reduction of weeds when between row spacing was decreased from 75 cm to 30 cm (Linares a nd Scholberg, unpublished). However, after death, sunn hemp residue significantly incr eased winter weed biomass (Figure 6.1C), likely because the unmowed stems settled dow n linearly along the rows (rather than spreading in random directi ons) leaving much exposed gr ound while residue released large amounts of N (Chapter 3). Broadcast planting and/or mowing may help spread residue more uniformly. On the other hand, a more vigorous stand of winter GMs or economic crops planted directly into living sunn hemp or immediately after death may provide highly effective weed contro l after sunn hemp no matter the residue management. Even relatively poor stands of vetch provided significant weed suppression (Figure 6.1C), and continuation of this project using a multi-species mixture as a winter GM appeared to be much more successful in providing uniform winter coverage and weed suppression (Avila and Scholberg, unpublished). Initial changes in both weed biomass and N over time appeared to reflect simultaneous changes in weed species co mposition. Grass species heavily dominated weeds in 2002 following a clean-plow of the fi eld, but after a few growing seasons under reduced and zero-tillage, weed species shif ted to non-leguminous dicots in 2003. These changes may help explain reduced weed bi omass and increased weed N concentration

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160 from the first (October 2001) to the s econd sampling (October 2002; Figure 6.1A,B). Reduced tillage may have acted to suppress or change patterns of weed seed germination through lower disturbance of the soil seed bank and lower light penetration through the increasing litter layer. Sunn hemp in 2002 (whi ch was 50% larger than in 2001) appeared to exert a greater competitive effect for N than in 2001, significantly reducing weed N content relative to plots wit hout sunn hemp. Decomposing re sidue in our reduced tillage system may also have made gr eater contributions to weed N content in plots not planted to sunn hemp. Short-term changes in weed species composition, biomass and N content may therefore reflect aspects of the system related to reduced tillage, biomass additions, and competitive effects of GMs. As shown in Figure 6.1D, weeds under sunn hemp in both years showed significantly higher N concentra tion than in conventional ( non-GM) plots, especially after a full year of reduced tillage. Evidence from other studies shows non-legumes growing in shaded environments tend to have higher N concentration than when grown in full sun (for example, Senanayake 1995 a nd Wilson 1996). An interesting possibility deserving more study would be to combine sunn hemp with a low growing, less massive non-legume for more complete weed suppression Should a farmer not desire or be able to narrow sunn hemp row spacing, or should sunn hemp suffer from pest, disease, or other environmental stress, such a mixture might offer some buffering ability for weed control. Preliminary results from further studies related to th is project shows mixtures of two or more winter GMs provided far more uniform coverage and weed control than winter legume monocrops used previ ously (Avila and Scholberg, unpublished). Additionally, residue from a low growing “carpeting” GM may complement sunn hemp

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161 by suppressing weeds in open spaces otherwis e left empty between sunn hemp stem residue. Such voids created major opportunity for weeds after sunn hemp death during the course of the study. Conclusions During the two years of this study, cropping system and GMs significantly affected parasitic nematode populations as well as we ed production and N characteristics. Rootknot, lesion, stubby-root, and ring nematodes all showed greatest increases following corn. Sunn hemp or its residues periodically exhibited suppressive effects on root-knot and stubby-root nematodes, while lupin and vetch showed mixed impacts on lesion and stubby-root nematodes. Sunn hemp and ve tch significantly reduced weed biomass production at the end of their respective gr owing seasons. However, data and anecdotal evidence suggest that changes in management may improve weed control, especially during winter and early spring.

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162 0.0 1.5 3.0 4.5 SHConvFinal Weed DW (Mt ha-1) A ba 0.0 1.0 2.0 3.0 4.0 SHConvFinal Weed DW (Mt ha-1) B ba 0.0 0.4 0.8 1.2 1.6123Weed N Concentration (%) SH Conv Oct-01 Oct-02Apr-03ab ab aa D 0.0 0.5 1.0 1.5 2.0 SH+LSHLConvFalFinal Weed DW (Mt ha-1) C bab c Figure 6.1. Final weed dry weight s and N concentrations. (A) Fi nal weed dry weight after sunn hemp in October 2001 and (B) in October 2002; (C) final weed dry weight after vetch in April 2003; (D) final weed N concen tration (%) after sunn hemp (October 2001 and 2002) and ve tch (April 2003). Error bars reflect standard errors; columns within GM leve ls (A, B and C) or sample dates (D) with identical lower case letters not significantly different to the = 0.05 level according to Duncan’s Multiple Range Test.

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163 Table 6.1. Nematode soil populat ion counts (individuals 100 cm-3) from selected treatments at selected dates. Root-Knot Lesion Stubby-Root Ring Oct. 2001 X X Conv <1 3 1 a 19 b SH <1 <1 <1 b 42 a Mar. 2002 Conv <1 <1 3 7 L 2 <1 3 11 Apr. 2002 Conv 5 <1 <1 2 L 199 2 3 1 SH = sunn hemp; L = winter legume; Conv = conventional (no green manure) X, = Significantly different from Conv at p 0.10 and 0.05 levels, respectively.

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164Table 6.2. Nematode soil populat ion counts (individuals 100cm-3) at selected dates. Root-Knot Lesion Stubby-Root Ring Jul. 02 Mar. 03 Jun. 03 Jul. 02 Mar. 03 Jun. 03 Jul. 02 Mar. 03 Jun. 03 Jul. 02 Mar. 03 Jun. 03 GM NS NS NS NS NS NS ** NS NS NS NS SH+L 57 21 85 6 b† 4 88 † 6 11 b† 11 2 9 b 35 SH 32 23 112 † 5 b 7 82 † 1 1 c 8 16 24 a 119 † L 90 † 29 89 † 12 b† 2 59 † 7 42 a† 11 16 16 ab 41 Conv 95 † 28 129 † 29 a† 1 45 † 9 1 c 6 19 8 b 71 Fallow 5 15 56 2 1 1 3 1 4 10 16 12 N-Rate NS NS NS NS NS NS ** NS NS NS NS NS 0N 154 27 136 30 2 44 4 cb 1 4 b 19 7 57 67N 82 31 125 11 <1 55 2 c 1 4 ab 13 10 87 133N 95 25 126 29 1 35 9 ab 1 11 ab 19 8 69 200N 98 14 178 30 1 38 11 a <1 9 ab 2 9 22 267N 47 29 183 15 1 91 5 ab 1 12 a 11 8 18 SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4NO3-N ha-1; NS means not significantly different at p 0.05 level; *,**,*** means signi ficantly different at p 0.05, 0.01, and 0.001 level, respectively; † mean significantly different than Fallow at p 0.05 level; means in vertical columns for GM or N -rate followed by the same letter do not differ at p 0.05 according to Duncan’s Multiple Range Test.

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165 CHAPTER 7 CONCLUSIONS Review and Synthesis of Findings Green manure (GM) approaches are inhe rently dynamic and system oriented. Unlike chemical inputs, GMs are biological or ganisms both affecting and affected by the cropping system. The breadth of useful GM species, growing environments and management strategies summarized in Table 1.1 highlights the complexity of options for such approaches to crop production. Yet proper assessment of GM techniques requires an even greater understanding of the site-specific relationships between the life-cycles of the plant species used (both GMs and economic cr ops), the production environment (climate, weather, soil, and pests), and management options (for example: type, patterns, and timing of tillage, planting, irrigation, and fert ility and pest control inputs, as well as production goals). Although such detailed info rmation often remains lacking, the wholesystems nature of GMs presents opportunity to develop integrated a pproaches to nutrient supply, soil properties, a nd pest management using on-farm, biologically-based resources. This study focused on a GM approach to a particularly ch allenging production system and environment: a spring planted, hi gh-N demanding vegetable crop (sweet corn; Zea mays var Rugosa) in north Florida (see Chapte r 1 for discussion). The overall project, funded by the USDA-SARE (grant number LS02-140, “A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems”)

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166 and involving collaborative studies in s outh Georgia and south Florida production environments, is envisioned to continue for another 2-3 years depending on external funding. Results of this study and preliminary data from the parallel studies (Phatak et al. unpublished, Roe et al. unpublished) reinforce the patterns seen in the sc ientific literature: that GM management often falls into three major categories based on regions of climate and soil. Especially in the southeast US, thes e three management zones are: (1) temperate regions with freezing winters but also fine-tex tured soil and cool, pred ictable climate; (2) tropical regions sometimes having coarse-tex tured soil but remaining free from winter freezes; and (3) transitional zones with coarse-t extured soil, freezing winter temperatures, and variable climate. In the first (temperate) region, soil organic matter (SOM) may be more readily increased, allowing greater storage potential for nitrogen (N) derived from decomposing residues (see Chapter 5 for review). Additi onally, cool-weather le gumes of temperate origin appear better adapted to the climate and soil of this region. In the second (tropical) region, vigorous tropical legumes and the crops that follow them are never limited by freezing temperatures. Living plants can therefore store N year-round, and the low potential SOM of these regions can be counter ed if crops and GMs ar e rotated quickly or intercropped so as to prevent N leaching lo ss (see Chapters 1 and 2 for reviews). The third (transitional) region, however, has none of the advantages of the other two. Here, coarse soils and hot, humid climate limit – or at least slow – potential SOM accumulation (Chapter 5); freezing winter temperatures inte rrupt use of tropical legume GMs for spring crop production; and temperate legumes appear poorly adapted to the variable climate, sandy soils, and pests of the region, especia lly when grown as a monocrop (Chapter 2).

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167 Green manure management options from te mperate and tropical regions, which can be relatively straightforward, may therefore be inappropriate for transitional regions. Our experience indicates leguminous GMs decompose very rapidly on warm, humid, sandy soils in north Florida, with gr eatest GM N-loss occurring too quickly (within 2-4 weeks) to match peak N demand from a subsequent crop. Production of large amounts of recalcitrant stem biomass did not result in net N-immobilization during leguminous GM decomposition, likely becau se the residue was not homogenized (Chapter 2). We also found that the microbial mode of decomposition is not associated with biomass C increases, and thus may not be as N-limited, as in other environments (Chapter 5). This indicates potential microbial N-immobiliza tion via increased C and/or lignin inputs may not be as much as found elsewhere. Nitrogen immobilization demonstrated by roots remained relatively sm all and unimportant due to low root biomass of our selected GMs. On the other hand, most winter-hardy GM monocrops in north Florida appear incapable of accumulating enough N to satisfy the requirements of many spring crops (Chapters 1, 2 and 3). Our results suggest producers and research ers in our region should consider several GM alternatives including: changes in rotation order (planting a fall/winter economic crop immediately after sunn hemp or movi ng sunn hemp to the spring prior to a subsequent summer economic crop); change s in planting method and GM termination (direct planting into living sunn hemp, using vehicle action to break stems and open the canopy); changes in GM residue manageme nt (homogenization of residue through mowing, or use of warm season legumes for which pods can be harvested to remove potentially labile N and deliver economic benefit); changes in GM species choice

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168 including those with greater below ground production (to increase N-immobilization); and/or use of winter GM mixt ures of legumes, small grains cool-season grasses and nonleguminous dicots (instead of monocrops). Nevertheless, we sometimes found that tissue growth, N content, and leaf indicators for sweet corn am ended with combined chemical N and GM residue remained comparable to chemically fertilized, high-N corn – but with reduced final ear yield at season’s end – even though the combined GM/chemical approach applied 90-110 kg N ha-1 less than the high-N chemical approach (Chapter 3). The competitiveness of the GM/chemical approach despite lower applied N, and the relative decline in ear yields for these treatments compared to high-N chemi cal treatments, appears partly explained by patterns of root growth (Chapter 4). Amen dment with GMs in a no-till system and band application of chemical N to the in-row ar ea apparently encouraged root proliferation close to the soil surface and near the plant by creating an environment of greater water (and probably N) availability during the first ha lf of the season. However, increased root growth in this area may have increased vulnera bility to water stress in the second half of the season. Because total applied N was much lower for GM approaches relative to the high-N chemical only approach, increased root growth in the upper soil area may also have brought on N-depletion for GM treatments toward the end of the season. Because it exhibits such low organic matter and nutrient retention, our sandy soil may have further encouraged these rooting patterns and furthe r exacerbated water and N-stress than would have fine-textured soils mo re often found elsewhere. Near season’s end, root length density fa r from the plant was significantly greater for high-N chemically fertilized corn compar ed to the combined GM/chemical approach.

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169 Other studies (Coelho and Or 1999) show these “far” root s may contribute significantly to water uptake and N-uptake, offering some explanation for differences in late-season ear-fill. Roots from high-N chemically fertiliz ed corn may have also been more likely to grow further from the plant during late seas on if some significant amount of the “extra” 90-110 kg N ha-1 still remained in between-row areas and at lower soil depths (30-60 cm). Estimates of effective rooting depth (40-60 cm) suggest such N would have been available (Chapter 4). Because such late-season changes leave pr oducers without adequate time to detect need for and implement adjustments, manage ment of GM approaches in the Florida environment must be “preventative” rather th an “therapeutic.” Great est predictive power for end-season yields came from N-content of GM residues and chemical N applied to corn. Afterwards, statistical anal ysis of plant growth characte ristics at early to mid-season could only distinguish gross disparity in yi eld potential despite significant final yield differences among treatments with closer N application rates. Soil incorporation of GM resi due to help encourage deeper root growth under these circumstances would be favorab le if subsequent nutrient lo ss from decomposition did not negate the benefits. However, in warm humi d areas with coarse textured soil, reduced tillage is often desired to slow organi c matter decomposition and nutrient loss and improve low soil water retention. In such redu ced tillage systems, improved use of GMs may necessitate different irrigation manage ment, including drip lines buried below surface residue to increase infiltration. Us e of GMs or GM mixtures with more substantial below ground production may be an even less expensive and laborious way to create a better rooting envir onment at deeper depths. Early season deficit irrigation to

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170 encourage root exploration or use of vari eties of economic crops with deeper root systems may help, but providing GMs with N-c ontent closer to the optimal rate applied with chemical fertilizer must remain a pr iority. Given our particular management strategies, the GM species chosen, and th e environment, we found a GM benefit of roughly 50-70 kg N ha-1 in terms of final sweet corn ear yields (Chapter 3). Corn amended with GMs remained more competitiv e with conventionally fertilized corn during a year with lower plant population ( 2002). Organic approach es to crop production relying heavily on GM N may be less risky with lower crop plant populations and with crops having lower N demand and price premiums not requiring large fruit size. Besides the short-term consequences on cr op growth, we found our GM approaches also affected soil and pest properties, possibly with longterm implications for the efficacy of the system within the context of the north Florida “transition zone.” Sweet corn cropping systems with combined use of sunn hemp and winter legume or sunn hemp alone contributed roughly 20 Mt dry matter ha-1 annually to the field. These GM approaches significantly increase d pools of the soil C and N that we would consider most indicative of recent changes in soil organic matter inputs – even on very sandy (95-97%) soil under reduced tillage and after only 2 y ears. Although best reso lved when evaluating the entire 0-15 cm soil layer, changes probabl y occurred mostly in the upper 7.5 cm of soil. Analysis also revealed a decline in total soil C (TC) for all treatments over time, possibly reflecting historical conversion from pasture to row-crop system. Depending on the size of future organic matter transfers into the soil fractions, we predict trends in total soil C may diverge, with integrated GM/c hemical approaches possibly slowing or

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171 reversing TC loss and conventional chemical approaches possibly speeding it. Size of future soil transfers will determine whether any differences from GMs result in practical changes of soil organic matter (on the orde r of 1% or more). Such a result would probably be unprecedented in agronomic resear ch in sandy Florida soils, and might also affect future root growth, N and water uptake potential and yield patt erns of sweet corn (or other crops) as investigated in Chapte rs 3 and 4. Notwithstanding, these results show short-term boosts in soil organic matter on th e order of 1% are unlikely in our production environment without even higher additions of organic amendments. Immediate goals of GM management techniques in the Flor ida environment should instead focus on delivering adequate N to economic crops. Cropping approaches that ill-su it pest pressures face litt le hope for farmer adoption. On the other hand, low-cost, on-farm based a pproaches delivering multiple benefits of nutrient supply, increased yields and enhancem ent of agricultural soil properties as well as partial pest control may be economically desirable or even necessary. Even during the first two years of this study, cropping syst em and GMs significantly affected parasitic nematode populations as well as weed produc tion and N characteristics (Chapter 6). Root-knot, lesion, stubby-root, and ring nematode all showed greatest increase following corn with strongest responses coming from root-knot and lesion. Populations of these pest nematodes also appeared to climb ove r time in all treatmen ts with sweet corn compared to complete fallow, with rootknot nematode maintaining highest population counts. Sunn hemp or its residues periodical ly exhibited suppressive effects on root-knot and stubby-root nematodes, while lupin and ve tch appeared to be hosts for lesion and stubby-root nematodes, respectively. However, in one of two years, soil under corn

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172 following any GM showed lower populations of all monitored nematodes, with significant reduction for lesion nematode desp ite possible host stat us of lupin. This suggests indirect effects, such as increased biological activity following GM additions, may also have played a role. The increase of parasitic nematodes under corn may have masked further repressive effects from GMs. Trends based on GMs may therefore become more detectable as nematode populat ions increase toward their potential over time and become more spatially uniform within plots. Sunn hemp and vetch significantly reduced weed biomass production at the end of their respective growing seas ons. However, data and anec dotal evidence suggest that changes in management may further improve we ed control, especia lly during winter and early spring. Broadcast planting and narrowe r row spacing, as well as GM mixtures (of sunn hemp with a carpeting legume or non-leguminous dicot during summer and of legumes, grasses, small grains, and/or nonleguminous dicots during winter), show promise and should be further investigate d. Short-term changes in weed species composition from grasses to non-leguminous dicots, reduced biomass and increased N concentration may also reflect aspects of the system related to reduced tillage and biomass additions. Future changes in these f actors could possibly serve as indicators for developmental changes in GM and conventional approaches. Future Work As mentioned earlier, pote ntial usefulness and adoption of GM approaches to cropping systems probably depend on reducing need for supplementary inputs for nutrient supply and pest (weed, herbivore/parasite, and disease) control, concomitant with an overall reduction in operation costs. Tr ue “whole-systems” economic evaluation of GM approaches should therefore include as many aspects as possi ble of real production.

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173 Having gained more understan ding of GM management in our region, future projects should probably conduct medium to long-te rm input and economic evaluation of well planned GM cropping systems on fi eld scale (rather than smalle r plot scale) in research and eventually on-farm. This approach, simila r to that of Phatak et al. (1999), would allow for more accurate assessment of pest pr essures operating on larger scales as well as better evaluation of management needs representative of actual farmers.

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174 APPENDIX A CHARACTERIZATION OF DOMINANT SOIL TYPES PRESENT IN FIELD Characterization data from representative soils provided by Carlisle et al. (1988). Table A.1. Selected characteristics from a La ke Fine Sand; Typic Quarzipsamments, hyperthermic, coated; Citrus County, FL. Depth Horizon Sand Silt Clay Organic C pH Base Sat cm % of all particles 2 mm % 1:1 H2O % 0-18 Ap 97.0 0.6 2.4 0.84 4.6 3 18-68 C1 96.3 1.4 2.3 0.25 4.3 2 68-102 C2 96.5 1.0 2.5 0.14 4.3 3 102-142 C3 96.4 0.9 2.7 0.06 4.7 5 142-203 C3 96.4 0.7 2.9 0.05 4.7 4 C = carbon; 1:1 H2O = equal weights soil and water; Base Sat = base saturation. Table A.1. Continued. Depth CEC Sat Hydr Cond H2O Cont (% by weight) Bulk Density cm meq 100g-1 cm hr-1 1/10 Bar 1/3 Bar 15 Bar g cm-3 0-18 7.96 16.4 8.0 5.3 2.3 1.50 18-68 3.91 17.4 6.0 3.8 1.5 1.54 68-102 3.52 26.6 5.5 3.1 1.6 1.47 102-142 2.91 25.9 5.2 3.0 1.3 1.53 142-203 2.79 26.3 5.3 2.9 1.3 1.52 CEC = cation exchange capacity; Sat Hydr Cond = saturated hydr aulic conductivity; H2O Cont = water content at specified tension.

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175 Table A.2. Selected characteristics from a Ca ndler Fine Sand; Typic Quarzipsamments, hyperthermic, uncoated; Alachua County, FL. Depth Horizon Sand Silt Clay Organic C pH Base Sat cm % of all particles 2mm % 1:1 H2O % 0-15 Ap 98.3 0.3 1.4 0.48 5.5 17 15-41 E1 98.2 0.8 1.0 0.18 5.3 7 41-71 E2 98.4 0.6 1.0 0.09 5.1 8 71-145 E3 98.1 0.5 1.4 0.05 5.2 5 145-178 E4 98.5 0.3 1.2 0.03 5.3 3 178-208 E/B 98.9 0.1 1.0 0.01 5.2 13 C = carbon; 1:1 H2O = equal weights soil and water; Base Sat = base saturation. Table A.2. Continued. Depth CEC Sat Hydr Cond H2O Cont (% by weight) Bulk Density cm meq 100g-1 cm hr-1 1/10 Bar 1/3 Bar 15 Bar g cm-3 0-15 2.52 26.0 7.1 5.1 1.2 1.49 15-41 1.96 33.2 4.8 3.2 0.7 1.53 41-71 1.06 37.8 4.3 3.0 0.6 1.52 71-145 0.55 37.1 4.1 2.7 0.6 1.49 145-178 0.68 37.4 4.6 3.0 0.6 1.50 178-208 0.38 33.5 4.5 3.0 0.4 1.49 CEC = cation exchange capacity; Sat Hydr Cond = saturated hydr aulic conductivity; H2O Cont = water content at specified tension.

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176 APPENDIX B CONTINUOUS MEASUREMENTS Table B.1. Continuously measur ed environmental factors. Radiation* Tmax Tmin Rainfall* Irrigation RH w m-2 C C cm cm % SH 2001 178 35.5 11.9 23 NA 77.3 L 2001-02 143 30.9 0.3 20 NA 74.7 Corn 2002 217 35.4 19.2 14 NA 68.8 SH 2002 176 35.3 20.4 33 NA 79.0 L 2002-03 137 24.0 6.0 57 NA 59.0 SH = sunn hemp; L = winter legume; Tmax = average maximum daily temperature; Tmin = average daily minimum temperature; RH = average relative humidity; radiation and rainfall data provided by Florida Automated Weather Network (2004); NA = not available.

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APPENDIX C SELECTED TISSUE FACTORS AND LEAF INDICATORS FOR SWEET CORN, 2002 AND 2003

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178Table C.1. Corn applied nitrogen, unaccounted for applied nitroge n and chlorophyll meter readings by green manure and nitrogen rate, sweet corn, 2002. NUE Corn Applied N† UAN Total UAN Chlorophyll meter readings kg kg-1 kg N ha-1 kg N ha-1 kg N ha-1 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS GM *** *** *** NS NS NS NS NS SH+L 0.32 ab 123 a 75 a 152 a 39.0 38.7 42.6 44.7 44.4 a SH 0.37 a 78 b 35 b 121 b 39.7 38.4 45.5 43.6 40.6 ab L 0.22 b 124 a 83 a 100 c 37.4 38.7 42.4 45.2 40.3 ab Conv 0.37 a 67 c 27 b 50 d 37.5 38.1 44.2 45.4 37.5 b N-Rate NS *** *** *** ** *** *** *** *** 0N 29 c 12 c 61 c 34.2 c 30.8 b 33.4 b 33.7 b 32.4 c 67N 0.34 97 b 50 b 101 b 38.7 b 41.3 a 47.4 a 48.4 a 40.8 b 133N 0.30 168 a 103 a 154 a 42.4 a 43.3 a 50.7 a 52.0 a 48.9 a GM: green manure. N-rate: chemical nitrogen rate (kg N ha-1). NUE: nitrogen uptake efficiency (calculated without 0N treatments). UAN: Unaccounted for applied nitrogen. Total UAN includes N content of sunn hemp and weeds prior to winter decomposition. WAE: weeks after emergence † Corn Applied N includes contributions from GMs and chemical N. NS: means within columns not significantly different to the 0.05 level. *, **, ***: means within columns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s MRT comparisons a t the 0.05 level.

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179Table C.2. Specific leaf area and spec ific leaf nitrogen by green manure and nitrogen rate, sweet corn, 2002. Specific Leaf Area (cm2 g-1) Specific Leaf N (g N cm-2) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS NS NS SH+L 371 257 208 182 160 45.3 47.3 55.3 ab 55.5 52.3 SH 372 257 216 183 163 44.8 45.4 46.2 b 50.4 55.1 L 379 243 223 193 166 43.1 51.6 68.4 a 53.6 52.6 Conv 379 247 214 190 163 41.3 51.2 46.2 b 52.3 50.6 N-Rate ** * NS *** *** *** *** *** 0N 388 a 263 a 241 a 195 a 170 a 39.2 b 35.8 b 35.5 b 42.7 b 42.8 c 67N 375 b 250 ab 202 b 182 b 163 ab 42.7 b 53.4 a 58.2 a 53.5 a 51.4 b 133N 362 c 240 b 203 b 184 b 156 b 49.1 a 57.3 a 68.3 a 62.6 a 63.8 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level. Table C.3. Stem dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2002. Stem Dry Weight (kg ha-1) Stem N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS SH+L 24 ab 255 a 2203 ab 2374 a 2101 a 0.5 5.9 a 24.1 12.8 11.5 a SH 28 a 260 a 2265 ab 2279 ab 1990 ab 0.6 5.9 a 21.6 11.2 9.5 a L 25 ab 216 ab 2244 a 2168 ab 1769 ab 0.5 5.1 ab 24.4 10.2 8.8 a Conv 19 b 185 b 1976 b 1959 b 1983 b 0.4 4.2 b 21.7 10.3 8.8 b N-Rate *** *** *** *** *** *** *** *** *** *** 0N 18 b 104 c 804 b 1272 b 1227 c 0.3 c 1.5 c 7.8 c 5.6 c 6.0 c 67N 25 a 257 b 2781 a 2495 a 2044 b 0.5 b 6.0 b 26.4 b 10.9 b 8.7 b 133N 30 a 326 a 2931 a 2819 a 2613 a 0.7 a 8.4 a 34.7 a 16.9 a 14.2 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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180Table C.4. Root dry weight and ni trogen content by green manure a nd nitrogen rate, sweet corn, 2002. Root Dry Weight (kg ha-1) Root N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE † 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS NS NS NS SH+L 16 a 78 348 390 347 0.2 0.9 2.2 1.7 1.3 SH 16 ab 92 344 402 355 0.2 1.0 2.1 1.2 1.2 L 15 ab 79 380 342 298 0.2 0.9 2.5 1.3 1.1 Conv 13 b 70 322 342 314 0.2 0.8 2.2 1.1 1.0 N-Rate *** *** *** *** *** *** *** *** *** *** 0N 11 b 38 b 139 b 174 c 178 c 0.1 c 0.3 c 0.8 c 0.7 c 0.7 c 67N 16 a 95 a 436 a 424 b 359 b 0.2 b 1.0 b 2.5 b 1.2 b 1.1 b 133N 17 a 106 a 470 a 509 a 449 a 0.2 a 1.3 a 3.4 a 2.0 a 1.6 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level; † estimated. Table C.5. Ear dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2002. Ear Dry Weight (kg ha-1) Ear N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS GM NS NS ** NS NS SH+L 242 ab 1823 1978 a 4.6 22.2 21.1 a SH 350 a 1805 1835 a 5.9 21.4 18.3 ab L 279 ab 1636 1902 a 5.0 20.2 19.8 a Conv 313 b 1597 1597 b 5.3 19.3 15.7 b N-Rate *** *** *** *** *** 0N 0 b 482 c 437 c 0.0 c 6.0 c 4.6 c 67N 309 a 2110 b 2121 b 5.2 b 24.2 b 19.7 b 133N 579 a 2552 a 2926 a 10.3 a 32.1 a 31.9 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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181Table C.6. Stem and root nitrogen concentrations by green manure and nitr ogen rate, sweet corn, 2002. Stem N Concentration (g kg-1) Root N Concentration (g kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE† 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS NS SH+L 19.6 21.1 10.4 ab 5.1 5.4 a 10.3 10.3 6.1 4.2 a 3.8 a SH 19.6 20.5 9.5 b 4.8 4.8 ab 9.9 9.9 6.0 3.2 b 3.6 ab L 20.3 22.1 10.5 ab 4.7 5.0 ab 10.5 10.5 6.3 3.8 ab 3.7 ab Conv 19.3 21.8 11.2 a 5.1 4.4 b 11.3 11.3 6.7 3.5 b 3.3 b N-Rate *** *** ** ** *** *** *** *** *** *** 0N 16.5 b 14.3 c 9.9 b 4.5 b 4.9 b 8.2 c 8.2 c 5.6 b 4.2 a 4.2 a 67N 18.6 b 23.5 b 9.5 b 4.4 b 4.3 c 10.4 b 10.4 b 5.8 b 2.9 b 3.1 c 133N 24.0 a 26.4 a 11.8 a 5.9 a 5.5 a 12.8 a 12.8 a 7.3 a 4.0 a 3.6 b WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level. † Estimated. Table C.7. Ear and total nitrogen concentrations by green manure a nd nitrogen rate, sweet corn, 2002. Ear N Concentration (g kg-1) Total N Concentration (g kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS GM NS NS NS NS NS NS SH+L 19.9 12.2 10.4 16.4 14.8 10.5 ab 7.9 7.4 a SH 18.3 11.9 9.9 16.3 14.1 9.8 b 7.3 7.0 ab L 18.0 12.8 10.6 16.4 15.3 10.6 ab 7.7 7.4 a Conv 18.9 12.3 9.6 15.9 14.9 10.8 a 7.9 6.4 b N-Rate * *** *** *** *** *** 0N 12.8 a 10.3 ab 14.3 c 10.5 c 8.9 c 6.7 c 6.3 b 67N 17.9 11.4 b 9.3 b 15.7 b 15.9 b 10.1 b 7.5 b 6.7 b 133N 19.6 12.7 a 10.8 a 18.7 a 17.8 a 12.2 a 9.0 a 8.2 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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182Table C.8. Pairwise contrasts of chlorophyll meter readings and specific leaf ar ea, sweet corn, 2002. Chlorophyll Meter Readings (unitless) Specific Leaf Area (cm2 g-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 46.9 45.4 48.7 53.8 53.5 347 240 194 174 149 Conv 267N 42.3 47.8 54.4 53.1 56.1 369 238 192 177 158 SH+L 67N 36.6* 41.0* 47.5 50.3 44.7*† 374* 263 192 171 160 SH 67N 39.7* 41.6* 49.9 50.8 44.1*† 366 260 214 182 153 L 67N 36.5* 42.0* 46.7* 47.4 37.9*† 387* 246 203 189 175* SH+L 133N 43.8 44.4 48.1 49.8 55.9 367 236 206 181 156 SH 133N 43.3 43.6 49.5 50.8 44.0*† 351 251 181 180 154 L 133N 42.5 42.3* 50.7 51.9 49.3† 362 236 224 195 159 *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.9. Pairwise contrast s of Stem Dry Weight and N Content, sweet corn, 2002. Stem Dry Weight (kg ha-1) Stem N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 30 342 3901 2936 2479 0.8 9.7 61.0 21.4† 19.2† Conv 267N 27 396 2989 3556 2766 0.7 11.3 41.6 34.0* 25.2* SH+L 67N 24 263† 2674 2898 2037*† 0.4* 6.1*† 26.9* 13.5*† 9.3*† SH 67N 29 307 2979 2321† 2114†† 0.6 6.7*† 22.9* 10.4*† 10.0*† L 67N 21 234† 2663 2363† 1827*† 0.4* 5.9*† 25.2* 10.5*† 7.7*† SH+L 133N 30 377 2886 3003 2705 0.7 9.7 36.4 19.6† 17.6† SH 133N 38 360 3072 2878† 2514 0.9 9.5 34.8* 16.4† 12.1*† L 133N 31 306 3183 2838† 2430 0.8 7.9† 39.3 14.2*† 13.0*† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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183Table C.10. Pairwise contrasts of root dry we ight and nitrogen content, sweet corn, 2002. Root Dry Weight (kg ha-1) Root N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 17 105 595 526 390 0.2 1.4† 5.6 2.8† 1.7† Conv 267N 17 127 580 648 479 0.2 1.8* 5.0 4.2* 2.9* SH+L 67N 17 84† 406*† 477† 340† 0.2 0.9*† 2.4*† 1.5*† 0.9*† SH 67N 16 116 446*† 388† 428 0.2 1.3† 2.4*† 1.0*† 1.4† L 67N 16 94 495 400† 334† 0.2 1.1 3.0*† 1.2*† 1.0*† SH+L 133N 18 112 475* 522 475 0.2 1.5 3.3*† 2.6† 1.8† SH 133N 18 122 457*† 613 464 0.2 1.5 3.0*† 1.9*† 1.4† L 133N 17 99 498 465† 410 0.2 1.2† 3.8*† 2.1† 1.6† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.11. Pairwise contrasts of ear dry wei ght and nitrogen content, sweet corn, 2002. Ear Dry Weight (kg ha-1) Ear N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 140† 2959 3431† 12.5 39.2 36.8† Conv 267N 889* 3218 3889* 16.5 43.2 45.2* SH+L 67N 270† 2283† 2283*† 6.1† 27.2*† 21.9*† SH 67N 224† 2224† 2219*† 7.7† 25.9*† 21.2*† L 67N 263† 2093† 2111*† 5.4† 23.0*† 19.3*† SH+L 133N 268† 2670 3185† 7.6† 33.3 36.6† SH 133N 512† 2835 2891*† 10.1 34.0 29.7*† L 133N 149† 2277† 2968*† 9.5 30.8† 32.9† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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184Table C.12. Pairwise contrasts of stem and r oot nitrogen concentra tions, sweet corn, 2002. Stem N Concentration (g N kg-1) Root N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 26.7 288 142 7.4† 7.7 13.3 NA 9.4 5.5 4.3† Conv 267N 25.2 287 138 9.6* 9.1 14.4 NA 8.6 6.4 6.0* SH+L 67N 18.0*† 23.5*† 10.0*† 4.7*† 4.6*† 10.4*† NA 6.0*† 3.2*† 2.8*† SH 67N 17.8*† 21.6*† 7.8*† 4.5*† 4.7*† 10.6*† NA 5.4*† 2.6*† 3.4*† L 67N 18.2*† 25.3 9.5*† 4.5*† 4.2*† 11.2† NA 6.1*† 3.0*† 3.0*† SH+L 133N 25.1 25.6 12.8 6.4† 6.6† 13.0 NA 7.2† 4.9† 3.9† SH 133N 24.1 26.6 11.3*† 5.7† 4.9*† 12.2 NA 6.7*† 3.1*† 3.1*† L 133N 24.9 26.5 11.7* 4.8*† 5.3*† 12.0 NA 7.6*† 4.4*† 4.0† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.13. Pairwise contrasts of ear and to tal nitrogen concentra tions, sweet corn, 2002. Ear N Concentration (g N kg-1) Total N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 20.0 13.3 10.7 20.1 18.8 14.4 10.4 9.3† Conv 267N 18.7 13.5 11.6 19.3 19.7 14.1 11.4 10.5* SH+L 67N 19.4 11.9 9.6 15.4*† 16.2*† 10.7*† 8.0*† 7.0*† SH 67N 16.4 11.6† 9.5 15.7*† 14.8*† 9.1*† 7.6*† 7.1*† L 67N 18.1 11.1*† 9.1† 13.6*† 17.1† 10.3*† 7.6*† 6.7*† SH+L 133N 20.3 12.4 11.5 19.1 17.6† 12.9 9.2† 9.0† SH 133N 20.3 11.9 10.2 19.0 17.9 11.5*† 8.7*† 7.7*† L 133N 17.9 13.7 11.0 19.0 17.7† 12.2*† 8.6*† 8.4† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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185Table C.14. Corn applied nitrogen, unaccount ed for applied nitrogen a nd chlorophyll meter readings by green manure and nitrogen rate, sweet corn, 2003. NUE Corn Applied N UAN Total UAN Chlorophyll meter readings kg kg-1 kg N ha-1 kg N ha-1 kg N ha-1 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS GM NS *** *** *** NS NS NS NS SH+L 0.22 118 a 92 a 212 a 37.8 ab 41.9 38.8 a 42.4 a 35.5 a SH 0.22 97 b 72 b 195 a 39.8 a 41.0 35.8 ab 40.3 ab 32.6 ab L 0.21 88 b 68 b 94 b 36.6 ab 41.3 35.9 ab 40.0 ab 30.3 b Conv 0.18 71 c 51 c 73 c 34.9 b 41.9 32.0 b 37.2 b 32.9 ab N-Rate NS *** *** *** *** *** *** *** *** 0N 26 c 18 c 86 c 29.3 c 32.1 c 24.1 c 26.2 c 22.7 c 67N 0.22 91 b 66 b 142 b 38.4 b 44.1 b 38.5 b 41.1 b 33.2 b 133N 0.19 163 a 127 a 204 a 44.1 a 48.4 a 44.3 a 52.6 a 42.6 a NUE: N use efficiency (calculated without 0N treatments). UAN: Unaccounted for applied N. Total UAN includes N content of sunn hemp and weeds prior to winter decomposition. WAE: weeks after emergence. † Corn Applied N includes contributions from GMs and chemical N (chemical N = 67 kg N ha-1). NS: means within columns not significantly different to the 0.05 leve l. *, **, ***: means within columns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Mu ltiple Range Test comparisons at the 0.05 level.

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186Table C.15. Specific leaf area and sp ecific leaf nitrogen by green manure and nitrogen rate, sweet corn, 2003. Specific Leaf Area (cm2 g-1) Specific Leaf N (g N cm-2) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS NS NS NS SH+L 324 255 200 183 169 b 43.4 20.5 30.7 a 35.0 30.7 SH 311 240 211 184 181 a 46.9 24.2 28.5 ab 33.4 27.0 L 309 259 193 179 177 ab 45.1 22.4 29.5 a 33.3 26.5 Conv 349 245 208 185 177 ab 43.6 21.8 24.7 b 31.2 27.3 N-Rate NS NS NS NS *** *** *** *** *** 0N 358 a 248 212 185 180 a 35.4 c 14.7 b 18.0 c 20.2 c 22.1 c 67N 309 b 247 197 180 169 b 46.6 b 23.6 a 30.2 b 33.2 b 28.1 b 133N 303 b 254 200 183 180 a 52.3 a 28.3 a 36.8 a 46.4 a 33.4 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level. Table C.16. Stem dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2003. Stem Dry Weight (kg ha-1) Stem N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS SH+L 45 255 a 2937 a 3187 ab 3540 a 0.5 2.0 a 6.0 a 5.6 ab 3.7 SH 46 260 a 3038 a 3874 a 3572 a 0.6 1.9 ab 6.0 a 6.4 a 3.8 L 45 216 ab 2738 ab 3084 b 3521 a 0.5 1.5 ab 5.1 b 4.6 b 3.4 Conv 44 185 b 2414 b 2979 b 2912 b 0.5 1.4 b 4.6 b 4.4 b 3.2 N-Rate *** *** *** *** *** *** *** *** *** *** 0N 35 b 104 c 1365 c 1956 b 2238 b 0.3 c 0.5 c 2.5 c 2.9 b 2.2 c 67N 47 a 257 b 3230 b 4157 a 3981 a 0.5 b 1.8 b 5.6 b 6.4 a 3.6 b 133N 53 a 326 a 3751 a 3730 a 3940 a 0.7 a 2.8 a 8.3 a 6.5 a 4.7 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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187Table C.17. Root dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2003. Root Dry Weight (kg ha-1) Root N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS * SH+L 27 78 655 ab 640 755 a 0.2 0.4 1.6 a 1.3 ab 1.5 a SH 29 92 643 ab 677 785 a 0.2 0.4 1.3 ab 1.2 a 1.2 b L 30 79 760 a 591 627 b 0.2 0.3 1.5 a 1.0 b 1.2 b Conv 25 70 501 b 577 596 b 0.2 0.3 1.0 b 1.1 b 1.0 b N-Rate *** *** *** *** *** ** *** *** *** *** 0N 22 c 38 b 282 b 322 b 386 b 0.1 b 0.1 b 0.5 c 0.6 c 0.8 b 67N 29 b 95 a 767 a 793 a 823 a 0.2 a 0.4 a 1.5 b 1.3 b 1.3 a 133N 34 a 106 a 871 a 748 a 863 a 0.2 a 0.5 a 1.9 a 1.6 a 1.6 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level. Table C.18. Ear dry weight and nitrogen content by green manure and nitrogen rate, sweet corn, 2003. Ear Dry Weight (kg ha-1) Ear N Content (kg ha-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS GM NS NS NS NS NS SH+L 244 1799 2564 a 2.5 11.3 16.0 SH 238 1371 2017 ab 2.6 9.5 12.8 L 352 1242 2037 ab 3.0 7.2 13.5 Conv 162 1281 1892 b 1.6 8.0 12.5 N-Rate ** *** *** ** *** *** 0N 11 b 192 c 347 c 0.1 b 1.2 c 2.3 c 67N 319 a 1575 b 1960 b 2.9 a 9.5 b 11.6 b 133N 417 a 2503 a 4075 a 4.2 a 16.2 a 27.3 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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188Table C.19. Stem and root nitrogen concentrations by green manure and nitrogen rate, sweet corn, 2003. Stem N Concentration (g kg-1) Root N Concentration (g kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS NS NS NS NS GM NS NS NS NS NS NS NS NS NS SH+L 10.9 7.1 2.0 1.6 1.0 6.2 4.3 2.4 a 2.1 2.1 SH 12.0 6.7 2.0 1.6 1.1 6.5 4.0 2.0 a 1.9 1.7 L 11.3 6.7 1.9 1.6 1.0 6.4 3.6 2.0 a 1.9 1.9 Conv 11.7 7.3 1.9 1.4 1.1 6.3 3.8 1.9 b 1.9 1.8 N-Rate *** *** * *** NS NS NS *** 0N 9.6 c 4.9 c 1.9 ab 1.5 b 1.0 b 5.6 3.2 1.9 2.0 ab 2.3 a 67N 11.5 b 7.2 b 1.7 b 1.5 b 0.9 c 6.6 4.2 2.1 1.7 b 1.6 c 133N 13.3 a 8.7 a 2.2 a 1.8 a 1.2 a 7.0 4.4 2.2 2.1 a 1.9 b WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level. Table C.20. Ear and total nitrogen concentrations by green manure and nitr ogen rate, sweet corn, 2003. Ear N Concentration (g kg-1) Total N Concentration (g kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS NS GM * NS NS NS NS NS SH+L 10.1 6.1 b 6.0 c 11.7 8.3 3.3 3.4 3.2 SH 10.9 6.7 a 6.3 bc 12.5 7.7 3.2 3.2 3.0 L 8.3 6.0 b 6.7 ab 11.8 7.8 3.1 3.1 3.0 Conv 9.6 6.2 b 6.9 a 12.1 8.1 2.9 3.0 3.0 N-Rate NS ** *** *** ** *** *** 0N 6.3 ab 6.8 a 10.2 c 5.8 c 2.4 c 2.2 c 2.2 c 67N 9.3 6.0 b 5.9 b 12.2 b 8.5 b 3.0 b 3.1 b 2.8 b 133N 10.0 6.4 a 6.8 a 13.6 a 9.6 a 3.9 a 4.3 a 4.1 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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189Table C.21. Pairwise contrasts of chlorophyll meter readings and specific leaf area, sweet corn, 2003. Chlorophyll Meter Readings (unitless) Specific Leaf Area (cm2 g-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 45.4 54.3 48.5 54.3 51.0 288 255 207 182 182 Conv 267N 46.6 50.2 53.5 55.4 52.5 298 243 197 183 177 SH+L 67N 38.8† 44.6* 38.6*† 44.5*† 33.5*† 326 258 204 180 155*† SH 67N 37.8*† 41.1*† 36.7*† 42.2*† 33.4*† 309 233 197 181 172 L 67N 41.0 45.9* 42.3† 41.4*† 31.7*† 304 256 188 175 175 SH+L 133N 43.9 47.0* 48.1 54.9 44.9 293 280† 194 182 184 SH 133N 46.5 49.7 45.5 51.2 42.3 296 237 208 183 184 L 133N 45.3 46.9* 41.6† 51.3 38.8*† 320 251 190 184 170 *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.22. Pairwise contrasts of stem dry we ight and nitrogen content, sweet corn, 2003. Stem Dry Weight (kg ha-1) Stem N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 61 342 3302 3616 3468 0.9 3.2 9.2 7.2 5.5 Conv 267N 60 396 2920 3790 3528 1.0 4.0 10.8 9.6 5.6 SH+L 67N 47 263† 3206 3967 4161 0.6*† 2.0*† 5.4*† 6.6 4.0*† SH 67N 49 307 3444 4519 4356 0.5*† 1.8*† 6.0*† 7.2 4.2*† L 67N 49 234† 3371 4147 4098 0.5*† 1.8*† 6.1*† 5.3† 3.5*† SH+L 133N 55 377 3749† 3841 3753 0.7 3.4† 9.4 7.6 4.9 SH 133N 53† 360 4096*† 4070 3710 0.8 3.3† 9.5 7.2 4.4 L 133N 55 306 3663 3428 4394* 0.7 2.4†† 7.0*† 6.1† 4.4 *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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190Table C.23. Pairwise contrasts of root dry we ight and nitrogen content, sweet corn, 2003. Root Dry Weight (kg ha-1) Root N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 33 105 921 791 853 0.2† 0.5 2.9† 1.8† 1.6 Conv 267N 42 127 664 761 848 0.3* 0.7 2.2* 2.3* 1.9 SH+L 67N 32 84† 738 788 921 0.2 0.4 1.9* 1.5† 1.6 SH 67N 24 116 715 800 1066 0.2† 0.5 1.3*† 1.3*† 1.5 L 67N 33† 94 1041† 814 594 0.2 0.4† 2.0* 1.2*† 0.9*† SH+L 133N 31† 112 837 814 842 0.2 0.6 2.2* 1.9 2.0 SH 133N 39 122 857 727 784 0.3 0.5 1.9* 1.5† 1.3† L 133N 34 99 989† 680 954 0.2 0.4† 1.8* 1.2*† 1.7 *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.24. Pairwise contrasts of ear dry we ight and nitrogen cont ent, sweet corn, 2003. Ear Dry Weight (kg ha-1) Ear N Content (kg ha-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 640 3051 4573 6.2 19.8 36.2 Conv 267N 738 2743 4762 7.5 20.0 37.4 SH+L 67N 277† 1831* 1926*† 2.7† 10.8† 10.4*† SH 67N 282† 1607*† 2170*† 2.8† 11.5† 13.0*† L 67N 614 1829* 1946*† 5.3 9.6†† 11.8*† SH+L 133N 410 3292 4984*† 4.4 21.4* 32.9 SH 133N 433 2252 3674 4.9 15.3 24.2*† L 133N 443 1790* 3838 3.6† 11.1† 26.2*† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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191Table C.25. Pairwise contrasts of stem and r oot nitrogen concentra tions, sweet corn, 2003. Stem N Concentration (g N kg-1) Root N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 15.4 9.1 2.8† 1.9 1.7 5.7 4.4 3.2 2.2† 1.9† Conv 267N 16.7 9.9 3.7* 2.6 1.6 6.6 5.3 3.3 3.2* 2.4* SH+L 67N 11.4*† 7.3† 1.7*† 1.6† 1.0*† 6.5 4.6 2.6* 1.9† 1.8† SH 67N 11.5*† 5.8*† 1.8*† 1.6† 1.0*† 6.7 3.9 2.0*† 1.8† 1.5† L 67N 10.9*† 8.0† 1.8*† 1.3† 0.9*† 6.7 4.2 1.9*† 1.5† 1.6† SH+L 133N 13.0† 9.0 2.5† 2.0 1.3*† 6.6 5.3 2.6† 2.4† 2.4 SH 133N 14.5 9.2 2.4† 1.8† 1.2*† 7.3 4.0 2.2*† 2.0† 1.7† L 133N 12.5*† 7.9† 1.9*† 1.9 1.0*† 6.9 3.7 1.9*† 2.0† 1.8† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.26. Pairwise contrasts of ear and to tal nitrogen concentra tions, sweet corn, 2003. Ear N Concentration (g N kg-1) Total N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 9.7 6.5† 7.8 14.5 9.8 4.7† 4.5 5.3 Conv 267N 10.2 7.3* 7.9 14.6 10.1 5.9* 5.2 5.4 SH+L 67N 9.6 5.9† 5.4*† 11.9*† 8.8† 3.1*† 3.4† 2.6*† SH 67N 10.0 7.1 5.9*† 12.4*† 7.5*† 3.0*† 3.4† 2.9*† L 67N 8.7 5.3*† 6.0*† 12.1*† 8.9 3.3*† 2.9† 2.8*† SH+L 133N 10.6 6.5† 6.6*† 13.6 9.8 4.2† 4.6 4.5*† SH 133N 11.4 6.7† 6.6*† 14.2 9.9 4.3† 4.1† 4.0*† L 133N 8.1 6.2† 6.8*† 12.9*† 9.1 3.3*† 4.1† 3.8*† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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192 Table C.27. Leaf nitrogen concentration by gr een manure and nitrogen rate, 2002 (g N g-1) Leaf N Concentration (g N kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS GM NS NS NS NS NS SH+L 16.7 12.0 11.0 10.0 8.3 SH 16.5 11.5 10.3 9.1 8.7 L 16.3 12.5 11 .0 10.2 8.7 Conv 15.5 12.2 11.2 9.8 8.1 N-Rate ** *** *** *** *** 0N 15.1 b 9.4 b 8.7 b 8.2 c 7.2 c 67N 16.0 b 13.1 a 11.7 a 9.7 b 8.2 b 133N 17.7 a 13.7 a 12.3 a 11.4 a 9.9 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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193Table C.28. Pairwise contrasts of leaf nitrogen concentration, sweet corn, 2002. Leaf N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 18.8 13.8 13.6 12.8 10.6 Conv 267N 17.7 15.1 14.4 13.2 11.9 SH+L 67N 16.0* 13.3† 12.0† 10.7*† 7.7*† SH 67N 16.2* 12.0*† 11.4*† 8.5*† 9.1† L 67N 15.7* 13.8 11.9† 10.4*† 8.9*† SH+L 133N 18.0 13.4† 13.4 11.3† 10.4† SH 133N 18.1 14.1 11.0*† 11.4† 9.3† L 133N 17.9 13.7 11.8† 11.5 10.4† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence. Table C.29. Leaf nitrogen concentration by gr een manure and nitrogen rate, 2003 (g N g-1) Leaf N Concentration (g N kg-1) 2WAE 4WAE 6WAE 8WAE 9WAE GM x N-Rate NS NS NS NS NS GM NS NS NS NS NS SH+L 13.8 9.7 6.1 a 6.4 5.2 SH 14.5 9.1 5.8 ab 6.1 4.9 L 13.8 9.4 5.6 ab 6.0 4.6 Conv 13.8 9.6 5.1 b 5.7 4.8 N-Rate *** *** *** *** *** 0N 11.9 c 7.0 c 3.7 c 3.7 c 4.0 c 67N 14.3 b 101 b 5.9 b 5.9 b 4.7 b 133N 15.7 a 112 a 7.3 a 8.5 a 6.0 a WAE: weeks after emergence. NS: means within columns not signific antly different to the 0.05 level. *, **, ***: means within co lumns significantly different to the 0.05, 0.001, and 0.0001 level, respectively. Letters reflect vertical Duncan’s Multiple Range Test comparisons at the 0.05 level.

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194 Table C.30. Pairwise contrasts of leaf nitrogen concentration, sweet corn, 2003. Leaf N Concentration (g N kg-1) 2 WAE 4 WAE 6 WAE 8 WAE 9 WAE Conv 200N 16.5 11.3 8.3 8.8 9.4 Conv 267N 16.4 11.5 9.6 9.3 11.2 SH+L 67N 13.9*† 10.4 6.1*† 6.4*† 5.3*† SH 67N 14.3* 9.2*† 6.0*† 6.5*† 6.3*† L 67N 14.4* 10.6 6.5*† 5.8*† 5.0*† SH+L 133N 15.9 11.2 7.8† 8.8 8.8*† SH 133N 16.4 11.5 7.9† 8.1 8.4*† L 133N 15.0 11.0 6.7*† 8.3 8.1*† *,† mean of GM treatment significantly different from mean of Conv 200N and Conv 267N, respectively, to the 0.05 level; WAE= weeks after emergence.

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APPENDIX D TABLES OF INTERACTIONS FOR ROOT LENGTH DENSITY BY LOCATION, 8 WEEKS AFTER EMERGENCE, SWEET CORN 2003

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196Table D.1. Root length density (cm cm-3) interaction between depth and position with green manure and chemical nitrogen rate held constant, 8 weeks after emergence, sweet corn, 2003. Depth Conv 0N Conv 133N (cm) Position Position IR 0 IR 0.5 BR IR 0 IR 0.5 BR 0-15 1.30 Aa 0.95 Aab 0.78 Ab 3.57 Aa 2.89 Aa 2.04 Aa 15-30 0.88 Ba 1.06 Aa 0.16 Bb 1.76 Ba 1.84 Ba 0.31 Ab 30-60 0.23 Ca 0.22 Ba 0.08 Bb 0.56 Ca 0.61 Ca 0.17 Bb SH+L 0N SH+L 133N Position Position IR 0 IR 0.5 BR IR 0 IR 0.5 BR 0-15 2.58 Aa 1.94 Aab 1.42 Ab 4.73 Aa 3.42 Aab 1.50 Ab 15-30 1.35 Ba 0.90 Bab 0.38 Bb 2.70 Ba 2.00 Ba 0.18 Bb 30-60 0.44 Ca 0.27 Cab 0.16 Bb 0.55 Ca 0.41 Ca 0.13 Bb Conv = conventional (no green manure); SH+L = sunn hemp plus winter legume green manure; N = kg NH4NO3-N ha-1 to sweet corn; IR0 = in-row next to plant; IR0.5 = in-row and half-way between plants; BR = between row; capital and lower-case letters reflect vertical and horizo ntal groupings, respectively, by Duncan’s Multiple Range Test at = 0.05.

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197Table D.2. Root length density (cm cm-3) interaction between depth a nd chemical nitrogen rate with green manure and position held constant, 8 weeks after emergence, sweet corn, 2003. Depth IR0, Conv IR0.5, Conv BR, Conv (cm) Chemical N-rate Chemical N-rate Chemical N-rate 0N 133N 0N 133N 0N 133N 0-15 1.30 Ab 3.57 Aa 0.95 Ab 2.89 Aa 0.78 Ab 2.04 Aa 15-30 0.88 Bb 1.76 Ba 1.06 Aa 1.84 Ba 0.16 Bb 0.31 Ba 30-60 0.23 Cb 0.56 Ca 0.22 Ba 0.61 Ca 0.08 Bb 0.17 Ba IR0, SH+L IR0.5, SH+L BR, SH+L Chemical N-rate Chemical N-rate Chemical N-rate 0N 133N 0N 133N 0N 133N 0-15 2.58 Ab 4.73 Aa 1.94 Ab 3.42 Aa 1.42 Aa 1.50 Aa 15-30 1.35 Ba 2.70 Ba 0.90 Bb 2.00 Ba 0.38 Ba 0.18 Ba 30-60 0.44 Ca 0.55 Ca 0.27 Ca 0.41 Ca 0.16 Ba 0.13 Ba Conv = conventional (no green manure); SH+L = sunn hemp plus winter legume green manure; N = kg NH4NO3-N ha-1 to sweet corn; IR0 = in-row next to plant; IR0.5 = in-row and half-way between plants; BR = between row; capital and lower-case letters reflect vertical and horizo ntal groupings, respectively, by Duncan’s Multiple Range Test at = 0.05. Table D.3. Root length density (cm cm-3) interaction between depth and green manure with position and chemical nitrogen rate held constant, 8 weeks after emergence, sweet corn, 2003. Depth IR0, 0N IR0.5, 0N BR, 0N (cm) Green Manure Green Manure Green Manure Conv SH+L Conv SH+L Conv SH+L 0-15 1.30 Aa 2.58 Aa 0.95 Ab 1.94 Aa 0.78 Ab 1.42 Aa 15-30 0.88 Ba 1.35 Ba 1.06 Aa 0.90 Bb 0.16 Ba 0.38 Ba 30-60 0.23 Ca 0.44 Ca 0.22 Bb 0.27 Ca 0.08 Bb 0.16 Ba IR0, 133N IR0.5, 133N BR, 133N Green Manure Green Manure Green Manure Conv SH+L Conv SH+L Conv SH+L 0-15 3.57 Aa 4.73 Aa 2.89 Aa 3.42 Aa 2.04 Aa 1.50 Aa 15-30 1.76 Ba 2.70 Ba 1.84 Ba 2.00 Ba 0.31 Ba 0.18 Bb 30-60 0.56 Ca 0.55 Ca 0.61 Ca 0.41 Ca 0.17 Ba 0.13 Ba Conv = conventional (no green manure); SH+L = sunn hemp plus winter legume green manure; N = kg NH4NO3-N ha-1 to sweet corn; IR0 = in-row next to plant; IR0.5 = in-row and half-way between plants; BR = between row; capital and lower-case letters reflect vertical and horizo ntal groupings, respectively, by Duncan’s Multiple Range Test at = 0.05.

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198Table D.4. Root length density (cm cm-3) interaction between position and chemical nitrogen rate with green manure and depth held constant, 8 weeks after emergence, sweet corn, 2003. Position 0-15 cm, Conv 15-30 cm, Conv 30-60 cm, Conv Chemical N-rate Chemical N-rate Chemical N-rate 0N 133N 0N 133N 0N 133N IR0 1.30 Ab 3.57 Aa 0.88 Ab 1.76 Aa 0.23 Ab 0.56 Aa IR0.5 0.95 ABb 2.89 Aa 1.06 Aa 1.84 Aa 0.22 Aa 0.61 Aa BR 0.78 Bb 2.04 Aa 0.16 Bb 0.31 Ba 0.08 Bb 0.17 Ba 0-15 cm, SH+L 15-30 cm, SH+L 30-60 cm, SH+L Chemical N-rate Chemical N-rate Chemical N-rate 0N 133N 0N 133N 0N 133N IR0 2.58 Ab 4.73 Aa 1.35 Ab 2.70 Aa 0.44 Aa 0.55 Aa IR0.5 1.94 ABb 3.42 Aa 0.90 ABa 2.00 Aa 0.27 ABa 0.41 Aa BR 1.42 Ba 1.50 Ba 0.38 Ba 0.18 Ba 0.16 Ba 0.13 Ba Conv = conventional (no green manure); SH+L = sunn hemp plus winter legume green manure; N = kg NH4NO3-N ha-1 to sweet corn; IR0 = in-row next to plant; IR0.5 = in-row and half-way between plants; BR = between row; capital and lower-case letters reflect vertical and horizo ntal groupings, respectively, by Duncan’s Multiple Range Test at = 0.05. Table D.5. Root length density (cm cm-3) interaction between position and green manure with chemical nitrogen rate and depth held constant, 8 weeks after emergence, sweet corn, 2003. Position 0-15 cm, 0N 15-30 cm, 0N 30-60 cm, 0N Green Manure Green Manure Green Manure Conv SH+L Conv SH+L Conv SH+L IR0 1.30 Aa 2.58 Aa 0.88 Aa 1.35 Aa 0.23 Aa 0.44 Aa IR0.5 0.95 ABb 1.94 ABa 1.06 Aa 0.90 ABb 0.22 Ab 0.27 ABa BR 0.78 Bb 1.42 Ba 0.16 Ba 0.38 Ba 0.08 Bb 0.16 Ba 0-15 cm, 133N 15-30 cm, 133N 30-60 cm, 133N Green Manure Green Manure Green Manure Conv SH+L Conv SH+L Conv SH+L IR0 3.57 Aa 4.73 Aa 1.76 Aa 2.70 Aa 0.56 Aa 0.55 Aa IR0.5 2.89 Aa 3.42 Aa 1.84 Aa 2.00 Aa 0.61 Aa 0.41 Aa BR 2.04 Aa 1.50 Ba 0.31 Ba 0.18 Bb 0.17 Ba 0.13 Ba Conv = conventional (no green manure); SH+L = sunn hemp plus winter legume green manure; N = kg NH4NO3-N ha-1 to sweet corn; IR0 = in-row next to plant; IR0.5 = in-row and half-way between plants; BR = between row; capital and lower-case letters reflect vertical and horizo ntal groupings, respectively, by Duncan’s Multiple Range Test at = 0.05.

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210 BIOGRAPHICAL SKETCH Corey Cherr was born on October 7, 1977 in Ta llahassee, Florida. Like his mother and father, he developed an interest in th e living world and its simultaneous complexity and simplicity. He earned his BS in physics from Florida State University, and while attending the University of Florida helped manage the student-run Collegiate Living Organization and the Graduate Student Council in addition to his professional research responsibilities. During this time, Corey met Aisha Goodman, a kindred soul in the journey of life. They were married on May 22, 2004, and happily expect their first child near Corey’s 27th birthday.


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Permanent Link: http://ufdc.ufl.edu/UFE0006501/00001

Material Information

Title: Improved Use of Green Manure as a Nitrogen Source for Sweet Corn
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0006501:00001

Permanent Link: http://ufdc.ufl.edu/UFE0006501/00001

Material Information

Title: Improved Use of Green Manure as a Nitrogen Source for Sweet Corn
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0006501:00001


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Full Text












IMPROVED USE OF GREEN MANURE AS A NITROGEN SOURCE FOR SWEET
CORN
















By

COREY CHERR


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Corey Cherr

































To my new family and my old, may you help me to find balance in my life and make me
a better person than I could ever be alone.















ACKNOWLEDGMENTS

I would like to acknowledge the wonderful help of Johan Scholberg, Andy

Schreffler, Brian Jackson, Sam Willingham, Vony Petit-Frere, Lily Chang-Chien, Holly

Nelson, Amy Van Scoik, John McQueen, Dipen Patel, Robert Wanvestraut, Alicia

Lusiardo, and Huazhi Liu, as well as the staff of the UF-IFAS Plant Science Research

and Education Unit in Citra. This research was funded by grants from the Sustainable

Agriculture Research and Education program of the United States Department of

Agriculture (grant number LS02-140, "A System Approach for Improved Integration of

Green Manure in Commercial Vegetable Production Systems") and the Center for

Cooperative Agricultural Programs (grant also titled "A System Approach for Improved

Integration of Green Manure in Commercial Vegetable Production Systems").
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ................ ................ ............. ...... .... ........ ..... ix

LIST OF FIGURES ......... ....... .................... .. ....... ........... xiv

A B STR A C T ..................... .................................................................. ...... ...... xv

CHAPTER

1 IN TR O D U C T IO N ............................................................. .. ......... ...... .....

O v e rv iew .................................................................................. 1
In tro d u ctio n .................................................................................. 1
R a tio n a le .................................................................................. 1
Green M anure M anagem ent .................................................................. ...........
A p p ro ach ............................................... 7
H ypotheses ................................................ 8
O bjectiv e s ..................................................................................................... . 9
G general Set-U p and D esign ....................................................... 9
M easurem ents ................................................................................................... ........ 10

2 GREEN MANURE GROWTH AND DECOMPOSITION ........... ...............20

Introduction and Literature Review ................................................. 20
M materials an d M eth od s .......................................................................................... 2 7
S et-u p an d D e sig n ........................................................................................... 2 7
Timeline of Operations ...................... .............. .........27
2001-02 ......... ......... .......... ...............27
2002-03 ......... ......... ......... ............. 28
Measurements .................. ......... .................. 28
2001-02 ......... ......... .......... ...............28
2002-03 ......... ......... ......... ............. 29
A n a ly sis .............................................................................3 0
R e su lts ..................... .............................. ................................................................ 3 0
Sunn H em p 200 1 ........................................................................................... ........ 30
G ro w th .................30.............................................
D ec o m p o sitio n ........................................................................................ 3 2


v









Sunn H em p 2002 ........................ .................. ................... .. ...... 34
G ro w th ............................................................................... 3 4
D ecom position ......................... .. .................... .. .. ...... ........... 36
Lupin 2001-2002 .................. ..................................... .................. 38
V etch 2002-2003 .......................................... .. .. .... ........ ......... 40
D isc u ssio n ............................................................................................................. 4 2
S u n n H em p ................................................................4 2
G ro w th ............................................................................... 4 2
D ec o m p o sitio n ....................................................................................... 4 5
L upin and V etch .............................................................47
C o n c lu sio n s ........................................................................................................... 5 1

3 GROWTH, YIELD, AND N-UPTAKE EFFICIENCY RESPONSE OF CORN
TO AMENDMENT WITH GREEN MANURES .................................... ................57

Introduction and Literature Review ................................................. 57
M materials an d M eth od s ......................................................................................... 63
Set-U p and D esign................................................... 63
Timeline of Operations ......... .. ............... ..........63
2001-02 .............. ......... .................. 63
2002-03 .......... ..... ............................64
Procedures and M easurements ................................ ............... 64
Analysis of Data ........................ .................. 66
Results ................ ...... .................. ...... ...............................67
N A p p lied to C o rn ......................................................................................... 6 7
E ar Y field s, 2 002 .............................................................67
E ar Y field s, 2 003 .............................................................6 8
Growth Analysis, 2002 ............ ... ......... ..... .........69
Leaf indicators ............................................... 69
Tissue characteristics ............... .............................. 70
G row th A analysis, 2003 ......................................................... 72
Leaf indicators ............................................... 72
Tissue characteristics ............. ........ ........................ .... 73
Nitrogen Uptake Efficiency and Unaccounted Applied Nitrogen ...................75
D isc u ssio n ...................... .. ............. .. ......................................................7 6
C o n c lu sio n s........................................................................................................... 8 2

4 EFFECTS OF GREEN MANURE AMENDMENT ON SWEET CORN ROOT
LENGTH DENSITY AND DISTRIBUTION............................. 94

Introduction and Literature Review ................................................. 94
M materials an d M eth od s ......................................................................................... 9 8
S et-u p an d D e sig n .......................................................................................... 9 8
Field and Lab Procedures ................. ...............................98
D ata A n a ly sis ................................................................................................. 9 9
Results .............. ...................................................... 102
Overall Root Length Density ................. .......... ........... 102









Root Length Density by Location ........................................ ............... 103
Relative Root Length by Location ....................................... ............... 105
R oot Length D density by Proxim ity................................................ ..............106
Relative Root Length by Proximity ............. .......................................107
E effective R ooting D epth ........................................................ ............... 107
Soil W ater Potential ..................................... ............... ........... 108
D iscu ssio n ................................................................................................ ..... 10 9
C o n clu sio n s.................................................... ................ 1 14

5 EFFECTS OF A GREEN MANURE APPROACH TO SWEET CORN
FERTILIZATION ON SOIL PROPERTIES ................................. ...............122

In tro du ctio n .................................................................................................. ..... 12 2
M materials and M methods ........................................... ....................................... 128
Set-U p and D design ................................................................. .. 128
Procedures and M easurements ...................................... .......................128
D ata A n aly sis ...................................................... 130
R e su lts ........................................................................ 1 3 1
D ry M atter A addition s ........................................... ....................................... 13 1
Microbial Biomass Carbon................ .. ...........................132
Total and Particulate Carbon and Nitrogen pools ..........................................132
S o il p H ..................................................... 13 4
D isc u ssio n ........................................................................................ 1 3 5
C on clu sion s...................................................... 14 0

6 EFFECTS OF GREEN MANURE APPROACHES ON CROP PESTS:
PARASITIC NEMATODES AND WEEDS .............. ..................146

Introduction and Literature Review ............................................................ 146
M materials and M methods ...........................................................150
Set-Up and Design ..... ......... .... ........ ...........150
Procedures and Measurements ................................................... ............ 151
Results ............... ...... ..........................................152
N em atodes .......................................................................................... 152
O ctober 2001 ......................... .......................................152
M a rc h 2 0 0 2 .......................................................................................... 1 5 3
A p ril 2 0 0 2 ............................................................................................ 1 5 3
Ju ly 2 0 0 2 .............................................................................................. 1 5 3
M a rc h 2 0 0 3 .......................................................................................... 1 5 4
Ju n e 2 003 .....................................15...............................
W eeds ............................................. ......... 155
Sunn hem p, October 2001 ................. .............................. .......... 155
Sunn hemp, October 2002 .................. ............................ ........... 156
V etch, A pril 2003 ................. ........................................................... ..... 157
Discussion ..................... ................................157
C conclusions .......................................... 161









7 CONCLUSIONS ....................................... .......... .. .............165

R eview and Synthesis of Findings....................................... ......................... 165
F u tu re W o rk ........................................................................................................ 1 7 2

APPENDIX

A CHARACTERIZATION OF DOMINANT SOIL TYPES PRESENT IN FIELD.. 174

B CONTINUOUS M EASUREM ENTS............................................ .....................176

C SELECTED TISSUE FACTORS AND LEAF INDICATORS FOR SWEET
C O R N 2002 A N D 2003............................................. ........................................ 177

D TABLES OF INTERACTIONS FOR ROOT LENGTH DENSITY BY
LOCATION, 8 WEEKS AFTER EMERGENCE, SWEET CORN 2003..............195

LIST OF REFEREN CES ........................................................... .. ............... 199

B IO G R A PH IC A L SK E T C H ........................................ ............................................210
















LIST OF TABLES


Table page

1.1 Review of green m anure studies. ............. ................... ....... ................ 11

1.2 Overview of experimental treatm ents. .......................................... ...... ......... 19

2.1 Sunn hemp nitrogen concentration by tissue type, 2001............... ..................54

2.2 Selected sunn hemp growth indicators, 2001...............................................54

2.3 Sunn hemp nitrogen concentration by tissue type after death, 2001-02. ...............54

2.4 Sunn hemp nitrogen concentration by tissue type, 2002.......................................55

2.5 Selected sunn hemp growth indicators, 2002.............. .... .................55

2.6 Sunn hemp nitrogen concentration by tissue type after death, 2002-03 .................55

2.7 Lupin nitrogen concentration by tissue type, 2001-02 .......................... ..........56

2.8 Selected lupin growth indicators, 2001-02.... ....................................56

2.9 Vetch tissue nitrogen concentration, 2002-03................ .... .................56

2.10 Selected vetch growth indicators, 2002-03 ......... ........ .. ..... ............... 56

3.1 Pairwise contrasts of selected nitrogen factors and ear yields, 2002 .....................84

3.2 Pairwise contrasts of selected nitrogen factors and ear yields, 2003 ....................84

3.3 Ear yields at final harvest, 2002 and 2003 ...... .................................................85

3.4 Leaf area index, 2002. ......................................... ............... ...........85

3.5 Pairwise contrasts of leaf area index and specific leaf nitrogen, 2002. ...................86

3.6 L eaf dry w eight. ............................................. .. .. .......................87

3.7 Total dry w eight, 2002. ................................................ ................................ 87

3.8 Leaf nitrogen content, 2002. ....................................................................... .............88









3.9 Total nitrogen content, 2002. .............................................................................. 88

3.10 Pairwise contrasts of leaf dry weight and nitrogen content, 2002. ..........................89

3.11 Pairwise contrasts of total dry weight and nitrogen content, 2002. .........................89

3.12 Leaf area index, 2002. ...... ........................... ..........................................90

3.13 Pairwise contrasts of leaf area index and specific leaf nitrogen, 2003 ...................90

3.14 L eaf dry w eight, 2003 ......................................................................................... 91

3.15 Total dry w eight, 2003 ........................................................................................ 91

3.16 L eaf nitrogen content, 2003 ..................................................................................... 92

3.17 Total nitrogen content, 2003. ........................................................ ............ ... 92

3.18 Pairwise contrasts of leaf dry weight and nitrogen content, 2003 ..........................93

3.19 Pairwise contrasts of total dry weight and nitrogen content, 2003 .........................93

4.1 Pairwise contrasts against Conv 267N for overall sampled root length density,
0 -6 0 cm ...................................... ....................................................1 18

4.2 Significance of green manure, nitrogen rate, position and depth and sub-effects
when constituting linear model for sampled root length density ...........................118

4.3 Various interactions with depth for root length density at 3 and 5 weeks after
em ergence............................................................... ... .... ......... 119

4.4 Significance of treatment, position and depth when constituting linear model for
sam pled root length density ........................................................ ............. ..119

4.5 Pairwise root length density comparisons against Conv 267N by depth and
position at 5 weeks after emergence ............................................ ...............119

4.6 Pairwise root length density comparisons against Conv 267N by depth and
position at 8 w eeks after em ergence ........................................... ............... 120

4.7 Significance of green manure, nitrogen rate, and proximity to plant when
constituting linear model for sampled root length density............... ................120

4.8 Significance of green manure, nitrogen rate, and proximity to plant when
constituting linear model for sampled root length density................................ 120

4.9 Interactions between nitrogen rate and proximity for root length density ...........121









4.10 Pairwise root length density comparisons against Conv 267N by proximity at 8
w weeks after em ergence. ........................................... ......................................... 12 1

5.1 Significance of green manure, nitrogen rate, and year in balanced analysis of
variance for soil carbon and nitrogen pools, July 2002 and June 2003 ................142

5.2 Significance of treatment and year in full analysis of variance and pairwise
contrasts of selected treatments for soil carbon and nitrogen pools, July 2002
and June 2003 .................................... ................................ ........143

5.3 Analysis of variance for all treatments and pairwise contrasts of selected
treatments within years for particulate organic carbon and nitrogen...................144

5.4 Significance of date, green manure and nitrogen rate for pH of sampled soil.......145

6.1 Nematode soil population counts from selected treatments at selected dates........163

6.2 Nematode soil population counts at selected dates. .............................................164

A. 1 Selected characteristics from a Lake Fine Sand; Typic Quarzipsamments,
hyperthermic, coated; Citrus County, FL .................................. ....................174

A.2 Selected characteristics from a Candler Fine Sand; Typic Quarzipsamments,
hyperthermic, uncoated; Alachua County, FL. ................... ............................. 175

B.1 Continuously measured environmental factors. .............................................. 176

C. 1 Corn applied nitrogen, unaccounted for applied nitrogen and chlorophyll meter
readings by green manure and nitrogen rate, sweet corn, 2002 ...........................178

C.2 Specific leaf area and specific leaf nitrogen by green manure and nitrogen rate,
sw eet corn 2 002 ................................................................. 179

C.3 Stem dry weight and nitrogen content by green manure and nitrogen rate, sweet
co rn 2 0 0 2 ............................................................................................ 17 9

C.4 Root dry weight and nitrogen content by green manure and nitrogen rate, sweet
co rn 2 0 0 2 .................................................................... ................. 18 0

C.5 Ear dry weight and nitrogen content by green manure and nitrogen rate, sweet
corn 2 002 ..................................................... ............ .. .............. 180

C.6 Stem and root nitrogen concentrations by green manure and nitrogen rate, sweet
corn 2 002 ........................... .................................. .. .. ... ............... 18 1

C.7 Ear and total nitrogen concentrations by green manure and nitrogen rate, sweet
co rn 2 0 0 2 ............................................................................................ 18 1









C.8 Pairwise contrasts of chlorophyll meter readings and specific leaf area, sweet
co rn 2 0 0 2 ............................................................................................ 182

C. 10 Pairwise contrasts of root dry weight and nitrogen content, sweet corn, 2002......183

C. 11 Pairwise contrasts of ear dry weight and nitrogen content, sweet corn, 2002. ......183

C. 12 Pairwise contrasts of stem and root nitrogen concentrations, sweet corn, 2002....184

C. 13 Pairwise contrasts of ear and total nitrogen concentrations, sweet corn, 2002......184

C. 14 Corn applied nitrogen, unaccounted for applied nitrogen and chlorophyll meter
readings by green manure and nitrogen rate, sweet corn, 2003 ...........................185

C. 15 Specific leaf area and specific leaf nitrogen by green manure and nitrogen rate,
sweet corn, 2003 ........... .............. .................... ................. 186

C. 16 Stem dry weight and nitrogen content green manure and nitrogen rate, sweet
corn, 2003 ............................ ..................... ............ ........... 186

C. 17 Root dry weight and nitrogen content by green manure and nitrogen rate, sweet
corn, 2003 .......... .. ... ... ............................................................... 187

C. 18 Ear dry weight and nitrogen content by green manure and nitrogen rate, sweet
corn, 2003 .......... .. ... ... ............................................................... 187

C. 19 Stem nitrogen concentration and root nitrogen concentration by green manure
and nitrogen rate, sw eet corn, 2003.............................................. ......................... 188

C.20 Ear nitrogen concentration and total nitrogen concentration by green manure
and nitrogen rate, sw eet corn, 2003.............................................. ......................... 188

C.21 Pairwise contrasts of chlorophyll meter readings and specific leaf area, sweet
corn, 2003 .............. .... ..... ......... ......................................189

C.22 Pairwise contrasts of stem dry weight and nitrogen content, sweet corn, 2003.....189

C.23 Pairwise contrasts of root dry weight and nitrogen content, sweet corn, 2003......190

C.24 Pairwise contrasts of ear dry weight and nitrogen content, sweet corn, 2003. .....190

C.25 Pairwise contrasts of stem nitrogen concentration and root nitrogen
concentration, sweet corn, 2003 .................................................... ....................191

C.26 Pairwise contrasts of ear nitrogen concentration and total nitrogen
concentration, sweet corn, 2003 .................................................... ....................191

C.27 Leaf nitrogen concentration by green manure and nitrogen rate, 2002............... 192









C.28 Pairwise contrasts of leaf nitrogen concentration, sweet corn, 2002. ....................193

C.29 Leaf nitrogen concentration by green manure and nitrogen rate, 2003................93

C.30 Pairwise contrasts of leaf nitrogen concentration, sweet corn, 2003.....................194

D. 1 Root length density interaction between depth and position with green manure
and chemical nitrogen rate held constant, 8 weeks after emergence, sweet corn,
2003 ........................................... .......................................... 196

D.2 Root length density interaction between depth and chemical nitrogen rate with
green manure and position held constant, 8 weeks after emergence, sweet corn,
2003 ............................... ........ .......... ..... ............... 197

D.3 Root length density interaction between depth and green manure with position
and chemical nitrogen rate held constant, 8 weeks after emergence, sweet corn,
2003 ............................. ......... ........... .............................. .............. 197

D.4 Root length density interaction between position and chemical nitrogen rate with
green manure and depth held constant, 8 weeks after emergence, sweet corn,
2003 ............. .......... .... ....................... ........... ........ ............... 198

D.5 Root length density interaction between position and green manure with
chemical nitrogen rate and depth held constant, 8 weeks after emergence,
sweet corn, 2003 .................................... .......................... .... ........ 198
















LIST OF FIGURES


Figure page

2.1 Sunn hemp dry weight and nitrogen content during growth and decomposition,
2001-02 .............. ............ .... ...... ....... ......................... 51

2.2 Leaf area index and dry weight of each green manure during growth.................52

2.3 Ratio of S:R-N to S:R-B of sunn hemp, lupin and vetch. ........................................52

2.4 Sunn hemp dry weight and nitrogen content during growth and decomposition,
2002-03 .................. .......... .... ............ .. ...................... 53

2.5 Lupin dry weight accumulation and nitrogen content during growth, 2001-02.......53

2.6 Vetch dry weight accumulation and nitrogen content during growth, 2002-03.......53

3.1 Marketable ear yields as fresh weight by treatment, 2002 and 2003 .....................83

4.1 Name, location and relative volume of root core samples. .............. ................ 115

4.2 Effect of amendment with SH+L on sampled sweet corn root length density....... 116

4.3 Effect of amendment with SH+L on sampled sweet corn root length density by
depth at 5 weeks after em ergnce. ................................................................ ..... 116

4.4 Effect of amendment with SH+L on sampled sweet corn root length density by
proximity class at 8 w eeks after em ergence...................................... ................ 117

4.5 Soil water potential at 15 cm and 60 cm during sweet corn growth. .................117

5.1 Dry matter additions by treatment (2001-2002, A; 2002-2003, B) and average
soil pH by GM over two years (C). .............................................. ............... 141

6.1 Final weed dry weights and N concentrations. ................... ................... .......... 162















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

IMPROVED USE OF GREEN MANURE AS A NITROGEN SOURCE FOR SWEET
CORN

By

Corey Cherr

August 2004

Chair: Johannes Scholberg
Major Department: Agronomy

A green manure (GM) is a crop used primarily as a soil amendment and a nutrient

source for future crops. Leguminous GMs may represent a substantial source of on-farm

nitrogen (N) also capable of increasing soil organic matter and suppressing weeds and

parasitic nematodes. In temperate and tropical environments, GMs such as hairy vetch

(Vicia villosa) and sunn hemp (Crotalariajuncea) have been found to accumulate 150-

250 kg N ha-1 while fully satisfying N-requirements of subsequent crops. However,

spring crop production with GMs in Florida remains particularly challenging because N

accumulation and subsequent GM benefits of temperate winter legumes are reduced

while tropical summer legumes cannot survive freezes and may experience unacceptable

levels of N-loss during winter fallow. Establishing a winter GM after the summer GM

may significantly reduce N leaching losses during winter and retain N benefits for spring

crops, but this has not been studied.









We conducted a 2-year field study to evaluate yield response of sweet corn (Zea

mays var. Rugosa) to GMs of sunn hemp and/or lupin (Lupinus angustifolius; winter

2001-02) and cahaba white vetch (Vicia sativa; winter 2002-03) and supplementation

with 0, 67, or 133 kg inorganic N ha-1. Unamended (non-GM) treatments receiving 0, 67,

133, 200 or 267 kg inorganic N ha-1 were used for comparison. Growth and N analyses

were conducted for all crops and decomposition monitored for overwintering sunn hemp.

These analyses revealed substantial growth and N-accumulation for sunn hemp (up to

12.2 Mt ha1 and 135 kg N ha-1), but rapid N-loss (60-66% 2-4 weeks after death)

occurred when senesced leaves and upright stems decomposed separately in our reduced

tillage and reduced mowing system. Winter GM growth (2-4 Mt ha-1 and 35-40 kg N

ha-1) was and not enhanced by following sunn hemp. Green manures resulted in N benefit

for sweet corn of -50-70 kg N ha-1, and ear yields for corn with sunn hemp plus winter

GM and 133 kg N ha-1 only were similar to unamended corn with recommended N-rate

(200 kg N ha-1) in either year. Amendment with GMs significantly increased corn root

length density, but did so in the upper 15 cm soil layer and close to the plant, possibly

interfering with late-season ear-fill by exposure to water and N-stress. Apparent N-

recovery was not affected by use of GMs in this system. Green manure approaches

significantly increased particulate organic C and N pools over two years, though it

remains unclear if these changes can create effective differences in soil organic matter.

Living sunn hemp reduced end-of-season weed biomass up to 80% and suppressed lesion

and stubby-root nematodes while winter legumes had mixed affects.














CHAPTER 1
INTRODUCTION

Overview

This chapter serves as a brief overview tying together the individual components of

this study. Detailed introductions, literature reviews, materials and methods, results,

discussions and conclusions are provided in relevant chapters. Green manure (GM)

growth, nitrogen (N) accumulation, decomposition and N-release are treated in Chapter 2.

Chapter 3 evaluates effects of GMs on sweet corn growth, N-accumulation, N-status

indicators, and ear yields. A sweet corn root study, conducted to complement information

from growth and yield analysis, is described in Chapter 4. Effects of GM approaches on

soil carbon (C) and N pools are treated in Chapter 5, with effects on weeds and plant

parasitic nematodes assessed in Chapter 6. Chapter 7 provides a review and synthesis of

findings, followed by Appendices (of selected tables) and References.

Introduction

Rationale

The last century of American agriculture has been characterized by a shift from

highly diversified low-input systems to highly specialized operations greatly depending

on external non-renewable resources. Dramatic increases in farm size, along with an

erosion of farm and crop diversity, have resulted in agroecosystems more vulnerable to

pressures of urbanization, climate change, and volatile global markets. Need exists to

provide farmers with economically viable alternatives that harness ecological processes,









farm and biological diversity, and on-farm resources (see Gold 1999, Dinnes et al. 2002

for discussions).

With a production area of 2.3 million acres and a crop value of 3.8 billion dollars,

vegetable, fruit and field crop production comprise major agricultural activities in Florida

(Florida Agricultural Statistics Service 2004). Many soils in this region possess little

organic matter (<1%) and exhibit poor water and nutrient retention (for examples, see

Appendix A or Carlisle et al. 1988), especially those experiencing regular disturbance

(through tillage) and low input rates for organic matter. Conventional cropping systems

on such soils therefore require continuous application of large amounts of external

nutrients and irrigation water, yet remain vulnerable to large losses of these inputs.

Producers, consumers, government agencies, and researchers have therefore expressed

increasing interest in alternative and/or organic production systems (Gold 1999, Dinnes et

al. 2002).

A green manure (GM) is a crop used primarily as a soil amendment and a nutrient

source for subsequent crops. In most production environments, lack of N limits plant

growth more than any other nutrient. Crop plants effectively satisfy their N requirement

only by acquisition of mineralized N (NH4+ and NO3-). Atmospheric N (N2), though

abundant, cannot be utilized by plants. Legumes, however, possess a symbiotic

relationship with rhizobial bacteria capable of transforming atmospheric N2 into plant-

usable form and may accumulate large amounts of N via this pathway. Legumes utilized

as GMs therefore represent a potentially renewable source of on-farm, biologically fixed

N. Unlike chemical N fertilizers, legumes may also fix and add large amounts of C to a

cropping system (Hargrove 1986, Sharma and Mittra 1988, Goyal et al. 1992). Legumes









can also correct phosphorus (P) imbalances typically associated with excess applications

of animal waste products because legume P levels are similar to those of other plants. The

slow release of N from decomposing GM residues may be better timed with plant uptake,

possibly increasing N-uptake efficiency and crop yield while reducing N leaching losses

(Bath 2000, Wivstad 1997). Green manure approaches may also drive long-term

increases of soil organic matter and microbial biomass, further improving nutrient

retention and N-uptake efficiency (for example, see Agustin et al. 1999). When used in

place of fallow, well-chosen GMs may reduce erosion and suppress weeds and specific

crop pests (Ross et al. 2001, McSorley 1999). Green manures may also offer habitat or

resources for beneficial organisms (Altieri and Letourneau 1982, Yeates et al. 1999,

Bugg et al. 1991a,b).

Presently, GM management is difficult relative to chemical fertilizer approaches.

Nitrogen release from plant residues depends on a large number of interactive factors

including chemical composition and N concentration, temperature, and water availability

(Schomberg et al. 1994, Andren 1992). Practical information about the composition and

N concentration of GMs as they change over a growing season is often lacking. Most

existing information on GM performance comes from studies conducted in temperate or

tropical environments on fine-textured soils, the results of which may not hold in north

Florida with its sub-tropical/sub-temperate climate and sandy soil. Especially for high N-

demanding crops, GMs may not supply adequate N if amount and timing of N-release do

not match crop demand. At times, GM-amended crops may require supplementary

inorganic N fertilizer to prevent yield reductions, but little or no information exists on

optimal supplementation levels. It also remains unclear if long-term improvements in soil









fertility can be achieved under Florida conditions. Current guidelines to GM use and

previous GM research have not yet developed management techniques necessary to

produce economic yields comparable to chemically fertilized, high-N demanding crops in

north Florida. Improved integration of GMs in such cropping systems will require more

precise and detailed information about GM growth and decomposition, subsequent crop

yield responses, and effects on soil and pests over time in specific production

environments.

Green Manure Management

Performance of GMs varies by species, growth environment (climate, soil, weather,

pests,etc.), and management (e.g., planting date, length of growing season,etc.). Table 1.1

summarizes the dry matter and N accumulation of about 50 GM species from 40 studies

and includes reported information about study location, soil type, and length of growing

season. Green manures generally fall into two categories: tropical ("warm weather") and

temperate ("cool weather"). Few, if any, tropical legumes can survive hard freezes (when

temperature drops below -2 C for several hours), though they can usually tolerate

temperatures in excess of 40 C. Temperate legumes, on the other hand, often decline at

temperatures over 25 C but may persist without injury at -10 C or lower. The most

widely used tropical GM legumes probably include those in genera Crotalaria (sunn

hemp), Glycine (soybean), Indigofera indigoss), Mucuna (velvetbean), Vigna cowpeaa),

Cajanus (pigeonpea), and Sesbania, while the temperate GM legumes often include

Trifolium cloverss), Vicia vetchess), Medicago (alfalfa, trefoils, and other medics), and

Lupinus lupinss). Typical non-legume temperate GMs consist of cereal rye (Secale

cereale), mustards (Brassica spp), radishes (Raphanus spp), buckwheat (Fagopyrum

esculentum), millet (Echinocloa spp), oats (Avena spp), and wheat (Triticum spp).









Legume GMs are often preferable to non-legumes because they supply their own N, but

in production scenarios where N is less limiting, where a specific GM service other than

high N supply (such as allelopathy) is sought, and/or where legumes do not perform well,

non-legumes or mixtures of legumes and non-legumes may be more desirable (for

example, see Karpenstein-Machan and Stuelpnagel 2000). Because they do not derive

direct sales profit, GMs are often chosen that require some acceptably low level of

nutrients, irrigation, and pest control and fit into otherwise unplanted fallow periods.

When biological N-fixation is not water or temperature limited, legumes are often

selected as GMs due to their N-fixation capacity. Probably because they are adapted to

and grown in warmer climates with higher light levels, tropical legumes often accumulate

biomass and N faster than winter legumes. Genetic differences (species and variety) also

may dictate that some legumes grow larger and accumulate more N than others.

Environment (temperature, soil type, nutrient and water availability) and management

(planting density and timing, mowing, pest control,etc.) may further alter performance of

individual GM species. For example, sunn hemp (Crotalariajuncea; a tropical legume)

generally grows more rapidly than temperate legumes, accumulates greater biomass and

N than most tropical legumes (likely because it is capable of growing upright and

becoming quite stemmy), with reduced performance on low-fertility soils and under

water stress (Seneratne and Ratnasinghe 1995, Ladha et al. 1996, Mansoer et al. 1997,

Jeranyama et al. 2000, Ramos et al. 2001, Steinmaier and Ngoliya 2001)

Climate probably limits GM selection more than any other single factor. In very

cold climates, temperate legumes survive during the spring, summer, and fall. As one

moves to warmer climates, increasing winter temperatures permit temperate legumes to









persist during winter months while tropical legumes become better suited during warmer

months. Where lowest "winter" temperatures remain above freezing, tropical legumes

may survive year-round, and high temperatures may begin to exclude use of temperate

legumes. However, precipitation, soil type, and pest pressures also interact with

temperature to determine how specific GMs will perform in a given location. Potential N

accumulation and growing season of a GM must fit a particular crop rotation. Desirability

of GMs may also include or exclude ability to reseed, growth habit (upright, prostrate,

viney,etc.), aggressiveness, and presence of toxic or allelopathic chemicals affecting

livestock, crops, and/or plant pests. For example, sunn hemp may be desirable preceding

a fall vegetable crop because it accumulates much N, thrives in high summer

temperatures, is killed easily by stem breakage, does not become weedy by reseeding

itself (at least in Florida), and may suppress parasitic nematodes. However, sunn hemp

may be ill-suited to grow near trees because its height and mass make it competitive for

light, water, and nutrients and it would require replanting on an annual basis.

Both tropical and temperate GMs may be used in north Florida, but their

production level and/or growing time is often restricted by variable temperatures and

low-fertility soils. Compared to temperate environments, effective N-accumulation from

temperate GM legumes in Florida may occur slowly and/or have limited potential due to

high temperatures and poor adaptation to sandy soils. More productive summer legumes

do not survive winter freezes in north Florida, and decomposition during this period can

result in heavy losses of residue N. If GMs do not supply adequate N to meet

requirements of subsequent crops, then supplementary inorganic N may be required to









prevent yield reductions. Crops with high N-demand planted just after winter therefore

pose a particularly acute challenge for GM use in north Florida.

During the winter, a temperate GM may take up significant amounts ofN from a

decomposing tropical legume, possibly boosting the growth and N accumulation of the

temperate legume and reducing N leaching losses. Overwintering residue with low N

content and/or high C:N ratio may also reduce N leaching losses (Stopes et al. 1996,

Wyland et al. 1996). In some systems, it may therefore be advantageous to follow a

stemmy, vigorous summer GM with a well-adapted winter GM, and to preserve as much

recalcitrant litter as possible by reducing tillage. However, excessive build-up of crop

residues may interfere with growth of a number of crops; selection of less sensitive crops

and/or periodic tillage may become important.

Approach

This project focused on a GM approach to production of a spring planted, high-N

demanding vegetable crop sweet corn (Zea mays var Rugosa) in north Florida. Based

on information from the University of Florida-Institute of Food and Agricultural Science

(UF-IFAS) Electronic Data Information Source (EDIS), extension recommendations for

sweet corn on sandy Florida soils include at least 180-200 kg N ha-1 (Hochmuth and

Cordasco 2000). Florida farmers generally use chemical fertilizers to satisfy the N

requirements of sweet corn. We conducted our study at the Plant Science Research and

Education Unit in Citra, Florida, primarily on Candler and Lake sands (see Appendix A

for soil characterization data).

To help develop improved GM management techniques appropriate for spring

crops in north Florida, a novel approach to a GM/sweet corn cropping system was

investigated. Sunn hemp was planted in late summer, grown for 12-14 weeks to optimize









N content, biomass and overall recalcitrance. Afterwards, blue lupin (Lupinus

angustifolius; winter 2001-02) or cahaba white vetch (Vicia sativa; winter 2002-03) was

planted into the standing sunn hemp residues to capture mineralized N and/or fix

additional N. Sunn hemp and lupin/vetch were also evaluated alone. After mowing of all

residues, sweet corn was then planted in the spring. All crops were planted using reduced

or zero-tillage. Several rates of supplementary inorganic N were applied to both GM-

amended and unamended (conventional) sweet corn. We believed the combined GM

approach would supply significant amounts ofN to sweet corn while also providing long-

term benefits by increasing soil organic matter and microbial biomass and suppressing

parasitic nematodes and weed production. The overall project is therefore planned to

continue for at least 5 years, depending on availability of external funding.

Hypotheses

1. Sunn hemp stem residues would immobilize a significant amount ofN during
winter decomposition (Chapter 2).

2. Growth of winter legumes following sunn hemp would be enhanced, reaching
levels similar to those reported for temperate environments (Chapter 2).

3. The double-GM approach would significantly reduce chemical N required by sweet
corn to achieve ear yields similar to an optimal level identified in the conventional
approach (Chapter 3).

4. Green manures would increase N recovery rates of sweet corn (Chapter 3).

5. Amendment with GMs would increase sweet corn root length density and
redistribute it nearer to the GM residue (Chapter 4).

6. GMs would increase total soil C and N as well as specific soil C and N pools often
indicative of recent organic additions including microbial biomass C and
particulate organic C and N (Chapter 5).

7. Green Manures would significantly suppress weed biomass and plant parasitic
nematode population (Chapter 6).









Objectives

The objectives of this research were as follows:

1. To generate detailed information about GM biomass and N accumulation by tissue
fraction during growth, and subsequent decomposition and N-release by the
summer GM during winter months (Chapter 2).

2. To gauge impacts of GMs on sweet corn growth, N-status indicators, and root
distribution patterns throughout the season (Chapters 3 and 4).

3. To estimate chemical N-supplementation needed to achieve acceptable sweet corn
ear yields for GM approaches (Chapter 3).

4. To determine if any GM approach can produce corn ear yields equivalent to the
conventional approach (Chapter 3).

5. To estimate N recoveries and losses for GM and conventional approaches (Chapter
3).

6. To evaluate effects of GMs on soil properties, weeds, and parasitic nematodes
having long-term implications for the efficacy of the system (Chapters 5 and 6).

General Set-Up and Design

Table 1.2 lists the 15 overall treatments of the study, which began in August 2001

and was conducted at the Plant Science Research and Education Unit near Citra, FL.

Candler fine sand (Typic Quarzipsamments, hyperthermic, uncoated; 98% sand in the

upper 15 cm) and Lake fine sand (Typic Quarzipsamments, hyperthermic, coated; 97%

sand in the upper 15 cm) were the dominant soil types (see Appendix A for detailed

characterization). Study design consisted of four randomized complete blocks with plots

7.6 m x 8.8 m (25 ft x 30 ft). Treatments were composed of two main effects: GM level

and chemical N-rate level (chemical N applied to sweet corn only). Green manure level

consisted of: a summer leguminous GM (sunn hemp) followed by a winter legume (blue

lupin, winter 2001-02; cahaba white vetch, year 2002-03) denoted as SH+L; summer

legume only (SH); winter legume only (L); and a conventional level with no GM (Conv).

Following summer and winter, a spring crop of sweet corn was planted in all plots and









supplemented with 0, 67, and 133 kg NH4NO3-N ha-1 for each GM level (denoted as ON,

67N, and 133N). Conventional GM level also possessed fertilization rates of 200 and 267

kg NH4NO3-N ha-1 (Conv 200N and Conv 267N) representing 3/3 and 4/3 the chemical

N-rate recommended for sweet corn in Florida by UF-IFAS extension. A final treatment

of complete fallow (Fal) receiving chemical weed control (identical to other treatments)

but no GM, sweet corn, chemical-N, or tillage was included for comparison purposes.

More detailed materials and methods are found in relevant chapters (see Overview,

above).

Measurements

* Measurements taken for all crops were: plant numbers at beginning of season; leaf
area and leaf, stem, root, and reproductive (flowers/pods/ears) fresh and dry
weights taken every 2-4 weeks and at final samplings, as well as N concentration
and content of all tissues.

* Measurements taken for sweet corn were: leaf chlorophyll readings every 2 weeks
and at final samplings, ear number and ear grade at final samplings, and root length
density for selected plots at selected dates in 2003.

* Measurements taken for soil (0-25 cm) were: Total Soil C and N, Particulate
Organic C and N, Microbial Biomass C, and pH at selected dates.

* Continuously taken measurements were: Precipitation, air temperature at Im, soil
temperature at a depth of 5 cm, and relative humidity using Watchdog Dataloggers
(Spectrum Technologies; Plainfield, IL; see Appendix B).

* Measurements of nematodes were: soil counts for selected plots at selected dates.

* Measurements taken for weeds were: total dry weight, N concentration and N
content in all plots at end of sunn hemp and vetch.










Table 1.1. Review of green manure studies.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Oats 3.3-4.3 Sandy loam, Maine
Avena sativa Dyck & Liebman 1995 80-82 3 months
Azolla 2.1-2.3 Silty Clay, Philippines
4zolla microphylla Ladha et al. 2000 61-75 5-9 weeks; flooded
Colza 2.1-4.6 Silt loam, Canada
Brassica campestris N'Dayegamiye & Tran 2001 59-99 4 months; 30 kg N ha' applied
Mustard 2.3-3.8 Silt loam, Canada
Brassica hirta N'Dayegamiye & Tran 2001 62-72 4 months; 30 kg N ha' applied
Pigeonpea 6.5-9.0 Clay or loam?, Philippines
Cajanus caqan Ladha et al. 1996 154-235 ~6 months; clipped to 20-30 cm 5 times
Canavalia 4.4 Sandy loam, Cuba
Canavalia ensiformis Ramos et al. 2001 58 8-9 weeks
Centro 1.2 Sandy loam, Zambia
Centrosema pubescens Steinmaier & Ngoliya 2001 27 14 weeks (?)
Rhodes Grass 14 Sandy loam, Zambia
, lil.. o gayana Steinmaier & Ngoliya 2001 167 14 weeks (?)
Clitoria 6.9-7.7 Clay or loam?, Philippines
Clitoria ternatea Ladha et al. 1996 256-306 ~6 months; clipped to 20-30 cm 2-3 times
Sunn Hemp 0.9-2.9 Loamy sand, Zimbabwe
Crotalaria juncea Jeranyama et al. 2000 23-82 5-7 weeks
7.6-7.8 Clay or loam?, Philippines
Ladha et al. 1996 277-279 ~6 months; clipped to 20-30 cm 2-3 times
4.8-7.3 Sandy loam, Alabama
answerr et al. 1997 120-138 9-12 weeks
11.1 Sandy loam, Cuba
_amos et al. 2001 195 8-9 weeks
6.1-9.6 NR, Sri Lanka
Seneratne & Ratnasinghe 1995 161-252 8-9 weeks
12.1 Sandy loam, Zambia
Steinmaier & Ngoliya 2001 227 14 weeks (?)
Crotalaria 5.0 (13 wks), 8.0 (19 wks) Loamy sand, Nigeria
Crotalaria ochroleuca Carsky et al. 1999 114 (13 wks), 137 (19 wks) 13 and 19 weeks; AAR = 1350 mm
2.0 (13 wks), 3.3 (19 wks) Clay loam, Nigeria
Carsky et al. 1999 52 (13 wks), 63 (19 wks) 13 and 19 weeks; AAR = 900 mm
AAR = average annual rainfall.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Sunn Hemp Marejea 14.6 Sandy loam, Zambia
Crotalaria zanzibarica cv Marejea Steinmaier & Ngoliya 2001 328 14 weeks (?)
Desmanthus 8.0-9.1 Clay or loam?, Philippines
Desmanthus virgatus Ladha et al. 1996 251-283 ~6 months
Millet 2.3-11.2 Silt loam, Canada
Echinochloa crus galli N'Dayegamiye & Tran 2001 65-139 4 months; 30 kg N ha' applied
Buckwheat 2.1-3.7 Silt loam, Canada
Fagopyrum esculentum N'Dayegamiye & Tran 2001 52-65 4 months; 30 kg N ha-' applied
Soybean 2.8-5.8 NR, Taiwan & Philippines
Glycine max Thonnissen et al. 2000a 106-141 2-2.5 months
Indigo 2.3-2.9 Clay loam, Philippines
[ndigofera tinctoria Agustin et al. 1999 56-57 5-6 months after death of other intercrops
0.2-2.0 NR, Taiwan & Philippines
Thonnissen et al. 2000a 5-44 2-2.5 months
Lablab
Lablab purpureusl 1.9 (13 wks), 2.0 (19 wks) Loamy sand, Nigeria
Dolichos lablab Carsky et al. 1999 71 (13wks), 47 (19wks) 13 and 19 weeks; AAR = 1350 mm
0.6 (13 wks), 1.8 (19 wks) Clay loam, Nigeria
Carsky et al. 1999 23 (13 wks), 49 (19wks) 13 and 19 weeks; AAR = 900 mm
0.7-1.7 Loamy sand, Mali
Kouyate et al. 2000 NR NR; AAR = 619 mm
0.6-2.0 Loam, Mali
Kouyate et al. 2000 NR NR; AAR = 619 mm
5.8 Sandy loam, Zambia
Steinmaier & Ngoliya 2001 115 14 weeks (?)
Black Lentil 2.3-2.7 Loam, Saskatchewan
Lens culinaris Brandt 1999 53-64 NR; AAR = 359 mm
1.0-2.2 Sandy loam, New Mexico
_uldan et al. 1996 34-58 -22 weeks; interseeded in sweet corn after 2 weeks
0.9-1.1 Sandy loam, New Mexico
Guldan et al. 1996 34-35 17 weels; interseeded in sweet corn after 7 weeks
Rye Grass 1.3-2.5 Loam, Ontario
Lolium multiflorum Dapaah & Vyn 1998 NR 7 months; intercropped with barley
0.7-17.5* Clay loam, England
topess et al. 1996 15-346* *6-25 months of growth; periodic mowing
AAR = average annual rainfall; NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Blue Lupin 5.3-6.7 NR (Sandy loam?), Tifton, GA
Lupinus 1, ,t,1.4il ,, Forbes 1970 NR NR
-1.0 Sand, Gainesville
Gallaher 1991 -20 24 weeks; 25 plants m-2
-1.8 Sand, Gainesville
Gallaher 1991 -30-35 24 weeks; 50 plants m-2
2.1 Sand, Gainesville
Gallaher 1991 36 24 weeks; 100 plants m-2
2.8-3.1 NR, South Carolina
Suman (in Forbes 1970) NR NR
Siratro 4.9-5.5 Clay or loam(?), Philippines
Aacroptilium atropurpureum Ladha et al. 1996 132-178 -6 months
2.4 Sandy loam, Zambia
Steinmaier & Ngoliya 2001 62 14 weeks (?)
Trefoil 0.6-20.4* Clay loam, England
Aedicago lupulina Stopes et al. 1996 15-459* *6-25 months of growth; periodic mowing
Burr Medic 1.1 (C), 1.6 (N) Loam, Michigan
Aedicago polymorpha Shresthra et al. 1999 NR 90 days; cut for forage at 60 days (C) or not (N)
Burr&Snail Medics 0.6-3.1 Loam, Michigan
Aedicago polymorpha, M. scutellata Jeranyama et al. 1998 17-75 9-11 weeks; 5 planting dates
0.1-1.3 Loam, Michigan
Jeranyama et al. 1998 2-32 9-11 weeks; 5 planting dates, intercropped with corn
Gamma Medic 1.4 Loam, Michigan
Aedicago rugosa Shresthra et al. 1999 NR 13 weeks
Alfalfa 3.7-5.7 Silt loam, Maine
ledicago sativa Griffin, et al. 2000 105-174 1 year
1.1-1.5 Sandy loam, New Mexico
Guldan et al. 1996 41-53 -22 weeks; interseeded in sweet corn after 2 weeks
0.5-1.2 Sandy loam, New Mexico
Guldan et al. 1996 21-43 -17 weeks; interseeded in sweet corn after 7 weeks
1.6 Loam, Michigan
Shresthra et al. 1999 NR 13 weeks
2.8-5.7 Sandy loam, Kansas
Singogo et al. 1996 107-138 7-8 months
NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Barrel Medic 2.4-4.5 Sandy loam, New Mexico
Medicago truncatula Guldan et al. 1996 72-131 -22 weeks; interseeded in sweet corn after 2 weeks
1.0-2.3 Sandy loam, New Mexico
Guldan et al. 1996 37-69 -17 weeks; interseeded in sweet corn after 7 weeks
1.4 (C), 3.2 (N) Loam, Michigan
Shresthra et al. 1999 NR 13 weeks; cut for forage at 60 days (C) or not (N)
Yellow Sweet Clover 3.1-5.4 Sandy clay loam, Alberta
Melilotus ,ttic iinalui.\ Blackshaw et al. 2001b NR NR; AAR = 387 mm; multiple intercrops
Mucuna 2.1 Sandy loam, Cuba
Mucuna aterrima Ramos et al. 2001 64 8-9 weeks
Velvet Bean
Mucuna pruriens / M. atropurpuriem 4.0 (13 wks), 6.2 (19 wks) Loamy sand, Nigeria
M. deeringiana Carsky et al. 1999 131(13 wks), 154 (19 wks) 13 and 19 weeks; AAR = 1350 mm
1.7 (13 wks), 3.4 (19 wks) Clay loam, Nigeria
Carsky et al. 1999 53 (13 wks), 85 (19 wks) 13 and 19 weeks; AAR = 900 mm
9.3 Sandy loam, Zambia
Steinmaier & Ngoliya 2001 183 14 weeks (?)
Glycine 0.9 Sandy loam, Zambia
Neonotonia wightii Steinmaier & Ngoliya 2001 21 14 weeks (?)
Winter/Field Pea Karpenstein-Machan and -4.8 Silty clay, Germany
Pisum sativum Stuelpnagel 2000 -200 4 months
1.1-3.5 (stover) Sandy loam, Alberta
Soon et al. 2001 8.2-28.3 (stover) NR; pea-wheat-canola-wheat rotation
Winter Pea + Rye Karpenstein-Machan and -6-12 Silty clay, Germany
Pisum sativum + Secale cereale Stuelpnagel 2000 -200 4 months; 3 different seeding mixtures
Winter Pea (Austrian) 3.2-7.6 Sandy loam, Kansas
Pisum sativum subsp arvense Singogo et al. 1996 107-230 7-8 months
Oilseed radish 2.5-3.5 Loam, Ontario
Raphanus sativus Dapaah & Vyn 1998 NR 3 months; intercropped with barley
Rye 3.5-4.0 (ON), 4.0-9.0 (140N) Silt loam, Kentucky
Secale cereale Cline & Silvernail 2001, 2002 28-43 (ON), 43-64 (140N) 8 months; 0 or 140 kg N ha-' for proceeding corn crop
4.1-6.6 Silt loam, Maine
Griffin, et al. 2000 52-66 9 months
AAR = average annual rainfall; NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Rye Karpenstein-Machan and -9-13.5 Silty clay, Germany
Secale cereale Stuelpnagel 2000 NR ~4 months
1.5-5.7 Loamy sand, Georgia
anells and Wagger 1996 17-64 5 months
2.7-3.4 (n), 6.4 (m) Silty clay loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
0.5-0.6 (n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
1.0-6.1 Loam, Ontario
Tollenaar et al. 1993 NR 8 months; 4 rye cultivars
Sesbania 7.1 Sandy loam, Zambia
Sesbania macrantha Steinmaier & Ngoliya 2001 124 14 weeks (?)
Sesbania 0.7-1.4 Loamy sand, Mali
Sesbania rostrata Kouyate et al. 2000 NR NR; AAR =619 mm
2.3-4.6 Loam, Mali
Kouyate et al. 2000 NR NR; AAR = 619 mm
Sesbania 3.2-4.6 Silty clay, Philippines
Sesbania rostrata Ladha et al. 2000 71-88 5-9 weeks; flooded
Stylo 4.3 Sandy loam, Zambia
Stylosanthes guianensis Steinmaier & Ngoliya 2001 88 14 weeks (?)
Teramnus 3.8 Sandy loam, Zambia
Teramnus uncinatus Steinmaier & Ngoliya 2001 80 14 weeks (?)
Berseem Clover 6.7-10.2 (n), 9.2 (m) Silty clay loam, Alberta
Trifolium alexandrinum Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
4.0-6.0 (n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
1.9 (C), 4.1 (N) Loam, Michigan
Shresthra et al. 1999 NR 13 weeks; cut for forage at 60 days (C) or not (N)
Kura Clover 6.2-10.7 Silt loam, Wisconsin
Trifolium ambiguum Zemenchik et al. 2000 NR NR; intercropped with corn, then grown alone
Alsike Clover 3.0-4.6 (n), 6.1 (m) Silty clay loam, Alberta
Trifolium hybridum Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
2.5-2.7 (n) Loam, Alberta
_oss et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
AAR = average annual rainfall; NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Crimson Clover 4.2-5.7 Sandy Loam, Maryland
Trifolium incarnatum Abdul-Baki et al. 1996 151 8 months
5.8-7.3 Sandy loam, Maine
Dyck & Liebman 1995 130-143 3.5 months
4.8-5.1 Sandy loam and silt loam, Maine
Dyck et al. 1995 117-123 2-2.5 months
Karpenstein-Machan and -4-10.5 Silty clay, Germany
Stuelpnagel 2000 -200 ~4 months
1.4-5.0 Loamy sand, Georgia
Ranells and Wagger 1996 35-134 6 months
Crimson Clover 2.1-4.0 (n), 5.7 (m) Silty clay loam, Alberta
Trifolium incarnatum Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
3.7-5.1 (n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
Crimson Clover + Rye Karpenstein-Machan and -6-12 Silty clay, Germany
Trifolium incarnatum + S. cereale Stuelpnagel 2000 -200 ~4 months; 3 different seeding mixtures
2.3-5.2 Loamy sand, Georgia
Ranells and Wagger 1996 42-111 6 months
Balansa Clover 2.5-4.5 (n), 7.2 (m) Silty clay loam, Alberta
Trifolium michelianum var balansae Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
2.3-3.5 (n) Loam, Alberta
_oss et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
Red Clover 2.4-3.7 Loam, Ontario
Trifolium pratense Dapaah & Vyn 1998 NR 10 months; intercropped with barley
1.5-3.0 Sandy loam, Maine
Davis & Liebman 2001 72-115 R; intercropped with wheat
0.8-1.9 Sandy loam, New Mexico
Guldan et al. 1996 29-49 22 weeks; interseeded in sweet corn after 2 weeks
0.3-0.6 Sandy loam, New Mexico
Guldan et al. 1996 13-16 17 weeks; interseeded in sweet corn after 7 weeks
0.6-0.7 Silt loam, Canada
N'Dayegamiye & Tran 2001 13 4 months; 30 kg N ha-' applied
1.7-2.9 (n), 5.2 (m) Silty clay loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
2.1-2.2 (n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
1.7-3.6 Sandy loam, Alberta
Soon et al. 2001 51-94 NR; red clover-wheat-canola-wheat rotation
0.8-25.4* Clay loam, England
Stopes et al. 1996 21-741* *6-25 months of growth; periodic mowing
White Clover 0.8-2.1 (n), 4.0 (m) Silty clay loam, Alberta
Trifolium repens Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
2.7-3.0(n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
0.6-25.0* Clay loam, England
Stopes et al. 1996 17-592* *6-25 months of growth; periodic mowing
Persian Clover 1.7-3.4 (n), 7.2 (m) Silty clay loam, Alberta
Trifolium resupinatum Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
3.7-4.6 (n) Loam, Alberta
Ross et al. 2001 NR 14-16 weeks; mowed at 7-8 wks (m) and non-mowed (n)
Wheat 4.9-9.8 Sandy loam, Kansas
Triticum aestivum Singogo et al. 1996 81-87 7-8 months;
Hairy Vetch 4.4-5.2 Sandy Loam, Maryland
Vicia villosa Abdul-Baki et al. 1996 167-197 8 months
3.5-4.0 (ON z 140N) Silt loam, Kentucky
Cline & Silvernail 2001, 2002 115-164 (ON 140N) 8 months; 0 or 140 kg N ha for proceeding corn crop
1.8-3.8 Sandy loam, New Mexico
_uldan et al. 1996 70-124 22 weeks; interseeded in sweet corn after 2 weeks
1.5-2.8 Sandy loam, New Mexico
Guldan et al. 1996 58-88 17 weeks; interseeded in sweet corn after 7 weeks
4.4 Silt loam, Pennsylvania
Puget & Drinkwater 2001 NR 8 months
2.9-4.8 Loamy sand, Georgia
Ranells and Wagger 1996 125-182 5 months
3.0-6.7 Sandy loam, Georgia
Sainju & Singh 2001 104-257 6 months; 3 tillage types, 2 kill dates
5.6-8.9 Sandy loam, Kansas
Singogo et al. 1996 233-247 7-8 months
NR = not reported.










Table 1.1. Continued.
Dry Weight (Mt ha-1) Environment
Green Manure Study N Content (kg ha-1) Growth Time; Management Notes
Hairy Vetch + Rye 5.9 Sandy Loam, Maryland
Vicia villosa + Secale cereale Abdul-Baki et al. 1996 120-162 8 months
4.0 (ON), 4.0-10.0 (ON) Silt loam, Kentucky
Cline & Silvernail 2001,2002 104-152 (ON), 141-149 (140N)8 months; 0 or 140 kg N ha-1 for proceeding corn crop
3.6-6.9 Silt loam, Maine
Griffin, et al. 2000 57-209 9 months
3.0-5.4 oamy sand, Georgia
Ranells and Wagger 1996 82-200 6 months
Black Gram 7.1-8.8 (stover) NR, Sri Lanka
Io-,,i mungo Seneratne & Ratnasinghe 1995 104-155 (stover) 11-12 weeks
Mung bean 3.1-5.5 (stover) NR, Sri Lanka
i;Toi. radiata Seneratne & Ratnasinghe 1995 30-88 (stover) 11-12 weeks; 2 cultivars
1.1 NR, Taiwan & Philippines
Thonnissen et al. 2000a 26 9-11 weeks
Cowpea 0.6 (13 and 19 wks) oamy sand, Nigeria
i;;,,,l unguiculata Carsky et al. 1999 16 (13 wks), 21 (19 wks) 13 and 19 weeks; AAR = 1350 mm
1.4 (13 wks), 2.3 (19 wks) Clay loam, Nigeria
Carsky et al. 1999 45 (13 wks), 58 (19wks) 13 and 19 weeks; AAR = 900 mm
0.6-4.6 Loamy sand, Zimbabwe
eranyama et al. 2000 15-154 11 weeks
1.5-2.5 Loamy sand, Mali
Kouyate et al. 2000 NR NR; AAR =619 mm
1.1-2.4 Loam, Mali
Kouyate et al. 2000 NR NR; AAR = 619 mm
3.7-8.5 NR, Sri Lanka
Seneratne & Ratnasinghe 1995 42-155 11-12 weeks; 3 cultivars
AAR = average annual rainfall; NR = not reported.









Table 1.2. Overview of experimental treatments.
Treatment Crop 1 Crop 2 Crop 3
July-November November-April April-July
1) SH+L ON Sunn hemp Lupin(Yearl) or Vetch(Year2) Sweet Corn + 0 N
2) SH+L 67N Sunn hemp Lupin(Yearl) or Vetch(Year2) Sweet Corn + 67 N
(3) SH+L 133N Sunn hemp Lupin(Yearl) or Vetch(Year2) Sweet Corn + 133 N
4) SH ON Sunn hemp Fallow Sweet Corn + 0 N
5) SH 67N Sunn hemp Fallow Sweet Corn + 67 N
(6) SH 133N Sunn hemp Fallow Sweet Corn + 133 N
(7) L ON Fallow Lupin(Yearl) or Vetch(Year2) Sweet Corn + 0 N
8) L 67N Fallow Lupin(Yearl) or Vetch(Year2) Sweet Corn + 67 N
(9)L 133N Fallow Lupin(Yearl) or Vetch(Year2) Sweet Corn + 133 N
(10) Cony ON Fallow Fallow Sweet Corn + 0 N
(11) Cony 67N Fallow Fallow Sweet Corn + 67 N
(12) Cony 133N Fallow Fallow Sweet Corn + 133 N
(13) Conv 200N Fallow Fallow Sweet Corn + 200 N
(14) Conv 267N Fallow Fallow Sweet Corn + 267 N
(15) Fal Fallow Fallow Fallow


Complete fallow;


SH = sunn hemp; L = winter legume; Conv = conventional; Fal =
N = kg NH4NO3-N ha1.














CHAPTER 2
GREEN MANURE GROWTH AND DECOMPOSITION

Introduction and Literature Review

Due to coarse texture, high temperatures and high rainfall, many Florida soils

contain little organic matter (less than 1-2%) and possess poor water and nutrient

retention. This is especially true for agricultural soils that experience regular tillage and

low carbon (C) input rates. Legumes utilized as green manures may be useful as a

component of sustainability in such production environments. A green manure (GM) is a

crop used primarily as a soil amendment and a nutrient source for future crops. Legumes

may add nitrogen (N) to the system through biological fixation and can correct

phosphorus (P) imbalances typically associated with excess applications of animal

manures. The slow release of N from decomposing GM residues may be better timed

with plant uptake (Bath 2000, Wivstad 1997). Unlike chemical fertilizers, legumes may

fix and add large amounts of C to a cropping system (Hargrove 1986, Sharma and Mittra

1988, Goyal et al. 1992) and may drive long-term increases of soil organic matter and

microbial biomas (Goyal et al. 1992, 1999). Green manures may provide other benefits

such as reduction of soil erosion, conservation of soil water, improved retention of other

crop nutrients, and control of plant pests, pathogens and weeds with less reliance on off-

farm chemical inputs (Bugg et al. 1991a McSorley 1999, Ross et al. 2001).

Effective use of GMs is often hampered by lack of precise information about N

availability for future crops. Nitrogen accumulation and subsequent release from

decomposing GMs depends largely on residue composition and N concentration,









temperature, water availability, and residue management (Schomberg et al. 1994, Andren

1992), which in turn depend on GM species, site environment (climate, soil, weather,

etc.), and cropping system. Table 1.1 outlines the dry matter and N accumulation of about

50 GM species from 40 studies. While not exhaustive, this review probably represents a

major cross section of recent GM studies, most of which took place in temperate (and

high latitude) or tropical (equatorial) regions on relatively fine-textured soils. Results of

such studies may not extend to intermediate regions such as Florida. In temperate

regions, the cooler and often less variable temperature regimes with longer daylight hours

may be more conducive to temperate legumes. Compared to tropical regions, much of

Florida experiences winter freezes that kill warm weather annuals used as both GMs and

many of the crops that follow them. Sandy soils and unique pest problems also make

establishment more challenging, especially for temperate GMs such as clovers and

medics.

Additionally, pertinent information about composition and N concentration of GMs

as they change over a growing season is often lacking. Almost all studies report end-of-

season GM biomass and N content/concentration only. For example, of the peer-reviewed

literature sampled in Table 1.1, only Karpenstein-Machan and Stuelpnagel (2000)

provide growth analysis of investigated GMs. This poses an obstacle to GM adoption

because growing time in an on-farm production system differs from that studied in

research, creating yet another way GM biomass and composition in an on-farm setting

may differ from reported findings.

Biological N-fixation and overall N-accumulation rates are primary factors

governing the adequacy of a GM as an N-source. Estimates of N accumulation for









leguminous GMs and the relative contribution of biological N fixation in this process

ranges broadly depending on soil fertility, water availability, and GM species. Generally

speaking, legumes will take up what N is available from the soil, but thereafter will

accumulate N from biological fixation to meet demand. For example, sunn hemp

(Crotalariajuncea) has been estimated to fix 27-39% (Ramos et al. 2001), 72-81%

(Ladha et al. 1996), and 91% (Senaratne and Ratnasinghe 1995) of its total N in different

study locations and conditions. Water stress and deficiency of nutrients other than N may

significantly reduce fixation, either directly or through reduced availability of assimilates

from photosynthesis. Although they did not report rainfall or temperature conditions,

Ladha et al. (1996) attributed a 9% differential in relative contribution of fixed N in sunn

hemp over two years to differences in weather patterns that directly affected plant

growth. Reduction of soil N through competition may increase rates of biological N-

fixation. Karpenstein-Machan and Stuelpnagel (2000) found that relative contribution of

N-fixation to hairy vetch (Vicia villosa) and crimson clover (Trifolium incarnatum) N

increased when intercropped with an increasingly larger proportion of cereal rye (Secale

cereale).

Nitrogen contributions from below-ground tissues of GMs (roots, root nodules) is

difficult to determine due to rapid turnover of these tissues and possible root exudation of

N. For example, Ramos et al. (2001) determined that 39-49% of all N accumulated by

Canavalia ensiformis and Mucuna aterrima GMs was belowground, and 10-12% of all

accumulated N transferred to the soil by root and nodule turnover and root exudation. In a

3-year study, Griffin et al. (2000) reported 56%, 46%, and 38% ofbiomass and 32%,

28%, and 19% of total N in roots at final sampling for alfalfa (Medicago sativa), cereal









rye, and hairy vetch plus rye intercrop. However, both of these studies occurred on fine

textured and/or high fertility soils.

Soil-based residue decomposition and N-release generally occur faster for residues

with lower C:N ratios and lignin and polyphenol contents, with optimum temperature and

water availability usually around 35 C and field capacity, respectively (Andren et al.

1992, Lomander et al. 1998, Vigil and Kissel 1995). Mathematically, these investigators

often characterize decomposition as a negative exponential decline in residue biomass or

C over time, with the rate affected by a "decay rate constant" that may depend on

temperature, water availability, N availability, and chemical quality of the residue.

Investigators may gain greater accuracy by statistically resolving decomposition into two

"pools" with faster and slower decay rate constants. For example, Somda et al. (date

unknown) used a litterbag study of a number of legumes and non-legumes; C:N and

lignin:N ratios were generally lower for legumes (8:1-27:1, and 2:1-9:1, respectively)

than for non-legumes (27:1-186:1, and 4:1-44:1, respectively) as were decay rate

constants of both fast and slow pools. Kuo and Sainju (1997) showed mixing hairy vetch

residue with increasingly large proportions of cereal rye and rye grass (Lolium

multiflorum) residues slowed the relative rate ofN release. Working in Georgia, Ranells

and Wagger (1996) also found faster decomposition and N-release for hairy vetch and

crimson clover grown alone than when grown with cereal rye, but still found no net N-

immobilization in any treatment (including rye alone).

For materials with low lignin:N, C:N may control decomposition, while lignin:N

ratio may become more important as it becomes higher. Decomposition of mixed

materials over time may therefore involve control by C:N initially, then lignin:N as









recalcitrant material makes up more of the remainder (Mueller et al. 1998). Palm and

Sanchez (1991) also found that polyphenol concentration may exert more control over

breakdown rates than lignin and N concentrations for residues high in polyphenols. In a

review of previously published data from 11 studies, Seneviratne (2000) found that: when

N availability is limiting (typically, plant residues with N concentrations less than 2%) a

positive linear relationship existed between N-release and N concentration (r2 = 0.63);

when N concentration was non-limiting (typically, residues with N concentration greater

than 2%) N concentration does not affect N-release; C:N ratio was a good predictor of N

release over a wide range of N contents; and polyphenol content better predicted N-

mineralization than lignin:N in low N residues in tropical environments.

Leaf C:N ratio and lignin content is generally much lower than stems or roots of the

same plant, and in most studies leaf decomposition and N release occurs significantly

faster than for other tissues. Prolonged periods of N-immobilization (when

decomposition results in a net accumulation of N) are often recorded for recalcitrant

stems and roots (Collins et al. 1990, Cobo et al. 2002). Cobo et al. (2002) characterized

decomposition, N, P, K, Ca, and Mg release, as well as C, lignin, polyphenol, and cell

wall contents of leaves and stems of about one dozen different tropical legumes. On

average leaves decomposed five times faster than stems, decomposition was closely

related to cell wall content, and N release most dependent on lignin:N ratio. Cobo et al.

(2002) found that decomposition and N-release was faster for stems mixed with leaves

than for stems alone, and slower for leaves mixed with stems than leaves alone. Both

Cobo et al. (2002) and Collins et al. (1990) showed the decomposition rate of different

tissue types decomposing together was faster than predicted by summing individual









decomposition rates. These studies suggest fungal decomposers may redistribute N from

leaves to more recalcitrant tissues during decomposition.

Puget and Drinkwater (2001) used 13C to compare the decomposition of root and

shoot derived C in hairy vetch residues on a silt loam in Pennsylvania. In their study,

shoot and root biomass for vetch was 3.71 and 0.89 Mt ha-1 respectively, lignin:N six

times higher in roots, and non-structural carbohydrates (eg, sugars and starches) was four

times higher in shoots. Twenty-two weeks after soil-incorporation, 13% of shoot C and

49% of root C remained in the soil, making the overall mass of shoot and root

contributions to soil organic C relatively equal at this time.

Soil incorporation of plant residues may speed decomposition and N release by

buffering temperature and water regimes relative to the surface. Hargrove et al. (date

unknown Schomberg et al. (1994), and Thonissen et al. (2000) showed more rapid

decomposition of soil incorporated residues vs surface residues in no-till systems.

Schomberg et al. (1994) also found greater N-immobilization potential for surface

sorghum (Sorghum bicolor) and wheat (Triticum aestivum) residue, although initial N-

immobilization was more rapid when the residues were buried. At peak immobilization

(5 months to 1 year or more), highly recalcitrant (sorghum and wheat) residues tied up

150-170% of their initial N content. For these low-N residues net N immobilization lasted

longer than one year on soil surface (study ended after 1 year) and only 1/3 year for

buried residues. Nitrogen immobilization ended and release began only when 45-55% of

the residue mass had decomposed. Bowen et al. (1993) found that 60-80% of N applied

within 10 legume GMs was released as inorganic-N within 120-150 days after soil

incorporation, while Thonnissen et al. (2000b) found similar levels of N-release to take









place faster (within 2-6 weeks) for soybean (Glycine max) and vetch. In Alabama,

however, Mansoer et al. (1997) found less than 50% of N remaining in surface and soil-

incorporated sunn hemp, respectively, at 16 weeks after plant death..

Most studies reviewed here found best correlation with two-pool exponential

models for decomposition and N-release (see Katterer et al. 1998 for a review). More

complex decomposition/N-release models exist that make use of residue quality, soil, and

weather data to predict decomposition, including the CERES and CENTURY models

used by DSSAT (Decision Support System for Agrotechnology Transfer, see Jones et al.

2003). These and similar models can often be adjusted to more accurately reflect actual

decomposition and N-release data obtained in field experiments (Bowen et al. 1993,

Hadas et al. 1993, Quemada et al. 1997).

As part of a larger study on improved use of GMs in vegetable cropping systems in

Florida, we investigated a GM sequence of sunn hemp (SH) followed by a winter legume

(L) of blue lupin (Lupinus angustifolius, winter 2001-02) and cahaba white vetch (Vicia

sativa, winter 2002-03) as an N-source for sweet corn (Zea mays var Rugosa). We

hypothesized that sunn hemp stem residues would immobilize a significant amount ofN

during winter decomposition, and that growth of winter legumes following sunn hemp

would be enhanced, reaching levels more typically seen in temperate environments. The

objectives of this particular study component were to generate detailed information about

GM biomass and N accumulation by tissue fraction during growth, and subsequent

decomposition and N-release by the summer GM over the winter.









Materials and Methods

Set-up and Design

This study consisted of nine of the 15 overall treatments related in Chapter 1 (Table

1.2). Only treatments with GM components in the rotation were investigated here: sunn

hemp followed by lupin (winter 2001-02) or vetch (winter 2002-03; treatment denoted as

SH+L), sunn hemp followed by fallow (SH), fallow followed by lupin or vetch (L). Only

methods relevant to GM growth, N-accumulation, decomposition and N-release are

presented in this chapter. Methods regarding GM effects on sweet corn growth analysis,

root dynamics, and yield, and effects on soil properties and plant pests are discussed in

relevant chapters.

Timeline of Operations

2001-02

On 7 August 2001, sunn hemp was planted following complete disking and

plowing of the field. Seed was inoculated with cowpea-type rhizobium and planted at 2-4

cm depth. In-row spacing was 3.12 cm (1.25 in), between-row spacing was 76 cm (30 in).

Sunn hemp emerged 11 August 2001 and grew until 31 October 2001 when it was killed

with an application of Gramoxone (Syngenta; Basel, Switzerland). Lupin was inoculated

with lupin-type rhizobium and planted on 19 November 2001 using a rip-strip planter and

with spacing identical to sunn hemp. Lupin emerged 22 November 2001 and grew until

12 April 2002. All plots were then mown and field treated with RoundUp (RoundUp;

Columbus, OH). Sweet corn (variety GS 0966, Syngenta) was planted 26 April 2002

using a rip-strip planter, with in-row spacing of 18 cm and between-row spacing of 75

cm.









2002-03

On 19 July 2002, inoculated sunn hemp was planted with a rip-strip planter at the

same spacing and depth as year 1, emerging 21 July 2002 and growing until 30 October

2002 when it was killed with Diuron/Touchstone. Cahaba white vetch was inoculated

with vetch-type rhizobium and planted 15 November 2002 with a zero-till grain-drill at a

rate of -35 kg ha-1 (30 lbs acre-'). Sweet corn (variety GS 0966, Syngenta) was directly

planted into vetch on 7 April 2003.

Measurements

2001-02

Sunn hemp was sampled from 8 of the 24 plots every two weeks after emergence

(WAE), and at final sampling all plots were sampled. Sunn hemp was also sampled at 4,

6, 10, 12, and 16 weeks after death (WAD). Decomposition was therefore quantified for

undisturbed material (not dried). Due to poor stand establishment, lupin was sampled

from 6 of 24 plots at 4, 8, 12, and 16 WAE, and at final sampling (20 WAE) all plots

were sampled. In each sampled plot, 61 cm (2 ft) of row length representative of the

entire plot and with uniform emergence was removed at each sampling, brought to the

UF Environmental Agronomy Lab (University of Florida, Gainesville, FL), refrigerated

before processing (no longer than one week). Entire plants were removed including roots.

In Gainesville, heights for all sampled plants were recorded, with plants subsequently

separated into leaves, stems, roots, and reproductive tissues (flowers and pods, where

existing). Roots were washed clean of soil and debris. Total sample leaf number and area

and leaf, stem, root, and reproductive (flowers/pods) fresh weights were taken for each

sample. Leaf area was determined with an LI-3000 (Li-cor; Lincoln, NE). Dry weights

were recorded for subsamples after oven-drying at 65 C for 72 hours. Afterwards, all









subsamples were ground in a Wiley mill to pass through a 2 mm screen, and a thoroughly

mixed 5 g portion of each grinding was subsequently stored. Grindings were then

subjected to a wet-acid Kjeldahl digestion, diluted, filtered, and analyzed for total

Kjeldahl N at the UF-IFAS Analytical Research Lab (University of Florida, Gainesville,

FL; EPA Method 351.2; Jones and Case 1991).

For each sample, shoot to root ratio of biomass (S:R-B) was calculated as the sum

of above ground dry matter divided by root dry matter (kg kg-1), and shoot to root ratio of

N (S:R-N) was calculated as shoot N content divided by root N content (kg kg-1). Specific

leaf area (SLA) was calculated as cm2 leaf g-1 leaf dry weight, and specific leaf N (SLN)

was calculated as tg N cm-2 leaf. Leaf area index (LAI) was determined by sample leaf

area divided by sampled area (sampled row length x between row space; m2 leaf m2

ground).

2002-03

Sunn hemp was sampled from all 24 plots at 2, 6, 10, and 14 WAE, and also from

treatments SH ON and SH 133N at 4, 8, and 12 WAE. Sunn hemp residue was sampled at

2, 4, 6, 8, 11, 14, and 18 weeks after death WAD. Vetch was sampled from all 24 plots

every 3 weeks after emergence. Row length sampled remained 61 cm. When plants

became large, all but 1-3 plants were clipped at ground level, weighed, and returned to

the plot. The subsample of 1-3 representative plants was excavated and taken to the

Environmental Agronomy Lab for measurement of the same growth parameters as

described for the previous year, with identical grinding and N analysis. Throughout both

years, continuous measurements of solar radiation, air temperature and relative humidity

at 1 m, rainfall/irrigation, and soil temperature at 12.5 cm were made using a Watchdog

datalogger (Spectrum Technologies; Plainfield, IL).









Analysis

Numerical trends and figures were developed using MS Excel. Using SAS

statistical software package (Statistical Analysis Systems; Cary, NC), a general linear

model was developed for final sampling data to assess the possibility of significant

differences due to GM combinations, sweet corn chemical N-fertilization rate, interaction

between GM type and N-rate, and replication effects for all measurements. Green manure

type, chemical N-rate, and the interaction of the two were insignificant at the a = 0.05

level of significance in either year. Results are therefore presented as averages of all

sampled treatments.

Results

Sunn Hemp 2001

Growth

As the initial crop of the experiment, all plots had identical histories (same previous

crops, same fertilization levels), and throughout the season no significant differences

existed for any factor of sunn hemp growth due to main or sub-effects. Figure 2. l(a)

illustrates the accumulation and subsequent decomposition of sunn hemp biomass by

tissue type for 2001. Figure 2.1(b) shows accumulation and subsequent loss of sunn hemp

N by tissue type. Sunn hemp produced a total of 8.00 + 0.40 Mt ha-1 and 76 4 kg N ha-1

by final sampling at 12 WAE. Of this, 6.95 0.37 Mt ha- (87%) and 72 4 kg N ha-1

(94%) was above ground. Maximum LAI of 3.59 0.25 occurring 10 weeks after

emergence (WAE) (Figure 2.2a). Average daily maximum temperatures remained around

36 C during August and September of 2001 and around 33 C during October of 2001.

In terms of biomass, leaves accounted for the largest tissue fraction in samplings

during the first 4 WAE (58% and 51% of total biomass at 2 and 4 WAE). By 6 WAE









stems became the largest single tissue fraction, accounting for over half of total dry

weight by 8 WAE. Stem and root dry weight increased throughout the entire growth

season reaching final values of 4.76 0.26 and 1.05 0.06 Mt ha-1 respectively; leaf

biomass increased for 10 weeks to a maximum value of 1.50 + 0.07 Mt ha-1. Flowers did

not appear until 8 WAE and increased to 0.76 0.05 Mt ha-1 by 12 WAE (Figure 2. la).

Leaves and flowers possessed relatively high N concentrations that changed little

throughout the season (20.1-21.8 g N kg-1 and 24.1-29.0 g N kg-1 respectively). Stems

and roots had much lower N concentrations that tended to decrease as a negative

exponential throughout the season (from 12.0 to 5.0 g N kg-1 and from 8.5 to 4.6 g N kg-1

respectively, with r2 = 0.93 and 0.97). Total N concentration showed an exponential

decay over time from 16.2 to 10.0 g N kg-1, reflecting increasing contribution from stems

and roots (Table 2.1).

Leaves formed the largest N pool of any tissue throughout the growing season,

reaching a maximum at 33 2 kg N ha-l at 8 WAE. Leaves and flowers (when flowers

existed) together accounted for most sunn hemp N throughout the season, beginning at

76% (2WAE) and decreasing down to 62% at final sampling (12 WAE). Flower N was

maximum at final sampling (18 1 kg N ha-1). In terms ofN content and as proportion of

total plant N content, stem N increased throughout the season reaching final values of 25

1 kg N ha-1 and 32%, respectively. Except at first sampling, roots typically formed the

smallest N pool (about 6-8% of total N), reaching a final N content of only 5 <1 kg N

ha-1 (Figure 2. Ib).

Shoot to root biomass ratio (S:R-B) increased linearly (r2 = 0.92) from 2.7 0.3 kg

kg- to 6.8 0.3 kg kg-1 over the 12 week growth season. Shoot to root N ratio (S:R-N)









also increased linearly (r2 = 0.95) from 6.2 0.6 kg kg-1 to 17.5 2.3 kg kg-1 (Table 2.2).

Both changes reflected the increasing amounts ofbiomass and N sent to shoots relative to

roots. The ratio of S:R-N to S:R-B remained almost unchanged throughout the season at

an average of 2.36 0.13 (Figure 2.3a), showing that partitioning of N and biomass to

roots and shoots remained consistent relative to each other. Specific leaf area (SLA)

varied between 209 and 297 cm2 g1 over the season and was somewhat described by a

polynomial behavior (r2 = 0.60), showing a maximum at 4-6 WAE. Because leafN

concentration changed very little over the season, specific leafN (SLN) was basically a

mirror image of SLA, varying from 74 to 97 [tg N cm-2 leaf, showing a minimum at 4-6

WAE and also being somewhat described by a polynomial (r2 = 0.50; Table 2.2).

Decomposition

Residue decomposition (loss of dry matter) and N-loss were greatest during the

first two weeks after death, after which reductions were much less rapid or even non-

detectable (Figure 2.1). Total plant decomposition and N-loss were 40% + 9% and 61% +

7%, respectively, at 2 weeks after death (2 WAD). Final residue dry weight at 16 WAD

was 52% + 14% of the original 8.00 Mt ha-1, but after the initial sharp drop at 2 WAD

there was no little change in dry weight. Final residue N content at 16 WAD was only

20% + 6% of the original 61 kg N ha-1, and although residue N-loss was also slow and

not always resolvable between sample dates final, N content was significantly lower than

N content at 2 WAD (Figure 2. lb).

Most rapid decomposition and N-loss occurred for leaves and flowers (which were

pooled together as they were too difficult to separate). Leaf and flower (combined) dry

weight and N content at 2 WAD were only 25% + 3% and 15% + 2%, respectively, of the

original amount before death. Rate of loss may have been inflated because two herbicide









applications were required to kill sunn hemp over a two-week period, during which

leaves generally died before stems and roots. However, no significant leaf and flower

material persisted by 12 WAD to sample. Roots also showed an initial flush of

decomposition, with only 50% + 8% and 25% + 8% of dry weight and N content,

respectively, remaining at 2 WAD (Figures 2.1 and 2.2). Root N concentration declined

from 3.7 0.2 g N kg-1 at death to 2.2 0.1 g N kg-1 at 2WAD and 2.0 + 0.3 g N kg-1 at 4

WAD (Table 2.3). Afterwards, roots showed little dry weight decomposition until final

sampling at 16 WAD when only 0.28 0.06 Mt ha-1 (26% + 6% of original) remained

(Figure 2.1). At the same time, roots showed a consistent trend towards N-

immobilization, with N concentration rebounding steadily to 3.0 g N kg-1 by final

sampling (16 WAD; Table 2.3) and root N content increasing back up to 40% + 11% of

the original at 12 WAD, though this trend amounted to immobilization of no more than 2

kg N ha-l at that time (with a total root N content of 2 < 1 kg N ha-l; Figure 2.2). By

final sampling at 16 WAD, root N content decreased to 1 + < 1 kg N ha-1 (28% + 6% of

the original; Figure 2.1b).

Stem decomposition and N-loss occurred more slowly than all other tissue types,

maintaining 77% + 14% and 89% + 19% of original dry weight and N content,

respectively, at 2 WAD. Stem dry weight remained stable after 2 WAD with little

changes, with 3.86 1.02 Mt ha-1 (81% + 22% of original) remaining at 16 WAD.

However, sample variability was quite high throughout the decomposition period (Figure

2.1). Average stem N content also showed little change after 2 WAD, though there was a

general decrease and by 12 WAD stem N content (14 3 kg N ha-1) was significantly

less than original. Stem N content at final sampling (11 + 3 kg N ha-1) was 56% + 15% of









original (Figure 2.1b). Stem N concentration after 2 WAD also declined steadily,

reaching 3.0 + 0.1 g N kg-1 at 16 WAD (see Table 2.3).

Sunn hemp 2002

Growth

Figure 2.4a illustrates the accumulation and subsequent decomposition of sunn

hemp biomass by tissue type for 2002, and Figure 2.4b shows accumulation and

subsequent loss of sunn hemp N by tissue type. Sunn hemp produced a total of 12.26 +

0.38 Mt ha-1 and 134 5 kg N ha- by final sampling at 14 WAE, exceeding 2001 final

production by 53% and 106% respectively (Figure 2.2b). Of 2002 production, 11.12 +

0.35 Mt ha-1 (91% of total) and 127 5 kg N ha-1 (95% of total) was above ground.

Maximum LAI of 6.07 0.28 occurred at 10 WAE (Figure 2.2a). Average daily

maximum temperatures remained around 36 C from planting until final sampling

throughout the 2002 season.

Although production was increased, dry matter partitioning among tissue fractions

was almost identical to 2001. Leaf and stem production in 2002 exceeded 2001 by 6

WAE. Leaves accounted for the largest tissue fraction in samplings during the first 4

WAE (59% and 53% of total biomass at 2 and 4 WAE). By 6 WAE stems became the

largest single tissue fraction, accounting for over half of total biomass by 8 WAE. Stem

and root biomass increased throughout the entire growth season reaching final values of

8.76 0.30 and 1.14 0.05 Mt ha-1 respectively; leaf biomass increased for 12 weeks to

a maximum value of 1.94 0.15 Mt ha-1, although changes in leaf biomass after 10 WAE

were not significant. Flowers did not appear until 10 WAE and increased to 0.61 0.05

Mt ha-l by 12 WAE (Figure 2.4a).









Total N concentration and content was much higher in 2002, perhaps reflecting

increased water availability (Figure 2.4b, Table 2.4). However, partitioning patterns

remained similar. Leaves and flowers again had relatively high N concentration

compared to stems and roots, although drops occurred in leafN concentration just before

flower appearance (10 WAE) and in flower N concentration at final sampling (probably

due to a contribution from pods, which were pooled with flowers). Stems and roots again

had lower N concentrations which tended to decrease as a negative exponential

throughout the season (from 15 to 6 g N kg-1 and from 23 to 4 g N kg-1, respectively, with

r2 = 0.90 and 0.94, respectively). Total N concentration also showed a negative

exponential trend decreasing over time from 30 to 12 g N kg-1 (Table 2.4).

Leaves again formed the largest N pool of any tissue throughout the growing

season, reaching a maximum at 63 3 kg N ha-1 at 10 WAE. Leaves and flowers (when

flowers existed) together accounted for most sunn hemp N throughout the season,

beginning at 75% (2 WAE) and decreasing down to 57% at final sampling (14 WAE).

Flower N-content was maximum at final sampling (15 1 kg N ha-1). In terms of N

content and as proportion of total plant N content, stem N increased throughout the

season reaching final values of 50 2 kg N ha-1 and 37%, respectively. Except at first

sampling, roots again formed the smallest N pool (about 4-9% of total N), reaching a

final N content of only 7 1 kg N ha-1 (Figure 2.4).

Biomass based shoot to root ratio (S:R-B) increased linearly (r2 = 0.99) from 3.2 +

0.2 kg kg-1 to 10.2 0.4 kg kg-1 over the 14 week season. Except at final harvest (when a

drop occurred), shoot to root N ratio S:R-N also increased linearly (r2 = 0.96) from 4.53 +

0.31 kg kg-1 to 36.0 1.9 kg kg-1 (Table 2.5). The ratio of S:R-N to S:R-B did not remain









constant as in 2001, but increased linearly (r2 = 0.89) except at final harvest when a drop

occurred. This ratio increased from 1.43 0.04 to 3.90 + 0.15 from 2 to 12 WAE (Figure

2.3a), showing that partitioning to shoot biomass did not keep pace with partitioning to

shoot N (relative to root N).

Specific leaf area and specific leaf N behaved similarly in 2002 as in 2001, but

were greater than in 2001 by an average 26% and 69% over the season, respectively.

Specific leaf area varied between 256 and 355 cm2 g1 over the season and was somewhat

described by a polynomial behavior (r2 = 0.62), showing a maximum at 6 WAE (Table

2.5). As in 2001, leaf N concentration changed very little over the season, and specific

leafN (SLN) was again a mirror image of SLA, varying from 105 to 135 [tg N cm2 leaf,

showing a minimum at 10 WAE and also being somewhat described by a polynomial (r2

= 0.72; Table 2.5).

Decomposition

Although residue decomposition and N-loss were again greatest during the first 2

WAD, residue decomposition proceeded more slowly and was less dramatic during this

time. Total plant decomposition and N-loss were 24% + 10% and 66% + 3%,

respectively, at 2 weeks after death (2 WAD; Figure 2.4). Final residue dry weight at 16

WAD was 6.91 + 1.10 Mt ha-1 (56% + 9% of the original 12.26 Mt ha-1), but there was

almost no significant change after 4 WAD (Figure 2.4a). Final residue N content at 16

WAD was only 21 kg N ha-1 (16% + 3% of the original 134 kg N ha-1), and, like

decomposition, overall residue N-loss was virtually complete after only 4 WAD (Figure

2.4b).

Most rapid decomposition and N-loss again occurred for leaves and flowers, but the

dynamics differed from 2001, with initial decomposition occurring more slowly (64% +









8% and 46% + 9% of original remaining at 2 and 4 WAD) and complete decomposition

occurring rapidly between 4 and 6 WAD (4-5 weeks earlier than in 2001; Figure 2.4a).

Nitrogen loss occurred more quickly than did decomposition, with only 28% + 3% and

15% + 3% of original leaf and flower N remaining at 2 and 4 WAD (Figure 2.4b).

Initial root N loss also proceeded more quickly than decomposition, with 34% +

3% and 86% + 12% of N content and dry weight, respectively, remaining at 2 WAD

(Figure 2.4). Root N concentration declined from 6.1 0.3 g N kg-1 at death to 3.3 0.2

g N kg-1 at 2WAD and 1.6 0.2 g N kg-1 at 6WAD (Table 2.6). After 6 WAD, roots

showed little dry weight decomposition through final sampling at 16 WAD (61% + 16%

remaining). As in 2001, roots eventually showed a consistent trend towards N-

immobilization, with N concentration rebounding at 8 WAD (3.3 0.3 g N kg-1) and

remaining at 2.2-2.5 g N kg-1 through final sampling (16 WAD). However, the increase

was generally not large enough to increase root N content in the face of decomposition

(Table 2.6).

Stem decomposition and N-loss again occurred more slowly than all other tissue

types, maintaining 82% + 12% and 54% + 7% of dry weight and N content, respectively,

at 2 WAD (Figure 2.4). Beginning at 6 WAD, stem N concentration remained relatively

unchanged at 0.25-0.30%, with only one sample date falling out of this range (Table 2.6).

Stem dry weight remained stable after 6 WAD, with 6.21 + 1.06 Mt ha-1 (71% + 12% of

original) remaining at 16 WAD, although sample variability was high throughout the

decomposition period (Figure 2.4a). Except for a single outlying date, stem N content

also remained relatively stable after 4 WAD, between 16 and 20 kg N ha-1 (Figure 2.4b).









Stems made up roughly 90% of total sunn hemp residue dry weight and N content after 8

WAD, with roots making up the remainder.

Lupin 2001-2002

Due to poor establishment, only treatments with lupin alone (L) were sampled until

the final sampling date (20 WAE) when all lupin plots (L and SH+L) were sampled. At

this sample date, a general linear model was developed to assess significance of previous

sunn hemp presence on growth factors. Duncan comparisons were made at the a = 0.05

level of significance, and effect of sunn hemp presence was non-significant. Lupin results

are therefore presented as average of all treatments.

Figure 2.5 illustrates the accumulation of lupin biomass and N, respectively, by

tissue type for 2001-02. Averaged over all treatments, lupin produced a total of 4.03

0.18 Mt ha-1 and 53 6 kg N ha- by final sampling at 20 WAE. Of this, 3.52 Mt ha- and

47 kg N ha- (87% and 90%) was above ground. Maximum LAI of 1.50 + 0.09 also

occurred at the final sampling at 20 WAE (Figure 2.2a).

Roots accounted for the largest tissue fraction at 4 WAE (44% of total), thereafter

leaves (40% and 47% of total dry matter at 8 and 12 WAE) followed by stems (46% and

56% of total dry matter at 16 and 21 WAE) became dominant (Figure 2.5a). Stem and

leaf dry matter increased throughout the entire growth season reaching final values of

2.25 0.12 and 1.21 0.03 Mt ha- respectively, although the increase in leaf dry matter

from 16 to 20 WAE was small. Root dry matter reached a maximum 0.60 + 0.13 Mt ha-

at 16 WAE. Pods did not appear until final sampling at 20 WAE when they accounted for

only 0.06 0.01 Mt ha-l. Large increases in biomass occurred between 8-12 and 12-16

WAE, with total dry weight more than quadrupling between each sampling and occurring









simultaneously with heavy root nodulation. However, biomass and N accumulation up to

12 WAE was relatively low (0.78 Mt ha-1 and 10 kg N ha-1; Figure 2.5).

Leaves possessed relatively high N concentration, increasing logistically from 15.2

to 22.6 g N kg-1 by 16 WAE (Table 2.7). And although lupin stems and roots generally

had lower N concentrations, stem N concentration increased linearly (r2 = 0.91) from 5.5

to 7.5 g N kg-1 over the season, while root N concentration increased exponentially from

3.8 g N kg-1 at 4 WAE to a peak of 17.1 g N kg-1 at 16 WAE, then dropping to 11.0 g N

kg-1 at 20 WAE. Total N concentration increased from 7.7 g N kg-1 (4 WAE) to 14.2 v

(16 WAE), with a small drop at final sampling (20 WAE; Table 2.7). These trends in N

concentration probably reflected the large increases in root nodulation seen around mid-

season, followed by a general die-off of root nodules by final sampling.

Leaves formed the largest N pool of any tissue throughout the growing season,

reaching a maximum at 27 1 kg N ha-1 by 16 WAE and representing 52-68% of total

plant N throughout the season (Figure 2.5b). Root N content as a fraction of total plant N

showed an initial decrease from 4 to 8 WAE (22% to 11%) followed by an increase from

8 to 16 WAE (11% to 22%) and a final drop back to 11% from 16-20 WAE (Table 2.9);

root biomass also decreased from 16 to 20 WAE (Figure 2.5b). Again, root trends were

probably related to increased root nodulation at mid-season, with late-season nodule die

off and pod production responsible for the drop in N concentration of other tissues. As

fraction of total plant N, stems showed the opposite trend (increase from 20% to 32%

from beginning to end of season, with a drop to 17% at 12 WAE). Maximum root and

stem N contents were 11 3 and 17 3 kg N ha-1 at 16 and 20 WAE respectively. Roots









generally formed the smallest N pool (about 11-22% of total N), although this was

greater than the relative N pool of sunn hemp roots (Figure 2.5b).

As in sunn hemp, S:R-B increased linearly (r2 = 0.93) from 1.6 to 7.2 kg kg-1 over

the 20 week growing season (Table 2.8). However, S:R-N saw periods of early and late

increase (4-8 WAE and 16-20WAE) around a period of linear decrease (8-16 WAE),

probably reflecting nodulation patterns. The overall range for lupin S:R-N was 3.7 to 10.8

kg kg-1. As a result, ratio of S:R-N to S:R-B shows a linear (r2 = 0.96) decrease for the

first 16 weeks, followed by an increase from 16-20WAE. This ratio showed a range of

0.8 to 2.8 (Figure 2.3b), which was similar to that seen for sunn hemp in 2002 but not

2001 (Figure 2.3a).

Specific leaf area (SLA) varied between 154 and 103 cm2 g1 over the season, the

pattern of change being well described by a negative polynomial function (r2 = 0.99) with

the lowest measurement made at 16 WAE. Specific leafN (SLN) increased linearly (r2 =

0.99) for the first 16 WAE, followed by a small drop at 20WAE (probably due to pod

formation), varying from 98 to 219 tg N cm-2 throughout the season (Table 2.8).

Vetch 2002-2003

Due to variable stand performance, results for vetch are presented for the best 10

plots (of 24 total) beginning at 12 WAE unless otherwise noted. These 10 plots showed

dry weight production at or above 1.00 Mt ha-1 by final sampling. Because these 10 plots

were evenly distributed across all GM and N-rate levels without apparent trend, and

because vetch growth was so variable, previous GM plantings and sweet corn chemical N

applications produced no significant differences by final sampling.

Figure 2.6 shows the accumulation of vetch dry weight and N content, respectively,

by tissue type for 2002-03. Averaged over the best 10 plots, vetch produced a total of









1.95 0.25 Mt ha-1 and 37 6 kg N ha-1 by final sampling at 18 WAE. Of this, 1.52 Mt

ha-1 and 34 kg N ha1 (78% and 93%) was above ground. Maximum LAI of 1.02 0.13

also occurred at the final sampling at 18 WAE (Figure 2.2a). Two plots with excellent

performance produced over 3.0 Mt ha-1 and 60 kg N ha-1 at final sampling. Roots

accounted for the largest tissue fraction for the first 9 WAE (41-52% of total), thereafter

leaves (44% at 12 WAE) followed by stems (45% and 43% at 15 and 18 WAE) became

the largest tissue fraction. Stem, leaf and root biomass increased throughout the entire

growth season reaching final values of 0.85 0.13, 0.68 + 0.11 and 0.43 0.05 Mt ha-1,

respectively at final sampling (Figure 2.6a). LeafN concentration remained low (22-27 g

N kg-1) until a linear (r2 = 0.97) increase began after 9 WAE, bringing leafN

concentration to 36 g N kg-1 at 18 WAE. Stem N concentration (11-17 g N kg-1) remained

relatively constant, while root N concentration decreased linearly (r2 = 0.96). Total N

concentration remained constant between 16-20 g N kg-1 (Table 2.9).

Except at first sampling, leaves formed the largest N pool of any tissue throughout

the growing season, reaching a maximum at 24 3 kg N ha-1 at 18 WAE and

representing 37-65% of total plant N throughout the season (Figure 2.6b). Stem N content

increased throughout the season reaching a maximum at final sampling of 12 2 kg N

ha-l. Stems accounted for an increasing fraction of total plant N over time (17% at 3

WAE to 34% at 18 WAE). Root N content as a fraction of total plant N decreased

throughout the season, but was marked by an initial period of relative importance for the

first 9 WAE (30-46% of total plant N) followed by a large drop (6-8% of total plant N) as

shoot growth increased. Maximum root N content reached only 3 1 kg N ha-l (18

WAE; Figure 2.6b).









Neither S:R-B nor S:R-N experienced much change during the first 6-9 weeks of

the season (values ranging between 0.9-1.4 kg kg-1 and 1.2-2.3 kg kg-1, respectively;

Table 2.10). However, from 6-9 WAE until final sampling both indices increased

logarithmically (r2 = 0.92 and 0.99, respectively) to 4.8 kg kg-1 (S:R-B) and 19.9 kg kg-1

(S:R-N). As a result, ratio of S:R-N to S:R-B showed a linear (r2 = 0.98) increase at

during this period, going from 1.1 to 4.1 by final sampling (Figure 2.3b).

Vetch SLA varied between 141 and 314 cm2 g- over the season, the pattern of

change being well described by a negative logarithmic function (r2 = 0.94) beginning at 9

WAE. Vetch SLA was generally higher than lupin SLA and comparable to that of sunn

hemp. Vetch SLN was highly variable, showing no overall trend and ranging from 87.4
-2
to 326 pg N cm-2 over the season (Table 2.10), likely related to variable performance.

Discussion

Sunn Hemp

Growth

Sunn hemp appeared quite well adapted to the sandy soils and hot summer

temperatures of north Florida with rapid nodulation and little damage from pests or

disease until the end of the second year. Biomass in both years (8.00 and 12.26 Mt ha-l;

Figues 2.1 and 2.5) and N accumulation in 2002 (134 kg ha-l; Figure 2.6) was higher than

that achieved by Mansoer et al. (1997; 5-6 Mt ha-1 and up to 120 kg N ha-1) in Alabama,

but similar to findings by Seneratne and Ratnasinghe (1995) and Steinmaier and Ngoliya

(2001) under tropical conditions. Nitrogen accumulation in 2001 (76 kg N ha-l; Figure

2.6) appears similar to that found by Jeranyama et al. (2000) under low precipitation

conditions. Sunn hemp also produced greater dry matter than that of other summer

legumes including cowpea evaluated in another on-going study at the same site









(including cowpea (Vigna unguiculata), hairy indigo (Indigofera hirsuta), and velvet

bean (Mucuna atropurpureum); Linares and Scholberg, unpublished), although its size

and stemmy nature (reaching 2.6 m in 2002) may make it inappropriate for some forms of

intercropping, agroforestry, or plastic mulch systems if sunn hemp is allowed to grow

past 4-8 weeks.

Increased biomass and N accumulation in 2002 compared to 2001 may have

resulted from both longer growing season as well as deeper root systems and improved

water availability following mechanical "ripping" of a plow-pan after 2001. Although

production remained excellent, water stress during the 2001 appeared to reduce sunn

hemp biomass, LAI (through reductions in both leaf dry matter and SLA) as well as N

concentration and SLN while decreasing both S:R-B and S:R-N (Tables 2.1, 2.2, 2.4, and

2.5). Sunn hemp dry weight in 2002 exceeded that in 2001 at similar sample dates by 6

WAE. Sunn hemp is reportedly capable of becoming extremely large (up to 20 Mt ha-1),

and it does appear that sunn hemp was able to take advantage of the extra two weeks

from earlier planting in 2002. Final sunn hemp dry weight and N accumulation was 53%

and 76% greater, respectively, in 2002 than in 2001 (Figures 2.1 and 2.4).

In both years, leaf material dominated early growth of sunn hemp (>50% of total

plant dry weight for the first 4 WAE; Figures 2. la and 2.4a) and stems retained relatively

high-N through 4-6 WAE (Tables 2.1 and 2.4). Leaves and flowers made up the largest N

pool throughout the growing season (57-76% of total plant N) and decomposed most

quickly after death (6-12 WAD; Figures 2. lb and 2.4b). However, large decreases in total

plant N concentration occurred after 6-8 WAE when stems began to dominate biomass

(up to 71% in 2002) and stem N concentration became relatively low. Root contributions









to biomass and especially total plant N were relatively low and decreased with growth -

sometimes to less than 10% and 3% of total biomass and N, respectively. Such root

contributions were lower than that found by Griffin et al. (2000) working with temperate

GMs in a cooler climate (Maine) with fine textured soil (silt loam). Relative root

contributions were higher (S:R-B and S:R-N were lower) during 2001, when water

availability was apparently diminished. Although root turnover or exudation was not

accounted for, the extremely consistent linear increases seen in S:R-B and S:R-N over the

season and the small root biomass and N-content appear to be in line with findings by

Thonnissen et al. (2000a) in a tropical environment. Because it occurred so consistently

in different years, this shoot-dominated growth behavior appears genetic, although

differences in water availability show capability of exerting some changes (Tables 2.2

and 2.5).

Differences between years also occurred in slope of the S:R-N to S:R-B ratio over

time, with slope being nearly zero in 2001 but distinctively positive in 2002 (Figure 2.3).

This suggests that water stress in 2001 also created a situation where shoot growth was

more N-limited. In 2001, increases in shoot N partitioning were associated with a

consistent biomass partitioning response to shoots. In 2002, with higher N concentration

and higher S:R-N, increases in shoot N partitioning were not "kept up with" by similar

increases in biomass partitioning to shoots, suggesting biomass partitioning to shoots was

decreasingly N-limited in 2002.

It therefore appears that both longer growth time and greater water availability

strongly increased N accumulation and dry weight accumulation, but that the relative

sizes of tissue pools were more strongly affected by growing time while water









availability may have exerted more control over tissue N concentrations and leaf

characteristics (SLA and SLN).

Decomposition

Previous studies on surface applied residue and mixed residues of different

recalcitrance show more rapid N-mineralization when overall plant N-concentration is

high (C:N < 20 or %N > 2%), with low-N residue (C:N > 30-40) exhibiting N-

immobilization and even immobilizing N from nearby, high-N sources (Kuo and Sainju

1997, Ranells and Wagger 1996, Schomberg et al. 1994, Collins et al. 1990, Mansoer et

al. 1997). However, in our study we found that leaves and flowers decomposed rapidly

while low-N stems showed net N-release at all times, rather than N-immobilization as

expected (Figures 2. lb and 2.4b). The spatial separation between stems (which remained

somewhat upright or raised above the soil surface during decomposition) and leaves and

flowers (which decomposed primarily on the ground) in our reduced tillage and reduced

mowing approach may have prevented movement of N from areas of high availability to

low availability. The initial N flush exhibited by stems may reflect decomposition of the

relatively succulent stem-tips which others (Marshall 2002) have shown to possess high

N-concentration. Lack of homogenization may have also prevented movement of N

between these stem fractions.

On the other hand, root dry weight stabilized and slight N-immobilization occurred

beginning at about 8 WAD in both years. The initial flush in root decomposition and N-

release was probably due to decomposition of finer roots and nodules. Net N-

immobilization probably occurred when only the more recalcitrant large roots remained,

and also because availability of N within the soil was likely much greater than on the

surface. However, the relatively small pool of root biomass could not immobilize more









than 2 kg N ha-1. Residue N losses from sunn hemp totaled 60% and 66% of initial N in

the first 2-4 WAD in 2001-02 and 2002-03, respectively. Final residue N-losses were 49

and 123 kg N ha-1 (80% and 84%) in 16 weeks of 2001-02 and in 18 weeks of 2002-03,

respectively (Figures 2.2 and 2.6). Dry weight decomposition losses (44-48% in 16-18

WAD) were much less than those of N, but also demonstrated most losses during the first

2-4 WAD. While leaves and flowers decomposed rapidly, stem dry weight loss at all

dates after 2-4 WAD remained within one standard error of each other, making

decomposition unresolvable. Root dry matter loss was almost as slow as that of stems

(Figures 2.la and 2.4a).

Mansoer et al. (1997) homogenized sunn hemp residue by mowing but experienced

similar levels of N-loss over the winter. Because it may help buffer water and

temperature in a decomposing litter layer, it is unclear if mowing as a means of

homogenization would lead to net N-immobilization in our environment. Had sunn hemp

had much greater root production, one could also speculate greater immobilization might

occur based on our results. Given the findings of numerous other investigators (for

example, Schomberg et al. 1994 and Thonnissen et al. 2000b), it appears that soil

incorporation would unacceptably intensify long-term N-loss of overwintering residue;

Mansoer et al. (1997) found much greater N-loss from soil-incorporating sunn hemp

residue in Alabama during the winter. Although the high N-losses from overwintering

sunn hemp residue were contrary to our management goals, our findings suggest this

reduced-tillage and reduced-mowing system may provide a "double" benefit if sunn

hemp (or another legume with similar growth habit) is followed immediately by one or

more economic crops. In this way, sunn hemp could provide quickly available N from









decomposing leaves and flowers; at a later date, the recalcitrant nature of left-over sunn

hemp stems (once mowed or pushed down to the ground) may immobilize surface

applied N, improving synchrony of N-release from chemical or animal manure sources

(see Chapter 3). On the other hand, sunn hemp accumulated more than half of its N

between 6 and 8 WAE in both study years, which was also around the time when leaves

and stems were equally dominant in terms of dry weight. If moderate N supply with less

residue is desired, our results suggest that killing sunn hemp back at this time would yield

30-70 kg N ha-1 and 3-7 Mt dry matter ha-1 (Figures 2.1 and 2.4) depending on growing

conditions.

Sunn hemp is a crop which may be easily killed without pesticide by using a roller,

which kills the plants by breaking their stems. Our experience and the experience of

others suggests sunn hemp may be planted into directly ("live mulched") prior to cold

weather. Tractor tires tend to "roll over" many of the rows and open up the canopy for a

new crop. At the onset of freezing temperatures, the rest of the sunn hemp will die. This

method of planting may make more efficient use of sunn hemp N by delaying much

decomposition until another crop is already established beneath the sunn hemp,

eliminating "gap" time that occurs between "wholesale" death of sunn hemp (from

herbicide or mowing) and subsequent planting and growth of another crop.

Lupin and Vetch

As cool-weather legumes, blue lupin and cahaba white vetch behaved quite

differently than sunn hemp. Their growth appeared controlled by time required for

effective nodulation to begin (when growth and N-accumulation increased) and

subsequent time until reproduction and rising temperatures (when total plant growth and

N-accumulation slowed and root nodules died off). These legumes may require longer









periods of cool temperature combined with longer daylength during fall and spring than

found in north Florida. Unseasonably warm weather (lupin, 2001-02) and poor adaptation

to sandy soil (vetch, 2002-03) may also have reduced performance of these legumes, but

as these conditions are typical in north Florida this may also point out general weaknesses

of cool-weather legumes in the region. Lupin and vetch still accumulated 4.03 and 1.95

Mt ha-1 dry weight and 53 and 39 kg N ha-1, respectively, similar to other results from

Florida (Gallaher 1991) and New Mexico (Guldan et al. 1996; see Figures 2.5 and 2.6).

However, results were highly variable and much lower than findings in temperate regions

with finer soils and longer growing times (Forbes et al. 1970, Abdul-Baki et al. 1996,

Cline and Silvemail 2001 and 2002, Puget and Drinkwater 2001, Ranells and Wagger

1996, Sainju and Singh 2001, Singogo et al. 1996). That neither lupin nor vetch were

significantly affected by presence of sunn hemp residue probably reflects the heavy initial

N-loss from sunn hemp, but also indicates that sunn hemp had no apparent allelopathic

effect on either crop as well.

In our study, linear growth phase of these species probably initiated far too late (8-9

WAE) for them to significantly reduce N losses from sunn hemp residue, and any N

benefit from sunn hemp residue early in the season became insignificant by final

samplings. Even in lupin, biomass and N production up to 12 WAE was relatively low

(0.78 Mt ha-1 and 10 kg N ha-1). However, although these species are not always as

productive as some summer legumes, they may still provide a significant source ofN. As

a monocrop, lupin appeared better adapted to our environment than vetch. Because its

performance was generally poor, cahaba white vetch was terminated at 18 weeks. Only

13 of 24 plots produced greater than 1 Mt ha-l, and at final sampling plants exhibited









severe root decay (likely caused by nematodes) and nutrient deficiency. However, some

plots of cahaba white vetch produced over 3.0 Mt ha-1 and 60 kg N ha-1, closer to

reported production of 3-9 Mt ha-1 elsewhere but still quite lower than potential N

accumulation (100-250 kg N ha-1; see Table 1.1). Performance of lupin and vetch were

quite variable. Nearby trials of other winter legumes including lupin, vetch, and clovers

indicate lupin may be the most productive as a monocrop (Linares and Scholberg,

unpublished), and the seemingly low growth of lupin and (the better plots of) vetch may

simply be near potential for winter legumes in north Florida. Results from continuation of

this study with a mixture of hairy vetch and cereal rye, and anecdotal evidence from 4-

way mixtures of vetch, rye, crimson clover and radish (Raphanus sativus) in the same

field suggest combinations of legumes, grasses, and/or non-leguminous dicots may

provide for more uniform and productive winter GMs in our area (Lavila and Scholberg,

unpublished).

Compared to sunn hemp, lupin and vetch roots accounted for a greater fraction of

total biomass in the first 4-9 weeks (41-52%), but thereafter the emphasis of growth on

leaves followed by stems was similar (Figures 2.1 and 2.4-2.6). Unlike sunn hemp, total

N concentration increased (lupin) or remained relatively constant (vetch) over the season

despite increases in stem tissue fraction, primarily because stem production was relatively

low (never more than 56% for lupin and 45% for vetch) and exhibited higher N

concentration than sunn hemp. Allowing these winter legumes to become more "stemmy"

did not lead to an apparent increase in their recalcitrance, although lupin and vetch

differed in their N concentration. Individual tissue and total N concentration of lupin was

relatively low and was comparable to that found in sunn hemp during the dry summer of









2001. Vetch leaf and whole plant N concentrations were higher than lupin and

comparable to sunn hemp in the rainier summer of 2002 (Tables 2.1, 2.4, 2.7 and 2.9).

Total plant and root biomass, N-concentration and N-content for vetch and lupin showed

exponential increases followed by a leveling off or decline near the end of the season,

apparently following patterns in nodule initiation and nodule death around the onset of

reproduction (Figures 2.6 and 2.7, Tables 2.7 and 2.9).

Because they die back at the onset of warm weather and/or reproduction, and

because they are not extremely large, cool-weather legumes in north Florida may be good

candidates for live mulch during spring. Mowing, strip tillage, strip herbicide, or tractor

traffic may be used to create openings for a spring crop planted into a cool-weather

legume. As mentioned earlier, this may reduce N losses from decomposition by reduction

of "gap time" between the two crops. In our experience, sweet corn strip tilled into vetch

suffered no adverse effects, and others (Phatak et al. 1999) have shown good results for

cotton no-till planted into live clover. Mixtures of leguminous GMs with non-legumes

capable of earlier growth and better "N-scavenging" may be desirable. Rye, oats, or

mustards may be well suited for such mixtures (for example, Abdul-Baki et al. 1996,

Cline & Silvernail 2001 and 2002, Griffin et al. 2000, Ranells and Wagger 1996,

Karpenstein-Machan & Stuelpnagel 2000). Our preliminary experience with mixtures of

multiple winter GMs shows great promise and should be investigated more, as

performance of any one GM (especially legumes) may be variable in north Florida

winters. These mixtures may also be more appropriate for systems requiring lower

growing, less stemmy, and/or less aggressive GMs than sunn hemp.









Conclusions

Results of all crops from both years highlight the dynamic nature of legume cover

crops. Patterns of biomass accumulation, N content and N concentration change over the

course of a season, but these patterns are quite different between cool and warm season

legumes, between different species growing at the same time of year and within the same

species growing in different years.


10 -A 0 Flower 80 -FBowe
0 Leaf I M c0loe
S8 Stem 0 0 Leaf
H Root 0 Stem
6 60
Fg u e Stem
d40 2 Root
44
c 20
S2 o
z 0
0 4 8 12 16 20 24 28 0 4 8 12 16 20 24 28
Weeks After Emergence Weeks After Emergence
Figure 2.1. Sunn hemp dry weight (A) and nitrogen content (B) during growth and
decomposition, 2001-02. Error bars reflect standard errors.











-D-SH 2001
-O-SH 2002
--- Lupin 2001-02
-4-Vetch 2002-03


CM
E


-j












0
I



'
Q


8 12 16 20
Weeks After Emergence


0 4 8 12 16 20 24
Weeks After Emergence
Figure 2.2. Leaf area index (A) and dry weight (B) of each GM during growth. Error bars
reflect standard errors.


5

4





1
-3
z
2


8 12


Weeks After Emergence


S---Lupin 2001-02
Vetch 2002-03







0 3 6 9 12 15 18 21
Weeks After Emergence


Figure 2.3. Ratio of S:R-N to S:R-B of sunn hemp (A) and lupin and vetch (B). Error bars
reflect standard errors.


0

0 4


14 B
12
10
8
6
4
2
0


0 4


----SH 2001 A-
-0-SH 2002


0 LV'


~pP"










-14
*12
10
- 8
6
S4
2 o
0


"150
120

90
1 60

0 30
o 0


0 4 8 12 16 20 24 28 32 0 4 8 12 16 20 24 28 32
Weeks After Emergence Weeks After Emergence

Figure 2.4. Sunn hemp dry weight (A) and nitrogen content (B) during growth and
decomposition, 2002-03. Error bars reflect standard errors.


5

4

-3


1

0


-60

z45

S30

C15
0
o
z 0


0 4 8 12 16 20 0 4 8 12 16 21
Weeks After Emergence Weeks After Emergence

Figure 2.5. Lupin dry weight accumulation (A) and N content (B) during growth, 2001-
02. Error bars reflect standard errors.


-3
"r,
2

e -
I1

a 0


- 45
e-r
z
S30


S15
0
0
o
z 0


0 3 6 9 12 15 18 0 3 6 9 12 15 18
Weeks After Emergence Weeks After Emergence

Figure 2.6. Vetch dry weight accumulation (A) and nitrogen content (B) during growth,
2002-03. Error bars reflect standard errors.









Table 2.1. Sunn hemp nitrogen concentration by tissue type, 2001.
WAE N Concentration
Leaf Stem Root Flower Total
gN kg1
2 21.1 + 0.7 12.0 + 0.6 8.5 + 0.6 16.2 + 0.6
4 21.8 0.5 8.2 0.4 7.2 0.4 14.9 0.4
6 21.3 + 0.5 6.2 + 0.3 6.1 + 0.3 12.1 + 0.3
8 21.8 + 0.4 5.3 + 0.2 4.9 0.2 27.3 0.4 10.3 0.2
10 21.7 + 0.3 5.0 + 0.2 4.7 0.2 24.1 + 0.5 10.0 + 0.1
12 20.1 + 0.4 5.2 + 0.2 4.6 0.2 29.0 + 4.3 10.1 + 0.5
WAE = weeks after emergence.

Table 2.2. Selected sunn hemp growth indicators, 2001.
WAE SLA SLN S:R-B S:R-N
cm2 g-1 [tg cm-2 kg kg-1 kg kg-1
2 222 8 96 4 2.7 0.33 6.1 0.6
4 297 8 74 2 3.8 0.24 9.0 + 0.7
6 287 6 75 3 5.0 + 0.20 10.9 0.6
8 235 6 93 3 4.5 0.19 11.0 + 0.8
10 238 7 92 3 6.3 0.49 14.6 1.1
12 209 5 97 3 6.8 0.33 17.5 2.3
WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B =
biomass-based shoot to root ratio; S:R-N = N-based shoot to root ratio.

Table 2.3. Sunn hemp nitrogen concentration by tissue type after death, 2001-02.
WAD N Concentration
Leaf Stem Root Total
g N kg-
0 16.1 0.3 4.2 0.1 3.7 0.1 7.7 0.1
2 10.8 0.2 4.7 0.3 2.2 0.2 3.5 0.2
4 9.8 1.0 3.6 0.2 2.0+ 0.3 3.8 0.1
8 9.0+ 0.5 3.2 0.1 2.4 0.3 3.5 0.1
12 3.2 0.1 2.9 0.3 3.2 < 0.1
16 3.0+0.1 3.0+<0.1 3.0+0.1
WAD = weeks after death.









Table 2.4. Sunn hemp nitrogen concentration by tissue type, 2002.
WAE N Concentration
Leaf Stem Root Flower Total
gN kg1
2 37.7 + 1.0 15.3 + 0.3 23.2 + 0.7 30.4 0.8
4 39.0 + 0.4 14.7 0.1 14.7 + 1.7 27.6 0.4
6 40.3 + 0.5 12.0 + < 0.1 10.1 + 0.4 22.4 0.2
8 34.3 0.6 9.0 + < 0.1 6.6 0.8 16.9 0.2
10 32.9 + 0.5 6.0 + < 0.1 5.4 + 0.4 40.2 + 0.9 13.1 + 0.2
12 31.8 0.6 6.0 +< 0.1 4.0 +0.6 42.7 4.8 12.5 0.5
14 31.3 0.6 5.7 < 0.1 6.1 0.3 21.1 < 0.1 11.7 0.7
WAE = weeks after emergence.

Table 2.5. Selected sunn hemp growth indicators, 2002.
WAE SLA SLN S:R-B S:R-N
cm2 g-1 [tg cm-2 kg kg-1 kg kg-1
2 281 8 135 3 3.2 0.2 4.5 0.3
4 331 8 118 3 5.0 + 0.2 12.3 0.3
6 355 4 114 2 6.1 0.2 15.6 1.0
8 288 7 121 7 7.3 0.3 22.9 3.5
10 318 7 105 3 8.5 0.3 25.8 2.9
12 284 7 113 4 9.2 1.0 36.0 6.8
14 256 7 124 4 10.2 0.4 21.0 1.9
WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B =
biomass-based shoot to root ratio; S:R-N = N-based shoot to root ratio.

Table 2.6. Sunn hemp nitrogen concentration by tissue type after death, 2002-03.
WAD N Concentration
Leaf Stem Root Total
g N kg-1
0 31.3 0.0.6 5.7 < 0.1 6.1 0.3 11.7 0.7
2 11.2 0.0.6 3.4 0.2 3.3 0.2 4.4 0.2
4 7.3 0.1.2 2.8 0.2 1.8 0.1 3.1 0.3
6 2.7 0.3 1.6 0.2 2.6 0.2
8 1.7 0.2 3.3 0.3 1.9 0.2
11 2.6 + 0.3 2.2 + 0.5 2.5 0.3
14 2.5 0.1 2.5 0.3 2.5 0.1
18 3.0 + 0.3 2.5 0.4 2.9 + 0.3
WAD = weeks after death.









Table 2.7. Lupin nitrogen concentration by tissue type, 2001-02
WAE N Concentration
Leaf Stem Root Pod Total
gN kg1
4 15.2 0.2 5.5 0.2 3.8 < 0.1 7.7 0.3
8 15.6 < 0.1 6.2 0.5 4.7 0.7 9.6 0.4
12 19.1 + < 0.1 6.8 + 0.4 10.4 + 1.4 13.1 + 0.6
16 22.5 + 0.4 6.7 + 0.2 17.1 + 0.8 14.2 0.4
20 22.6 0.7 7.5 0.4 11.0 + 0.7 35.0 13.0 + 0.4
WAE = weeks after emergence.


Table 2.8. Selected lupin growth indicators, 2001-02
WAE SLA SLN S:R-B
cm2 g-1 pg cm-2 kg kg-1
4 154 4 98 3 1.6 0.3
8 118 8 135 7 3.4 0.1
12 107+ 8.9 185 17 4.2 0.6
16 103 3 219 6 4.6 0.2
20 124 3 183 7 7.2 0.5
WAE = weeks after emergence; SLA = specific leaf area; SLN
biomass-based shoot to root ratio; S:R-N = N-based shoot to ro


S:R-N
kg kg-1
4.5 + 0.8
8.3 + 0.8
5.9 1.0
3.7 + 0.2
10.8 + 2.0
= specific leaf N; S:R-B
ot ratio.


Table 2.9. Vetch tissue nitrogen concentration, 2002-03.
WAE N Concentration
Leaf Stem Root Total
gN kg-1
3 27.3 + 1.3 16.6 + 0.5 17.8 + 0.5 20.2 + 0.7
6 22.0 1.0 11.3 0.3 15.5 0.5 16.5 0.6
9 21.8 + 0.9 13.0 + 0.5 12.9 + 0.8 16.1 + 0.4
12 27.5 2.5 15.0 1.1 7.8 0.5 18.8 1.8
15 29.7 + 2.4 13.3 + 1.0 5.8 + 0.3 17.0 + 1.5
18 35.6 + 1.5 14.5 + 0.6 6.4 + 0.8 20.3 1.3
WAE = weeks after emergence.

Table 2.10. Selected vetch growth indicators, 2002-03.
WAE SLA SLN S:R-B S:R-N
cm2 g-1 Lg cm-2 kg kg1 kg kg1
3 259 8 105 3 0.9 <0.1 1.2 0.1
6 252 5 87 4 1.3 0.1 1.4 0.1
9 314 30 326 29 1.4 0.1 2.3 0.3
12 210 5 130 10 3.5 + 0.5 10.0 + 1.5
15 141 + 14 250 + 45 4.8 + 1.0 16.9 + 3.7
18 141 15 239 10 7.2 1.6 19.9 3.7
WAE = weeks after emergence; SLA = specific leaf area; SLN = specific leaf N; S:R-B =
biomass-based shoot to root ratio; S:R-N = N-based shoot to root ratio.














CHAPTER 3
GROWTH, YIELD, AND N-UPTAKE EFFICIENCY RESPONSE OF CORN TO
AMENDMENT WITH GREEN MANURES

Introduction and Literature Review

Utilized as green manures (GMs), legumes may represent a substantial source of

on-farm nitrogen (N) for subsequent crops. In temperate environments on fine textured

soils, winter legumes such as vetch (Vicia spp.), clover (Trifolium spp.), and medics

(Medicago spp.) are capable of accumulating large amounts of biomass (7-10 Mt ha-1)

and N (150-250 kg N ha-1) and delivering substantial N benefit to subsequent crops. On a

silt loam soil in Maine, Griffin et al. (2000) found alfalfa (M. sativa) and winter rye

(Secale cereale) plus hairy vetch (Vicia villosa) combinations as GMs capable of

satisfying the N requirements of sweet corn (Zea mays var Rugosa) in two of three study

years. In Kentucky, sweet corn N requirements were fully met when vetch N was equal to

or greater than 166 kg N ha-1 (Cline and Silvernail 2002).

In tropical environments, warm weather legumes such as sunn hemp (Crotalaria

juncea), cowpea (Vigna unguiculata), and mungbean (V. radiata) may also accumulate

large amounts of biomass and N. Because no freezes occur in tropical environments,

these legumes may be followed immediately by frost-sensitive crops. For example,

studies in Asia have shown such GMs capable of supplying the N-requirements of rice

(Oryza sativa; Ladha et al. 2000, Agustin et al. 1999, Aulakh et al. 2000). However, few

GM studies have been conducted with high-N demanding spring crops under north

Florida conditions (sandy soils, sub-temperate climate). In this environment, the two









greatest challenges to leguminous GM approaches for spring cropping systems remain

accumulation of adequate N by GMs and delay ofN-release during the winter to better

match timing of spring crop uptake.

As discussed in more detail in Chapter 2, temperate legumes often do not perform

as well in north Florida, while the rotation from tropical legume to spring crop is

interrupted by feezing temperatures over the winter. Based on ear leaf N concentration,

Gallaher (1993) reported N-substitution values by blue lupin (Lupinus angustifolius),

hairy vetch and crimson clover (Trifolium incarnatum) of only 67 kg N ha-1 compared to

chemical N for a variety of different residue management systems. In another study near

Gainesville, Florida, Gallaher and Eylands (1985) reported N substitution value for blue

lupin near 56 kg N ha-1 based on sorghum (Sorghum bicolor) grain yields. In a low-input

system on a loamy sand in Zimbabwe, Jeranyama et al. (2000) found relatively low

fertilizer N equivalency for sunn hemp (36 kg N ha-1). Need exists to develop GM

management techniques for north Florida and like environments that deliver N benefits

similar to those achievable elsewhere.

The slow release of N from decomposing GM residues may be better timed with

plant uptake (Bath 2000, Wivstad 1997). Indeed, some researchers have found N-

substitution values for GMs in excess of their actual N accumulation, suggesting that GM

N is either used more efficiently than chemical fertilizer N, that GMs modify the soil

environment and/or crop growth such that greater crop N uptake is possible, or that GMs

also supply some other nutrient which is limiting crop growth (such as phosphorus). In a

low-land rice system, Agustin et al. (1999) found 58 kg N ha-1 from indigo (Indigofera

tinctoria) comparable to 120 kg N ha-1 from urea and speculated that all of the above









mentioned factors may have been involved. Also studying rice, Aulakh et al. (2000)

found application of 84 kg N ha-1 in the form of cowpea and sesbania (Sesbania rostrata)

equivalent to 104 kg N ha-1 applied as chemical-N. Green manures may provide other

benefits such as reduction of soil erosion, recycling of other crop nutrients, and control of

plant pests, pathogens and weeds with less reliance on off-farm chemical inputs (see

Chapters 5 and 6).

In a 3-year study using several different GMs on a silt loam in Canada,

N'Dayegamiye and Tran (2001) found yield benefits for wheat (Triticum aestivum) of 30-

90 kg N ha-1 and an increase in fertilizer N recovery with GM use. Recovery rates for

GM derived N in the same study ranged from 19-36%, which were lower than many of

the fertilizer N recovery rates (25-52%). Lower recovery rates for GM-derived N may

have been due to stabilization of N in organic forms rather than loss through leaching,

volatilization, and denitrification. In another study by N'Dayegamiye (1990), 15% of red

clover (T. repens) N applied to maize was taken up, while 19% and 28% were recovered

in microbial biomass and soil organic fractions respectively. Steinmaier and Ngoliya

(2001) evaluated the use of 11 GMs as N sources for maize on a sandy loam in Zambia.

Although a formal control was not used, comparison to low producing GMs suggests a N

benefit of around 50 kg N ha-1 or more from sunn hemp and velvet bean (Mucuna

pruriens). On a sandy clay loam in India, Sharma et al. (2000) reported N-replacement of

60 kg N ha-1 for rice when a mungbean GM was plowed in prior to planting. Studying

fertigated, mulched tomatoes (Lycopersicon esculentum), Abdul-Baki et al. (1996) found

that plastic-mulch with 112 kg N ha-1 (recommended rate) produced lower yields than

hairy vetch, crimson clover, and hairy vetch plus rye live mulches with only 56 kg N ha-1.









Synchrony between GM-N availability and subsequent crop N-demand remains

difficult to achieve. Many investigators have shown more rapid decomposition for plant

residues when soil incorporated (see Chapter 2). Rapidly growing crops immediately

following GMs may benefit from soil-incorporation of GM residues, especially in cool

climates and/or on fine-textured soils with high N-retention. For example, Shrestha et al.

(1998), finding no N-replacement benefit for winter canola (Brassica napus) in Michigan

even when a spring GM produced over 100 kg N ha-1, hypothesized that GM-N release

occurred as the canola crop went into dormancy. Griffin and Hesterman (1991) found that

legume GMs increased biomass, N uptake and N concentration of potato (Solanum

tuberosum) but did not benefit tuber yields. They concluded that N from GMs became

available too late to benefit tuber growth. However, in warm environments GM-N release

more often occurs so rapidly that peak availability takes place well before peak N

demand from a subsequent crop. Potential N-leaching losses under these circumstances

may eliminate advantages of GMs. In Georgia, Sainju and Singh (2001) showed greater

corn N-uptake and ear yield following hairy vetch for no-till compared to conventional

tillage, although the opposite trend occurred for corn following (highly recalcitrant)

winter wheat. In these cases reduced or zero-tillage may better synchronize leguminous

GM-N release with subsequent crop demand.

The N loss incurred by overwintering of decaying residues may negate any N

benefit to a subsequent spring crop. For example, on a loam soil in Saskatchewan, Brandt

(1999) saw no N benefit from production of less than 3 t ha-1 of black lentil (Lens

culinaris) on a subsequent crop of wheat. In Alabama, Mansoer et al. (1997) found close

to 2/3 N loss in mowed, overwintering sunn hemp. Reduced tillage as means of slowing









GM decomposition during winter months may increase GM-N availability during the

spring.

Modification of residue quality (especially C:N ratio or N concentration) may also

control timing of residue N-release. Some field grown grass-legume mixtures have shown

potential to increase GM C:N and total GM N content relative to legumes alone,

improving both the amount and synchrony of GM N-release (Ranells and Wagger 1996).

Nitrogen accumulation of such mixtures may be reduced, however, if legume seed rate is

too low (Karpenstein-Machan and Stuelpnagel 2000, Cline and Silvernail 2002). Based

on subsequent ear yields, small grain GMs do not appear capable of satisfying corn N

demand (Griffin et al. 2000, Karpenstein-Machan and Stuelpnagel 2000, Cline and

Silvernail 2002, Gallaher and Eylands 1985). Alternatively, overwintering residue with

low N concentration and/or high C:N ratio may also highly reduce N leaching losses

(Stopes et al. 1996, Wyland et al. 1996). Selecting leguminous GMs capable of

accumulating large fractions of stemmy, low-N biomass may create opportunity for both

high GM-N accumulation and improved N-retention. Green and Blackmer (1995) found

N-immobilization (followed by N-release) by soybean (Glycine max) residue helped

explained N benefits to subsequent corn.

Establishing a winter GM after the summer GM may significantly reduce N

leaching losses and enhance performance of the winter GM, but the effective N benefit to

a subsequent spring crop from such a double-GM approach has not been studied. In some

systems, it may therefore be advantageous to follow a vigorous and stemmy summer GM

with a well established winter GM, and to preserve as much recalcitrant litter as possible

by reducing tillage.









If GMs do not supply adequate N to meet requirements of subsequent crops, then

supplementary inorganic N may be required to prevent yield reductions. Many studies

have compared use of GMs alone against synthetic fertilizers (for example, see Carsky et

al. 2000), and others have also investigated GMs used in combination with synthetics,

(for example, see Ladha et al. 2000). However, these studies usually do not establish

optimums for chemical N rate whether used alone or with GMs, making it difficult to

assess how much (if any) chemical N is required "on top of' GMs for optimal production.

A number of studies (such as Prasad et al. 2002) do so for "cut and carry" systems where

GMs are not grown in place, but this does not reflect common agricultural practice in

developed countries.

As part of a larger study on improved use of GMs in vegetable cropping systems in

the southeast US, we investigated a GM sequence of sunn hemp followed by a winter

legume (blue lupin, winter 2001-02; cahaba white vetch, Vicia sativa, winter 2002-03) as

an N-source for sweet corn. Details of GM growth and decomposition patterns can be

found in Chapter 2. In these studies, sunn hemp followed by winter legume produced a

cumulative 12-15 Mt dry matter ha-1 and up to 170 kg N ha-1. We evaluated sweet corn

growth and leaf characteristics throughout the season for GM amended and unamended

corn supplemented with multiple chemical N-rates. We hypothesized the double-GM

approach would significantly reduce chemical N required by sweet corn to achieve ear

yields similar to an optimal level identified in the conventional approach, and that GMs

would increase N-uptake efficiency of sweet corn. Objectives of the study were to gain

greater understanding of the impacts of GMs on sweet corn growth throughout the season









and to estimate chemical N-supplementation needed to achieve acceptable sweet corn ear

yields.

Materials and Methods

Set-Up and Design

This study consisted of 14 of the 15 overall treatments related in Chapter 1 (Table

1.2). Treatments consisted of sweet corn following rotations of sunn hemp (summer) and

lupin (winter 2001-02) and vetch (winter 2002-03), denoted as SH+L; sunn hemp alone,

denoted as SH; winter legume (lupin 2001-02, vetch 2002-03) alone, denoted as L; and

unamended corn denoted as Cony (for conventional). Each GM level received

supplementation with 0, 67, or 133 kg inorganic N ha-1 (ON, 67N, and 133N). Other

unamended (Conv) treatments also received 200 or 267 kg inorganic N ha-1 (Cony 200N

and Cony 267N). Only methods relevant to corn growth and N-accumulation analysis are

considered here. Methods regarding GM growth and accumulation, root dynamics, and

effects on soil properties and plant pests are discussed in relevant chapters. Please see

Chapter 1 for overview.

Timeline of Operations

2001-02

On 7 August 2001, sunn hemp was planted following complete disking and

plowing of the field. Seed was inoculated with cowpea-type rhizobium and planted at 2-4

cm depth. In-row spacing was 3.12 cm (1.25 in), between-row spacing was 76 cm (30 in).

Sunn hemp emerged 11 August 2001 and grew until 31 October 2001 when it was killed

with an application of Gramoxone (Syngenta; Basel, Switzerland). Lupin was inoculated

with lupin-type rhizobium and planted on 19 November 2001 using a rip-strip planter and

with spacing identical to sunn hemp. Lupin emerged 22 November 2001 and grew until









12 April 2002. All plots were then mowed and field treated with Round-Up (RoundUp;

Columbus, OH). Sweet corn (variety GS 0966 Syngenta) was planted 26 April 2002

using a rip-strip planter, with in-row spacing of 18 cm and between-row spacing of 76

cm. Corn emerged 1 May 2002. For each treatment, chemical N was applied as NH4NO3

in three equal applications: at emergence and 3 and 5 weeks after emergence (WAE).

2002-03

On 19 July 2002, inoculated sunn hemp was planted with a rip-strip planter at the

same spacing and depth as in 2001, emerging 21 July 2002 and growing until 30 October

2002 when it was killed with Diuron/Touchstone. Cahaba white vetch was inoculated

with vetch-type rhizobium and planted 15 November 2002 with a zero-till grain-drill at a

rate of roughly 40 kg ha-1 (35 lbs acre-'). Sweet corn (variety GS 0966) was directly

planted into vetch on 7 April 2003 with the same planter, spacing, and depth as 2002.

Corn emerged 15 April 2003. For each treatment, chemical N was again applied as

NH4N03 in equal applications at emergence and 3 and 5 weeks after emergence (WAE).

Procedures and Measurements

At emergence a plant count was made to determine an average plant population.

In both years, sweet corn biomass was sampled five times (2, 4, 6, 8, and 9 WAE) and

ears were harvested at maturity (9 WAE). The final biomass samplings were taken the

day before harvesting ears. Ear harvest was conducted in an inner area of the plot kept

free from destructive biomass and soil sampling. This inner area was roughly 4.6 m (15

feet) by 4.6 m, allowing harvest of the central 4.6 m of row length from each of the six

inner rows of corn (out of a total of 10 rows in each plot). Representative subsamples of

ears from harvest were graded using USDA standards (United States Department of

Agriculture 1997).









Biomass sampling was conducted outside this inner area but away from plot edges

using three feet of row length representative of the entire plot in plant number, size,

spacing, and appearance. Within each sample, one representative subsample plant was

dug from the ground with a shovel to include roots (except at 2 WAE when all plants

were dug out). All other plants were clipped at ground level. Clipped plants were

weighed and counted. Roots were cut from the subsample plant and stored separately,

and the subsample plant top was then weighed and refrigerated until further processing in

Gainesville. The clipped plants were returned to the plots when possible. Relative

humidity, air temperature at 1 m, soil temperature at 5 cm depth, and precipitation were

recorded continuously with a Watchdog datalogger (Spectrum Technologies; Plainfield,

IL).

At the UF Environmental Agronomy Lab (University of Florida, Gainesville, FL),

height and total plant leaf numbers were taken for each subsample plant. Chlorophyll

meter readings (CMR), taken with a Minolta SPAD-502 (Spectrum Technologies;

Plainfield, IL), were made on the two most recently matured leaves (before tasseling) or

the third and fourth leaves from the top (after tasseling). Plants were then separated into

tissue type: leaves, stem, dead leaves, and ears (where applicable). Roots were washed

clean of soil and debris and fresh weights were taken for all tissues. Leaf area was

determined for each subsample plant using an LI-3100 (Li-cor; Lincoln, NE). All tissues

were then bagged and dried for 72 hours at 65 C and then reweighed. Afterwards, all

tissues were ground in a Wiley mill to pass through a 2 mm screen, and a thoroughly

mixed 5 g portion of each grinding was subsequently stored. Grindings were then

subjected to a wet-acid Kjeldahl digestion, diluted and filtered. The diluted samples were









then analyzed for total Kjeldahl N (TKN) at the UF Analytical Research Laboratory

(University of Florida, Gainesville, FL; EPA Method 351.2; Jones and Case 1991).

Nitrogen applied to corn (NAC) for each plot was calculated as: NACx =

Chemical-Nx + Residue-Nx; where Chemical-Nx = N applied as NH4N03 to corn in plot

"x" and Residue-Nx = TKN present in any winter GM, winter weeds, and sunn hemp

residue in plot "x" at the final sampling prior to corn planting. Nitrogen-uptake efficiency

(NUE) was calculated as: NUEx = (Total N Contentx Total N Contentconv ON) / NACx;

where Total N Contentx = TKN present in total corn biomass in plot "x" and Total N

Contentcon ON = average TKN present in total corn biomass of Conv ON treatment.

Unaccounted applied N (UAN) was calculated as: UANx = NACx Total N Contentx.

Analysis of Data

A balanced analysis of variance (ANOVA) was conducted to assess the effect of

GM application on corn growth, yield, and N-uptake efficiency responses, as well as the

effect of chemical N-rate and possible interactions of chemical N-rate with GMs. For all

measured values, this ANOVA was conducted on SAS software (Statistical Analysis

Systems; Cary, NC) using data from all treatments receiving N rates of 0, 67 or 133 kg N

ha-1 (4 GM levels x 3 N-rates = 12 treatments; a = 0.05). Measured values were regressed

with a linear model (PROC GLM) based on GM level, N-rate, GM x N-rate interaction,

and block. A randomization term for block was included. Significance of main effects

and the interaction term (GM x N-rate) are shown. Where interaction of the two main

effects is non-significant, Duncan multiple range test (a = 0.05) of pooled averages are

shown for main effects. Interaction between GM level and N-rate was never significant.

Pairwise contrasts were conducted to assess the parity of GM treatments

supplemented with 1/3 (67N) or 2/3 (133N) the recommended N-rate for sweet corn with









conventional treatments receiving 3/3 (Conv 200N) or 4/3 (Conv 267N) of the

recommended N-rate (6 GM treatments compared with 2 Conv treatments = 12

contrasts). Using SAS software, contrasts were made with an ANOVA based on a linear

model (PROC GLM) of treatment and block. All 14 treatments were included in the

ANOVA. Possible error from the high number of contrasts was mitigated as much as

possible by evaluating relevance of contrast results within the context of the overall

statistical, numerical, and graphical trends.

Results

N Applied to Corn

In 2002, average amounts of N applied to corn (NAC) derived from GM residues

of SH+L (56 kg N ha-1) and L (57 kg N ha-1) were statistically similar to each other and

greater than SH (11 kg N ha-1; see Appendix C, Table C. 1). In 2003, SH+L (51 kg N ha-)

applied significantly more N to corn than both SH (30 kg N ha-)) and L (21 kg N ha-l; see

Appendix C, Table C.14). As a result, NAC in both years was numerically greater for

Cony 200N and Cony 267N compared to any GM with 67N or 133N, with differences

significant everywhere except SH+L 133N similar to Conv 200N in 2002 (Tables 3.1 and

3.2).

Ear Yields, 2002

For all treatments, marketable ear yields (fresh weight) for 2002 are shown in

Figure 3.1(A). Amendment with GM increased end-season marketable, fancy, and total

ear yields by 30-45%, 46-68%, and 15-24% respectively, with no differences between

GM types (Table 3.3). Yields for grades No. 1 and No.2 as well as non-marketable ears

were not affected by GM application (data not shown). Reduction in N-rate from 133N

resulted in much lower yields for marketable ears (5% and 60% for ON and 67N,









respectively, compared to 133N), fancy ears (2% and 50% for ON and 67N, respectively),

and total ears (17% and 73% for ON and 67N, respectively). Interactions between GM

and N-rates were non-significant in all cases (Table 3.3). For all residue levels, fraction

of ear yield as fancy and marketable increased as chemical N-rate went up.

Optimal marketable ear yields were achieved with Conv 200N (within 4% of

maximum yielding Conv 267N). Amendment with SH+L 133N produced statistically

similar marketable and fancy ear yield to amendment with Conv 200N or 267N, and

similar total ear yields to 200N, though yields with SH+L 133N were numerically less in

all cases. Corn with SH 133N also produced similar fancy ear yields to Conv 200N.

Otherwise, marketable, fancy, and total ear yields were significantly greater with Conv

200N or 267N than with any GM plus 67N or 133N (Table 3.1).

Ear Yields, 2003

For all treatments, fresh weight of marketable ears for 2003 are shown in Figure

3. 1B. Due to earlier planting date and nearly 50% higher plant population, overall ear

yields compared to 2002 increased for treatments receiving 133N or more and decreased

for treatments receiving ON or 67N. Unlike 2002, optimal ear yield for the conventional

treatment was not reached at Cony 200N (Cony 267N was greater than Cony 200N by

15%).

Only amendment with SH+L significantly increased end-season marketable, fancy,

and total ear yields relative to Conv (Table 3.3). Yields for grades No. 1 and No.2 as well

as non-marketable ears were unaffected (data not shown). Increase in N-rate again

resulted in much higher yields for marketable, fancy, and total ears. Interaction between

GM and N-rates was non-significant in all cases (Table 3.3). Pair-wise contrasts showed

that SH+L 133N produced similar marketable, fancy, and total ear yield to Conv 200N,









but lower yields in all cases when compared to Conv 267N. Otherwise, marketable,

fancy, and total ear yields were greater with Conv 200N or 267N than with any GM plus

67N or 133N (Table 3.2).

In 2003 compared to 2002, fraction of ear yield as USDA fancy grade decreased

while fraction as USDA No. 1 and No.2 increased for all treatments (except Conv ON

which had no fancy ears in either year). Decrease in fraction as fancy was particularly

high for nearly all GM treatments (12.3-28.7 percentage points lower in 2003 than in

2002), but less for all Conv treatments as well as SH+L ON and SH ON (2.1-9.7

percentage points lower in 2003 than in 2002). For highest yielding treatments (Conv

267N, Conv 200N, SH+L 133N, SH 133N, and L 133N) fraction of ear yield as

marketable ears decreased slightly, from 0.6-7.6% percentage points lower in 2003 than

in 2002, depending on the treatment (Tables 3.1 and 3.2).

Growth Analysis, 2002

Leaf indicators

For nearly all treatments, leaf area index (LAI), chlorophyll meter readings

(CMR), and specific leafN (SLN) showed linear or logarithmic increases up through the

time of ear appearance (6 WAE) or the following sample date (8 WAE; see Tables 3.4

and 3.5 and Appendix C, Table C.2). Green manures affected these indicators weakly

with statistical differences appearing primarily at the beginning (2-4 WAE) or end (9

WAE) of the growing season. However, LAI, CMR, and SLN for GM-amended corn

usually showed numerical advantage compared to Conv throughout the entire season.

Advantages were strongest for SH+L and SH and most pronounced in LAI, often ranging

from 30-45% for all the leaf indicators (except SLA) during the first 2-4 WAE and up to

16% afterwards. Specific leaf area showed no consistent response to GMs. Chemical N-









rate strongly affected LAI, CMR, and SLN at almost every sample date, always

producing increases from ON to 67N (significant at all dates) and from 67N to 133N

(significant in about half of sample dates) with most pronounced benefits in the middle 4

weeks of the season (4-8 WAE). Greatest response to increasing N-rate occurred for LAI

(46-126% and 70-145% increases for 67N and 133N, respectively, compared to ON), with

response from CMR and SLN on the order of 15-50% (67N compared to ON) and 25-

60% (133N compared to ON; Tables 3.4, 3.5 and Appendix C, Table C.2). Specific leaf

area (SLA) decreased in response to chemical N, but effects were less strongly significant

than for other leaf indicators.

In terms of LAI, SH+L 133N and SH 133N showed numerical advantage over

Conv 200N and Conv 267N at 2 and 4 WAE (Table 3.5). Otherwise, Conv 200N and

Conv 267N generally showed numerical advantage over all GMs with 67N and 133N for

LAI, SLN and CMR, though these differences did not become significant until 8-9 WAE.

In terms of LAI, CMR, and SLN, Conv 267N showed more frequent and more significant

advantages over GM treatments than Conv 200N, and SH+L 133N remained the only

GM treatment statistically similar to Conv 267N and Conv 200N throughout the season

(Table 3.5 and Appendix C, Table C.8).

Tissue characteristics

Dry weights and N contents for leaf, stem, and total plant increased logarithmically

or exponentially during the first 6 WAE (time of ear appearance; Tables 3.6-3.9 and

Appendix C, Tables C.3-C.5). During this time, benefit from GM application generally

ranged from 5-45%. Consistently significant benefit from GMs occurred for dry weights

and N-contents of leaf and stem tissue and (to a lesser extent) for the total plant. At or

after ear appearance, GM benefit for dry weight and N content of vegetative factors









became somewhat reduced (less than 20%), but more pronounced for ears themselves

(15-30%). Compared to leaf, stem, and ear dry weights, GM benefits were somewhat

lower and less consistently significant for root dry weight as well as tissue N contents

(generally, 5-30%) and had little effect on tissue N concentrations (see Tables 3.6, 3.8

and Appendix C, Tables C.6-C.7 and C.27). Advantages were typically greatest for SH+L

and SH, although SH+L generally showed greatest tissue dry weights by late season and

greatest N content throughout the season.

In terms of all tissue N contents and dry weights, SH+L 133N and SH 133N

showed numerical advantage over Conv 200N and Conv 267N during the first 2-4 WAE.

Otherwise, Conv 200N and Conv 267N maintained greater tissue dry weights and N

contents than all GM treatments, though differences did not become significant until late

season (6-8 WAE and 8-9 WAE relative to GMs with 67N and GMs with 133N,

respectively). Compared to other GM treatments, late-season differences against Conv

200N and Conv 267N treatments were generally less dramatic for SH+L 133N (see

Tables 3.10, 3.11 and Appendix C, Tables C.9-C.11). In regards to tissue N

concentration, pairwise contrasts showed consistent statistical advantage for Conv 200N

against GMs with 67N throughout the season, but not until the end of the season when

compared to GMs with 133N. As with other growth factors, tissue N concentration for

SH+L 133N remained closer to Conv 200N and Conv 267N than any other contrasted

GM treatment (see Appendix C, Tables C.12-C.13 and C.28).

Chemical N-rate strongly affected all tissue characteristics (dry weights, N

contents, and N concentrations) on all sample dates, with tissue N contents showing

strongest response. Application of ON and 67N (compared to 133N) reduced vegetative









tissue N content by roughly 60-80% and 25-40%, respectively, while reductions for

vegetative dry weights were about 40-70% and 10-20%, respectively, with greatest

differences occurring just before or at ear appearance (4-6 WAE; Tables 3.6-3.9 and

Appendix C, Tables C.3-C.5). At final biomass sampling, application of ON and 67N

resulted in ear N content of 14% and 62%, respectively and ear dry weight by 15% and

72%, respectively, compared to 133N (Appendix C, Table C.5). Vegetative tissue N

concentrations showed similar patterns to dry weight and N content before ear

appearance, though reductions due to lower chemical N-rate were generally less (not

more than 50%). After ear appearance, root, stem and ear N concentrations typically

remained lowest for corn with 67N even compared to ON primarily due to "dilution

effect" (biomass increases outpaced increases in N accumulation) with stronger N

remobilization to ears from vegetative tissues possibly playing a role as well (Appendix

C, Tables C.6-C.7 and C.27).

Growth Analysis, 2003

Leaf indicators

Leaf indicators showed similar behavior in 2003 compared to 2002, although GM

effects were weaker. In terms of LAI, CMR, and SLN, benefit from GM amendment

typically remained within 20-30%, with significant differences less consistent than in

2002. However, as in 2002 greatest GM benefits occurred for SH+L and (to a lesser

extent) SH (Table 3.12 and Appendix C, Table C. 15). Neither GM nor chemical N-rate

significantly affected SLA. Chemical N-rate again strongly affected LAI, CMR and SLN

throughout the season with significant increases from 0 to 67N and from 67N to 133N at

all sample dates, and smallest relative benefits at 2 WAE. As in 2002, LAI response to

increased chemical N-rate (26-107% and 44-134% for 67N and 133N, respectively,









compared to ON) was greater than for CMR and SLN (generally, 30-70% and 50-100%

for 67N and 133N, respectively, compared to ON; see Table 3.12 and Appendix C, Table

C. 15). Relative to 2002, LAI values were greater by 30-60% and SLN values lower by

30-45% within treatments at similar samples date in 2003. Values for SLA and CMR

changed little from 2002, especially for treatments with 133N or more.

Both SH+L 133N and SH 133N maintained similar LAI compared to Conv 200N

and Conv 267N, although LAI for SH 133N dropped in comparison at final sampling

(Table 3.13). Values of CMR for SH+L 133N and SH 133N were statistically similar,

though numerically less, than those of Conv 200N and Conv 267N. Contrasted GM

treatments did not demonstrate consistent early-season numerical advantage against Conv

200N and Conv 267N in terms of CMR and SLN (see Appendix C, Table C.21).

Tissue characteristics

Amendment with SH+L and SH consistently increased tissue dry weights and N

contents by 10-45% throughout the season (Tables 3.14-3.17; see also Appendix C,

Tables C.16-C.18). Like 2002, relative advantages from GMs generally peaked at or just

prior to ear appearance (4-6 WAE) and thereafter declined. However, more dramatic

declines in some characteristics for Cony at final sampling (9 WAE) created apparent

benefits for GMs similar to those seen at 4-6 WAE. Relative advantages within each GM

level remained qualitatively similar across all tissue dry weights and N contents.

Advantages were again stronger for SH+L and SH compared to L, and tissue N

concentration again showed almost no effect from GM amendment (see Tables 3.14-3.17

and Appendix C, Tables C.16-C.18).

Changes in chemical N-rate in the ON to 133N range also produced effects

qualitatively similar to 2002. Chemical N-rate strongly affected most tissue









characteristics at all sample dates, with tissue N contents showing strongest response.

Application of ON and 67N (compared to 133N) lowered vegetative tissue N content by

roughly 50-80% and 15-40%, respectively, while reductions of vegetative dry weights

were about 40-70% and 10-15%, respectively, with greatest differences again occurring

just before or at ear appearance (4-6 WAE; for examples, see Tables 3.14-3.17 and

Appendix C, Tables C.16-C.18). At final biomass sampling, application of ON and 67N

decreased ear N content to 8% and 42%, respectively, and ear dry weight to 9% and 48%,

respectively, compared to 133N (Appendix C, Table C.18). Vegetative tissue N

concentrations showed similar patterns to dry weight and N content before ear

appearance, though decreases due to lower chemical N-rate were generally less (as in

2002, not more than 50%). After ear appearance, root, stem and ear N concentrations

again remained lowest for corn with 67N (see Appendix C, Tables C.19-C.20).

Tissue and total dry weights generally remained numerically superior throughout

the season for SH+L 133N compared to Conv 200N and Conv 267N, although

differences were often non-significant. Tissue and total N contents for SH+L 133N also

remained similar to Conv 200N and Conv 267N throughout most of the season.

Additionally, tissue dry weights and N contents of SH 133N were rarely less than Conv

200N and Conv 267N until a decline at final sampling (Tables 3.18 and 3.19 and

Appendix C, Tables C.22-C.24). Tissue N concentrations for GMs with 133N again

stayed lower than those of Conv 200N and Conv 267N, becoming significantly lower at

or after 6 WAE but remaining closest for SH+L 133N. Stem dry weights for all GMs

with 67N were often numerically (sometimes significantly) greater than for Conv 200N

and Conv 267N (Appendix C, Table C.22), likely as a result of much lower ear









production by GMs with 67N. Root, leaf, and total dry weights for GMs with 67N also

remained statistically similar (though numerically less) to Conv 200N and Conv 267N

until final harvest. Statistical advantage of Conv 200N and Conv 267N over GMs with

67N were detected for the first 6 WAE for stem N content and N concentration and

afterward for leaf and ear N contents and N concentrations (Tables 3.18-3.19, Appendix

C, Tables C.24, C.26 and C.30).

Values for tissue dry weights of all treatments in 2003 (compared to values from

similar sample dates in 2002) increased 20-80%, while values for total N content in 2003

fell by 40-50% for most treatments beginning at 4 WAE. Values from TKN may have

been reduced by a percentage consistent for all samples, which are being resubmitted for

analysis. New N data will likely be proportional to that listed here, which will therefore

not change statistical trends for tissue N contents and concentrations. Also, unlike 2002,

when little or no net N uptake occurred after 6 WAE (Table 3.11), tissue samples in 2003

revealed 40-45% of total plant N uptake occurred between 6 WAE and final harvest (9

WAE) for highest yielding treatments (Conv 267N, Conv 200N, and SH+L 133N; Table

3.19).

Nitrogen Uptake Efficiency and Unaccounted Applied Nitrogen

With few exceptions, N- uptake efficiency (NUE) was not significantly affected

by GMs or chemical N-rate in either year, nor did any statistical differences for NUE

exist between Conv 200N and Conv 267N compared to GMs with 67N or 133N (Tables

3.1-3.2 and Appendix C, Tables C.1, C.14). In 2002 and 2003 NUE showed a decreasing

trend as chemical N-rate increased beyond 67N, with decreases also occurring from Conv

133N to Conv 200N and from Conv 200N to Conv 267N. Nevertheless, none of these

trends were significant.









Corn amended with SH+L 133N and L 133N showed unaccounted applied N

(UAN; defined as N applied at or after corn planting not accounted for in corn tissues)

similar to Conv 200N in both years. Otherwise, UAN was significantly greater for Conv

200N and Conv 267N compared to all GMs with 67N or 133N. However, when one

includes the N accumulated by SH and weeds during the fall of each year but lost before

corn planting, UAN from SH+L 133N and SH 133N become similar to or greater than

Conv 200N and Conv 267N while L 133N, SH+L 67N and SH 67N become similar to

Conv 200N (Tables 3.1-3.2). However, it must be remembered that this UAN pool

includes any N still present in non-corn residues or in the soil and therefore does not

necessarily indicate loss from the system.

Discussion

Amendment with GMs resulted in ear yield, growth and N accumulation benefit

for sweet corn. However, GM approaches in this particular management system delivered

only 13-51 kg N ha-1, with SH+L supplying highest N in both years (Tables 3.1-3.2).

Benefits from GMs were usually greatest early in the season (2-4 WAE), strongest from

the combination of SH+L and weakest for L alone (Tables 3.3-3.4, 3.6-3.9, 3.12, 3.14-

3.17), and required chemical N supplementation at least two-thirds (133 kg N ha-1 or

more) of the recommended N-rate (200 kg N ha-1) to achieve ear yields similar to the

conventional approach with recommended inorganic N (Figure 3.1[A,B]). Results were

similar to other experiments where winter-decomposed residues from tropical GMs

(Brandt et al. 1999) or low-performing temperate GMs (Gallaher 1993, Gallaher and

Eyelands 1985) could not satisfy N demand for spring crops. As suggested by our GM

growth and decomposition study (Chapter 2) as well as other studies of sunn hemp

decomposition (Mansoer et al. 1997) and potential use of hairy vetch GMs (Sainju and









Singh 2001) in the southeast US, our GM approaches were limited by low N

accumulation and/or rapid N loss during winter-time GM decomposition.

In 2002, growth of sweet corn amended with any GM plus two-thirds the

recommended N-rate (133N) generally fell behind that of unamended corn with the full

or high N-rate (Conv 200N and Conv 267N) after showing initial advantage during the

first 2-4 WAE (Tables 3.5, 3.10-3.11). In 2003, Conv 200N and Conv 267N showed

advantage over SH+L 133N for final ear harvest only, with almost no differences

occurring throughout the season in terms of tissue dry weights, N contents and leaf

indicators (Tables 3.13, 3.18-3.19). Significant reduction in tissue and ear yields for SH

133N relative to Conv 200N and Conv 267N during 2003 also occurred only at 8-9

WAE. Corn with SH+L 133N produced ear yields statistically similar to, though

numerically lower than, Conv 200N (2002 and 2003) and Conv 267N (2002 only). All

other corn amended with GMs plus 67N or 133N produced ear yields significantly lower

than Conv 200N or Conv 267N (Tables 3.1 and 3.2). Griffin and Hesterman (1991)

showed similar results for potato, with greater GM benefit for vegetative growth than

reproductive yields.

We were unable to detect interesting differences in NUE based on GM or N-rate N,

and direct measures of N-loss (via suction lysimeter sampling; see Chapter 4) failed to

produce data. In terms of all N (plant and/or chemically-derived) applied to corn in each

treatment SH+L 133N remained similar to Conv 200N and less than Conv 267N.

However, when one includes N accumulated by SH and weed tissues but lost by

decomposition before corn planting, unaccounted applied N (UAN) for SH+L 133N

becomes similar to Conv 267N (Tables 3.1 and 3.2). Generally, NUE ranged from 25-









35% in 2002 and 15-25% in 2003. These results do not differ radically from those of

N'Dayegamiye and Tran (2001) and N'Dayegamiye (1999).

Results for growth factors between years were similar qualitatively, although

generally not quantitatively. Dry weights, N concentrations and N contents for all tissues

(leaf, stem, root, ears) and total plant as well as leaf indicators such as LAI, SLN, and

CMR revealed GM benefits up to 45% in the first 4 to 6 WAE in both years. Among leaf

indicators, LAI responded most strongly to GM application and N-rate, as did leaf and

stem dry weights and N contents among tissue factors (Tables 3.4, 3.6, 3.8, 3.12, 3.14,

3.16). Tissue N concentrations were somewhat variable, often showing lower values for

GM amended corn especially at mid-season when ears appeared. These lower values for

GM amended corn probably reflected greater N-stress and N remobilization to ears from

vegetative tissues. Specific leaf area (SLA) never displayed any consistent response to

GMs (see Appendix C, Tables C.2 and C.15).

In terms of pairwise comparisons at early and mid-season, no leaf or tissue

characteristic showed statistical differences predictive of the "finer" but significant

differences in final ear yield patterns among highest producing treatments (Conv 200N

and Conv 267N produced greater ear yields than all GMs with 67N or 133N except

SH+L 133N; Tables 3.1-3.2, 3.5, 3.10-3.11, and 3.18-3.19). Vegetative tissue and leaf

characteristics often did not display differences reflective of ear final yields until 8-9

WAE, far too late for a grower to generate a yield response by adjusting management.

Most net GM N release probably occurred within the first 2-4 weeks after emergence,

and total N delivered to corn via GMs was significantly less than the extra 67 and 133 kg

N ha-1 received by Conv 200N and Conv 267N likely resulting in a "running out" effect









after the first 4 WAE (Tables 3.1 and 3.2). Early season advantages and late-season

declines for GMs with 133N (especially in terms of ear-fill) may also have been the result

of rapidly changing root growth and proliferation patterns, explored in detail in Chapter

4. Taken together, this suggests N content of GM residues at planting may better indicate

required levels of N supplementation. Management of GM approaches to fertility may

thus need to be "preventative" rather than "therapeutic" because plant and root

characteristics will not provide adequate warning time to adjust management.

Despite 50% higher plant population, lower N recovery for all treatments occurred

during 2003 compared to 2002 (Tables 3.11 and 3.19). While values for tissue dry

weights of all treatments in 2003 (compared to values from similar sample dates in 2002)

increased 20-80%, values for total N content in 2003 decreased. As mentioned above, N-

values from all corn tissue samples in 2003 may have been underestimated by a constant

fraction (which would not change statistical findings). However, the decreased N may be

partly explained by rainfall distribution less than 25 mm (less than one inch) fell during

the first six weeks of corn growth in 2002 but greater than 100 mm (greater than four

inches) fell during the same period in 2003. Chemical N, applied at 0, 3, and 5 WAE, as

well as mineralized N from GM residues, may have suffered far more leaching in 2003,

especially if topsoil was already near field capacity from irrigation. Suction lysimeters

were installed to quantify such N-leaching losses in selected treatments, but due to coarse

soil texture sample extraction was far too inconsistent to yield results. Nonetheless,

unlike 2002, when little or no net N uptake occurred after 6 WAE, tissue samples in 2003

revealed 40-45% of total plant N uptake occurred between 6 WAE and final harvest (9

WAE) for highest yielding treatments (Conv 267N, Conv 200N, and SH+L 133N; Tables









3.11 and 3.19). Considering the possibly low N content of corn plants in 2003 and the

lack of yield plateau at 200N, such late-season N-uptake does not seem unreasonable.

With higher plant density and lower apparent N-recovery in 2003, differences in

size and location of the available N pool appear to have become more important.

Treatments supplemented with less chemical N showed lower total ear yield gains and/or

greater reductions in marketable ear yield as a fraction of total ear yield in 2003

compared to 2002 (Tables 3.1 and 3.2). The lower N-content of vetch residues in 2003

may have also become a greater liability. Even in the best 10 (of 24) plots where it was

planted, vetch N accumulation in 2003 was 10 kg N ha-1 less than lupin in 2002, with 30-

40 kg N ha-1 reductions in some of the worst plots. As a result, average N content of

SH+L residue at the time of corn planting was little more than 50 kg N ha-l, and N

released from decomposing SH+L residue during the corn growing season may have only

been some fraction of this total. Nitrogen from vetch residues which possessed highest

N concentrations of all GMs studied may have mineralized rapidly and been especially

vulnerable to leaching loss during early-season rains of 2003.

Increased root distribution near the soil surface and near the plant may have

ameliorated potential N and water stresses for SH+L 133N during early to mid season,

but may also have exacerbated them during late-season ear fill, especially as late-season

N-uptake appears to have been a factor (see Chapter 4). Taken together, lower GM N

content, greater release of N early in the corn growing season during higher rainfall,

higher plant population, and root patterns combined with continued N demand through

late season may have reduced the relative ear yield benefit of GMs in 2003 compared to

2002.









Decline in ear quality for GM treatments during 2003 was evidenced by greater

reductions in fancy ears as fraction of total yields compared to conventional (Tables 3.1

and 3.2). At final harvest in 2003, ears from GM treatments with 133N also appeared to

suffer more from overmaturity, which was not quantified but may have reduced apparent

yields and grade quality. Data from a collaborative study in Tifton (Phatak et al.

unpublished) suggests corn ears may indeed have matured earlier with GMs than without.

These changes in ear yield timing and/or quality may also be related to some combination

of rooting patterns and GM N release potential.

Given our particular management strategies, it appears insufficient N from our

summer leguminous GM was immobilized over the winter until spring corn planting, nor

did our winter leguminous GM perform well enough to accumulate N at levels similar to

those seen in temperate environments. Our results were therefore similar to those of

previous investigators in north Florida (Gallaher 1993, Gallaher and Eyelands 1985) or

similar environments (Mansoer et al. 1997, Jeranyama et al. 2000), with less benefit from

GMs than found in temperate (Griffin et al. 2000, Cline and Silvemail 2002) and tropical

(Ladha et al. 1996, Seneratne and Ratnasinghe 1995, Aulakh et al. 2000, Agustin et al.

1999) environments. Scheduling of chemical N supplementation, which delivered two-

thirds of applied NH4NO3 during the first 4 weeks of growth, may have conflicted with

simultaneous release of N from GM residues. In both years, conventional treatments with

200N and 267N may have gained advantage by receiving more N at final application date

(5 WAE) than GM treatments, especially during a year with high early season rainfall.

Notwithstanding possible long-term benefits for weed and pest control (see Chapter

6) or changes in soil properties conducive to crop production (see Chapter 5), we should









consider the following options: altering management of sunn hemp (or similar GMs) in

our reduced-tillage/reduced-mowing system to better immobilize N during winter

decomposition (see Chapter 2) and improve corn root growth patterns (see Chapter 4);

following sunn hemp with a fall/winter economic crop that will make better use of sunn

hemp N; moving sunn hemp to the spring and following it with a summer or fall

economic crop; and/or substituting sunn hemp with another legume for which seeds may

be harvested, thereby generating an economic benefit while removing "excess" N from

the system.

Substitution of our winter GM monocrop with mixtures of winter legumes, grasses,

small grains and/or non-leguminous dicots in the continuation of this project appears to

have dramatically improved winter GM potential (Lavila and Scholberg, unpublished; see

also Karpenstein-Machan and Stuelpnagel 2000, Cline and Silvemail 2002). We also

recommend better exploitation of direct planting winter GMs or economic crops into

living sunn hemp (or another easily broken GM) so as to eliminate gap time between

rapid sunn hemp decomposition and crop N uptake (see also Chapters 1 and 2). Chemical

(or animal manure) N supplementation in GM approaches in the north Florida

environment should probably deliver more N at mid season to avoid unnecessary

coincidence with early season GM N release and reduced crop N demand. Finally,

organic approaches to crop production relying heavily on GM N may be less risky with

lower crop plant populations and with use of crops having lower N demand and without

price premiums for large fruit size.

Conclusions

Green manure approaches to N fertilization of spring sweet corn in a north Florida

reduced tillage system significantly increased vegetative and reproductive tissue growth,









N accumulation and most leaf indicators. Greatest benefits often came during the first 2-4

weeks of growth, although performance of corn amended with SH+L and 133 kg N ha-1

and (to a lesser extent) SH and 133 kg N ha-1 rivaled that of corn with 200 or 267 kg N

ha-1 but fell behind in terms of final ear yields. Final ear yield trends were best predicted

by statistical differences in total N applied to corn at planting in the form of residue and

subsequent NH4N03 supplementation, but may also be related to timing of N availability

and dynamic changes in root growth patterns (explored in Chapter 4). Improvement of

GM benefits may require selection of different GMs or GM mixtures and modification of

management techniques including residue management, selected crop rotation, planting

procedure, and scheduling of N supplementation.




S16 Akg NH4NO3-N ha- 24 B kg NH4N03-N ha- -
O Oh.0
14 20
i 1 067 2 067
0133 16 T 0133
S10 -200 -0 200

6 .6 8
Z 4-
0 2
211 !5 I 111 i
g SH+L SH L Conv SH+L SH L Conv
Figure 3.1. Marketable ear yields as fresh weight by treatment, 2002 (A) and 2003 (B).
Error bars reflect standard errors.









Table 3.1. Pairwise contrasts of selected nitrogen factors and ear yields, 2002.
Treatment NUE NAC UAN UAN-Total" Fancy Ears
kg kg- kg N ha1 kg N ha1 kg N ha-1 Mt ha-1
Conv 200N 0.33 200 127' 129' 12.3
Conv 267N 0.28 267* 178* 187* 12.9


Marketable Ears
Mt ha-1
13.9
14.4


SH+L 67N 0.34 116*" 61*" 1381 6.5*" 8.1*" 9.9*1
SH 67N 0.40*" 79*1 32*" 123t 5.2*" 7.3*1 9.5*1
L 67N 0.24 127*" 81*" 96*" 5.3*1 7.3*1 9.3*1
SH+L 133N 0.28 211t 1391 218* 11.8 12.8 13.81
SH 133N 0.34 144*" 79*1 163* 10.7l 11.8*" 12.9*"
L 133N 0.19 209W 151' 168' 10.6*" 12.1* 12.9*"
SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4N03-N ha- ; NUE = N uptake efficiency;
NAC = N applied at or after corn planting; UAN = applied N not recovered by corn; # includes N from sunn hemp residue and weeds
prior to sunn hemp death; mean different from Conv 200N and Conv 267N at the p < 0.05 level, respectively.


Table 3.2. Pairwise contrasts of selected nitrogen factors and ear yields, 2003.
Treatment NUE NAC UAN UAN-Total"
kg kg- kg N ha1 kg N ha1 kg N ha-1
Conv 200N 0.24 203 149 184
Conv 267N 0.19 271 213 252


Fancy Ears
Mt ha-1
14.27
17.7*


Marketable Ears
Mt ha-1
17.0V
20.1*


SH+L 67N 0.15 113*" 86*" 2141 2.8*" 6.2*" 7.6*"
SH 67N 0.21 99*1 70*" 200W 2.4*" 5.7*1 7.0*"
L 67N 0.20 81*" 58*" 81*" 2.7*" 5.3*1 6.7*"
SH+L 133N 0.24 184*" 1391 261* 11.9 14.8 16.3t
SH 133N 0.20 163*" 128*" 258* 10.4* 13.3*" 14.9*"
L 133N 0.22 168*" 140' 170' 8.3*" 11.9*" 13.6*"
SH = sunn hemp; L = winter legume; Conv = conventional (no green manure); N = kg NH4N03-N ha- ; NUE = N uptake efficiency;
NAC = N applied at or after corn planting; UAN = applied N not recovered by corn; # includes N from sunn hemp residue and weeds
prior to sunn hemp death; mean different from Conv 200N and Conv 267N at the p < 0.05 level, respectively.


Total Ears
Mt ha1
14.8
15.4


Total Ears
Mt ha-1
18.9
21.7