<%BANNER%>

Screening Forage Crops Suitable for Remediating P-Impacted Soils in Florida

Permanent Link: http://ufdc.ufl.edu/UFE0025062/00001

Material Information

Title: Screening Forage Crops Suitable for Remediating P-Impacted Soils in Florida
Physical Description: 1 online resource (63 p.)
Language: english
Creator: Sui, Xiaolin
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: crop, elephantgrass, phosphorus, phytoremediation
Soil and Water Science -- Dissertations, Academic -- UF
Genre: Soil and Water Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Phosphorus (P) is an essential element for all livings and represents an important input in agriculture. However, when improperly managed, P from intensive agriculture operations may contribute to the non-point source pollution and surface water eutrophication problems. Phytoremediation, defined as the use of plant to extract P from soils, is considered an environmental sound alternative to reduce the risks associated with soil P buildup and subsequent transport to water bodies. From the economic perspective, synergies between biomass and bioenergy production can further enhance the importance of phytoremediation as a cost-effective alternative to remediate P in P-enriched soils. The objective was to evaluate suitable forage crops to remediate P-impacted soils in Florida. An experimental site impacted by continuous manure deposition for over fifty years was selected for the study. The project aimed to 1) screen an array of minor forage crops for their phytoremediation capacity potential, and to 2) evaluate the P removal capacity of four pre-chosen forage crops and their effects on groundwater quality. Fifteen forage species were selected based on dry matter yields, adaptability to Florida conditions and potential to be used as bioenergy crops. Soil samples were collected from the Ap, E, and Bh horizons and analyzed for Mehlich-1 and water-extractable P at the beginning and the end of the growing seasons. Forages were harvested periodically and analyzed for tissue P and N concentrations. Water samples were collected every 2 wks at 60- and 90-cm soil depths throughout the growing season and analyzed for ortho-P. Our data indicate significant effect of crop P uptake on Mehlich-1 soil P concentrations during the 2-year study. On average, Mehlich-1 P was reduced from 657 to 459 mg kg-1. Elephantgrass (Pennisetum purpureum) exhibited the greatest P-removal potential (139 kg P ha-1 yr-1) compared with other forage species (average = 65 kg P ha-1 yr-1). Groundwater quality also showed treatment effect. Results indicated that elephantgrass has the highest P removal potential and can be also used as a renewable energy source. Further investigation is needed to address the long-term effects of forage crops on water quality.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Xiaolin Sui.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Silveira, Maria L.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0025062:00001

Permanent Link: http://ufdc.ufl.edu/UFE0025062/00001

Material Information

Title: Screening Forage Crops Suitable for Remediating P-Impacted Soils in Florida
Physical Description: 1 online resource (63 p.)
Language: english
Creator: Sui, Xiaolin
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: crop, elephantgrass, phosphorus, phytoremediation
Soil and Water Science -- Dissertations, Academic -- UF
Genre: Soil and Water Science thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Phosphorus (P) is an essential element for all livings and represents an important input in agriculture. However, when improperly managed, P from intensive agriculture operations may contribute to the non-point source pollution and surface water eutrophication problems. Phytoremediation, defined as the use of plant to extract P from soils, is considered an environmental sound alternative to reduce the risks associated with soil P buildup and subsequent transport to water bodies. From the economic perspective, synergies between biomass and bioenergy production can further enhance the importance of phytoremediation as a cost-effective alternative to remediate P in P-enriched soils. The objective was to evaluate suitable forage crops to remediate P-impacted soils in Florida. An experimental site impacted by continuous manure deposition for over fifty years was selected for the study. The project aimed to 1) screen an array of minor forage crops for their phytoremediation capacity potential, and to 2) evaluate the P removal capacity of four pre-chosen forage crops and their effects on groundwater quality. Fifteen forage species were selected based on dry matter yields, adaptability to Florida conditions and potential to be used as bioenergy crops. Soil samples were collected from the Ap, E, and Bh horizons and analyzed for Mehlich-1 and water-extractable P at the beginning and the end of the growing seasons. Forages were harvested periodically and analyzed for tissue P and N concentrations. Water samples were collected every 2 wks at 60- and 90-cm soil depths throughout the growing season and analyzed for ortho-P. Our data indicate significant effect of crop P uptake on Mehlich-1 soil P concentrations during the 2-year study. On average, Mehlich-1 P was reduced from 657 to 459 mg kg-1. Elephantgrass (Pennisetum purpureum) exhibited the greatest P-removal potential (139 kg P ha-1 yr-1) compared with other forage species (average = 65 kg P ha-1 yr-1). Groundwater quality also showed treatment effect. Results indicated that elephantgrass has the highest P removal potential and can be also used as a renewable energy source. Further investigation is needed to address the long-term effects of forage crops on water quality.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Xiaolin Sui.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Silveira, Maria L.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-02-28

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0025062:00001


This item has the following downloads:


Full Text

PAGE 1

1 SCREENING FORAGE CROPS SUITABLE FOR REMEDIATING P-IMPACTED SOILS IN FLORIDA By XIAOLIN SUI 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 2009

PAGE 2

2 2009 Xiaolin Sui

PAGE 3

3 To my Mom and Dad

PAGE 4

4 ACKNOWLEDGEMENTS I wish to express m y sincere thanks to my advisor Dr. Maria Silvei ra for her continuous support for me and my research. I am much grateful for her guidance and mentorship throughout my research. I al so thank Dr. OConnor, who provided me with on-campus mentorship during my first-year stay in the United States. I thank Dr. OConnor for allowing me to join the weekly meeting of his resear ch group and providing me with inspirational ideas and valuable critiques for my research. I also thank my other committee members, Dr. Sollenberger and Dr. Vendramini, for their s upport and meaningful suggestions about my project. The environmental soil chemistry group provide d me with an enormous amount of help. I thank Sampson for patiently giving me advi ce in my lab work and for his continuous encouragement to me. I thank Augustine for assisting me and for his priceless advice for me in many aspects of life. Matt, Liz, Daniel, Ja ya, Manmeet are great friends and coworkers. Without them I could not have proceeded to the place I am. I also thank the faculty members and co-w orkers at the Range Cattle Research and Education Center. Ms. Cindy Holley is a great lab technician as well as a helpful co-worker, without whose help I could not fi nish my lab analysis. Daniel a nd Christine helped with part of my lab analysis. Dr. Vendramini and Carly an d many other people helped in the harvests. I thank Dr. Arthington, Christina and Toni for f acilitating many processe s that I went through in the station. Special tha nk goes to Joe who is a good technician and a good friend. I am deeply grateful to my parents for enc ouraging me to pursue an advanced education in the United States and for the comfort they extended to me in difficult times. I am also thankful to my girlfriend Jinghui, who helped me proofread this thesis and comforted me whenever I felt frustrated with work. Last but no t least, I thank all of my friends who are the most important treasure that I am ble ssed with in the course of my life.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGEMENTS.............................................................................................................4LIST OF TABLES................................................................................................................. ..........7LIST OF FIGURES.........................................................................................................................8LIST OF ABBREVIATIONS........................................................................................................ 10ABSTRACT...................................................................................................................................11 CHAP TER 1 INTRODUCTION..................................................................................................................13Environmental Issues Asso ciated With Phosphorus............................................................... 13Phytoremediation of High Phosphorus Soils.......................................................................... 14Concept.....................................................................................................................14Phytoremediation of Phosphorus.............................................................................16Potential Phosphorus Hyperaccumulator Species.................................................... 17Time Required to Remediate P-Enriched Soils........................................................ 18Synergies Between Phytoremediation and Bioenergy Production......................................... 18Hypotheses and Research Objectives..................................................................................... 192 MATERIALS AND METHODS........................................................................................... 22Site Description......................................................................................................................22Forage Selection and Experimental Design............................................................................ 22Small Plot Study..............................................................................................................22Large Plot Study.............................................................................................................. 23Fertilization and Harvest Schedule.........................................................................................23Soil Analyses..........................................................................................................................24Tissue Analyses......................................................................................................................25Water Sampling and Analyses................................................................................................ 25Statistical Analysis........................................................................................................... .......263 RESULTS...............................................................................................................................29Small Plot Study............................................................................................................... ......29Mehlich-1 Phosphorus..................................................................................................... 29Water-Extractable Phosphorus........................................................................................29Dry Matter Yield............................................................................................................. 30Tissue Phosphorus Concentrations.................................................................................. 30Tissue Nitrogen Concentrations......................................................................................31

PAGE 6

6 Phosphorus Uptake.......................................................................................................... 31Phosphorus Mass Balance............................................................................................... 32Large Plot Study............................................................................................................... ......32Mehlich-1 Phosphorus..................................................................................................... 32Water-Extractable Phosphorus........................................................................................33Crop Yield, Tissue Phosphorus/Nitrogen Concentration and Phosphorus Uptake......... 33Leachate P Concentrations.............................................................................................. 34Leachate N Concentrations.............................................................................................. 354 DISCUSSION.........................................................................................................................52Soil-Phosphorus Decrease, Phosphorus Uptake and Phosphorus Leaching........................... 52Dry Matter, Tissue Phosphorus Concen trations, and Phosphorus Uptake............................. 53Ground Water Effects........................................................................................................... ..545 CONCLUSIONS.................................................................................................................... 56LIST OF REFERENCES...............................................................................................................58BIOGRAPHICAL SKETCH.........................................................................................................63

PAGE 7

7 LIST OF TABLES Table page 1-1 Yield and P uptake for different crops............................................................................... 21 1-2 Biofuel crops adapted to Florida conditions...................................................................... 21 2-1 Species, cultivars, and plot numbers assigned to warm-season grasses screened for high capacity P-uptake in the small plot study............................................................. 27 2-2 Species, cultivars, and plot assigned to warm-season grasses selected for field evaluation of P accumulation in the large plot study......................................................... 27 2-3 Harvest protocols for the small plot and large plot studies................................................ 28 3-1 Summary of the initial soil test P va lues across the sma ll plot study area......................... 37 3-2 Average dry matter yield, P uptake and soil P removal from soil for ten forage species in the 2-yr small plot study.................................................................................... 37 3-3 Mean dry matter yields, tissue P concen trations, P uptake by year and by species in the large plot study.........................................................................................................37 3-4. Average dry matter yield, P uptake and soil P removal from soil for four forage species in the 2-yr large plot study.................................................................................... 38

PAGE 8

8 LIST OF FIGURES Figure page 2-1 Location of the experiment site.......................................................................................... 28 3-1 Initial water-extractable P concen tration in the small-plot study......................................39 3-2 Average soil P concentrations in th e small plot study over the study period measured as Mehlich-1 P and Water-Extractable P........................................................... 40 3-3 Average decreases of soil test P values during the 2-yr study by different treatment groups............................................................................................................... ..40 3-4 Average small plot soil test P decrease s after 2 years as affected by 15 forage cultivars...................................................................................................................... ........41 3-5 Average dry matter yields of 15 forage cultivars in the small plot study.......................... 42 3-6 Tissue total Kjeldahl P of 15 forage cultivars.................................................................... 42 3-7 Tissue total Kjeldahl N concentrations of 15 forage cultivars in the small plot study...................................................................................................................................43 3-8 Phosphorus uptake by 15 forage cu ltivars in the small plot study..................................... 43 3-9 Average soil P concentrations in th e large plot study across the study period measured as Mehlich-1 P and Water-Extractable P........................................................... 44 3-10 Average soil test P decrease in different horizons after the 2-yr large plot study as affected by species............................................................................................................ .45 3-11 Average dry matter yields for the four treatments in the large plot study......................... 46 3-12 Tissue Total Kjeldahl P concentrations of the four forage cultivars in the large plot study............................................................................................................................46 3-13 Tissue Total Kjeldahl N concentrations of the four forage cultivars in the large plot study............................................................................................................................47 3-14 Phosphorus uptake values for the four fo rage cultivars in the large plot study................. 47 3-15 Precipitation and water-table depth during the growing season of 2008........................... 48 3-16 Average leachate ortho-P concentrati ons at two depths collected across the growing season in the large plot study............................................................................... 49

PAGE 9

9 3-17 Average leachate ortho-P concentrations at the 60and 90-cm depths for different treatm ents in the large plot study....................................................................................... 49 3-18 Average leachate ammonium concentra tions at two depths collected across the growing season in the large plot study............................................................................... 50 3-19 Average leachate nitrate concentrati ons at two depths collected across the growing season...................................................................................................................50 3-20 Average leachate nitrate concentrations at 60and 90-cm depths for different treatment in the large plot study......................................................................................... 51

PAGE 10

10 LIST OF ABBREVIATIONS BHG Bahiagrass BMD Bermudagrass DM Dry Matter FAWN Florida Automated Weather Network IFAS Institute of Food and Agricultural Sciences K Potassium M1P Mehlich-1 Phosphorus N Nitrogen STG Stargrass STP Soil Test P TKN Total Kjeldahl Nitrogen TKP Total Kjeldahl Phosphorus P Phosphorus WEP Water-Extractable Phosphorus

PAGE 11

11 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 SCREENING FORAGE CROPS SUITABLE FOR REMEDIATING P-IMPACTED SOILS IN FLORIDA By Xiaolin Sui August 2009 Chair: Maria Silveira Major: Soil and Water Science Phosphorus (P) is an essential element for a ll livings and represents an important input in agriculture. However, when improperly managed, P from intensive agriculture operations may contribute to the non-point source polluti on and surface water eutrophication problems. Phytoremediation, defined as the use of plant to extract P from soils is considered an environmental sound alternativ e to reduce the risks associ ated with soil P buildup and subsequent transport to water bodies. From the economic perspective, synergies between biomass and bioenergy production can further enhance the importanc e of phytoremediation as a cost-effective alternative to remediate P in P-enriched soils. The objective was to evaluate suitable forage crops to remediate P-impacted soils in Florida. An experimental site impacted by continuous manure deposition for over fifty years was selected for the study. The project aimed to 1) screen an array of minor forage crops for their phytoremediation capacity potential, and to 2) evaluate the P removal capacity of four pre-chosen forage crops and their effects on gr oundwater quality. Fifteen forage species were selected based on dry matter yields, adaptability to Florida conditions and potential to be used as bioenergy crops. Soil samples were collect ed from the Ap, E, and Bh horizons and analyzed for Mehlich-1 and water-extractable P at the beginning and the end of the growing seasons. Forages were harvested periodically and analyzed for tissue P and N concentrations.

PAGE 12

12 Water samples were collected every 2 wks at 60and 90-cm soil depths throughout the growing season and analyzed for ortho-P. Our data indicate significant effect of cr op P uptake on Mehlich-1 soil P concentrations during the 2-year study. On average, Mehl ich-1 P was reduced from 657 to 459 mg kg-1. Elephantgrass (Pennisetum purpureum ) exhibited the greatest P-removal potential (139 kg P ha-1 yr-1) compared with other forage species (average = 65 kg P ha-1 yr-1). Groundwater quality also showed treatment effect. Results indicated that elephant grass has the highest P removal potential and can be also used as a rene wable energy source. Fu rther investigation is needed to address the long-term effects of forage crops on water quality.

PAGE 13

13 1CHAPTER 1 INTRODUCTION Environmental Issues Asso ciated With Phosphorus Phosphorus plays an im portant role in agri culture. Adequate P supplies are necessary for seed and root formation, straw strength in cereals, crop quality, and synthesis of microbial biomass in ruminant animals (Whitehead, 2000; Havlinet al., 2004). Phosphorus fertilization, including the use of commercial fertilizers a nd animal waste, has been widely used in agriculture to supply adequate amounts of P to crops. However, when improperly managed P fertilizers can also pose a potential threat to the environment. Once added to the soil, P can be sorbed by di fferent soil constituents such as Al and Fe oxides (Schmidtet al., 1997). Although P mobility in most soils is generally low, P can be easily transported from agriculture areas when P additions exceed the maximum soil retention capacity. It is estimated that over 50% of US soils exhibit high P status (Fixen, 2002). The high P levels are generally associated with repeated applications of fertilizers and manure. Soil P buildup may pose a threat to the environm ent, as P may be easily transported to the water bodies (Sharpleyet al., 1994). Phosphorus losses from agricultural areas cause serious problems for water quality and the environment (Sharpley, et al., 1994). A major problem is the accelerat ed eutrophication in aquatic environments. Eutrophication occurs when a water body rece ives excess nutrient input, resulting in unwanted growth of algae and aquatic weeds, causing anoxic conditions. (Sharpley, et al., 1994; Brady and Weil, 1999). Increasing number of harmful algae blooms [such as a dinoflagellate ( Pfiesteria spp. )] in water bodies has been linked to excess P input (Sharmaet al., 2007). Eutrophication is an importa nt environmental concern because it results in fish kills, unbalanced nutri ents status, and can make the water unsuitable for drinking. These negative effects may also restrict the us e of surface waters for aesthetics, fisheries, recreation, industry, and drinking, an d thus have serious local a nd regional economic impacts,

PAGE 14

14 including posing threats to human health (B urkholder and Glasgow, 1997). In most inland waters, P is the limiting nutrient for algae gr owth (Thomann and Mueller, 1987); thus even small P concentrations can negatively imp act water quality and cause eutrophication problems (Sharpley, et al., 1994). Phosphorus is transported to water bodies though two main pathways: i. runoff, and ii. leaching. Runoff is often the most important mechanism by which P is transported from agricultural land to surface water bodies (Sha rpley, et al., 1994). Factors affecting the potential of P runoff include so il erosion, soil test level, and P fertilizer application rate (Sharpley, et al., 1994). Leaching also represen ts an important pathway through which P can pose a threat to the aquatic environment. Although P is relatively immobile in most soils, long-term application of manure and P fertil izer could increase P leaching potential (Eghballet al., 1996). Research has shown that P leaching from P-impacted soils could continue for many years even when P additions are reduced or ceased (Nelsonet al., 2005). Phosphorus leaching can be a serious envir onmental threat, especially in sandy soils with shallow water tables (Nels on, et al., 2005). Florid a soils are particular ly susceptible to P leaching due to their sandy texture, low P re tention capacity, and fluctuating water table. Research has shown that P losses from spodos ols represent an important environmental concern in Florida (Reddyet al., 1995). In addi tion, lateral movement of water and nutrients induced by artificial drainage also increases the potential risks of P transport to surface water bodies. Overland flow may also represent an im portant source of nut rient loss to nearby streams, rivers, and lakes during the rainy season (Graetz and Nair, 1995). Phytoremediation of High Phosphorus Soils Concept Phytorem ediation refers to the direct use of living plants for in-situ remediation of contaminated soils, sludge, sediments, and ground water through contaminant removal,

PAGE 15

15 degradation, or containment (USEPA, 1999). Mo st of the research on phytoremediation has focused on the ability of plants to remedi ate trace metals or organic contaminants. Hyperaccumulator species, which by definition ar e those that can accumulate certain target contaminants at very high tissu e concentration (u sually > 10 g kg-1 DM), have been identified for remediatation of organic compounds a nd trace metals (Hogstad, 1996; Cunninghamet al., 1997; Maet al., 2001). The most important advantages of phytoremediation strategies are: a. a cost-effective and environmental friendly approach to reme diate soils, b. better public acceptance than traditional remediation approaches such as excavation, and c. capable of remediating a diverse range of contaminants (Maceket al., 2000). Estimated costs of phytoremediation for the organics and metal contaminated sites are $25,000 to $75,000 per ha, which is 20 to 50% of traditional capping (Sur esh and Ravishankar, 2004). There are also limitations a ssociated with phytoremediation. Because phytoremediation relies on plant roots to absorb the contaminants, the extension of plant roots is critical to the success of phytoremediation. The concentration of contaminants below the root zone is usually not affected by phytorem ediation strategies. Another lim itation is that plants cannot grow in highly toxic levels of contaminants, so phytoremediation is only suitable when contaminant concentrations are not excessively high. Disposal of harvested plant biomass also represents difficulties. Perhaps the most important limitation associated with phytoremediation is that it is usually a time-c onsuming process, and may require years to decades to reduce contaminant concentrations to acceptable levels (Cunningham and Berti, 1993). Salt et al. (1995) reporte d that even when effectiv e Ni and Zn hyperaccumulator species were used, phytoremediation required 13 to 14 yr to clean a contaminated site.

PAGE 16

16 Phytoremediation of Phosphorus Lim ited research has been conducted on the potential benefits of phytoremediation to minimize environmental problems associated with excess P (Delormeet al., 2000; Gastonet al., 2003). Before the 1980s, agronomists we re primarily concerned about the limited mobility of P in the soil and its effects on plant P nutrition. Because P was perceived to be highly immobile, recommendations of animal waste applications at rates based on crop N requirements (McDowellet al., 2001). Decades of excessive manure and fertilizer applications resulted in increased soil P concentration in many agricultural areas. Current approaches to reduce the off-site movement of P from agricultural areas include the utilization of soil chemical am endments and agricultu ral best management practices. Phosphorus can form insoluble comple xes with a variety of metal ions including aluminum, calcium, and iron. Chemical amendments such as alum, ferric chloride and lime have been applied to reduce the availability of water-soluble P in manure-impacted soils (Moore and Miller, 1994; Dao, 1999; Douet al., 2003). Recently, water treatment residuals have been evaluated as a potential soil am endment that can effectively decrease the water-soluble P concentration in P-impacted soils in both the short and long term (Silveiraet al., 2006; Agyin-Birikorang and O'Connor, 2007). Riparian buffers are also used to reduce transport of particle P into streams (Nova k and Chan, 2002). Agricultural practices that reduce surface runoff and erosion can also mini mize P movement into surface water bodies (Brady and Weil, 1999; Novak and Chan, 2002). Although many best management practices ha ve been shown to effectively reduce off-site movement of P into surface water bodies, they generally fail to prevent P accumulation or reduce total P concentrations in the soil. Thus, the threat of P movement and subsequent solubilization remains after implementation.

PAGE 17

17 Potential Phosphorus Hyperaccumulator Species Delorm e et al. (2000) suggested that various crop species exhibited extensive variation in the P-uptake, and suitable plants which ha ve above-average tissue P concentrations could be used to remove P from the soil. The averag e P concentration of plant tissue ranges from 2 to 4 g kg-1 of dry matter (Brady and Weil, 1999); a nd successful phytoremediation of excess soil P requires P-hyperaccumulator species that exhibit tissue P concentrations between 8 and 14.5 g kg-1 (dry matter basis) (Novak and Cha n, 2002). The majority of previous P hyperaccumulator studies were conducted unde r greenhouse conditions (Sharma and Sahi, 2005; Sharma, et al., 2007). However, development of P-hyperaccumulator plants under greenhouse or small plot conditi ons does not guarantee a successful performance at field scale (Novak and Chan, 2002). No P hyperaccumula tor crop has been identified at the field scale (Pantet al., 2004). Annual removal of P by typical forage species ranges from about 20 kg ha-1 (by red clover, Trifolium pratense) to about 85 kg ha-1 [by johnsongrass, Sorghum halepense (L.) Pers.] (Pierzynski and Logan, 1993) Table 1-1 compiles the annual yield and P removal potential of several crops (Pierz ynski and Logan, 1993; Delorme, et al., 2000; Novak and Chan, 2002). Gaston et al. (2003) studied the P removal potential of five cool-season and five warm-season forages on a P-impacted Ruston so il (fine-loamy, siliceous, thermic Typic Paleudult). Bahiagrass ( Paspalum notatum Flugge) or bermudagrass [ Cynodon dactylon (L.) Pers.] followed by annual ryegrass ( Lolium multiflorum Lam.) seeded in the fall was identified as a potentia l strategy to remediate soil with el evated P concentrations. Ryan (2006) conducted a field study of the effectiveness of a double-cropped system (bermudagrass over-seeded with annual ryegrass) in reducing soil P from a P-impacted Ruston fine-sandy loam. Removal rates were nearly 200 kg P ha-1 over 4 yr. Sharma et al. (2007) studied several species, including vegetables, legume and he rb crops, for their suitability for P

PAGE 18

18 phytoremediation in the greenhouse. Cucumber ( Cucumis sativus ) and yellow squash ( Cucurbita pepo var. melopepo) accumulated P concentration > 10 g kg-1 dry weight, but no field evaluations were conducted to validate the greenhouse results. W oodard et al. (2007) evaluated five forage systems for their P-re moval capacity on a Kershaw sand soil (thermic, uncoated Typic Quartzipsamment). The greatest P removal (91 kg ha-1 cycle-1) was observed in a bermudagrass-rye ( Secale cereale L.) system in the first two years. The authors concluded that due to its hi gh yield and persistence, bermudagrass was likely the best warm-season forage to recover P from da iry sprayfields in northern Florida. Time Required to Remediate P-Enriched Soils Lacking a P hyperaccum ulator species, phytorem ediation of soils enriched with P is expected to require long times to reduce P to acceptable levels using common row crops and forage grasses (Sharpley, et al., 1994). Fo r example, McCollum (1991) estimated that reducing Mehlich-3 P in a Portsmouth soil (fine sandy over sandy or sandy-skeletal, mixed, thermic Typic Umbraquult) from 100 mg P kg-1 to the threshold agronomic level of 20 mg P kg-1 would require 16 to 18 yr of c onsecutive cropping with corn (Zea mays L. ) or soybean [ Glycine max (L.) Merr. ]. Mozaffari and Sims (1996) re ported that 16 yrs of corn and soybean cropping was required to redu ce Mehlich-1 P levels of 100 mg kg-1 to a crop response level on a Portsmouth fine sandy loam. Synergies Between Phytoremediation and Bioenergy Production As discussed before, phytorem ediation can be a feasible technique to reduce soil P concentrations. However, serious limitations associated with phytoremediation of P are (i) the disposal of the harvested plant biomass, (i i) possible high establishment and maintenance costs, and (iii) long remediation time that ma y be required. One alternative to improve the economic sustainability of phytoremediation st rategies is the use of plant biomass for

PAGE 19

19 bioenergy production. Biomass produced in the processes of phytoremediation can potentially be used as a renewable energy source. The concept of synergies between phytor emediation and bioenergy production is relatively new, especially for the phytoremedia tion of P. Van Ginneken et al. (2007) reported a phytoremediation project conducted in Belgium where plants were cu ltivated to extract metals from the soil, and then harvested for seed and biofuel production. The use of metal-accumulating plants for bioenergy produc tion is promising, despite some concerns relative to the fate of metals in the plant tissu e material. The plants accumulate heavy metals in the tissues which are furthe r harvested and processed to produce biofuel. When the biofuel is exhausted, or the plant ash disposed, it is unknown if there will be environmental risks associated with hazardous metal emission (Van Ginneken, et al., 2007). Jasinskas (2008) conducted a 3-year study of the potential of 8 species of tall perennial grasses for use as energy crops, and concluded that biomass of pe rennial grasses can be used as fuel. Among 35 species chosen for evaluation, switchgrass ( Panicum virgatum ) was the native perennial grass with the greatest potential for bioenergy production. Results from studies conducted in Florida indicate that biomass and biofuel production vary cons iderably among forage species (Newmanet al., 2008). The focus of this project was to evaluate the phytoremediation potential of various forage crops that could also be used as a renewable energy source. The study was not designed to evaluate the biofuel produc tion potential of th e various species. Hypotheses and Research Objectives The research objectives of this study are to: Specific Aim 1 : Screen an array of m inor forage cr ops for their phytoremediation capacity potential in soils with excessive P concentrations. Specific Aim 2 : Compare the P removal capacity of four forage crops [(elephantgrass ( Pennisetum purpureu), sugarcane (Saccharum officinarum ), switchgrass ( Panicum

PAGE 20

20 virgatum ), and stargrass ( Cynodon nluemfuensis )] grown on a manure-im pacted site and the impacts of forage P uptake on groundwater quality. Three hypotheses will be tested for the project: Hypothesis 1 : Soil test P concentrations will decrease over time in response to crop P uptake. Hypothesis 2 : P removal capacity will vary among different crop species Hypothesis 3 : Groundwater P concentrations will be reduced in respons e to P uptake of forage crops.

PAGE 21

21 Table 1-1. Yield and P uptake for different crops, adapted from Pierzynski and Logan (1993), De lorme et al. (2000) and Novak an d Chan (2002) Crop Yield (Mg ha-1 yr-1) P uptake (kg ha-1 yr-1) Corn silage ( Zea mays) 67.2 39.2 Coastal Bermuda grass ( Cynodon dactylon )22.4 70.6 Red clover ( Trifolium pratense) 9 22.4 Johnsongrass ( Sorghum halepense ) 26.9 93 Indian mustard ( Brassica juncea ) 18 84.6 Table 1-2. Biofuel crops adapted to Florida c onditions (adapted from Newman et al., 2008) Crop Scientific name Yield Biofuel Production Potential Environmental Concerns Production Costs Switchgrass Panicum virgatum 4500 kg ha-1 yr-1 13000 kg ha-1 yr-1 combustion/lignocellulosic ethanol little Not well known Miscanthus M. sacchariflorus x M. sinensis 11200 kg ha-1 yr-133600 kg ha-1 yr-1 Combustion could escape and spread Not known Sweet Sorghum Sorghum bicolor (L.) Moench 2900 kg ha-1 yr-1 3100 kg ha-1 yr-1 in sugar ethanol production none Limited information Soybean Glycine max 470 L ha-1 yr-1 560 L ha-1 yr-1 in biodiesel Biodiesel none Varies Peanut Arachis hypogaea 1100 ha-1 yr-1 1400 L ha-1 yr-1 in biodiesel Biodiesel Little $2000 ha-1 $2500 ha-1 Canola Brassica napus 1500 L ha-1 yr-1 in biodiesel biodiesel None Varies Elephantgrass Pennisetum purpureum 31000 kg ha-1 yr-145000 kg ha-1 yr-1 methane generation/ co-firing with coal nitrate leaching / invasiveness $1500 ha-1 Sugarcane Saccharum spp. 2000 L ha-1 yr-1 2500 L ha-1 yr-1 in ethanol ethanol byproduct of vinasse $370 ha-1 $600 ha-1

PAGE 22

22 2CHAPTER 2 MATERIALS AND METHODS Site Description Field experim ents were conducted from 2007 to 2008 on a P-impacted site, located in the Lake Okeechobee basin in South Florida ( 27 32' 17"N, 81 51' 31"E; Figure 2-1). The site is a private dairy farm and has been cont inuously occupied by dairy cows for over 50 yr. The area selected for the research project wa s approximately 0.6 ha in size. The study area was fenced to prohibit access of cattle, sprayed to kill the existing stargrass ( Cynodon nlemfuensis Vanderyst) pasture before planting. Forage Selection and Experimental Design Small Plot Study To accom plish the first specific aim, 15 forage cultivars adapted to South Florida conditions were selected for study (Table 2-1) Treatments consisted of the fifteen cultivars replicated three times in a completely randomized design for a total of 45 plots. Control plots (bare ground) were also included, one per replic ate. Forage species used in the study were selected based on the following criteria: High forage production potential (yield and quality) Expected relatively high P uptake Adapted to South Florida conditions Seed/planting material widely available Potential to be used as bioenergy crops Plot area was 3 x 2 m with a 2-m ai sle between plots. Elephantgrass (Pennisetum purpureum ) and sugarcane ( Saccharum officinarum ) were planted on 1 Feb. 2007. Bahiagrass ( Paspalum notatum ), switchgrass ( Panicum virgatum), and mulato ( Brachiaria sp. ) were seeded on 5 Apr. 2007. The remaining species were established between late June and early July, 2007. Plots were closely monitored to asse ss need for weed control. Mechanical and chemical weed control was performe d as needed during the 2-yr study.

PAGE 23

23 Large Plot Study The second specific aim was addressed using large scale (10 x 10m) plots to more accurately evaluate forage DM yields, P remova l capacity of various species, and the impacts of P uptake on groundwater quality. Sixteen larger plots were es tablished on the experimental site using four forage species believed to have the greatest P removal potential. The species include: (i) sugarcane ( Saccharum officinarum ) cv. CP 78-1620, (ii) elephantgrass ( Pennisetum purpureum) cv. Merkeron, (iii) switchgrass ( Panicum virgatum ) cv. Alamo, and (iv) stargrass cv. Florona. Treat ments consisted of the four fo rage species replicated four times on a completely randomized design for a total of 16 plots (Table 2-2), and no bare plots were included. Surgarcane and elephantgras s were established on 1 Feb. 2007. Switchgrass and stargrass planting was delayed because of drought conditions, and planting finally occurred between late June a nd early July of 2007. As in the small plot study, mechanical and chemical weed control was performed on a regular basis throughout the 2-yr study. Two lysimeters were installed at 60and 90-cm depths in th e center of each large plot for a total of 32 sample points. The lysimeters were positione d at these depths to monitor water quality above and below the Bh horizon (66to 90-cm), respectively. A pressure transducer was also installed in the field to monitor fluctuations in the groundwater level. Precipitation was monitored by Ona station of the Florida Automated Weather Network (FAWN). Fertilization and Harvest Schedule Crops were fertilized accor ding to IFAS recomm endations (Mylavarapuet al., 2009). All experimental units in both studies (excluding elephantgra ss and sugarcane plots) were fertilized with ammonium nitrate (NH4NO3) at a rate of 90 kg ha-1 N after every harvest (every 6 wk). The elephantgrass plots received 180 kg ha-1 N after every harvest (every 12

PAGE 24

24 weeks). The sugarcane plots received 200 kg ha-1 N after every cut (every 12 months). No P and K fertilizer was added during the study. During the growing seasons (May to Nove mber) each year, plots were periodically harvested to determine DM yields, and tissue N and P concentrations determined (Table 2-3). All the plots, except for the elephantgrass and sugarcane plots, were harvested at 6-wk intervals. Elephantgrass plots were harveste d every 12 wk, and sugarcane was harvested once a year. Forage sub-samples were collected and placed in pre-weighed cloth bags and weighed for fresh weight. Sub-samples were oven-dried at 60C for approximately 48 h until constant dry weight. Oven-dried samples were ground to pass through a 1-mm stainless-steel screen and stored for further analysis. Soil Analyses Soil in the p lots was of the Pomona seri es (sandy, siliceous, hyperthermic Ultic Alaquods (United States. Natural Resources Co nservation Service., 1996). Soil samples were taken from the Ap (0-15 cm) horizon in the sma ll plot study at the be ginning and the end of each growing season during the 2-yr study. In the large plot study, soil samples were collected from Ap (0-15 cm), E (15-60 cm) a nd Bh (66-90 cm) horizons Five 5-cm soil core samples (four from near each corner, one in the center) within each plot were taken and mixed thoroughly to constitute a representative composite sample. Soil samples were aired dried, sieved (2 mm) and stored for further analysis. Soil P concentrations were determined using different extraction methods. Water extractable P (WEP) has been widely used in some European countries (Ehlertet al., 2003) as an indicator of runoff P potential (Kleinmanet al., 2002). In this project, WEP was used as an indicator of the labile P that would be readily subject to leaching. Water-Extractable P was determined by adding 20 mL of water to 2 g of soil and shaken at 3.6 g for 1 h. The mixture was centrifuged at 2147 g for 10 min, a nd filtered through 0.45-m membranes

PAGE 25

25 (Luscombeet al., 1979; Pageet al., 1982). Me hlich-1 P (Mehlich, 1953) was determined by adding 20 mL of Mehlich-1 extractant to 5-g soil samples and shaken at 3.6 g for 5 minutes, and filtered through Whatman grade No.2 filter paper (Mehlich, 1953). Phosphorus in the extracts was analyzed colorimetrically (E PA Method 365.1) using a Seal AQ-2 discrete analyzer (2006 SEAL Analytical Ltd. Mequon, WI). Tissue Analyses The forage tissue sam ples from each harvest were analyzed for total Kjeldahl P (TKP) (EPA Method 365.4), and total Kjeldahl N (TKN) (EPA Method 351.1). Briefly, tissue samples were heated in a digestion block in the presence of H2SO4, K2SO4 and CuSO4. The digestion converts P species to ortho-phosphate and converts N compoun ds to ammonia. The residue from digestion was cooled and filte red through Whatman grade No.2 filter paper for analysis. Nitrogen and P analyses were perf ormed using a Seal AQ2 discrete analyzer. Phosphorus uptake by forage crops was calcu lated as the product of plant tissue P concentration and dry matter yield. Water Sampling and Analyses Lysim eter (water) samples were collected from 30 May 2008 to 23 Nov. 2008 at 14-d intervals. Samples were collect ed using hand-pumped syringes connected with long sampling tubes. Caps of the lysimeters were remove d then sampling tubes were extended into the lysimeters for the water. Lysimeter samples we re contained in 20-mL bottles in the field and were filtered through 0.45-m membrane in the laboratory. Samples were analyzed for ortho-P (EPA Method 365.3), NO2-N plus NO3-N (EPA Method 352.1) and NH4-N (EPA Method 350.1) using a Seal AQ-2 discrete analyzer At times in the beginning and the end of rainy season, less water was colle cted from the field and not e nough for all the analyses; then ortho-P was analyzed as first priority.

PAGE 26

26 Statistical Analysis Data were analyzed using SAS softwa re (L ittelet al., 1996; SAS Inc., 2001). Differences among treatments were statistically analyzed with a completely randomized design (CRD) using the general linear mode l (PROC GLM) of the SAS software. The statistical model tested the effects of fo rage species, year and their interactions on soil P concentrations, P uptake, tissue P and N concentrations and water quality. Because initial soil P concentrations are expected to aff ect the effectiveness of crops to remediate P, initial P levels were included in the statistical analysis as a covariate. Mean separations were performed using Fishers protected LSD (least significant difference) method at significance ( ) level of 0.05.

PAGE 27

27 Table 2-1. Species, cultivars, and plot numbers assigned to warm-season grasses screened for high capacity P-uptake in the small plot study Treatment ID Common name Scientific Name Cultivar Plot No. 1 Bahiagrass Paspalum notatum Pensacola 11,29,37 2 Bahiagrass Paspalum notatum Argentine 6,22,38 3 Mulato Brachiaria sp. Mulato 5,32,42 4 Stargrass Cynodon nluemfuensis Ona 7,15,33 5 Stargrass Cynodon nluemfuensis Florico 4,24,27 6 Bermudagrass Cynodon dactylon Tifton 85 16,31,40 7 Bermudagrass Cynodon dactylon Jiggs 17,19,44 8 Bermudagrass Cynodon dactylon Florakirk 10,25,35 9 Bermudagrass Cynodon dactylon Coastcross-22,21,39 10 Elephantgrass Pennisetum purpureum Merkeron 1,14,34 11 Sugarcane Saccharum oficcinarum CP 78-16209,28,41 12 Limpograss Hermathria altissima Floralta 8,13,43 13 Guineagrass Panicum maximum Mombaca 12,20,30 14 Digitgrass Digitaria eriantha Pangola 18,23,36 15 Switchgrass Panicum virgatum Alamo 3,26,45 Table 2-2. Species, cultivars, and plot assign ed to warm-season grasses selected for field evaluation of P accumulation in the large plot study Treatment ID Common name Scientific Name Cultivar Plot No. 1 Sugarcane Saccharum officinarum CP 78-16203,10,11,16 2 Elephantgrass Pennisetum purpureum Merkeron 2,7,12,13 3 Switchgrass Panicum virgatum Alamo 1,6,8,14 4 Stargrass Cynodon nlemfuensis Ona 4,5,9,15

PAGE 28

28 Table 2-3. Harvest protocols for the small plot and large plot studies. Harvest No. Date Protocol Note 1 7 Sep. 2007 II. 2 17 Oct. 2007 I 3 27 Nov. 2007 II 4 12 Mar. 2008 III 5 24 Apr. 08 I 6 5 Jun. 08 II 7 17 Jul. 08 I 8 28 Aug. 08 II 9 9 Oct. 08 I In 2008, several plots in the small plot study were infested with excessive weed, and were not harvested. Switchgrass plots were not harvested throughout the year. 10 20 Nov. 08 II* Sugarcane plots were harvested on 12 Mar. 2009 I denotes that all plots ex cept sugarcane and elephantgrass plots were harvested. II denotes that all plots except s ugarcane plots were harvested. III denotes that all plots were harvested. Figure 2-1. Location of th e experiment site

PAGE 29

29 3CHAPTER 3 RESULTS Small Plot Study Mehlich-1 Phosphorus There was c onsiderable spatial variability associated with the initial soil P concentrations in the Ap horizons of the small plots (Figure 3-1). This was likely due to pasture management (i.e. location of f eed and water trough) and non-uniform manure deposition over the field by the dairy operations. Average initial soil test P in the Ap horizon (695 mg kg-1) was considered very high according to the IFAS soil test interpreta tion (Mylavarapu and Kennelley, 2002). Mean Mehlich-1 P (M1P) concentrations decreased from 695 to 506 mg kg-1 (as much as 27%) over the 2-yr study period ( P < 0.01; Figure 3-2A). Greater decreases in soil-test-P values were observed in 2008 compared to 2007 ( P = 0.02). There was no significant effect of the interaction between fora ge species and year ( P = 0.83). During the 2-yr study, plots cultivated with forage species exhibited an average M1P decrease of 198 mg kg-1 compared to 55 mg kg-1 for the control plots ( P = 0.01; Figure 3-3A). Significant differences in M1P were observe d among the cultivars (Figure 3-4A). Mulato plots exhibited the largest decreases in M1P values, whereas sugarcane and limpograss plots exhibited the smallest decreases. Water-Extractable Phosphorus Average WEP across all 48 of the sm all plots decreased from 97.1 to 53.0 mg kg-1 from the beginning of 2007 to the end of 2008 ( P < 0.01; Figure 3-2B). A larger decrease in WEP values was observed in 2007 than in 2008 ( P < 0.01). Similar to M1P data, the interaction between species and year was not significant ( P = 0.69). In contrast to the Mehlich-1 P data, ther e was no treatment effect on WEP values ( P = 0.14; Figure 3-3B). Regardless of the forage spec ies, water-extractable P values in all plots

PAGE 30

30 was reduced after the 2-yr study (Figure 3-4B). The lack of WEP response to forage uptake was likely because WEP was designed as an environmental soil test, not an agronomical soil test related to the crop response. Because WEP is an indicator of soil labile P, the decrease of soil WEP across the study area may suggest that labile P may had been leached out of the surface horizon. The fact that the decrease in 2007 was greater than 2008 suggests that a significant amount of labile P was leached out in the first year and less labile P was available to be leached out in the second year. The hypothesis that significant amounts of P were leached from the Ap horizon was further investigated in the large plot study. Dry Matter Yield Average crop DM yields by year varied considerably am ong the various species ( P < 0.01; Figure 3-5). Average DM yield ranged from 11 Mg ha-1 yr-1 for Pensacola bahiagrass to 44 Mg ha-1 yr-1 for elephantgrass. Dry matter yields we re, in general, consistent with those reported by Woodard and Prine (1993), Newman et al. (2008), Newman (2008a,b), Mislevy (2006), Woodard et al. (2007) and Legendre a nd Burner (1995). Switchgrass showed poor persistence and plots were not harvested in 2008 because of excessive weed infestation. The two cultivars of bahiagrass (Pen sacola and Argentine) did not exhibit significant differences in DM yield; neither did the two cultivars of st argrass (Florico and Ona stargrass) or the four cultivars of bermudagrass. Elephantgrass and sugar cane, also referred to here as tall grasses, had significantly greater DM yields (44 Mg ha-1yr-1 and 42 Mg ha-1 yr-1 respectively) than the other species. The effect of year and the in teraction between year and species were not significant ( P = 0.14). Tissue Phosphorus Concentrations Tissue P con centration is a useful indicat or of P removal potential. Total tissue P concentrations varied from 1.5 g P kg-1 for sugarcane to 4.4 g P kg-1 for guineagrass (Figure

PAGE 31

31 3-6). On average, shorter grasses exhibite d tissue P concentrati ons of around 4 g P kg-1. The two tall-grass species had signi ficantly lower tissue P concentrations than the other forage species. Elephantgrass had an averag e tissue P concentr ation of 2.8 g P kg-1, and sugarcane averaged 1.5 g P kg-1. Goorahoo et al. (2005) evaluated the nutri tion and growth of elephantgrass (Pennisetum sp .) on 1.2-ha plots in California a nd reported an average tissue P concentration of 7 g P kg-1 in a 60-d study. However, the DM yields of 3800 kg ha-1 observed by Goorahoo et al. (2005) were much less than observed here, which could explain the much greater tissue P concentration than what was found here. Tissue Nitrogen Concentrations The shorter-growing grasses and elephant grass were fertilized at 360 kg N ha-1 in 2007 and at 540 kg N ha-1 in 2008, whereas sugarcane plots received 200 kg N ha-1 yr-1 in both years. The relatively high levels of N fer tilization recommended by IFAS are typically not used by the majority of producers because of costs. The high N fertilization rates used in this study were intended to maximize DM yield and, consequently, P removal rates. Total tissue N concentrations varied from 7 g N kg-1 for sugarcane to 23 g N kg-1 for guineagrass (Figure 3-7). Similar to tissue P data, tissue N concentr ations for the shorter-growing grasses (around 20 g N kg-1) were greater than those for the ta ller grasses. Elephantgrass tissue N concentrations averaged 16 g N kg-1, and sugarcane averaged 7 g N kg-1. The tissue N concentrations for elephantgrass were simila r to values reported by Goorahoo et al. (2005), average tissue N concentration of 20 g N kg-1 during a 60-d growth period. Phosphorus Uptake Phosphorus uptake was significan tly affected by forage species and varied from 24 kg ha-1 yr-1 for switchgrass to 126 kg ha-1 yr-1 for elephantgrass (Fi gure 3-8). As observed by Gaston et al. (2003), P uptake varied among spec ies and was not necessarily related to tissue P concentration. Although elephantgrass exhib ited lower tissue P concentrations than the

PAGE 32

32 shorter-growing grasses, P uptake was greater because elephantgrass produced greater DM yields than the shorter grasses. Sugarcane, however, did not exhi bit greater P removal compared to the shorter grasses. The phytoremed iation benefit of high DM yield of sugarcane was undermined by its low tissue P concentrat ions. Gaston et al. (2003) observed that bahiagrass ( Paspalum notatum Flugge), bermudagrass [ Cynodon dactylon(L.) Pers.] and switchgrass ( Panicum virgatum L.) removed 122, 128, and 146 kg P ha-1 yr-1 P respectively. The P removal rates observed by Gaston et al. (2 003) were much greater than those found in our study, possibly due to optimal conditi ons for DM production and P uptake under greenhouse conditions. Phosphorus Mass Balance The overall results from the small plot st udy were compiled into Table 3-2, including DM yield, P uptake, and gross P change from soil for 10 forage species. Based on the P removal rate, elephantgrass appears to be the more suitable species in removing P from the soils. Roughly 10 yr (695 mg kg-1 observed STP 50 mg kg-1 target STP / 67 mg P kg-1 yr-1 ~= 10 yr) is the estimated time required to lower th e soil test P to the agronomy threshold. Large Plot Study Mehlich-1 Phosphorus Unlike in th e small plot study, there was no significant spatial variability associated with M1P concentrations was observed in the large plot study (data not presented). On average, the initial soil-test-P va lue in the Ap horizons was 232 mg kg-1, which is considered very high according to the IFAS soil test in terpretation (Mylavarapu and Kennelley, 2002). Mean Mehlich-1 P concentrations in th e Ap horizons decreased from 232 mg kg-1 to 119 mg kg-1 (a 49% decrease) over the 2-yr study period ( P < 0.01; Figure 3-9A). Sugarcane plots exhibited smaller decrease of M1P values in the Ap horizons th an other treatments ( P = 0.03; Figure 3-10A). This trend is consistent with the small pl ot study (Figure 3-4A). There

PAGE 33

33 were no differences observed among species in M1P decrease in the Bh and E horizons (Figure 3-10A). Water-Extractable Phosphorus Mean W EP values for Ap horizons across all 16 plots decreased from 50 to 24 mg kg-1 from the beginning of 2007 to the end of 2008 ( P < 0.01; Figure 3-9B). Conversely, significant increases in average WEP values (from 16 to 24 mg kg-1) were observed in the Bh horizons ( P = 0.01). The simultaneous decreases of WEP values in the Ap horizon and increase of WEP values in the Bh horizon supported the hypothesis th at a significant amount of P was leached to deeper soil depths. No significant change in WEP in the E horizon was observed from the beginning to the end of th e study, because E horizons have the least P retaining capacity (Brady and Weil, 1999). Sugar cane plots exhibited the smallest decreases in WEP values in the Ap horizon (Figure 3-10B). No grass species eff ects were observed in the Bh and E horizons. Crop Yield, Tissue Phosphorus/Nitrogen Concentration and Phospho rus Uptake As in the small plots, elephantgrass and sugarcane produced greater yields than the other forage species (Figure 3-11). Dry matte r yields by elephantgrass, sugarcane and stargrass (Ona) were similar with those observed in the small plot study, and comparable with the literature. Unlike in the small plots, switchgrass survived the 2-yr large-plot study, therefore DM yield by switchgrass in the large pl ots were greater than those observed in the small plots. Year significan tly affected DM yields ( P < 0.01), and the year by species interaction was significant (P < 0.01; Table 3-3). Elephantgra ss and stargrass yielded more in 2008 than 2007, likely occurred because more harvests were conducted in 2008. Sugarcane was harvested once per year and DM yields in 2007 and 2008 were similar. More harvests in 2008 did not increase switchgrass DM yiel ds possibly due to poor persistence.

PAGE 34

34 Data from the large-plot study were consistent with the small-plot study and confirmed that tall grasses (elephantgr ass and sugarcane) have sign ificantly lower tissue P and N concentrations than other switchgrass and stargrass (Figure 3-12, Figure 3-13). Phosphorus uptake was significan tly affected by forage species. Phosphorus uptake was greater by elephantgrass than the other specie s (Figure 3-14). Despite high DM yields, P uptake by sugarcane was less than by stargrass, because of low sugarcane tissue P concentrations. Year and the interaction year x species significantly affected P uptake (Table 3-3). The species P uptake followed a si milar pattern as species DM yield. The overall results from the large plot study are compiled into Table 3-4, including DM yield, P uptake, and soil P change by the four forage species. In summary, the large-plot study confirme d the findings observed in the small plot study that elephantgrass produced greater DM yields and reduced soil P concentrations. Sugarcane, due to its low tissue P concentratio n, exhibited P removal capacity similar to the small grasses. The study also confirmed that af ter the 2-yr study, soil P remained above the agronomic sufficiency level. No P addition was needed to maintain adequate forage production. Leachate P Concentrations There were significan t differences in mean ortho-P concentrations in the samples collected from the 60and the 90-cm depth across the sampling dates, where ortho-P concentrations at the 60-cm depth were on aver age greater than those at the 90-cm depth. ( P < 0.01; Figure 3-16). At the 60cm depth, ortho-P concentrations varied from 0.012 to 7.9 mg L-1, and were not affected by sampling dates. Species effects were significant for ortho-P concentrations at the 60-cm depth ( P = 0.01; Figure 3-17). At the 90-cm depth, ortho-P concentrations ranged from 0.0057 to 6.6 mg L-1, and were not affected by species or by sampling dates. The pattern of ortho-P concentr ations at the 60-cm depth appeared inversely

PAGE 35

35 correlated with the pattern of P uptake by the sp ecies (Figure 3-14). This suggested that the P uptake by species did reduce the leachate P concentration. In cont rast, no relationship between leachate-P and forage P upt ake was observed at the 90-cm depth. Leachate N Concentrations In 2008, small-grass plots and elephantgras s plots received a total of 540 kg N ha-1 fertilization and sugarcan e plots received 200 kg N ha-1. These are the highest levels of N fertilization recommended by IFAS and are greate r than those used by forage producers. Such N fertilization levels were chosen to maxi mize the DM yield and, consequently, P uptake rates. Such N application rates could result in contamination of groundw ater, so the impact of high-level N fertilization on N losses was investigated. Leachate NH4 concentrations remained relatively low across the sampling period (Figure 3-18). Species and the sampling events had no significant effects on NH4 concentrations at the 60and 90-cm depth. The levels of NOx at both depths were very high, ranging from 32 to 150 mg L-1 for the 60-cm depth and from 32 to 121 mg L-1 for the 90-cm depth. The NO3 concentrations observed in this study far exceed the U.S. Envi ronmental Protection Agency drinking water standard of 10 mg L-1. Baker and Johnson (1981) summarized results of a 4-yr study with corn that related the tile drainage NO3 concentrations to the N fertilization rate. Nitrate concentrations for the plot receiving 45 to 50 kg N ha-1 yr-1 averaged 20 mg L-1; for the plot receiving 120 kg N ha-1 yr-1, the NO3 concentration was 40 mg L-1. In our project, the N fertilization rate was 540 kg N ha-1 yr-1, four times greater than th e high-level rate used by Baker and Johnson (1981), thus the high NO3 concentrations are expected. With exception of the data collected in 7 August, 2008 when the field was flooded (Figure 3-15), the NO3 concentrations at both depths appear to follow a decreasing trend over time (Figure 3-19). However, due to the high vari ability within the data, statistical analysis

PAGE 36

36 failed to confirm the trend. Species had an effect on NO3 concentrations at either depth ( P < 0.01). Switchgrass plots exhi bited the greatest NO3 concentrations of al l treatments at the 60-cm depth. This could be partially due to th e low DM yield resulting in low N uptake of switchgrass. At the 90-cm depth s ugarcane plots had the greatest NO3 concentrations of all treatments. Sugarcane plots were fertili zed once in March 2008 at the 200 kg N ha-1 rate, whereas the other treatm ents received 90 kg N ha-1 (elephantgrass plots received 180 kg N ha-1) per harvest, therefore a significant amount of N had leached into deep horizons after the fertilization for the sugarcane plots. To summarize the water quality data, we obs erved that ortho-P concentrations in the leachates collected at the 60-cm depth appeared to be related to the P uptake of the species. The more P a species could take up, the greater ability it had to lowe r the ortho-P in the shallow (60-cm) water. The high N rate of fertili zation used in this study led to substantial N leaching, which could cause other issues su ch as N contamination of the groundwater. Further studies on the long-term effects of N fertilization on N losses are needed.

PAGE 37

37 Table 3-1. Summary of the initial soil test P va lues across the small plot study area (n = 48) Soil P Mean Std errorMedianStd devMinimumMaximum Range mg kg-1 Water-Extractable P 97 4.393 30 42 159 117 Mehlich-1 P 695 56 652 389 190 1520 1330 Table 3-2. Average dry matter yield, P uptake and soil P removal from soil for ten forage species in the 2-yr small plot study Crop No. of Harvests Dry matter Yield (Mg ha-1) Tissue P conc. (g kg-1) P removal (kg ha-1) STP decrease(mg kg-1) Bahiagrass 10 22 () cd 4.4 (.1) abc96 () cd 166 () b Bermudagrass 10 47 () b 4.1 (.1) cd 184 () b 228 () ab Digitgrass 10 24 () cd 4.7 (.2) ab 108 () c 180 () ab Elephantgrass 6 89 () a 3.0 (.2) e 253 () a 160 () b Guineagrass 10 26 () cd 4.8 (.3) a 113 () c 195 () ab Limpograss 10 24 () cd 4.3 (.2) bcd98 () c 96 () b Mulato 10 34 () bc 4.3 (.2) bcd128 () c 347 () a Stargrass 10 33 () bc 4.3 (.1) bc 138 () bc 244 () ab Sugarcane 2 83 () a 1.7 (.1) f 128 () c 99 () b Switchgrass 4 13 () d 3.9 (.3) d 47 () d 161 () b Values followed by the plus/minus sign represents one standard error. Means with the same letter in a column are not significantly different (Fishers LSD test, P<0.05) Note: Soil-test-P change was measured by th e gross change of Mehlich-1 P from the beginning of 2007 to the end of 2008 Table 3-3. Mean dry matter yields, tissue P co ncentrations, P uptake by year and by species in the large plot study Species Dry matter (Mg ha-1) Tissue Pconc. (g kg-1) P uptake (kg ha-1) 2007 2008 2007 2008 Elephantgrass 35 (.4) a 63 (.0) b2.8 (.2) b 102 () a 176 () b Stargrass 15 (.3) a 30 (.7) b 4.0 (.1) a 64 () a 116 () b Sugarcane 38 (.2) a 35 (.2) a1.8 (.1) c 71 () a 58 () a Switchgrass 15 (.3) a 17 (.4) a3.9 (.1) a 59 () a 64 () b Values followed by the plus-minus sign represents one standard error. Year means within a row and response variab le are not different if followed by the same letter (Fishers LSD test, P > 0.05)

PAGE 38

38 Table 3-4. Average dry matter yield, P uptake an d soil P removal from soil for four forage species in the 2-yr large plot study Species Dry Matter Yield (Mg ha-1) Tissue P conc. (g kg-1) P removal (kg ha-1) STP decrease (mg kg-1) Elephantgrass 98 (.0) a 2.8 (.2) b 279 () a 125 () a Stargrass 45 (.9) c 4.0 (.1) a 180 () b 137 () a Sugarcane 73 (.4) b 1.8 (.1) c 129 () c 76 () b Switchgrass 32 (.4) d 3.9 (.1) a 123 () c 113 () a Values followed by the plus/minus sign represents one standard error. Means within a column followed by the same lett er are not significantly different (Fishers LSD test, P > 0.05) Note: Soil-test-P change was measured by th e gross change of Mehlich-1 P from the beginning of 2007 to the end of 2008

PAGE 39

39 Figure 3-1. Initial water-extractable P concentr ation in the small-plot study. Map created using ArcMapTM 9.2 software package.

PAGE 40

40 A Soil Sampling Date 02/200711/200711/2008Mehlich-1 P (mg kg -1 ) 0 200 400 600 800 1000 a ab b B Soil Sampling Date 02/200711/200711/2008Water Extractable P (mg kg -1 ) 0 20 40 60 80 100 120 a b b Figure 3-2. Average soil P concentrations in the small plot study over the study period measured as A) Mehlich-1 P and B) Wa ter-Extractable P. Each data point represents the average of 48 samples. E rror bars indicate one standard error. Values labeled with the same letters are not statistically different ( P > 0.05; Fisher's Least Signi ficant Difference). ControlTreatmentsDecrease of Mehlich-1 P (mg kg-1) 0 50 100 150 200 250 a bA ControlTreatmentsDecrease of Water Extractable P (mg kg-1) 0 10 20 30 40 50 60 a aB Figure 3-3. Average decreases of soil test P values during the 2-yr study by different treatment groups. A) Decrease of Me hlich-1 P and B) Decrease of Water-Extractable P. Means followed by the same letter are not significant different ( P > 0.05) (Fisher's Least Significant Difference). Bars represent the average of 3 samples (control) and 45 samples (treatments), respectively. Error bars indicate standard error.

PAGE 41

41 A Mulato Florakirk (BMD) Tifton 85 (BMD) Florico (STG) Ona (STG) Guineagrass Digitgrass Pensacola (BHG) Coastcross-2 (BMD) Switchgrass Elephantgrass Argentine (BHG) Jiggs (BMD) Sugarcane Limpograss Decrease of Mehlich-1 P (mg kg-1) 0 100 200 300 400 500 Controla ab ab abc abc abc abc abc abc abc abc bc bc c c B Tifton 85 (BMD) Sugarcane Florakirk (BMD) Florico (STG) Elephantgrass Mulato Jiggs (BMD) Coastcross-2 (BMD) Switchgrass Guineagrass Argentine (BHG) Ona (STG) Pensacola (BHG) Digitgrass Limpograss Decrease of Water Extractable P (mg kg-1) 0 20 40 60 80 100 Controla a a a a a a a a a a a a a a Figure 3-4. Average small plot soil test P decr eases after 2 years as affected by 15 forage cultivars. A) Decrease of Mehlich-1 P a nd B) Decrease of Water-Extractable P. Means followed by same letter ar e not significantly different ( P >0.05). Error bars indicate standard error of the mean with in the group. Control lines represent the average decrease of soil P in the control plots.

PAGE 42

42 Elephantgrass Sugarcane Jiggs (BMD) Florakirk (BMD) Tifton 85 (BMD) Coastcross-2 (BMD) Florico (STG) Mulato Ona (STG) Guineagrass Digitgrass Limpograss Argentine (BHG) Pensacola (BHG) Switchgrass Dry Matter Yield (103 kg ha-1 yr-1) 0 10 20 30 40 50 60 a a b bc bc c cd cde def ef efg efg fg fg g Figure 3-5. Average dry matter yields of 15 fora ge cultivars in the small plot study. Bars represent the average of all samples colle cted in all harves ts over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicate one standard error of the mean within the group. Guineagrass Digitgrass Ona (STG) Pensacola (BHG) Florakirk (BMD) Argentine (BHG) Limpograss Florico (STG) Tifton 85 (BMD) Coastcross-2 (BMD) Mulato Jiggs (BMD) Switchgrass Elephantgrass Sugarcane Tissue P Concentrations (g kg-1) 0 1 2 3 4 5 6 a a ab ab ab ab abc abc bc c c c c d e Figure 3-6. Tissue total Kjeldahl P of 15 fora ge cultivars. Bars represent DM-weighted average TKP of all samples collected in all harvests over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicate one standard error of the mean within the group.

PAGE 43

43 Guineagrass Tifton 85 (BMD) Coastcross-2 (BMD) Florico (STG) Pensacola (BHG) Florakirk (BMD) Jiggs (BMD) Digitgrass Argentine (BHG) Ona (STG) Limpograss Mulato Switchgrass Elephantgrass Sugarcane Tissue N Concentrations (g kg-1) 0 5 10 15 20 25 30 a ab ab ab ab ab abc bc bc bc cd cd d d e Figure 3-7. Tissue total Kjeldahl N concentrations of 15 forage cultivars in the small plot study. Bars represent DM-weighted average TKN of all samples collected in all harvests over 2 yr. Means followed by same letter are not signif icantly different ( P >0.05). Error bars indicate one standard error of the mean within the group. Elephantgrass Jiggs (BMD) Florakirk (BMD) Tifton 85 (BMD) Coastcross-2 (BMD) Florico (STG) Sugarcane Mulato Ona (STG) Guineagrass Digitgrass Limpograss Argentine (BHG) Pensacola (BHG) Switchgrass Phosphorus Uptake (kg ha-1 yr-1) 0 20 40 60 80 100 120 140 160 a b cb cb cd cde fde fde fde fe f f f f g Figure 3-8. Phosphorus uptake by 15 forage cultiv ars in the small plot study. Bars represent the average of all samples collected in all harvests. Means followed by same letter are not significantly different ( P >0.05). Error bars indica te one standard error of the mean within the group.

PAGE 44

44 ASoil Sampling Date 02/200711/200711/2008Mehlich-1 P (mg kg-1) 0 50 100 150 200 250 300 Ap E Bh BSoil Sampling Date 02/200711/200711/2008Water Extractable P (mg kg-1) 0 10 20 30 40 50 60 Ap E Bh Figure 3-9. Average soil P concentrations in the large plot study across the study period measured as A) Mehlich-1 P and B) Wa ter-Extractable P. Each date point represents the average of 16 samples. E rror bars indicate one standard error.

PAGE 45

45 A ElephantgrassStargrassSugarcaneSwitchgrassDecrease of Mehlich-1 P (mg kg-1) -100 -50 0 50 100 150 200 Ap E Bh a a a b B ElephantgrassStargrassSugarcaneSwitchgrassDecrease of Water Extractable P (mg kg-1) -20 -10 0 10 20 30 40 Ap E Bh ab a ab b Figure 3-10. Average soil test P decrease in di fferent horizons after th e 2-yr large plot study as affected by species. A) Decrease of Mehlich-1 P and B) Decrease of Water-Extractable P. Means followed by sa me letter are not significantly different ( P >0.05) in Ap horizon. There were no significant differences in Bh and E horizons ( P >0.05). Error bars indicate standard error of the mean within the group.

PAGE 46

46 ElephantgrassSugarcaneStargrassSwitchgrassDry Matter Yield (103 kg ha-1 yr-1) 0 10 20 30 40 50 60 70 a b c d Figure 3-11. Average dry matter yields for the f our treatments in the large plot study. Bars represent the average of all samples colle cted in all harves ts over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicate one standard error of the mean within the group. StargrassSwitchgrassElephantgrassSugarcaneTissue P Concentrations (g kg-1) 0 1 2 3 4 5 a a b c Figure 3-12. Tissue Total Kjeldahl P concentrations of the four forage cultivars in the large plot study. Bars represent DM-weighted av erage TKP of all samples collected in all harvests over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicat e one standard error of the mean within the group.

PAGE 47

47 StargrassSwitchgrassElephantgrassSugarcaneTissue N Concentrations (g kg-1) 0 5 10 15 20 25 a a b c Figure 3-13. Tissue Total Kjeldahl N concentrations of the four forage cultivars in the large plot study. Bars represent DM-weighted av erage TKN of all samples collected in all harvests over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicat e one standard error of the mean within the group. ElephantgrassStargrassSugarcaneSwitchgrassPhosphorus Uptake (kg ha-1 yr-1) 0 20 40 60 80 100 120 140 160 180 a b c c Figure 3-14. Phosphorus uptake values for the f our forage cultivars in the large plot study. Bars represent the average of all samples collected in all harv ests over 2 yr. Means followed by same letter are not significantly different ( P >0.05). Error bars indicate one standard error of the mean within the group.

PAGE 48

48 -140 -120 -100 -80 -60 -40 -20 0 20 40 4/12/20086/1/20087/21/20089/9/200810/29/200812/18/2008 DateWater Table (cm)0 2 4 6 8 10 12 14 16 18Rainfall (cm) Water Table (cm) Rainfall (in) Figure 3-15. Precipitation and water-table depth during the growing season of 2008.

PAGE 49

49 Sampling Date May-08 Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Ortho-P Concentrations (mg L-1) 0.0 0.5 1.0 1.5 2.0 2.5 60 cm 90 cm Figure 3-16. Average leachate ortho-P concentr ations at two depths collected across the growing season in the large plot study. Each data point represents 16 samples collected in every sampling event. Error bars indicate one sta ndard error of the mean within the group. Differences among sampling events are not significant for both depths ( P > 0.05). ElephantgrassStargrassSugarcaneSwitchgrassOrtho-P concentrations (mg L-1) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 60 cm 90 cm a a ab b A AB B B Figure 3-17. Average leachate ortho-P concen trations at the 60and 90-cm depths for different treatments in the large plot study. Bars represent the mean ortho-P concentrations of all sampling dates fo r each treatment. Means followed by same letter are not significantly different in each depth ( P >0.05). Error bars indicate one standard error of th e mean within the group.

PAGE 50

50 Sampling Date Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 NH4 Concentrations (mg L-1) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 60 cm 90 cm Figure 3-18. Average leachate ammonium concen trations at two depths collected across the growing season in the large plot study. Each data points represents 16 samples collected in every sampling event. Error ba rs indicate standard error of the mean within the group. Differences among sampling events are not significant for either depth ( P > 0.05). Sampling Date Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 NOx Concentrations (mg L-1) 0 20 40 60 80 100 120 140 160 180 200 60 cm 90 cm Figure 3-19. Average leachate nitrate concentrations at two depths collected across the growing season. Each data points represen ts 16 samples collect in every sampling event. Error bars indicate standard erro r of the mean within the group. Differences among sampling events are not significant for both depths ( P > 0.05).

PAGE 51

51 StargrassElephantgrassSugarcaneSwitchgrassNOx concentrations (mg L-1) 0 20 40 60 80 100 120 140 160 180 60 cm 90 cm A A AB B a ab c b Figure 3-20. Average leachate nitrate concentra tions at 60and 90-cm depths for different treatment in the large plot study. Bars re present the mean ortho-P concentrations of all sampling dates for each treatment. Means followed by same letter are not significantly different at each depth ( P >0.05). Error bars indicate one standard error of the mean within the group.

PAGE 52

52 4CHAPTER 4 DISCUSSION Soil-Phosphorus Decrease, Phosphorus Uptake and Phosphorus Leaching Results of the project indicated that P upt ake only accounts for part of the soil-test-P decrease (Table 3-2). Poor m ass balance wa s observed between crop P uptake and soil P decrease (15 79% of P uptake relative to decrea se of soil test P). Similar results have been reported in the literature. For instance, Brown (2006) studied the winter forage-corn silage system and found that P uptake only accounted fo r half of the soil P decrease. The authors suggested that the natural P-so rption processes also contributed to reduction in soil-test-P concentrations. Similarly, McDowell and Sharpley (2001) studied the loss of P in subsurface flow from three cultivated soils [Alvira ( Aeric Fragiaquult ), Berks ( Typic Dystrudept ) and Watson ( Typic Fragiudult ) channery silt loams]. They suggest ed that poor P balance could be induced due to the large amount of P lost by leaching. Nair et al. (1995) studied soil P fractions of dairy systems in South Florida's Lake Okeechobee watershed. They that no major shifts in labile P (defined as 1 M NH4Cl-extractable P) to more stable soil P forms were observed even after the manureimpacted soil had been abandone d for 12 yr. Therefore, given the soil and environmental conditions where our study was conducted, it is reasonable to believe that P leaching rather than shifts in P forms in the soil is main reason for the poor mass balance observed in our study. The simultaneous decrease of WEP in Ap horizons and increase of WEP in Bh horizons in the large plots furt her supported the hypot hesis that significan t amounts of P were leached to deeper soil depths. Th e leaching of labile P from Pimpacted surface soil has been well documented in the literature. Phosphor us leaching through sandy soils receiving excessive amounts of manure was also observed by (Novaket al., 2000). Heckrath et al. (1995) found that P leached increased rapidly when soil test P exceeded a certain point (60 mg Olsen P kg-1 in their study), which they called the chan ge point. Their model suggested that as

PAGE 53

53 soil-test-P concentrations increase, high-ener gy sorption sites become increasingly saturated, reducing a soils overall P-retaining strength and increasing the P-leaching potential. Although in our project we did not measure the change point, the extremely high soil-test-P values at the research location suggest that P was likely subject to leaching. Dry Matter, Tissue Phosphorus Concentrations, and Phosphorus Uptake The effectiveness of phytorem ediation depends on the total P removed from the target field, thus it is important to increase the DM yield or the tissue P c oncentrations or both (Novak and Chan, 2002). Results in the projects s how that DM yields, tissue P concentrations and P uptake differed among the forage species. Tall grasses yielded greater DM but had lower tissue P concentrations than short gra sses. Thus, elephantgrass took up more P than other forages because of its high DM yield. Phosphorus uptake by sugarcane was similar to that of the short grasses due to the low sugarcane tissue P con centrations. It appeared that switchgrass was not adapted to intermittent high water table cond itions commonly found during the summer months in so uth Florida (Figure 3-15) and wa s not persistent with weeds. Other factors not fully tested in this projec t, such as soil drainage, planting timing, row spacing, weed control, fertilization and harvest interval could also affect potential DM yields of forage crops (Stricker, 2003). For example, wider row spacing would result in lower DM yield of tall grasses and sugar canes, while narrower spacing incr ease stand density resulting in higher DM yield (Stricker, 2003). Though the study of most of the parameters was out of scope of the project, but fertilization management was considered. Guevara et al. (2000) studied th e effect of N fertilization on forage yields in Midwest Argentina and found that each kilogram of N applied (at rate of 25 kg N ha) increased rangeland forage production by 12.4 kg. Andrade et al. (2000) studied the effects of N and K fertilization on elephantgrass (cv. Napier) yields in an incomplete factorial design (N rates of 20, 50, 100, 200, 300, 350 and 380 kg ha-1 x K rates of 16, 40, 80, 160, 240, 280 and 304 kg

PAGE 54

54 ha-1). Elephantgrass biomass yields increased 86% in response to N and K fertilizers. Because DM yield is critical for phytoremediation and bioenergy production, continuous N fertilization could be require d, which would be expensive and may increase the risk of N leaching into groundwater. In this project, no hyperaccumulator of P wa s identified. Short grasses had higher tissue P concentrations (around 4 g P kg-1) than tall grasses (3 g P kg-1 for elephantgrass, 1.6 g P kg-1 for sugarcane). Novak and Chan (2002) sugges ted that breeding approach could be used to increase the tissue P concentration in DM of plants. For example, Delhaize and Randall (1995) found that a mutant strain of Arabidopsis thaliana (L.) could accumulate as high as 14.5 g P kg-1 of DM in leaves, whereas a normal strain accumulated 6.8 g P kg-1. However, no study has been conducted to validate the adapta bility of such mutant strains to the field conditions. Although tissue P concentration is important in terms of phytoremediation, in the two-fold use of forage crops to remediate P-impacted soils and to produce bioenergy, DM yield is more important. The more DM a forage can produce, the more P can be taken up and the more biomass available for energy producti on. The results this project suggest that elephantgrass can be the most suitable sp ecies for both P remediation and bioenergy purposes. Ground Water Effects The surface soils of the Lake Okeechobee watershed are m ainly sands, with very low nutrient retention capacity. Wate r table fluctuated between the subsurface horizon and the soil surface for extended periods each year (Gr aetz and Nair, 1995). Thus, there is significant potential for subsurface nutrient movement when groundwater carries nutr ients vertically or laterally.

PAGE 55

55 High N-fertilization rates were used in the project to promote high DM yield. However, results suggested that such high N rates lead to substantial N leaching to the shallow groundwaters. Therefore, further studies are need ed to address the impacts of N fertilization on water quality and DM yield. Forage species showed different capacit ies to accumulate nutrients and reducing nutrient leaching. Ortho-P concen trations in the leachates co llected at the 60-cm depth appeared to be inversely rela ted to the P uptake of the speci es. Elephantgrass took up more P than other species and reduced the ortho-P c oncentrations more than the other species. Species effects were also significant on leachate NO3 concentrations. Switchgrass plots showed highest leachate NO3 concentrations, possibly due to low N-uptake by switchgrass. Further studies are needed to determine l ong-term effects of forage grasses on the groundwater quality.

PAGE 56

56 5CHAPTER 5 CONCLUSIONS The phytorem ediation of P-impacted soils may require a long time to reduce soil P concentrations to acceptable levels. Despite th e smaller cost associated with establishment and maintenance of phytoremediation schemes compared to other remediation approaches, phytoremediation represents a costly alternative for the majority of the beef cattle producers. Thus, the two fold use of selected forage cr ops for both the phytorem ediation of P-impacted soils and the use of resulting biomass as a renewable energy source represent a potentially feasible alternative. Results from the 2-yr study showed a declin e in Mehlich-1 P in the surface horizon in response to crop uptake. Significant amounts of P were leached from Ap horizon to the subsurface horizon. Forage species differed in DM yield and P uptake potential. Among the shorter grasses, DM yield ranged from 11 to 28 Mg ha-1 yr-1, and P uptake rate ranged from 48 to 102 kg ha-1 yr-1. Elephantgrass resulted in greater DM yield (49 Mg ha-1 yr-1) and P uptake (139 kg ha-1 yr-1) than the other crop s. Although sugarcane DM yields were greater th an the shorter grasses, lower tissue P concentrations resulted in overall P uptake (64 kg ha-1 yr-1) that was similar to the shorter grasses. Forage species significantly affected water ortho-P concentrations at the 60-cm depth. Elephantgrass, due to its high P uptake, resulted in lower water ortho-P concentrations than other species. On the other hand, NO3 analysis suggested that high N rate of fertilization promoted substantial N leaching into the subs urface horizons, which could cause other issues such as N contamination of the groundwater. The first hypothesis of this project that soil P concentrations decrease over time in response to P uptake of forage crops was accep ted (in the small plot study). The second hypothesis that P removal capacity differs among grass species was also accepted.

PAGE 57

57 Elephantgrass exhibited the greatest annual DM yield and P uptake. Sugarcane produced greater DM yields, yet its lo w tissue P concentration undermin ed its P-uptake capacity. There was no conclusive evidence to support and/or reject the third hypothesis that groundwater quality (in terms of P concentrations) improves in response to P uptake of forage crops. Shallow groundwater P concentrations were less in the elephantgr ass plots than other treatments, but water sampling was conducte d for only one year. Additional study is necessary to investigate th e long-term effects of forage uptake on water quality. An evaluation of the conversion efficiency of the various forage species into bioenergy production was not part of this study. We assu med that DM yield was a good indicator of the amount of bioenergy producible. Thus, we c onclude that elephant grass is the most appropriate species to phytoremediate P-impact ed soils as well as to produce bioenergy. One concern with the growth of elephantgrass is the large amount of N required to maintain adequate DM yields, and possible environmental degradation associated with large N losses. Therefore, long-term studies evaluating the im pacts of N fertilization on elephantgrass DM yield, P removal potential, envi ronmental impacts and net energy balance are critically needed.

PAGE 58

58 LIST OF REFERENCES Agyin-Birikorang, S., O'Connor, G.A., 2007. Labili ty of drinking water treatm ent residuals (wtr) immobilized phosphorus: Aging and ph ef fects. Journal of Environmental Quality 36, 1076-1085. Andrade, A.C., da Fonseca, D.M., Gomide, J.A., Alvarez, V.H., Martins, C.E., de Souza, D.P.H., 2000. Elephant gra ss napier cv. Mass producti on and nutritive value under increasing levels of nitrogen and potassi um fertilizers. Revista Brasileira De Zootecnia-Brazilian Journal of Animal Science 29, 1589-1595. Baker, J.L., Johnson, H.P., 1981. Nitrate-nitrogen in tile drainage as aff ected by fertilization. Journal of Environmental Quality 10, 519-522. Brady, N.C., Weil, R.R., 1999. The nature and pr operties of soils, 12th ed. Prentice Hall, Upper Saddle River, NJ. Brown, B.D., 2006. Winter cereal-corn double crop forage production and phosphorus removal. Soil Science Society of America Journal 70, 1951-1956. Burkholder, J.M., Glasgow, H.B., 1997. Pfiest eria piscicida and ot her pfiesteria-like dinoflagellates: Behavior, impacts, and environmental controls. Limnology and Oceanography 42, 1052-1075. Cunningham, S.D., Berti, W.R., 1993. Remediation of contaminated soils with green plants an overview. In Vitro Cellular & Developmental Biology-Plant 29P, 207-212. Cunningham, S.D., Shann, J.R., Crowley, D.E., Anderson, T.A., 1997. Phytoremediation of contaminated water and soil. ACS Symposium Series 664, 2 7-2 7. Dao, T.H., 1999. Coamendments to m odify phosphorus extractability and nitrogen/phosphorus ratio in feedlot manure and composted manure. Journal of Environmental Quality 28, 1114-1121. Delhaize, E., Randall, P.J., 1995. Characteriza tion of a phosphate-accumulator mutant of arabidopsis-thaliana. Plant Physiology 107, 207-213. Delorme, T., Angle, J., Coale, F., Chaney, R., 2000. Phytoremediation of phosphorus-enriched soils. International Journal of Phytor emediation 2, 173-181. Dou, Z., Zhang, G.Y., Stout, W.L., Toth, J.D ., Ferguson, J.D., 2003. Efficacy of alum and coal combustion by-products in stab ilizing manure phosphorus. Journal of Environmental Quality 32, 1490-1497. Eghball, B., Binford, G.D., Baltensperger, D. D., 1996. Phosphorus movement and adsorption in a soil receiving long-t erm manure and fertilizer application. Journal of Environmental Quality 25, 1339-1343.

PAGE 59

59 Ehlert, P., Morel, C., Fotyma, M., Destain, J., 2003. Potential role of phosphate buffering capacity of soils in fertilizer management strategies fitted to environmental goals. Journal of Plant Nutrition and Soil Science 166, 409-415. Fixen, P.E., 2002. Soil test levels in north america. Better Crops 86, 12-15. Gaston, L.A., Eilers, T.L., Kovar, J.L., Cooper, D., Robinson, D.L., 2003. Greenhouse and field studies on hay harvest to remediate high phosphorus soil. Communications in Soil Science and Plant Analysis 34, 2085-2097. Goorahoo, D., Cassel, F., Adhikari, D., Ro thberg, M., 2005. Update on elephant grass research and its potential as a forage cr op, California Alfalfa and Forage Symposium. UC Cooperative Extension, Agronomy Research and Extension Center, Plant Sciences Department, University of Calif ornia, Davis, Visalia, CA. Graetz, D.A., Nair, V.D., 1995. Fate of phosphorus in florida spodosols contaminated with cattle manure. Ecological Engineering 5, 163-181. Guevara, J.C., Stasi, C.R., Estevez, O.R., Le Houerou, H.N., 2000. N and p fertilization on rangeland production in midwest argentin a. Journal of Range Management 53, 410-414. Havlin, J.L., Tisdale, S.L., Nelson, W.L., Beat on, J.D., 2004. Soil fertility and fertilizers: An introduction to nutrient management. Pr entice Hall, Upper Saddle River, NJ Heckrath, G., Brookes, P.C., Poulton, P.R., Goulding, K.W.T., 1995. Phosphorus leaching from soils containing different phosphorus conc entrations in the broadbalk experiment. Journal of Environmental Quality 24, 904-910. Hogstad, O., 1996. Accumulation of cadmium, copper and zinc in the liver of some passerine species wintering in centr al norway. Science of the Total Environment 183, 187-194. Jasinskas, A., Zaltauskas, A., Kryzeuiciene, A., 2008. The investigation of growing and using of tall perennial grasses as ener gy crops. Biomass & Bioenergy 32, 981-987. Kleinman, P.J.A., Sharpley, A.N., Wolf, A.M., Beegle, D.B., Moore, P.A., Jr., 2002. Measuring water-extractable phosphorus in manure as an indicator of phosphorus in runoff. Soil Sci Soc Am J 66, 2009-2015. Legendre, B.L., Burner, D.M., 1995. Bioma ss production of sugarcane cultivars and early-generation hybrids. Bi omass & Bioenergy 8, 55-61. Littel, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. Sas system for mixed models. SAS Inst., Cary, NC Luscombe, P.C., Syers, J.K., Gregg, P.E.H ., 1979. Water extraction as a soil testing procedure for phosphate. Commun. Soil Sci. Plant Anal 10, 1361-1369.

PAGE 60

60 Ma, L.Q., Komar, K.M., Tu, C., Zhang, W., Cai, Y., Kennelley, E.D., 2001. A fern that hyperaccumulates arsenic. Nature 409, 579-579. Macek, T., Mackova, M., Kas, J., 2000. Exploitation of plants for the removal of organics in environmental remediation. Biotechnology Advances 18, 23-34. McCollum, R.E., 1991. Buildup and decline in soil-phosphorus 30-year trends on a typic umprabuult. Agronomy Journal 83, 77-85. McDowell, R.W., Sharpley, A.N., 2001. Phosphorus losses in subsurface flow before and after manure application to intensively farm ed land. Science of the Total Environment 278, 113-125. McDowell, R.W., Sharpley, A.N., Condron, L.M., Haygarth, P.M., Brookes, P.C., 2001. Processes controlling soil phos phorus release to runoff and im plications for agricultural management. Nutrient Cycling in Agroecosystems 59, 269-284. Mehlich, A., 1953. Determination of p, ca, mg, k, na and nh4. North Carolina Soil Test Division., Raleigh, NC Moore, P.A., Miller, D.M., 1994. Decreasing phosphorus solubili ty in poultry litter with aluminum, calcium, and iron amendments. Journal of Environmental Quality 23, 325-330. Mozaffari, M., Sims, J.T., 1996. Phosphorus transformations in poultry litter-amended soils of the atlantic coastal plain. Jour nal of Environmental Quality 25, 1357-1365. Mylavarapu, R., Wright, D., Kidder, G., Ch ambliss, C.G., 2009. Uf/ifas standardized fertilization recommendations for agronom ic crops. the Soil and Water Science Department, Florida Cooperative Extension Se rvice, Institute of Food and Agricultural Sciences, University of Florida. Mylavarapu, R.S., Kennelley, E.D., 2002. Uf/ifas extension soil testing laboratory (estl) analytical procedures and training manual. Soil & Water Science, Cooperative Extension Service, IFAS. Nair, V.D., Graetz, D.A., Portier, K.M., 1995. Forms of phosphorus in soil profiles from dairies of south florida. Soil Scienc e Society of America Journal 59, 1244-1249. Nelson, N.O., Parsons, J.E., Mikkelsen, R.L ., 2005. Field-scale ev aluation of phosphorus leaching in acid sandy soils receiving swine waste. Journal of Environmental Quality 34, 2024-2035. Newman, Y., Erickson, J., Vermerris, W., Wr ight, D.L., Woodard, K.R., Rainbolt, C., 2008. Production of biofuel crops in florida series. Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricu ltural Sciences, University of Florida.

PAGE 61

61 Novak, J.M., Chan, A.S.K., 2002. Development of p-hyperaccumulator plant strategies to remediate soils with excess p concentrations. Critical Re views in Plant Sciences 21, 493 509-493 509. Novak, J.M., Watts, D.W., Hunt, P.G., Stone, K.C., 2000. Phosphorus movement through a coastal plain soil after a d ecade of intensive swine manure application. Journal of Environmental Quality 29, 1310-1315. Page, A.L., Miller, R.H., Keeney, D.R., 1982. Methods of soil analysis.. Pt. 2,. Chemical and microbiological properties. American Societ y of Agronomy, Soil Science Society of America. Pant, H.K., Adjei, M.B., Scholberg, J.M.S., Chambliss, C.G., Rechcigl, J.E., 2004. Forage production and phosphorus phytoremediation in manure-impacted soils. Agronomy Journal 96, 1780-1786. Pierzynski, G.M., Logan, T.J., 1993. Crop, soil, and management effects on phosphorus soil test levels. Journal of Production Agriculture 6, 513-520. Reddy, K.R., Diaz, O.A., Scinto, L.J., Agami, M., 1995. Phosphorus dynamics in selected wetlands and streams of the lake okeec hobee basin. Ecological Engineering 5, 183-207. Ryan, V.A., 2006. Phytoremediation of a high phosphorus soil by summer and winter hay harvest., Agricultural and Mechanical College. Louisiana State University. Salt, D.E., Blaylock, M., Kumar, N.P.B.A., Dushenkov, V., Ensley, B.D., Chet, I., Raskin, I., 1995. Phytoremediation: A novel strategy for th e removal of toxic metals from the environment using plants. Bio-Technology 13, 468-474. SAS Inc., 2001. Sas/stat user's guide. SAS Institute, Cary, NC. Schmidt, J.P., Buol, S.W., Kamprath, E.J., 1997. Soil phosphorus dynamics during 17 years of continuous cultivation: A method to es timate long-term p availability. Geoderma 78, 59-70. Sharma, N.C., Sahi, S.V., 2005. Characteriza tion of phosphate accumulation in lolium multiflorum for remediation of phosphorus-e nriched soils. Environmental Science & Technology 39, 5475-5480. Sharma, N.C., Starnes, D.L., Sahi, S.V., 2007. Phytoextraction of excess soil phosphorus. Environmental Pollution 146, 120-127. Sharpley, A.N., Chapra, S.C., Wedepohl, R ., Sims, J.T., Daniel, T.C., Reddy, K.R., 1994. Managing agricultural phosphorus for protecti on of surface waters: Issues and options. Journal of Environmental Quality 23, 437-451.

PAGE 62

62 Silveira, M.L., Miyittah, M.K., O'Connor G.A., 2006. Phosphorus release from a manure-impacted spodosol: Effects of a water treatment residual. Journal of Environmental Quality 35, 529-541. Stricker, J.A., 2003. Energy from crops : Production and management of biomass/energy crops on phosphatic clay in central florida. University of Florida Cooperative Extension Service Institute of Food and Agricultur al Sciences EDIS, Gainesville, Fla. Suresh, B., Ravishankar, G.A., 2004. Phytoremediation a novel and promising approach for environmental clean-up. Critical Reviews in Biotechnology 24, 97-124. Thomann, R.V., Mueller, J.A., 1987. Principles of surface water quality modeling and control. Harper & Row, New York. United States. Natural Resources Conservation Service., 1996. Keys to soil taxonomy, 7th ed. U.S., Washington, D.C. USEPA, 1999. Phytoremediation resource guide U.S. Environmental Protection Agency Office of Solid Waste and Emergenc y Response Technology Innovation Office, Washington, DC. Van Ginneken, L., Meers, E., Guisson, R., Rutte ns, A., Elst, K., Tack, F.M.G., Vangronsveld, J., Diels, L., Dejonghe, W., 2007. Phytoremedia tion for heavy metal-contaminated soils combined with bioenergy production. Journal of Environmental Engineering and Landscape Management 15, 227-236. Whitehead, D.C., 2000. Nutrient elements in gr assland: Soil-plant-animal relationships. CABI. UK Woodard, K.R., Prine, G.M., 1993. Dry matter accumulation of elephantgrass, energycane, and elephantmillet in a subtropical climate. Crop Sci 33, 818-824. Woodard, K.R., Sollenberger, L.E., Sweat, L. A., Graetz, D.A., Rymph, S.J., Joo, Y., 2007. Five year-round forage systems in a dairy effluent sprayfield: Phosphorus removal. Journal of Environmental Quality 36, 175-183.

PAGE 63

63 BIOGRAPHICAL SKETCH Xiaolin Sui was born in 1984 a sm all city in China. Xiaolin received his Bachelor of Science in environmental science in May 2007 at Nankai University of China. After he graduated from college, he decided to dig furt her in the area of environmental science. In August 2007, Xiaolin joined the Soil and Water Science Department at University of Florida and started his graduate study under the supervisi on of Dr. Silveira. Most of his research was conducted at Range Cattle Research and Educa tion Center. His master's project focused on the phytoremediation of P-impacted soils. Xiaolin received his Master of Science degree in August 2008.