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Soil Carbon Sequestration and Stabilization in Tree-Based Pasture Systems in Florida

Permanent Link: http://ufdc.ufl.edu/UFE0021415/00001

Material Information

Title: Soil Carbon Sequestration and Stabilization in Tree-Based Pasture Systems in Florida
Physical Description: 1 online resource (131 p.)
Language: english
Creator: Haile, Solomon Ghebremussie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: agroforestry, bahiagrass, carbon, elliottii, florida, fractionation, isotope, notatum, paspalum, pasture, pine, pinus, sequestration, silvopasture, size, slash, soil, spodosol, stable, ultisol
Forest Resources and Conservation -- Dissertations, Academic -- UF
Genre: Forest Resources and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Silvopasture that integrates deep-rooted trees in pasture production systems is likely to enhance soil carbon (C) sequestration in deeper soil profiles, compared to tree-less pasture systems. The total soil C content at six depths (0 ? 5, 5 ? 15, 15 ? 30, 30 ? 50, 50 ? 75, and 75 ? 125 cm) were determined in silvopastures of slash pine (Pinus elliottii) + bahiagrass (Paspalum notatum), and adjacent open pastures at two sites, representing Spodosols and Ultisols, in Florida. The C contents within three fraction-size classes (250 ? 2000, 53 ? 250 and < 53 ?m) of each soil profile were determined, and using stable C isotope signatures, the plant sources (C3 vs. C4 plant) of C fractions were traced. Total soil organic carbon (SOC) in whole soil was higher under silvopasture by an average of 33% near trees (SP-T) and 28% in the alleys (SP-A) as compared to adjacent open pastures. Moreover, new SOC in macroaggregate fraction increased by 39% in SP-A and 20% in SP-T. The SOC protected in microaggregates were 12.3% and 18.8% more in SP-A and SP-T respectively than in open pastures. Isotopic-ratio analysis suggested that the SOC increase in silvopasture could largely be due to accumulation of new C3-derived SOC in macroaggregates fractions, retention of older C3-derived SOC in microaggregates, and retention of C in silt + clay fraction of soil ( < 53 ?m). C3 plants (slash pine trees) seemed to have contributed more C in the silt + clay fraction than C4 plants (bahiagrass), particularly in lower soil depths, in all sites, and the Spodosol contained more C in the < 53?m fraction at and below the spodic horizon (on average 50 cm deep) in silvopasture compared to open pasture. In both soil orders, the C3 plant contributed more C in the smallest soil fraction ( < 53?m) than the C4 plant, particularly at the lowest zone studied (75 ? 125 cm). The results support the hypothesis that compared to open pastures, silvopasture contains more stable C in deeper soil profiles under similar ecological settings.
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 Solomon Ghebremussie Haile.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Nair, Ramachandr P.
Local: Co-adviser: Nair, Vimala D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021415:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021415/00001

Material Information

Title: Soil Carbon Sequestration and Stabilization in Tree-Based Pasture Systems in Florida
Physical Description: 1 online resource (131 p.)
Language: english
Creator: Haile, Solomon Ghebremussie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: agroforestry, bahiagrass, carbon, elliottii, florida, fractionation, isotope, notatum, paspalum, pasture, pine, pinus, sequestration, silvopasture, size, slash, soil, spodosol, stable, ultisol
Forest Resources and Conservation -- Dissertations, Academic -- UF
Genre: Forest Resources and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Silvopasture that integrates deep-rooted trees in pasture production systems is likely to enhance soil carbon (C) sequestration in deeper soil profiles, compared to tree-less pasture systems. The total soil C content at six depths (0 ? 5, 5 ? 15, 15 ? 30, 30 ? 50, 50 ? 75, and 75 ? 125 cm) were determined in silvopastures of slash pine (Pinus elliottii) + bahiagrass (Paspalum notatum), and adjacent open pastures at two sites, representing Spodosols and Ultisols, in Florida. The C contents within three fraction-size classes (250 ? 2000, 53 ? 250 and < 53 ?m) of each soil profile were determined, and using stable C isotope signatures, the plant sources (C3 vs. C4 plant) of C fractions were traced. Total soil organic carbon (SOC) in whole soil was higher under silvopasture by an average of 33% near trees (SP-T) and 28% in the alleys (SP-A) as compared to adjacent open pastures. Moreover, new SOC in macroaggregate fraction increased by 39% in SP-A and 20% in SP-T. The SOC protected in microaggregates were 12.3% and 18.8% more in SP-A and SP-T respectively than in open pastures. Isotopic-ratio analysis suggested that the SOC increase in silvopasture could largely be due to accumulation of new C3-derived SOC in macroaggregates fractions, retention of older C3-derived SOC in microaggregates, and retention of C in silt + clay fraction of soil ( < 53 ?m). C3 plants (slash pine trees) seemed to have contributed more C in the silt + clay fraction than C4 plants (bahiagrass), particularly in lower soil depths, in all sites, and the Spodosol contained more C in the < 53?m fraction at and below the spodic horizon (on average 50 cm deep) in silvopasture compared to open pasture. In both soil orders, the C3 plant contributed more C in the smallest soil fraction ( < 53?m) than the C4 plant, particularly at the lowest zone studied (75 ? 125 cm). The results support the hypothesis that compared to open pastures, silvopasture contains more stable C in deeper soil profiles under similar ecological settings.
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 Solomon Ghebremussie Haile.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Nair, Ramachandr P.
Local: Co-adviser: Nair, Vimala D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021415:00001


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1 SOIL CARBON SEQUESTRATION AND STABILIZATION IN TREE-BASED PASTURE SYSTEMS IN FLORIDA By SOLOMON GHEBREMUSSIE HAILE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

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2 2007 Solomon Ghebremussie Haile

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3 To my beloved wife Helen G. Weldelibanos, my cherished children Delina, Nathan and Joshua, my adored mother Medhin T. Haile, and to th e memory of my father, Ghebremussie Haile

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4 ACKNOWLEDGMENTS My sincere gratitude goes to my major ad visor Dr. P.K. Ramachandran Nair for his endurances and much-needed support. His invalu able scientific guidance and stimulating discussions made my study program and research project fruitful. In addition to his scientific guidance, his advice and comments in structuring of this dissertation has been extremely helpful. Without his vivacious support, the work could not have the quality it has. I owe Dr. Vimala Nair, my co-advisor, many thanks for her much-needed assistance and advice through out the course of my research. My apprecia tion goes to Dr. Tim Marin a nd Dr. Michael Bannister for encouragement and advice in addition to being on my committee. I thank to Dr. Lynn Sollenberger and Dr. Michael Binford for their comments and serving in my graduate committee. While undertaking soil sample collection in the field and analyzing it in the lab, I had the support of many individuals. I tha nk all the individuals who helped in this project especially, Dr. Sam Allen, Dr. Alain Michel and Subhrajit Saha (SFRC, UF) for their help during soil sample collection, Willie Wood (SFRC), Dawn Lucas, at th e Soils Lab (SWSD, UF), and Bill Pothier Senior Chemist, and Dr Patrick Inglett at the isotope lab who greatly facilitated my work. Many thanks goes to Mr. Fred Clark (Alachua), Mr Bruce Goff (Suwannee), and Mr. Harris Hill (Osceola ) who allowed me to collect soil samples and other field data from their farms, without their cooperation and support, this study would have not been r ealized. I thank for all my agroforestry lab mates for their greater encouragement and being all-time friends. Finally, I am grateful for the continued love and support of my family. I thank to my wife, Helen Weldelibanos for her resilience, much ne eded support and encouragement, to my dear children Delina, Nathan and Joshua. They had to endure my absence for long period during the first three years of my study program. I am also very grateful to my mom Medhin T. Haile and my sister Selam for their ceaseless care and all-time love to my children.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........7 LIST OF FIGURES................................................................................................................ .........8 ABSTRACT....................................................................................................................... ............12 CHAPTER 1 INTRODUCTION..................................................................................................................14 Objectives of the Study...................................................................................................17 Organization of the Dissertation......................................................................................17 2 LITERATURE REVIEW.......................................................................................................19 Southeastern USA: State of Agriculture, Forestry, and Agroforestry....................................19 Soil C Sequestration in Treebased Pasture Systems.............................................................23 Soil C Sequestration: Process and Significance..............................................................23 Land-use and Soil C Storage...........................................................................................25 Below-ground Inputs and C Sequestration......................................................................27 Soil Structure and C Stabilization...................................................................................27 Using Stable C Isotopes as an Indicato r of Change: Background and Methodology.............30 Summary........................................................................................................................ .........32 3 SOIL CHARACTERISTICS OF TREE-BA SED AND OPEN PASTURE SYTEMS..........35 Introduction................................................................................................................... ..........35 Materials and Methods.......................................................................................................... .36 Study Area..................................................................................................................... ..36 Soils of Study Area..........................................................................................................36 Descriptions of Pasture Systems.....................................................................................37 Plant Components............................................................................................................38 Soil Sampling..................................................................................................................39 Bulk Density and pH Determination...............................................................................39 Statistical Analysis..........................................................................................................39 Results........................................................................................................................ .............40 Soil pH........................................................................................................................ .....40 Bulk Density................................................................................................................... .40 Discussion..................................................................................................................... ..........40 Conclusion..................................................................................................................... .........41

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6 4 CARBON STORAGE IN DIFFERENT SIZE FRACTIONS OF SOILS IN SIVOPASTORAL SYSTEMS OF FLORIDA.......................................................................45 Introduction................................................................................................................... ..........45 Materials and Methods.......................................................................................................... .47 Study Area..................................................................................................................... ..47 Soil Sampling..................................................................................................................47 Physical Fractionation.....................................................................................................48 Chemical Analysis...........................................................................................................48 Statistical Analysis..........................................................................................................49 Results........................................................................................................................ .............49 SOC Storage in Whole Soil.............................................................................................49 SOC in Macroaggregates (250 m 2000 m)...............................................................50 SOC in Microaggregates (53 m 250 m)...................................................................51 SOC in Silt and Clay (<53 m).......................................................................................51 Discussion..................................................................................................................... ..........52 Conclusions.................................................................................................................... .........57 5 SOIL CARBON SEQUESTRATION BY TREES AND GRASS IN SILVOPASTORAL SYSTEMS: EVIDENCE FROM STABLE ISOTOPE ANALYSIS...77 Introduction................................................................................................................... ..........77 Materials and Methods.......................................................................................................... .80 Study Area..................................................................................................................... ..80 Soil Sampling..................................................................................................................80 Size Fractionation............................................................................................................81 Chemical Analysis...........................................................................................................81 Stable C Isotope Analysis................................................................................................81 Results........................................................................................................................ .............82 Changes in the Natural Abundance of 13C SOC..............................................................82 Whole Soil Sample...................................................................................................82 Fractionated Samples...............................................................................................83 Plant Sources of SOC in Whole Soil Sample..................................................................83 Plant Sources of SOC in 250 to 2000 m Fraction.........................................................84 Plant Sources of SOC in 53 to 250 m Fraction.............................................................84 Plant sources of SOC in <53 m Fraction.......................................................................84 Discussion..................................................................................................................... ..........85 Conclusions.................................................................................................................... .........88 6 SUMMARY AND CONCLUSIONS...................................................................................113 LIST OF REFERENCES.............................................................................................................118 BIOGRAPHICAL SKETCH.......................................................................................................131

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7 LIST OF TABLES Table page 2-1 Agroforestry practices in the Southeastern U.S.A.............................................................33 3-1 Climatic and edaphic char acteristics of silvopasture (S P) and open pasture (OP) in four study sites Florida, USA.............................................................................................42 3-2 Soil pH (H2O) values of sampling locations across soil depth in the four study sites......43 3-3 Soil bulk density for two land-use treatmen t locations across soil depth in the four study sites.................................................................................................................... .......43 5-1 13C values of whole soil SOC in different la nd-use locations at four sites across soil depth.......................................................................................................................... .........90

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8 LIST OF FIGURES Figure page 3-1 Location of soil sampling sites..........................................................................................44 4-1 Changes in SOC with depth at three pasture locations [(silvopasture: the center of the alley (SP-A) and in-between tree rows (SPT); and open pasture (OP)] for whole-soil of Alachua site................................................................................................................ ...58 4-2 Changes in SOC with depth at three pasture locations [(silvopasture: the center of the alley (SP-A) and in-between tree rows (SPT); and open pasture (OP)] for whole-soil of Suwannee site............................................................................................................... .59 4-3 Changes in SOC in whole-soil with depth at three pasture loca tions [(silvopasture: the center of the alley (SP-A) and in-bet ween tree rows (SP-T); and open pasture (OP)] for whole-soil of Hardee site...................................................................................60 4-4 Changes in SOC in whole-soil with depth at three pasture loca tions [(silvopasture: the center of the alley (SP-A) and in-betw een tree rows (SP-T)]; and open pasture (OP) for whole-soil of Osceola site...................................................................................61 4-5 Changes in SOC in macroaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Alachua site...................................................................................62 4-6 Changes in SOC in macroaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Suwannee site...............................................................................63 4-7 Changes in SOC in macroaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Hardee site....................................................................................64 4-8 Changes in SOC in macroaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Osceola site...................................................................................65 4-9 Changes in SOC in microaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Alachua site...................................................................................66 4-10 Changes in SOC in microaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Suwannee site...............................................................................67

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9 411 Changes in SOC in microaggregates with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Hardee site....................................................................................68 412 Changes in SOC in microaggregates with depth at three past ure locations [(SP-A, SP-T and OP)] for Osceola site..........................................................................................69 4-13 Changes in SOC in silt+clay fraction with depth at three pasture locations at Alachua site........................................................................................................................... ...........70 4-14 Changes in SOC in silt + clay fraction with depth at three pasture locations for Suwannee site.................................................................................................................. ...71 4-15 Changes in SOC in silt+clay fraction with depth at three pasture locations for Hardee site........................................................................................................................... ...........72 4-16 Changes in SOC in silt+clay fraction with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Osceola site...................................................................................73 4-17 Changes in mean SOC across the Spodosol locations (Hardee and Osceola) in A) whole soil, B) macroaggregates C) microa ggregates, and D) silt + clay fraction down the soil profile depths at three pa sture locations (SPA, SP-T; and OP).................74 4-18 Changes in mean SOC across the Ultisol locations (Alachua and Suwannee) in A) whole soil, B) macroaggregate C) microaggr egates, and D) silt + clay fraction down the soil profile depths at three past ure locations (SP-A, SP-T; and OP)...........................75 4-19 Changes in overall mean SOC across all locations in A) whole soil, B) macroaggregate C) microaggregates, and D) si lt + clay fraction down the soil profile depths at three pasture locations (SP-A, SP-T; and OP)....................................................76 5-1 Changes in 13C values of SOC in macroaggrega te fraction in different land-use locations at four sites acro ss soil depth: A) Alachua, B) Suwannee C) Hardee and D) Osceola across soil depth...................................................................................................91 5-2 Changes in 13C values in microaggregates size in sites: A) Alac hua, B) Suwannee C) Hardee and D) Osceola across soil depth.....................................................................92 5-3 Changes in 13C values in silt + clay size fr action for sites: A) Alachua, B) Suwannee C) Hardee and D) Osceola across soil depth....................................................93 5-4 Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Alachua site....................................94 5-5 Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Suwannee site.................................95

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10 5-6. Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Hardee site......................................96 5-7 Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Osceola site....................................97 5-8 Changes in percent of C3-derived C in 250 2000 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Alachua site................98 5-9 Changes in percent of C3-derived C in 250 2000 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Suwannee site.................................99 5-10 Changes in percent of C3-derived C in 250 2000m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Hardee site....................................100 5-11 Changes in percent of C3-derived C in 250 2000m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Osceola site..................................101 5-12 Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Alachua site..................................102 5-13 Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Suwannee site...............................103 5-14 Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Hardee site....................................104 5-15 Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pa sture (OP) in Osceola site..................................105 5-16 Changes in percent of C3-derived C in <53 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open past ure (OP) in Alachua site.............................................106 5-18 Changes in percentage of C3-derived C in the <53 m fraction with soil depth on silvopasture (SP-T and SP-A) and on an adja cent open pasture (OP) in Hardee site......108 5-19 Changes in percent of C3-derived C in <53 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open past ure (OP) in Osceola site.............................................109 5-20 Changes in overall mean C3-derived a nd C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggreg ates, and D) silt + clay fraction down the soil profile depths in Ultisols at thr ee pasture locations (SP-A, SP-T; and OP)........110 5-21 Changes in overall mean C3-derived a nd C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggreg ates, and D) silt + clay fraction down the soil profile depths in Spodosols at th ree pasture locations (S-A, SP-T; and OP)......111

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11 5-22 Changes in overall mean C3-derived and C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggreg ates, and D) silt + clay fraction down the soil profile depths at three past ure locations (SP-A, SP-T; and OP).........................112

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12 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SOIL CARBON SEQUESTRATION AND STABILIZATION IN TREE-BASED PASTURE SYSTEMS IN FLORIDA By Solomon Ghebremussie Haile August 2007 Chair: P.K. Ramachandran Nair Cochair: Vimala D. Nair Major: Forest Resources and Conservation Silvopasture that integrates deep-rooted trees in pasture production systems is likely to enhance soil carbon (C) sequestration in deeper soil profiles, compared to tree-less pasture systems. The total soil C content at six de pths (0 5, 5 15, 15 30, 30 50, 50 75, and 75 125 cm) were determined in silvopastures of slash pine ( Pinus elliottii ) + bahiagrass ( Paspalum notatum ), and adjacent open pastures at two site s, representing Spodosols and Ultisols, in Florida. The C contents within three fractio n-size classes (250 2000, 53 250 and <53 m) of each soil profile were determined, and using stab le C isotope signatures, the plant sources (C3 vs. C4 plant) of C fractions were traced. To tal soil organic carbon (SOC) in whole soil was higher under silvopasture by an average of 33% near trees (SP-T) and 28% in the alleys (SP-A) as compared to adjacent open pastures. Mo reover, new SOC in m acroaggregate fraction increased by 39% in SP-A and 20% in SP-T. Th e SOC protected in microaggregates were 12.3% and 18.8% more in SP-A and SP-T respectively than in open pastures. Isotopic-ratio analysis suggested that the SOC increase in silvopasture could largely be due to accumulation of new C3derived SOC in macroaggregates fractions, retention of older C3-derived SOC in microaggregates, and retention of C in silt + clay fraction of soil (<53 m). C3 plants (slash pine

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13 trees) seemed to have contributed more C in the silt + clay fraction than C4 plants (bahiagrass), particularly in lower soil dept hs, in all sites, and the Spodosol contained more C in the <53m fraction at and below the spodic horizon (on aver age 50 cm deep) in silv opasture compared to open pasture. In both soil orders, the C3 plant co ntributed more C in the smallest soil fraction (<53m) than the C4 plant, particularly at the lowest zone studied (75 125 cm). The results support the hypothesis that compared to open pastur es, silvopasture contains more stable C in deeper soil profiles under si milar ecological settings.

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14 CHAPTER 1 INTRODUCTION Increasing concentrations of the so-called gr eenhouse gases (GHGs) in the atmosphere and associated global warming are issues of seri ous environmental concern today. The most abundant among the GHGs are oxides of car bon, particularly carbon dioxide (CO2). It is generally agreed that, if th e world emission of GHGs con tinues unabated, atmospheric CO2 concentration may double by the end of the 21st century. This doubling of CO2 concentration is predicted to increase the world average surface air temperature by between 1.5 and 4.5 C (Kattenberg et al., 1996). Responding to this concern, the Kyoto protocol ( UNFCCC, 1997) was embraced to stabilize the greenhouse gas concentrati ons in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate. The Kyoto protocol legally binds the developed nations (annex-1 countries) to reduce emission of GHGs by 5 % below the 1990 levels (Baker and Barrett, 1999; Faeth and Greenhalgh, 2000). Besides reduction in industrial emissions, the Kyoto Protocol also proposes se questering carbon (C) in terrestrial sinks. Nevertheless, the effects of anthropogenic ac tivities on C stocks of the world are still inadequately understood. The idea of using soils to sequester C and mitig ate risk of accelerated greenhouse effect is rather novel. Soil organic carbon (SOC) is a major pool in the terrestrial C cycle and it is linked to atmospheric CO2 through inputs from plants and losses via decomposition. With approximately 1500 Pg C stored to one meter dept h of the worlds soil (B atjes, 1996; Bruce et al., 1999), shifts in land cover and/or land-use pract ices that affect pools and fluxes of SOC have large implications for the C cycle fluxes on a wi de-reaching scale. Different models have been developed to calculate the C s ource vs. sink relationship between soil and atmosphere. However, disparity exists between model predictions and measurements of annual fluxes of C among the

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15 various C pools even with the latest improved model accuracy, resulting in ambiguities in global C budget. Lal et al. (1995) suggested that the disparity is ascribed to terrestrial ecosystems, of which soils are a major component. Estimating the global C budget has been diffi cult and remains controversial due to uncertainties in the quantitative aspects of C st orage and dynamics in terrestrial ecosystems. Changes in soil C stocks that occur following a sh ift from one vegetation type to another is one of the great concerns in this context: land-use change may lead to a depletion of soil organic carbon and consequent incr ease in atmospheric CO2. However, no clear conclusion has been reached whether terrestrial ecosystems are net C sources or sinks (Detwiler and Hall, 1988). Although some evidence points to a large terres trial C pool as sink (H oughton et al., 1999; Pacala et al., 2001), the geographic location as well as C accumulation rates and mechanisms controlling this terrestrial sink are still largely non-quantified (Houghton et al., 1999). However, it is widely believed that the amount of C accu mulated following tree integration in to nonforested ecosystems is a potentially large, yet uncertain sink for atmospheric CO2 (Houghton et al., 1999; Pacala et al., 2001). Silvopasture agroforestry systems that integrate trees into pastures have been studied as an alternative land-use approach to intensive open-pasture grass syst em for beef cattle production in the southeastern USA, mainly to diversify the enterprise and improve e nvironmental quality of land (Garrett et al., 2000; Nair et al., 2007; Michel et al., in pr ess). Such tree-based land-use systems may potentially store C in the soil be neath them. Information on soil C storage and dynamics of silvopasture is, however, inadequa te. Furthermore, studies on soil C storage potential of silvopastoral and such other systems that integrate tree s into pastoral and agricultural landscapes have the potential to substantiate and get better output from model of SOC by

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16 allowing extrapolation of results from the ecosystem-level to the broader scales needed to predict terrestrial ecosystems role in the global C cycl e including potential of te rrestrial ecosystems for long-term soil C sequestration. Predicting the effects of land-cover change s induced by human activities on C dynamics and assessing future land-use opti ons associated with integration of trees in pasture or other treebased land-use systems in the region re quires detailed information on SOC dynamics. Information on the mechanisms of organic C stor age in the different so il C pools, especially those with longer residence time, is essential in understanding whether terrestrial ecosystem C sinks will continue to seques ter C in the future. A major problem is that SOC is very heterogeneous and it comprises various fractions th at differ in rate and extent of decomposition. Different constituents of SOC have relative stability ranging from labile to stable forms (Carter 1996). The different compartments cannot be de termined directly by chemical or physical fractionation procedures (Paustia n et al. 1992). Tiessen and Stew art (1983) and Feller et al. (1991) observed that sand-size organic matter (macroorganic matter, > 250 m) is often more labile than the organic C in th e clay and silt size fractions. Ch ristensen (1992) also found that by physically fractionating soils, specif ic particle sizes can be link ed to specific organic matter fractions and to soil structure and aggregation. These findings formed the fundamental basis for the widely used sizeand de nsityfractionation of soil. Tree-based pasture systems present a unique opportunity for using stable isotope methodology to study SOC dynamics following the shift in vegetation structure due to the integration of trees to open pasture. The diffe rences in isotope ratio of the plant community associated with the C3 vs. C4 plants in silvopasture system can be used to quantify the contribution of plants of each photosynthetic path way to soil organic matter (Balesdent et al.,

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17 1988). No study has employed the natural abundance of 13C to study SOC in pine-based silvopatoral system where C3 and C4 plants are grown simulta neously. The analysis of 13C combined with soil physical frac tionation could be used to quantif y and serve as an indication of plant sources of each C quality. Specifically, it will show whether SOC accumulation is the result of increases in the most recent organic matter inputs derived from trees or organic C derived from the grasses or previous pasture lan d, or from a mixture of both sources (assuming that there is little contribution from he rbaceous C3 plants in the system). Objectives of the Study The general objective of this study is to evaluate the consequences of integration of trees in pastures of cattle-beef production systems on the so il C cycles in Florida. The specific objectives are: Quantify some of the physical and chemical ch aracteristics of SOC and evaluate how these characteristics change following pa sture-to-silvopasture alteration; Estimate total SOC accumulation and sequest ration in whole soil and fractionated soil particle size classes following tree integration into pastur e or pasture-to-silvopasture conversion; Determine the relative importance of C derived from slash-pine trees (C3) vs. behaigrass vegetation (C4) in the silvopasture using the na tural isotopic difference between C4 grasses and C3 woody plants; and Elucidate specific physical protect ion mechanisms of soil C sequestration in this system by combining the plant sources and determining where C is stored relative to soil aggregate sizes. Organization of the Dissertation The dissertation is presented in systematic progression with the above-stated individual objectives as chapter topics. It begins by expl aining the rationale of th e study and concepts used to address the study objectives. Thus this chapter (Chapter 1) pr esents the general introduction, objectives and the over all organization of the disse rtation. Chapter 2 is devoted to describing the

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18 state and role silvopastural agro forestry and synthesizing current literatures on effects of tree integration into pasture on dynamics and storage of SOC. The results of investigations on soil characteristics in silvopasture and open pastur e land-use systems are presented in Chapter 3; these soil characteristic s are set out as basic background in formation for the study objectives presented in the subsequent chapters. The assessme nt of potential of silvop asture as opposed to open pasture in soil C storage and stabilization be gins in Chapter 4 by pr esenting the results of carbon storage in whole and fractio nated soil-size classes. Chapter 5 presents the results of analysis of stable carbon isotope ratio to identify the plant sources for SOC stored in each landuse system. Finally the summary and conclusions of the study are presented in Chapter 6.

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19 CHAPTER 2 LITERATURE REVIEW Southeastern USA: State of Agriculture, Forestry, and Agroforestry Small farms and non-industrial timber operations are significant driv ers of the economy in Florida and other parts of the s outheastern USA. Small farms, defined as having annual sales less than $250,000 (USDA, 2002), accounted for 88% of Fl orida farms, but they constitute 56% of total agricultural income in the state. About 84% of small farms are individualor family-owned (USDA, 2002). Similarly, out of the 6.6 milli on hectares (16.3 million acr es) of forestland in Florida, 52% is non-industrial private land. Ev idently, small-scale agricultural and timber operations constitute a major sector of the rural economy. The situati on is similar in other states of the southeastern region too. As in many parts of the USA, farm families in the Southeast are facing several new challenges. These are partly the result of ch anges related to urbanization of rural lands, agricultural land-use, agricultural and forestry inte nsification, availability and quality of water, climate change, and competition from foreign markets (Workman and Allen, 2004). The smallscale operations in particular are increasingly vulnerable because of capita l constraints in facing many environmental problems if not threats. The increasing impact of a rapidly urbanizing landscape, for example, creates significant chan ges in ecosystem characteristics such as increased fire danger, changes in water draina ge patterns leading to soil erosion and flooding, and fragmentation of wildlife ha bitat that could affect to the small-scale farms due their vulnerability. The Environmental Protection Ag ency (EPA) has shown that agricultural nonpoint source pollution is a significant cause of stream and lake contamination and prevents attainment of water quality goa ls specified in the Clean Wate r Act (USEPA, 1996). The problem of phosphorus (P) loss from soils is a major c oncern in fertilized agricultural and forestry

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20 enterprises, particularly in coarse-textured and poorly drained soils where drainage water ultimately mixes with a body of surface wate r (Nair and Graetz, 2004). Soils with these characteristics are prevalent in the southeastern states. Intensive highinput agriculture in a humid climate with frequent, hea vy rainfall and wide use of irri gation and draina ge would cause and/or accelerate P leaching in d eep sandy soils (Sims et al., 1998). Ranching is a significant land-use activity in Florida, with about 3.07 million ha (7.6 million acres) of rangeland. In the northern half of the state, where forests products are the primary source of income from the range, the land remains forested. However, millions of acres were improved with the fert ilization and establishment of introduced grasses, mainly bahiagrass ( Paspalum notatum ). Florida has approximately 1 million ha (2.5 million acres) of bahiagrass pastures (USDA/ERS, 2006b). Traditionally, pastur e (as opposed to range) was fertilized. The potential for P loss from fertilized pastures resulting in water quality degradation is a serious issue (Nai r and Graetz, 2004). Approximately 8 million ha (20 million acres ) of land under row crop production in the Southern USA were reported as marginal lands (Dangerfield and Harwell, 1990) with low profit potential and high erosion risk when cropped in rows. Although conversion to pasture or forests may protect these lands (Dangerfie ld and Harwell, 1990), the combin ation of trees and pasture is reported to be the most efficient and economi cal production schemes on marginal land in the southern USA (Zinkhan and Mercer, 1997). Under all these scenarios, there is mounting pr essure on land owners of small-scale farms to adopt land management practices that ar e economically and ecologically sustainable. Integrated systems such as agroforestry the integration of trees in to the farm or/and pasture lands that provide diversified ec onomic advantages as well as improved land environmental

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21 quality can be utilized as an appropriate strategy under these circumstances (Workman et al., 2003). Today there is an increasing body of knowledge on agroforestry as well as increasing awareness about its potential as a land-management approach. Agroforests are planned and managed agroecosystems (Sharrow, 1997). Increasing the overall productivity and efficiency of the land-use system and its sustainability are ma jor goals of agroforestry (Schroth, 1995). When trees and/or shrubs are deliberat ely combined with crops and/or livestock, due to the biophysical interactions of those components, the physical biological, ecological, economic, and social benefits are optimized (MacDicken and Vergara, 1990; Garrett et al., 1991; Nair, 1993; Leakey, 1996). A distinction has to be drawn here betw een optimization and maximization: it is well known that in most agroforestry situations, the producti on of a specific compone nt of the system may not be maximum compared to the production from the sole-stand system of that component (as reported in the case of beef-cattle production under si lvopasture in this study site: Kalmbacher and Ezenwa, 2005 ), however, considering the pro ductivity and other benefits derived from all components and the system as a whole over a longer pe riod of time than the production cycle of crops, the overall benefits from the combined system could be more on a unit-area basis than the combined production from sole stands of the components in over the same period of time (Nair 1993; Alavalapati et al ., 2004). Agroforestry uses specific structural and functional characteristics of natural ecosystems to create a sustainable agroecosystem (Winterbottom and Hazlewood, 1987; Vandermeer, 1995), and mimics the large patch scale dynamics and succession of natural ecosystems (Ong and Leakey, 1999). Thus, agroforestry has received considerable attention and importance as an alternative land-use practice to managing and protecting the land resour ces in a sustainable way.

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22 As a land-use practice, agroforestry is not completely new to the Southeast. With introduction of cattle to the coastal area of Florid a, grazing within pine forests was practiced as early as 1520 (Lewis, 1983). Duri ng the 1950s, tree-based pasture us ed to be practiced as "treepasture" or "pine-pasture" in which pine trees were planted in improved pasture as Conservation Reserve Soil Bank Program (Nowak and Long, 2003). Silvopasture that provides an opportunity for managing trees for high-value sawlogs and at th e same time generates an annual income from livestock grazing is considered attractive to noni ndustrial private forest landowners and livestock operators in southeastern USA who want to diversify their enterprise (Nair et al. 2005). In the southeastern USA, five major agrofo restry practices are id entified: alley cropping, forest farming, riparian forest bu ffers, windbreaks and silvopasture (Table 2-1) (Garrett et al., 2000; Nair 2001). Silvopasture th e integration of trees into fora ge or/and livestock is the most prevalent form of these agroforestry prac tices in North America (Garrett et al., 2000; Nair et al., 2005). Silvopasture is usually established by planting trees in well established and managed bahiagrass ( Paspalum notatum ), bermudagrass, ( Cynodon dactylon) or other similar open pastures. This eliminates costs of forage es tablishment, shrub and brush control, or removal of timber harvest residues (Nowak et al., 2003; Nair et al. 2005). Available information suggests that silvopa sture can improve land environmental quality, especially as an approach to mitigating the pr oblem of nutrient pollution resulting from beefcattle pastures (Michel et al., in press; Nair et al., 2007). The t echnology is applicable on a wide range of geographic scale and is adaptable for small land holdings as well as operations with small numbers of animals. Silvopasture practi ces are known to have economic benefits and are good option for existing pastures in the sout heast (Clason and Sharrow, 2000). Studies on silvopasture in southeastern USA have found the pr actice to be an economi cal viable enterprise

PAGE 23

23 (Lundgren et al., 1983; Clason, 1995; Grado et al., 2001, Stainback and Alavalapati, 2004). Stainback et al. (2004), however found that combining slash pi ne with cattle production in Florida was not competitive with conventional ranching when the environmental costs and benefits were not considered in the analysis. Production of widely spa ced rows of trees for timber, especially of pines, e.g., longleaf pine ( Pinus palustris ), in combination with the benefits of shade for animals offsets loss of pasture ar ea and is attractive to a growing number of producers (Workman et al., 2003). As landowners are challenged to manage the natural resource in integrated approach, the conve rsion of pasture-to-silvopasture is expected to increase in the region. Silvopasture has multiple plant species and is applicable in both livestock production and reforestation. Due to its multiple species na ture, thus a more efficient production than monoculture, silvopasture could store substant ial carbon (C) in its pl ant-soil system. This potential of silvopastoral systems has, however not been even adequa tely studied, let alone exploited (Montagnini and Nair, 2004). It is believed that th e potential to increase aboveand below-ground productivity in tree-based system may lead to greater C sequestration in the system, particularly in soils and therefore proper design a nd management of silvopasture practices can make them effectiv e C sinks. Evidently, the effects of tree integration into pasture system or pasture-to-silvopast ure conversion on soil C, though seemingly substanial, has not been quantified. Detailed studies on storage a nd dynamics of soil organic carbon (SOC) in silvopasture ecosystem as opposed to open pasture are therefore warranted. Soil C Sequestration in Tree-based Pasture Systems Soil C Sequestration: Process and Significance Carbon sequestration has attained considerable prominence as a term and concept in the context of heightened interest in GHG mitigatio n and climate change, and it refers to the net

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24 removal of CO2 from the atmosphere into long-lived pool s of C in terrestrial ecosystems. The pools can be living, aboveground biomass (e.g., tr ees), products with a long and useful life created from biomass (e.g., lumber), living bioma ss in soils (e.g., roots and microorganisms), or recalcitrant organic and inorgani c carbon in soils and deeper s ubsurface environments. Although the soil C pool constitutes a major global C reserv e, the concept of soil C sequestration has not been widely appreciated because of the lack of understanding of the role soil processes play in the global C cycle (Lal, 2002). Furthe r, agriculture has been generally considered as a source of GHGs and other environmental pollutants. Carbon is sequestered in soils in two ways: di rect and indirect (SSSA 2001). Direct soil C sequestration occurs by inorganic ch emical reactions that convert CO2 into soil inorganic carbon compounds such as calcium and magnesium carbonates. Indirect plant C sequestration occurs as plants photosynthesize atmospheric CO2 into plant biomass. Some of this plant biomass is indirectly sequestered as SOC during decomposition processes. The amount of C sequestered at a site reflects the long-term balance between C uptake and release mechanisms. Because those flux rates are large, changes such as shifts in land cover and/or land-use practices that affect pools and fluxes of SOC have large implications fo r the C cycle and the earths climate system. Globally, soils play a vital role in the C cycl e. The soil C pool comprises SOC estimated at 1550 Pg (1 petagram = 1015 g = 1 billion ton [metric]) and soil inorganic carbon (SIC) about 750 Pg both to 1-m depth (Batjes, 1996). The total soil C pool of 2300 Pg is three times the atmospheric pool of 770 Pg and 3.8 times the ve getation pool of 610 Pg (Lal, 2001). About 5% of the global SOC pool, which is estimated to be 75 80 Pg C, is in soils of the USA (Waltman and Bliss, 1997, Bellamy et al. 2005). A reduction in soil C pool by 1 Pg is equivalent to an

PAGE 25

25 atmospheric enrichment of CO2 by 0.47 ppmv (Lal, 2001). Thus, any change in soil C pool would have a significant effect on the global C budget. Uncertainties in the quantitative aspects of C storage and dynamics in terrestrial ecosystems, however, have crea ted a controversy and delay in the global C budget assessments. Whether terrestrial ecosystems are net sources of C emission or sinks has been the center of the controversy (Detwiler and Hall, 1988). Several studies largely point to terrestrial ecosystems as a C sink (Houghton et al., 1999; Pacala et al., 200 1). The C storage rates and mechanisms controlling this terrestrial si nk and its geographic location ar e still largely unknown (Houghton et al., 1999). Land-use and Soil C Storage Tree-based land-use systems are expected to have better carbon se questration potential than most agricultural systems. It has been suggested that the amount of C accumulated following tree integration into non-forested ecosyst ems is a substantial, yet uncertain sink for atmospheric CO2 (Houghton et al., 1999; Pacala et al., 2001 ).It has been shown that land-use conversion from native prairies or forest vegetati on to row crop production agriculture leads to a decrease in the SOC pool (Brown and Lugo, 1990; Burke et al., 1989). Although the C amount in native ecosystem may not necessar ily represent the upper soil C limit (Six et al., 2002), notable SOC loss, due to inappropriate land-use and soil mismanagement practices has caused a decline in soil quality and grea ter emission of C into the atmosphe re. The SOC loss from croplands in the USA is estimated in the range of 30 to 50 Mg C/ha, or about 50% of the antecedent level (Lal, 2002). Sharrow and Ismail, (2004) reported that silvopasture system accumulated approximately 740 kg ha year more C than forests and 520 kg ha year more C than pastures. They concluded that the agroforest (silvopasture) ha d the advantage in term s of higher total annual

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26 biomass production and had active nutrient cycling patterns of both forest stands and grasslands, compared to those of pasture or timber stands al one. More recently, in a plantation with native tree species established in a degraded pasture of the Caribbean lowlands of Costa Rica, the highest SOC pool was measur ed under trees species Hieronyma alchorneoides and Vochysia guatemalensis [132 and 119 Mg C ha 1, respectively], whereas in treeless pasture it was 116 Mg C ha 1 (Jimenez et al., 2007). These studies support the proposition that agroforestry practices, such as silvopasture, may be more efficient at accumulating C than tree plantations or pasture monocultures. When trees are allowed to grow in grass-dom inated land such as an open pasture, some functional consequences are in evitable, most notably altera tions in aboveand below-ground total productivity, modifications to rooting dept h and distribution, and ch anges in the quantity and quality of litter inputs (Sc holes and Hall, 1996; Connin et al., 1997; Gill and Burke, 1999; Jackson et al., 2000; Jobbgy and Jackson, 2002). Th ese changes in vegetation component, litter, and soil characteristics modify the C dynamics a nd storage in the ecosystem and may lead to alterations of local and regional climate system s through feedback interactions (Schlesinger et al., 1990; Ojima et al., 1999). But not many studi es are available on the mechanisms and processes associated with C dynamics and storag e in tree-based grassland/pasture systems such as silvopasture, despite thei r local, if not regional, si gnificance (Jackson et al., 2000, 2002; Archer et al.. 2001, 2004; Hudak et al., 2003). Humification (conversion of biomass into humus), aggregation (formation of organomineral complexes as secondary particles), translocation of biomass into subsoil by deep roots and bioturbation, and leaching of soil inor ganic carbon into groundwater as bicarbonates are processes that lead to SO C sequestration (Lal, 2001). All thes e processes are operational in

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27 tree-based and-use systems. The impact of silvopastoral systems on soil C sequestration depends largely on the amount and quality of input provid ed by tree and grass components of the system, and on properties of the soils themselves, su ch as soil structure and their aggregations. Below-ground Inputs and C Sequestration Roots are an important part of the C balance, because they transfer large amounts of C into the soil. More than half of the C assimilated by the plant is eventually transported below ground via root growth and turnover, root exudates (of organic substances), and litter deposition. Therefore, soils contain the major stock of C in the ecosystem (Albrecht et al., 2004; Montagnini and Nair, 2004). Depending on rooting depth, a cons iderable amount of C is stored below the plow layer and is, therefore, bett er protected from disturbance, which leads to longer residence times in the soil. With some trees having rooting depths of more than 60 m, root C inputs can be substantial, although the amount d eclines sharply with soil depth (Akinnifesi et al., 2004). Most of the biomass of the roots of annual crops/ grasse s consists of fine roots (< 2 mm in diameter) whereas biomass of tree roots, which is a large proportion of the below-ground productivity, consists of coarse roots (> 2 mm diameter) (Alb recht et al., 2004, Akinnife si et al., 2004). Fine roots of both trees and crops have a relatively fast turnover (day s to weeks) (van Noordwijk et al., 1998), but the lignified coarse roots decompos e much more slowly an d may thus contribute substantially to below-ground C stocks (Vanlauwe et al 1996). Soil Structure and C Stabilization Soil structure can be described in terms of form and stability. Structural form refers to the arrangement of solid particles a nd void space between them whereas stability of the soil structure refers to the ability of the soil fabric to w ithstand the disrupting action of external forces, particularly drying and rewetting, and this ability is directly related to aggregation of soil particles. The most accepted model of aggregat ion involves two physical units, microaggregate

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28 <250 m diameter and macroaggregate >250 m dia (Edward and Bremener, 1967; Tisdall and Oades, 1982). Microaggregates are the build ing blocks of soil structure but both macroaggregates and microaggregates depend on or ganic carbon for their integrity and stability. Humic molecules become associated with clay minerals and amorphous aluminum (Al) and iron (Fe) oxides with the formation of micro aggreagtes (Brady and Weil, 2002). Through the cementing action of polysaccharides and humic s ubstances, they can become united with one another and with other component s such as fragments of decomposing organic material and sand particles to form macroaggreagte s. Macroaggregate may breakdow n when exposed to disrupting actions of external forces such as slaking or rewetting of soils. This is caused primarily by pressure exerted by the entrapped air inside the aggregates and rapid swelling of clays (Kemper and Rosenau, 1986). SOC contains a variety of fractions that differ in decomposability and are very heterogeneous in structure. The turnover of SOC is intimately linked with organic matter quality (Agren et al., 1996; Martens, 2000). Distinctiv e components of SOC have different residence times, ranging from labile to stable forms (Car ter, 1996). The amount of carbon sequestered in a soil pool compartment reflects th e long-term balance between i nputs from plants and release mechanisms via decomposition. This concept has le d to the suggestion that SOC can be viewed as having an active, labile pool (mean residence times [MRTs] 1 2 yr) a slow pool (MRTs 25 yr), and a passive, recalcitrant pool (MRTs 100 1000 yr) (Parton et al., 1987; Jenkinson, 1990; Schimel et al., 1994). Furt her, protection of SOC by silt and clay particles is well established (Sorensen, 1972; Ladd et al., 1985; Fe ller and Beare 1997; Hassink, 1997). It is also known that aggregation increases in less disturbed systems and that organic material within the

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29 soil aggregates, especially the microaggregates have lower decomposition rate than those located outside the aggregates (Oades, 1984; Elliott and Coleman, 1988; Six et al., 2000). Soil aggregation is the main process whereby SOC is retained in soil. Such retention can be characterized by both short-term storage in macroaggregates or long-term sequestration in microaggregates. Size and density fractionation has shown promise for physically dividing SOC into pools. It is widely believed that soil size fractionation is linked to specific SOC fractions, soil structure and aggregation char acteristics (Christensen, 1992). The fundamental basis for soil size fractionation is the observa tion that SOC associated wi th sand-size aggregates (or macroorganic matter > 250 m) is often more lab ile than SOC in the clay and silt fractions (Tiessen and Stewart, 1983; Feller et al., 1991) In fact, soil aggregat ion and SOC accumulation are interrelated: SOC or fractions thereof are basic to the aggregation process, while SOC sequestered within aggregates is protected against decompositi on. Both the degree of the soil aggregation and extent of SO C storage are influenced by land-use, and soil and crop management practices (Carter, 1996). The s undry compartments in non-living SOC cannot, however, be determined directly with chemical or physical fr actionation procedures, which has been a major problem in predicting the dyna mics of the SOC (Paustian et al., 1992). Analyzing soil C fractions provides very useful information, particularly with respect to monitoring changes in land-use management. In a review, van Noordwijk et al. (1997) showed that in land-use change where deforest ation was followed by long-term sugarcane ( Saccharum sp. ) cultivation the decline in forest-derived SO C continued during the 50 years of the study and that the apparent equilibrium value of the total SOC content of the soil was based on the balance between gradual build-up of sugarcane SOC and decay of forest-derived SOC. By comparison, when forest was converted to pasture, the declin e of labile forest-deriv ed SOC was much faster;

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30 however, the accumulation of labile pasture-deri ved C returned the total SOC content to its original level 7 year s after conversion. Generally, soil organic matter (SOM) becomes stable through three mechanisms: chemical recalcitrance of the SOM, physical protection, and chemical and biochemical stabilization (Christensen, 1996; Stevenson, 1994). Under the cu rrent pressures on land owners to improve the environmental quality of farm lands, silvopa sture agroforestry on a wide range of scales could be adaptable for small land holdings or small numbers of animals. As a strategy to mitigate accumulation of CO2 in the atmosphere, soil under silvop asture could potentially sequester C mainly because of the potential increase in aboveand belowground productivity. Many biogeochemical consequence in general and the st orage and dynamics of SOC in particular are not well understood when trees are integrated into pasture system or pasture-to-silvopasture conversions are undertaken. Using Stable C Isotopes as an Indicato r of Change: Background and Methodology Carbon occurs in two stable isotopes, 13C and 12C with natural abunda nce about 98.89% of 12C and 1.11% of 13C in atom % (Barrie and Prosser, 1996). However, due to isotopic fractionation during physical, chemi cal, and biological processes, the ratio of these two stable isotopes (13C/12C) in natural materials fluctuates slight ly around these proportions. Differences in C isotopes are relatively small in vegetation and SOC, with the most enriched (those highest in 13C) differing from the least enriched (those with lowest 13C) by only about 2%. Therefore, 13C/12C of the SOM must be measured with high pr ecision if these small differences are to be utilized. A gas isotopic ratio mass spectrometer equi pped with three ion beam collectors is commonly used for this purpose. Briefly, SOM is quantitatively converted into CO2 by combustion at about 900 C in an O2 atmosphere, the CO2 is isolated and purified by cryogenic

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31 distillation or gas chromatography, and the isotopic compositions of CO2 determined by simultaneous collection of masses 44 (12C16O16O), 45 (13C16O16O) and 46 (12C18O16O) in mass spectrometer. The isotopic composition of CO2 from the sample is compared to that derived from a standard with known 13C/12C ratio. Plants discriminate against 13CO2 during photosynthesis due to the biochemical properties of the primary C-fixing enzymes and limitations to CO2 diffusion into the leaf (Vogel, 1980; Leary, 1988; Farquhar, 1989). Plants with the C3 photosynthetic pathway reduce CO2 to phosphoglycerates, a 3-carbon compound vi a enzyme ribulose bisphospate carboxylase/oxygenase (RuBisCO). A bout 85 % of all plant specie s possess the C3 pathway of photosynthesis (Ehleringer et al., 1991), and C3 species dominate in most ecosystems from the boreal region to the tropics. Terrestrial C3 plants grown under natural conditions have a 13CPDB value ranging between 22 and 34 with a mean of 28 (Boutton, 1991; Vogel, 1993). C4 plants on the other hand reduce CO2 to aspartic or malic acid, both 4-carbon compounds, via the enzyme phosphoenolpyruvate (PEP) carboxylase. The C4 plant species comprises only 5 % of all plant species (Ehleringer et al., 1991) but they cover approxi mately 17 % of the terrestrial surfaces (Smith, 1979). Plants with the C4 photosynthesis pathway discriminate less against 13CO2 during photosynthesis and, ther efore, have larger values 13CPDB than C3 plants, ranging from approximately 17 to 9, with mean 13. (Boutton, 1991; Vogel, 1993). Thus the ranges of 13CPDB values for C3 and C4 plants do not overlap with an average difference of 15. Alteration of pasture-to-silvopasture, as the ca se in point, presents a unique opportunity to use stable isotope methodology to study SOC dynami cs following a shift in vegetation structure due to the integration of trees onto open pasture. In Florid a, the plant community of the silvopasture system comprises of C3 plants slash pine ( Pinus elliottii ) trees ( 13C -29.5 ;

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32 Parasolova et al., 2003) and C4 grass sp ecies as in open pa sture bahiagrass ( Paspalum notatum ) (Roots and shoots 13C -13.3 Nakano et al 2001). The 13C value ranges of C3 and C4 plants do not overlap; differences in isotope ra tio, therefore, can be used to quantify the contribution of each photosynthetic pathway to SOM (Balesdent et al., 1988). The isotopic difference between the plant community types allows determining the contribution of each plant type to the SOC accumulation following pastureto-silvopasture conversion, and for identifying the plant source for specific SOC fraction by whic h SOM is sequestered in silvopasture system. Few studies have employed the natural abundance of 13C to study SOC in mixed plant communities where C3 and C4 are grown simultaneously. Specifically, it will indicate whether SOC accumulations are the result of increases or decreases in the most recent organic matter inputs derived from trees or from organic C derive d from the grasses or previous pasture land, or from a mixture of both sources. Summary Land owners in southeastern USA are curr ently under pressure for change in land management to improve the environmental quality of the farm lands. Tree-integration practices on agricultural/pasture landscape such as silvop asture, intercropping, and riparian buffers are some of the available alternate land-use options and approaches. Silvopastoral practices that are known to have diversified economic and ecological benefits are marked as a good option for existing open pastures in the Southeast. In fact silvopastoral technologi es are applicable on a wide range of scale, and could be adaptable pa rticularly for small land holdings or operations with small numbers of animals. Silvopasture ha s the potential to increa se the total biomass productivity of the pasture system both above and below ground compared to open pastures, and consequently sequester more car bon, particularly in the soil in the long term. The functional

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33 consequences of tree integration into a pasture system or convers ion of pasture-to-silvopasture on many biogeochemical characteristics of this agroecosystems in general and the storage and dynamics of SOC in particular are, however, poorly understood. The impact of silvopastoral systems as functioning agroecosystems on C sequestration depends largely on the amount a nd quality of input provided by trees and grass components of the system, and on properties of the soils themselves, such as so il structure and its aggregation characteristics. The literature suggests that below-ground inputs in tree-based systems can be substantial. Soil C sequestration in soils is strong ly related to their clay + silt content. Waterstable aggregates provide physic al protection for C and reduce so il erodibility. Because soil size fractionation is found to be linke d to specific SOC fractions, so il structure, and aggregation characteristics, soil size frac tionation has shown promise for physically dividing SOC into pools. Furthermore, soil aggregation and SOC accumulati on are interrelated: SOC or fractions thereof are basic to the aggregation process, while SO C sequestered within aggregates is protected against decomposition. Both the degree of soil aggregation and extent of SOC storage are influenced by land-use, and the nature of soil and crop. Furthermore, integration of trees into open pasture (C4 grass) systems presents a unique opportunity for using stable isotope methodology to study SOC dynamics in the mixed system.

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34 Table 2-1 Agroforestry practices in the Southeastern U.S.A. Agroforestry practice Description Silvopasture Silvopasture is the intentiona l combination of trees, forage plants and livestock together as an integrated, intensively-managed system. Bermuda ( Cynodon dactylon )L.( Pers.) and bahia ( Paspalum notatum Flgg) mix with pines; winter grazing under pecans; poultry litter and manure application on trees/pasture; forage crops for cow/calf or fodder for confined operation; pastured poultry and free range with tree shade; fruit trees with animal pasture/hay; livestock-and fruit for biogas on family farm, plant browse species along fence lines; cattle or goats with scattered trees managed for shade; Alley cropping Alley cropping is the cultivation of food, forage or specialty crops between rows of trees. Pecans ( Carya illinoinensis )Wangenh.( K.Koch) with hay and/or clover; pecans with peaches ( Prunus persica ( L.) Batsch) for first 1012 years; Vegetables in alleys during pecan or citrus establishment; ornamentals with blueberries ( Vaccinium corymbosum L.); fruit or nut (e.g., persimmon, Diospyros virginiana L., or chestnut, Castenea dentata )Marshall( Borkh.) with intercrop (e.g., vegetables or cut flowers) Riparian Forest Buffers Riparian forest buffers are strips of trees, shrubs and grass planted between cropland or pasture and surface water courses. Including shrubs and trees for wildlife use and bee forage, managed timber or short rotation woody crops, managed along stream sides and in farm drainage ravines, shrubs and trees with deeper roots to aid nutrient absorption, artificial wetlands/add woody buffer for animal waste lagoons (including fish ponds) Forest Farming Forest farming is the intenti onal cultivation of edible, medicinal or decorative specialty crops beneath native or planted woodlands that are managed for both wood and understory crop production. Pi ne straw; N & P fertilization increase straw; farmer to chef herbs, mushrooms, specialty vegetables; growing edible and medicinal mushrooms (e.g ., on melaleuca); ferns under natural woodland (e.g., oak, Quercus laurifolia Michx.) shade; saw palmetto ( Serona repens ( W. Bartram Small manageme nt on native woodland range; ornamentals under shade trees; honey bees-(apiculture) and wildflowers grown for seed; and native medicinals/botanicals grown under forest shade: mosses, Queens delight, mints, mushrooms Windbreaks Windbreaks are linear plantings of trees and shrubs designed to enhance crop production, protect people and livest ock, and benefit soil and water conservation. Border plantings for vine yards, around citrus or other orchards, for protecting from frost, avocado, Persea americana Mill., for carambola, Averrhoa carambola L. (needs wind protection), palms on bunds in flooded rice (field rotation with vegetables), along lot lines increase assessed land value at sale, as barriers against pesticide drift, odor, noise, dust, or roadsides, and protection of animals from ocean winds and excess salt Source: modified from Workman et al. (2003).

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35 CHAPTER 3 SOIL CHARACTERISTICS OF TREE-BASED AND OPEN PASTURE SYTEMS Introduction Pastureland is an important land resource in the southeastern US A. This sector supports 6.5 million beef cattle and more than 990,000 dairy cows and covers more than 12.6 Million ha (12%) of the total land area the southeastern region (AL, FL, GA, KY, MS, NC, SC, TN, and VA) (USDA, 1992). Managing these agroecosystems and the natural resour ces in them in an economically and ecologically sustainable manner has become very challenging. This is partly due to problems caused by intensified chemical ag riculture leading to land -quality deterioration and ecosystem degradation, and social changes resulting from commerci alization leading to disappearance of family farms (Workman and Alle n, 2004). Tree integration into pasture land is increasingly becoming one of the prime candidate land management options in addressing these challenges in the rapidly changing social and economic conditions of the Southeast. Integration of trees into fo rage and pasture systems ch anges aboveand belowground productivity, modifies the rooting depth and distribution, and may re sult in a shift in the quantity and quality of litter inputs (Sc holes and Hall, 1996; Connin et al., 1997; Gill and Burke, 1999; Jackson et al., 2000; Jobbgy and Jackson, 2000). These changes in vegetation, litter, and soil characteristics are widely believed to alter some of the physical and chemical characteristics and dynamics of soil ecosystem. The need for a thorough understanding of these dynamics of the soil ecosystem and developing effective land management strategies warrants a systematic study of the important soil characteristics such as soil pH and bulk density in di fferent soil layers in pasture systems. The objective of this study was, therefore, to determine these characteristics and assess how these characteristics change followi ng tree integration into pasture systems.

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36 Materials and Methods Study Area The study was conducted in four sites, locate d in Alachua (29 N, 82W), Osceola (280 9 N, 810 10 W), Hardee (27 N, 82 W), and Suwannee (30 N, 830 0 W) counties (Figure 3.1 ) in Florida. Two sites represented priv ately owned farms of Mr. Fred Clark in Alachua and Mr. Harris Hill in Osceola counties; the other two were at the Florida Sherif Boys ranch in Live Oak and the IFAS Range Cattle Res earch and Education Center, Ona, Florida. For the sake of convenience, the si tes are designated by county names where the farms are located. Detailed climatic and edaphic charac teristics of the sites are given in Table 3-1. At each site, a silvopasture and an adjacent ope n pasture plot were selected from which soil samples were drawn. Slope, aspect, and soil seri es were uniform across plots in a site, ensuring that land-use system (pasture vs. silvopasture) was the primary factor influencing the soil C content in plots. Soils of Study Area The sites selected for this study represente d two out of the seven soil orders found in Florida. The soils in Alachua and Suwannee si tes are Ultisols whereas Hardee and Osceola are Spodosols. Spodosols and Ultisols are the most prev alent soil orders in the southeastern USA. Spodosols have a sandy A horizon, followed by an el uted E horizon underlying which is the Bh or spodic horizon, followed by the Bw horizon (So il Survey Staff, 1999). The Ap horizon of the Ultisols is about 15 cm thick and is followed by Bt horizons with sand grains coated and bridged with clay (Soil Survey Staff, 1999). The soil at the Alachua site is Kendrick in the series and consists of well drained, slowly to moderately slowly permeable soils formed as thick beds of loamy marine sediments on nearly level sloping areas in the coasta l plain. It has a taxonomic class of loamy, siliceous, semiactive and hyperthermic Arenic paleudults. The Blanton seri es soils of Suwannee site consists of very

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37 deep, somewhat excessively drained to moderately well drained, moderately to slowly permeable soils on uplands and stream terraces in the Coastal Plain. Such soils are formed in sandy and loamy marine or eolian deposits (NRCS, 2006). Soils in Hardee site were of the Ona series ch aracterized as poorly drained and moderately permeable, formed in thick sandy marine sediments with a sandy, siliceous, hyperthermic Typic Alaquods taxonomic class. Similar soils are found in the flatwood areas of central and southern Florida. Slopes range from 0 to 2 percent. The Osceola site consists of deep and very deep, poorly drained and very poorly drai ned soils that are formed in sa ndy marine sediments. This soil type occurs on flatwoods and in depressions of Peninsular Florid a. Slopes are dominantly 0 to 2 percent but range to 5 percent (NRCS, 2006). Descriptions of Pasture Systems The term silvopasture can be defined as intentio nal combination of trees, forage plants and livestock in an integrated and intensively managed system (Nair et al., 2005). In three of the sites (Suwannee, Hardee and Osceola), the silvopasture was established by planting slash pine trees in existing pastures of bahiagrass where as the silv opasture in Alachua site was under a pine forest and was grazed naturally. The silvopasture in Hardee site was esta blished in December 1991 by planting south Florida Slash pine at 1120 tree ha -1 in 1.2 X 2.4 X 12.2 m, double row configuration (Kalmbacher and Ezenwa, 2005). In Osceala slash pine was planted in 3.1 x 1.2 x 12.2 m spacing in double row configuration. In th e Alachua and Suwannee sites, tr ees were planted in a single row configuration with spacings of 1.5 x 3.0 m and 1.5 x 7.2 m respectively. No fertilizers were applied to the silvopastur e and the open pastures in both Osceola and Alachua sites. The pasture at Suwannee site had been fertilized twice with 19-5-19 NPK

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38 fertilizer at 336 kg ha-1 in 2003, and 224 kg ha-1 in 2004. Since 1978, dolomite had been applied to the open pasture at the rate of 4.5 Mg ha-1 once every 4 years. The pasture in Hardee was fertilized with N, P, and K at 55, 6, and 45 kg ha-1. Plant Components In all the study sites, the s ilvopastoral system consisted of a combination of slash pine ( Pinus elliottii ) + bahiagrass ( Paspalum notatum ) and an adjacent open pasture system of exclusively bahiagrass (Figure 3.1).Slash pine and bahiagra ss are important species in the southeastern USA for timber and forage respecti vely. Coniferous and br oadleaved tree species are also considered for establishment of silvopa sture. However, due to its light crowns and good self-pruning abilities, slash pine is the most suitable tree species among the southern slash pines. The species grows best on moderate to poorly dr ained sandy soils. Slash pine is one of the most commercially important pine species in the southern USA (Walker and Oswald, 2000). The species grows naturally from southern South Ca rolina to central Florida and west to eastern Louisiana, but has also been pl anted and direct-seeded in Louisiana and eastern Texas where it now reproduces naturally (Lohrey and Kossuth, 1990). Bahiagrass is a sod-forming, deep-rooted, warm-s eason perennial grass (Watson & Burton, 1985). It was introduced to the USA fr om Brazil in 1913 by the Bureau of Plant Industry at the Florida Agricultural Experiment Station, Gain esville (Scott, 1920). Several cultivars of P. notatum such as the hardy Pensacola or Parag uay strains, were introduced into North America for forage and erosion control. Pensa cola is the most common cultivar in Florida (Werner & Burton, 1991). Bahiagrass has been extensivel y planted for forage and soil stabilization in the southern USA, especially in Florida, where a 1 million hectares have been

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39 planted (Ipiales, 2003). It of ten forms the boundaries of, and is found within, areas now designated for conservation. Soil Sampling Soil samples were collected in August 2005 from three different sample sets: two locations in a silvopasture that includes locations between trees in a row (SP-T) and at the center of an alley (SP-A); and another set on an open pasture (OP). Each of these sample sets had stratified grid sampling points made by three rows with four sampling points in a row. At each sampling point, soils were collected from six soil depths 0 5, 5 15, 15 30, 30 50, 50 75, and 75 125 cm. While in the field, a composite for each depth interval was prepared by mixing soils of four sampling points in a row, resu lting in composite samp les of three per set (treatment); the total number of samples was 216 (6 depths 3 replication 3 location 4 sites). Bulk Density and pH Determination Soil bulk density ( d) for each layer was measured by th e core method (Blake and Hartge, 1986). Using a tube of stainless steel (5 cm in diameter and 75 cm deep) cores samples were collected from all depth intervals. Initial weight of soil core from each layer was measured in the lab immediately after collection. Simultaneou sly, soil moisture content was determined gravimetrically by oven-drying a sub-sample at 105.8 C for 48 h to calculate the dry bulk density. The pH (H2O) was determined in 1:2 soils: wate r (w/v) suspension using a pH-meter. Statistical Analysis Planned-comparison analysis of variance with Tukey's studentized range test (HSD) means comparison test was used to test for land-use treatments effect on soil pH, bulk density, SOC in whole, macroaggreagtes, microaggreg ates and silt + clay associated fractions at all four sites. Statistical analyses were carried on depth-wise data. All statisti cal tests were performed with

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40 SAS (1985) and differences were considered si gnificant when P<0.05. The composite sample in a row (grid line in open pasture) within each soil depth constituted the experimental unit of the analysis and each unit had th ree replications (rows). Results Soil pH The results showed that OP lands in Suwannee si te had higher soil pH value than SP-A or SP-T across all soil profiles. In the uppe r 5 cm of Osceola soil, however, pH increased in the order of SP-A < OP
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41 production or consumption of H+ or by exudation of organic acids and thereby induce changes in pH compared to the bulk soil. Whether H+ exudation or consumption dominates depends on the plant species, the nutritional status of the plants and the fo rm of nitrogen supply (Marschner et al., 1986). The mechanisms by which tree species influence soil acidity are and include interspecific differences in the uptake of ex changeable cations and anions (Alban, 1982). Biogeochemical processes that cau se plant-induced changes in so il pH included nitrogen fixation and ensuing nitrification (van Miegroet and Cole, 1984), production and soil deposition of litter high in organic acid concentr ation (Ovington, 1953), and stimulati on of mineral weathering (Tice et al., 1996). Furthermore, changes in aboveand belowground productivity, modification of rooting depth and distribution, and a likely shift in the quantity and quality of litter inputs in silvopasture could have result in a shift in soil reaction. The bulk density did not show significant difference between silvopasture and open pasture in most of the sites except in Suwannee sites at the lower depth profile and the upper 5 cm of the Osceola site. Generally, soil bulk dens ity increases with decrease in organic matter content and is higher in soils of sandy texture. Differences in soil texture between the silvopasture and the open pasture within a site ar e small, as expected. However, the SOC content in Suwannee was higher in the silvopasture than in the open pasture, particularly at the lower depth of the profile (Chapter 4). Thus, the lowe r bulk density of soils in silvopasture could be related to their higher SOC content. Conclusion Soil pH was lower in silvopasture compared to open pastures on the old-growth of silvopastoral stands (Suwannee, 40 yr), which is a reflection of the regular application of dolomite to pasture. Although minimum, pH change could also be caused due changes in species composition as a result of tree integration into pa sture systems and consequent changes in litter

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42 quality. Furthermore, this could be a reflection of the possible influence of previous land-use on subsequent biogeochemical processes in the soil. The long-term consequences of land conversion from open pasture or natural sta nds to silvopasture in terms of soil reaction and soil quality need further investigation. Table 3-1 Climatic and edaphic characteristics of silvopasture (SP) and open pasture (OP) in four study sites Florida, USA Site Land-use Size (ha) MAT( C) MAP (mm) Soil series Prior land-use Age (years) Maximum Minimum SP 28.3 25.7 -3.0 1332.3 Kendrick Grazed naturally 8 Alachua OP 16.2 25.7 -3.0 1332.3 Kendrick Agriculture (Corn field) 55 SP 8.5 28.1 3.3 1232.5 Immokalee Pasture (15 years) 12 Osceola OP 6.1 28.1 3.3 1232.5 Immokalee Florida Flatwoods 45-50 SP 17.2 35.9 -0.2 1346.7 Ona Bahiagrass pasture 14 Hardee OP 8.6 35.9 -0.2 1346.7 Ona Grazed naturally 22 SP 16.2 25.8 -3.2 1365.7 Blanton Agriculture, pasture 40 Suwannee OP 16.2 25.8 -3.2 1365.7 Blanton Agriculture 40 MAT = mean air annual temperatur e, MAP = mean annual precipitation

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43 Table 3-2 Soil pH (H2O) values of sampling loca tions across soil depth in the four study sites. Sites Alachua Suwannee Hardee Osceola Soil depth, (cm) OP SP-A SP-T OP SP-ASPTOPSP-ASP-TOP SP-A SP-T 0 5.6 5.6 5.8 6.9a5.6b5.3b4.95.04.95.8b 5.3c6.4a 5 5.7 5.8 5.8 6.6a5.6b5.3b5.25.25.25.6 5.15.3 15 6.0 6.0 6.1 6.6a5.7b5.5b5.75.25.35.5 5.75.5 30 6.2 6.0 6.1 6.6a5.8b5.7b5.75.25.25.6 5.95.6 50 5.9 5.9 5.8 6.7a5.6b5.6b5.75.05.05.7 5.85.7 75 5.6 5.6 5.7 6.6a5.7b5.5b5.35.04.95.8 5.55.4 Note: Lower case letters next to the error bars indi cate significant differences in SOC among pasture locations at a given depth Table 3-3 Soil bulk density for two land-use trea tment locations across soil depth in the four study sites. Bulk density (g/cm3) Alachua Suwannee Hardee Osceola Soil depth,(cm) OP SP OP SP OP SP OP SP 0 1.9 2.1 1.4 1.6 1.2 1.3 0.9b 1.7a 5 1.1 1.4 1.8 1.5 1.2 1.5 1.5 1.3 15 1.5 1.4 1.4b 1.7a 1.6 1.4 1.6 1.9 30 1.2 1.2 1.7a 1.4b 1.4 1.2 1.4 1.3 50 2.1 2.0 2.6a 2.2b 1.3 1.7 1.5 1.5 75 1.4 1.6 2.6a 1.8b 1.4 1.4 1.8b 2.0a Note: Lower case letters next to the error bars indi cate significant differences in SOC among pasture locations at a given depth

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44 Figure 3-1 Location of soil sampling sites Hardee Osceola Silvopasture Open-pasture Alachua Suwannee

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45 CHAPTER 4 CARBON STORAGE IN DIFFERENT SIZE FRACTIONS OF SOILS IN SIVOPASTORAL SYSTEMS OF FLORIDA Introduction Pastureland is an important la nd resource in the southeastern United States. It covers more than 12.6 million ha (12% of the total land area ) and supports 6.5 million beef cattle and more than 990,000 dairy cows, (USDA, 1992). These ag roecosystems and the people inhabiting them are challenged with natural res ource management problems due to farm intensification and rapid changes in land-use in of the region. As a resu lt the small-scale farm communities are at a crossroads in search of land management prac tices that are economically and ecologically sustainable to cope with the problems. Agro forestry land-use practices are among the prime candidates of alternate land management c hoices (Nair, 1993; Garrett et al., 2000). Silvopasture integrating trees into past ure and forage plant production is the most widespread form of agroforestry in North Am erica (Garrett et al., 2000; Nair et al., 2005). Available information suggests th at silvopatoral system is an ecologically sustainable and environmentally desirable approach to mitigate the problem of nutrient pollution resulting from open pastures (Nair et al., 2007) and other avenues of non-point sources of pollution. In areas where nutrient pollution is a very serious envi ronmental problem, e.g., in the coarse-textured soils of the southeastern USA, silvopasture could be an alterna tive land-use option particularly for non-industrial private forest la nd owners and lives tock operators. When silvopasture is established by integra ting trees into grass vegetation of pasture systems, aboveand belowground productivity, th e rooting depth and distribution, and the quantity and quality of litter i nputs to soil will change (Schol es and Hall, 1996; Connin et al., 1997; Gill and Burke, 1999; Jackson et al., 2000; Jobbgy and Jackson, 2000). However, the soil C dynamics and storage in a continuum of tree-gra ss systems in general and silvopastoral system

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46 in particular are poorly unde rstood (Jackson et al., 2000, 2002; Archer et al., 2001, 2004; Hudak et al., 2003). The fore mentioned changes in tree -grass systems are believed to impact the soil C dynamics and storage. Through feedback interacti ons, it may lead to alterations of local and regional climate systems (Schlesi nger et al., 1990; Ojima et al ., 1999). Presence of large land area of pasture lands and a tree -based non-forest land use prac tice warrants a detailed study of the soil C dynamics and storage of these ecosystem and implications of their importance in the global C cycle. Tree-based land-use systems are expected to ha ve better soil C seque stration potential than most row crop agricultural systems (Montagnini and Nair, 2004). Conversion of agricultural crop land to forest usually results in substantial increases in soil C (Johns on, 1992); a recent estimate assumed that 50 Mg C/ha would be sequestered in afforestated soils in 30 yr (Bouwman and Leemans 1995). On the other hand, land-use conversi on from native prairies or forest vegetation into cultivated agriculture lead s to a decline in the SOC pools (Brown and Lugo, 1990; Burke et al., 1989). Native C, however, may not necessarily represent the upper soil C limit (Six et al., 2002). Sharrow and Ismail (2004) reported from thei r studies in Oregon that silvopasture system accumulated approximately 740 kg ha year more C than forests and 520 kg ha year more C than pastures. They concluded that agrofo resty may produce more total annual biomass and have synchronized mixed nutrien t cycling patterns of both forest and grassland. Claims on C sequestration potential of agroforestry systems are based on the premise that the tree components in agroforestry systems can be significant sinks of atmospheric C due to their fast growth, high and long-term biomass stock, and ex tensive root systems (Montagnini and Nair, 2004) The purpose of the research reported here wa s to quantify C storage in the whole soil and size-fractionated soil pools in a slash pine-based silvopasture (SP) and an adjacent open pasture

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47 (OP) system across six soil depths, at four site s representing Spodosols and Ultisols of Florida. The specific objectives were: (1) quantify total SOC accumulation and sequestration of open and tree-based pasture systems; (2) quantify SOC stor ed in the different fractionated particle size classes; and (3) Elucidate specific physical pr otection mechanisms of soil C sequestration in open and tree-based pasture systems. Materials and Methods Study Area Soil samples were collected in August 2005 from four sites, located in Alachua (29 N and 82W), Osceola (280 9 N, 810 10 W), Hardee (27 N, 82 W), and Suwannee (30 N, 830 0 W) counties (Figure 3-1) in Florida. Two s ites represented privately owned farms of Mr. Fred Clark in Alachua and Mr. Harr is Hill in Osceola counties; the other two were at the Florida Sheriff Boys Ranch in Live Oak and the IF AS Range Cattle Research and Education Center, Ona, Florida. For the sake of convenience, the sites are designated by county names where the farms are located. Detailed clima tic and edaphic characteristics of the sites are given in chapter 3 (Table 3-1). At each site, a silvopastur e and an adjacent open pasture plot were selected from which soil samples were drawn. Slope, aspect, and soil series were uniform across plots in a site, ensuring that land use syst em (pasture vs. silvopasture) was the primary factor influencing the soil C content in plots. Soil Sampling Soil samples were collected from three different sample sets: two locations in a silvopasture that includes locations between trees in a row (SP-T) and at the center of an alley (SP-A); and another set on an ope n pasture (OP). Each of these sa mple sets had stratified grid sampling points made by three rows with four sa mpling points in a row. At each sampling point, soils were collected from six soil depths 0 5, 5 15, 15 30, 30 50, 50 75, and 75 125

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48 cm. While in the field, a composite for each depth interval was prepared by mixing soils of four sampling points in a row, resulting in composite samples of three per set (treatment); the total number of samples was 216 (6 depths 3 replications 3 locations 4 sites). Physical Fractionation All field moist composite samples were air dried and passed through a 2 mm sieve. Soils were physically fractionated by wet-sieving foll owing a procedure modified from Elliott (1986) and Six et al. (1998). The proced ure involved using disruptive for ces of slaking and wet-sieving through a series of two sieve sizes (250 and 53 m) to obtain three aggregate size classes. Briefly, a sub-sample of 100 g of the compos ite soil sample was submerged in deionized water as disruptive forces of slaking for about 5 min prior to placing it on top of 250 m sieve. The sieving was done manually by moving the si eve up and down approximately 50 times in 2 minutes. The fraction remaining on the top of a 250 m sieve was collected in a hard plastic pan and allowed to dry in oven at 65oC and weighed. Water plus soil < 250 m was poured through a 53 m sieve and the same sieving procedure was repeated. The overall wet sieving procedure yielded a water-stable fraction sizes of a macroaggregate-sized fraction 250 2000 m; a microaggregate-sized fraction 53 250 m, and silt +clay fraction size <53 m. The recovery of mass soil fractions after overall we t sieving procedure ranged from 96 to 99% of the initial soil mass. Chemical Analysis For chemical analysis, whole and frac tionated soil were oven-dried at 60 C for 72 h, and ground to fine powder using a ball m ill (Cianflone Scientific Instru ments, Pittsburgh, Pa.). Total soil organic C concentration was determined for whole and fractionated soil samples by dry

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49 combustion on an automated FLASH EA 1112 N C elemental analyzer (LECO Corporation, St. Joseph, Mich.). Statistical Analysis Planned-comparison analysis of variance with Tukey's studentized range test (HSD) means comparison test was used to test for landuse treatments effect on SOC in whole, macroaggreagtes, microaggregates and silt + clay asso ciated fractions at all four sites. Statistical analyses were carried out separa tely for all depth-classes. All statistical tests were performed with SAS (1985) and differences were cons idered significant when P<0.05. The composite sample in a row (grid line in open pasture) with in each soil depth constituted the experimental unit of the analysis and each unit had three replications (rows). Results SOC Storage in Whole Soil In all four sites, SOC storage declined with increase in soil dept h on both silvopasture and open pastures systems. At the Alachu a site, at soil depth interval 5 15 cm and at the lowest depth (75 125 cm), the SOC in the whole soil was significantly different ( p <0.05) across sampling locations with highest values in the SP-A, 22.6 and 2.2 kg/m2, respectively. In all other depths of Alachua site, SOC in whole soil was not different among the sampling locations (Figure 4-1 ). At the upper 5 cm of soil in Suwannee site, th e whole soil SOC was significantly higher in both SP-T (47.7 kg/ m2) and SP-A (41.8 kg/ m2) of the silvopasture than in the open pasture (28.5 kg/ m2). Similarly, at soil depths below 15 cm and above 75 cm, this value was higher in SP-T (range: 8 17 kg/ m2) than in the two other land-use elements (Figure 4-2). At the lowest

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50 soil profile (75 125 cm), however, the total SOC in the whole soil increased in order of SP-A< OP
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51 SOC in Microaggregates (53 m 250 m) The C accumulated in the microaggregate fr action did not show differences among the sampling locations in Alachua for all the depths although it consistently decreased with the increase in soil depth (Figure 4-9). Similarly, in Suwannee site (Figure 4-10) both silvopasture and open pasture did not differ at any depth exce pt for the lowest (75 125 cm) depth where SPT (4.35 kg m-2) had stored significantly more than SP-A and OP. SOC in Silt and Clay (<53 m) The C content in silt + clay did not show difference between land-use locations in all profiles of Alachua site, Hardee sites, a nd most sections of the Suwannee site (Figures 4-13, 414, and 4-15). At the lowest depth of the soil prof ile in Suwannee site, the open pasture retained slightly more C than at both locations of silvopast ure. In Osceola site, the C associated with silt + clay fraction of OP field was significantly higher than that of the SP-T at both surface (0 5 cm) and subsurface (5 15 cm) profile depth (Figure 4-16). On the contrary, at the lowest depth (75 125 cm), SP-T was significantly higher in C cont ent for the same fraction size. Generally, with an increase in soil depth, the C content of silt + clay fraction consistently decreased in all locations of Ultisol sites. At th e Spodosol sites, C content in the silt + clay C fraction followed a sporadic pattern with no trend across soil dept h; however, C content was higher in and around the spodic horizon than other horizons of the soil profile (40 55 cm). The SOC across the Spodosol and Ultisol locatio ns for the whole and fractionated soils are shown in Figures 4-17 and 4-18. In both the Spodosol and Ul tisol locations, higher SOC in whole soils was observed at the lower depth (bel ow 15 cm) in silvopasture (SP-T and SP-A) as compared to the open pasture. Similar patterns were also observed in the fraction sizes of macroaggregates (250 2000 m) (Figures 4-17B and 4-18bB) and the microaggregates (53

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52 250 m) (Figures 4-17C and 4-18C). However, at both soil orders no difference in SOC was observed between silvopasture ( SP-T and SP-A) and the open pastur e in the silt + clay fraction (<53 m) (Figures 4-17D and 4-18D). The over all mean storage of the SOC in whole soils (Figure 4-19A), macroaggregates (250 2000 m) (Figures 4-19B) an d microaggregates (53 250 m) (Figures 4-19C) showed higher values for the silvopasture at all depths but the surface (0 5 cm) as compared to the open pasture. Discussion The main objective of this section of the st udy was to assess the potential of tree-based pasture system to retain C in the soil. In orde r to accomplish this objective, it is important to understand the specific physical SOC protection mechanisms in open and tree-based pasture systems. Comparison of total SOC accumulation at any given soil depth, particularly at the deeper soil depths, for open and tree-based pastur e systems showed an increase in total C storage in whole soils of the silvopast ure in most of the sites (Figure 4-2, 4-3, and 4-4) except at the Alachua site, (Figure 4-1). The absolute quantities of such increases, however, varied at the three sites. The silvopasture plot in Alachua that was established only 8 years ago is the youngest of the sites. Evidence from physical soil fractionation showed that the SOC in macroaggregates was higher for silvopasture (SP-A or SPT) than open pasture, particular ly at lower soil profile at all but Suwannee site (Figure 4-5, 4-6, 4-7, and 4-8). Similarly, the SOC in microaggregates was higher under silvopasture than open pasture in both sites of Spodos ol. Except for Osceola site, where higher SOC storage in silt + clay fracti on was found, SOC in silt+clay fraction did not differ significantly among the three pasture loca tions across soil depth. The results demonstrate

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53 the potential of silvopasture to enhance tota l soil C storage. The amount of C accumulation varied in the different soil aggregate size cl asses. More C was accumulated in the largest aggregates. SOC in silvopasture increased by an overall average of 33 % in SP-T and 28 % in SP-A as compared to the adjacent open pasture (F igure 4-19). This indicates that the tree-based pasture system resulted in significant increase in C sequestration since its establishment. Integrating trees into open pasture increases temporal and spatial species diversity. Thus the system, compared with open pa sture system, could be closer to natural ecosystems in terms of better soil quality and increa sed C storage. Information on spec ific impact of integration of pine trees into pasture on the biogeochemistry of the ecosystem is limited. Some information that is available on C-storage of landuse systems where trees had encroached into a natural grassland ecosystem is relevant to this discussion. Most studies on this subject in a variety of ecosystems have shown greater concentrations of SOC in soils on sites where tr ees encroached a grass-dominated ecosystems compared with an adjacent grassland without any trees (Tiedema nn and Klemmedson, 1973; Virginia and Jarrell et al., 1983; Mordelet et al., 1993; Stock et al., 1995; San Jose et al., 1998; Geesing et al., 2000; Burrows et al., 2002; Reyes-Reyes et al., 2002; Yele nik et al., 2004). Results of the current study are consistent with those trends of increase in SOC. Some results from assessments of this nature, however, dispute whether such a phe nomenon, trees encroachme nt into grassland ecosystem, would lead to a net C sink. Jackson et al. (2002) studied the di rect effect of native woody vegetation on SOC at six sites in plot s of grassland invaded by common native woody species such as Prosopis (mesquite), Larrea (creosote), and Juniperus (juniper) spp. and an adjacent plot of grassland without trees in the southwestern USA. They predicted a decline in SOC in grassland invaded by woody vegetation fo r areas that received > 600 mm mean annual

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54 precipitation: The authors attributed this to hi gher rate of decomposition. In contrast, some studies show no changes in SOC following woody pl ant invasion of grasslands (McCarron et al., 2003; Smith and Johnson, 2003). Furthermore, it is not yet clear why responses to tree incorporation can range from net losses to net gains in soil C. Increase in SOC under silvopasture in the current study could be a direct manifestation of the higher rates of ne t primary productivity (NPP) in tree-based land-use relative to open grass pasture as reported by Belsky et al. (1993), Mo rdelet et al. (1993), Boutton et al. (1998), Archer et al. (2001), and Hibbard et al. (2001). Sharrow et al. ( 1996), for instance, reported that 10-year-old Douglas-fir ( Pseudotsuga menziesii )/grass/ clover pasture/sheep agroforests produced 1.6 times as much phytomass as did pastures or forests of the same age, on the same site. Typical rates of aboveground NPP in grass-dominated vegetation in the Rio Grande Plains of southern Texas, US A, were found to be 1.9 3.4 Mg ha -1 yr -1; in contrast, the rates of aboveground NPP in areas adjacent gr assland encroached by woody vegetation were 5.1 6.0 Mg ha -1 yr -1 (Archer et al., 2001; Hibbard et al., 2001 ). Other studies al so suggested that belowground productivity was accelerated followi ng tree establishment in grassland vegetation although rates of belowground NPP were not quan tified. Coarse and fine root biomass were found to be two-to-five times greater and showed significantly larger seasonal fluctuations in areas with trees than in grasslands (Boutt on et al., 1998, 1999; Hibba rd et al., 2001). Thus, increases in aboveand belowground primary production are potentially able to account for observed increases in SOC in this study in silv opastures relative to open pasture. However, a direct determination of NPP in both silvopasture and open pastur e which was not undertaken in this study may help to confirm this.

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55 Quantification of SOC stored in the different particle size classe s (fractions) showed that in both land-use systems highest SOC was stored in the largest size fraction (macroaggregates) followed by microgregates and then in the smalle st size fraction ( Figur e 4-17, 4-18 and 4-19). This suggests a hierarchical aggr egation process as reported by Oa des and Waters (1991). In this process of aggregation, the silt + clay build up to form the microagrrega tes and further building up of the microgreagtes forms the macroaggregates. Much of the C associated with larger size aggregates is of recent C deposition into the soil system. The macroaggregate is considered to be an important pool of bioavailable C. But, the amount of C in macroaggregates is sensitive to fluctuation in plant litter input and may show si gnificant seasonal and spatial variability in landuse systems (Karlen and Cambardella, 1996). Results from the fractionation in this study clearly showed increases in total SOC pools in silvop asture compared to adjacent open pasture (Figure 4-19). Further examination of the results show that this increase in SOC pools could be due to a number of factors: 1 Accumulation of new SOC in macroaggregates fraction: compared to open pasture, there was an overall mean increase of 39 % in the alleys of silvopastur e (SP-A) and 20 % in silvopasture near th e trees (SP-T) (Figure 4-19). This increase was as high as 114% in the alleys of silvopasture (SP-A) and 78 % in sil vopasture near the trees (SP-T) for the Hardee site (Figure 4-7). In the Ultisol site, however, this increase was observed only in the alleys of silvopasture (SP-A), an increase by 46 % in Alachua (Figure 4-9). 2 Retention of older SOC by protection in mi croaggregates: compared to open pasture, there was an overall mean increase of 12.3% in the alleys of silvopasture (SP-A) and 18.8 % in silvopasture near the trees (SP-T) (Figure 4-19). This increase was as high as 108% in the alleys of silvopasture (SP-A) and 111 % in silvopasture near th e trees (SP-T) for the Hardee site (Figure 4-11); whereas in the Ultisol si te, this increase was observed only in SP-A, an increase by 46 % in Alachua (Figure 4-9) 3 Association of SOC with silt + clay: compared to open pasture, there was an increase of 72% in the alleys of silvopast ure (SP-A) and 60% in silvopast ure near the trees (SP-T) at Hardee site (Figure 4-15). Although relatively small, there was an increase of 35% in the alleys of silvopasture (SPA) at Suwannee site.

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56 In most of the study sites, the amount of mi neral-associated C at a given depth did not differ significantly between land-use systems acr oss almost all the so il profiles. Although stabilization of soil organic C by association wi th mineral soil silt a nd clay particles is directly related to the silt plus clay content of the soil in a variety of ecosystems (Hassink 1997; Six et al. 2002), the soils at all sites of this study ar e characterized by high content of sand, more than 95% (Nair et al., 2007; Michel et al., in pess). The C in this fraction size is older and more stabilized in nature. Hence it is less likely th at the effects of changes in land use due to integration of tree in the curre nt land use will be indicated. In contrast, the Spodosol sites differed significantly from the Ultisols site on average by nearly 1.2 kg C m-2 (54%) (Figures 417 and 4-18) in silt + clay associated C. These so ils may have some potential to stabilize more soil C in organo-mineral complexes due to the sp odic horizon which differs from the other soil materials due to the prevalence of organically-ass ociated Al that has ve ry high surface area for retention (Eswaran et al., 2003). At least part of the C in largest fraction si ze is involved in the fo rmation and stabilization of the macroaggregates in the form of C within macroaggregate and C associated with clay. The bulk of available information is in agreement with current results in that the macroaggregate soil C presents a factor of potential use in assessi ng shorter-term changes in soil C storage induced by changes in soil management-, land-use-, and ve getation regimes. Thus, macroaggregation and physical protection of SOC are more closely linked to the abundan ce and turnover of larger-sized fraction of C than to the whole soil C level. Clearl y, in this case larger-s ized fraction C must be considered to encompass an active SOC pool. Fu rthermore, other studies indicated that the larger-sized class fraction C is quantitatively more important in sandy than clayey soils (Greenland and Ford, 1964; Leuschner et al., 1981)

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57 Generally, water-stable aggreg ates provide physical protect ion for C and reduce soil erodibility. The formation of thes e aggregates is enhanced by r oot and faunal activity. Most of the increased differences in total SOC in silvopa sture relative to an ope n pasture were in the lower depth of the soil profile suggesting that i nputs from the deep root systems of trees in silvopasture could be significant. The deep rooting nature is, how ever, a conjecture; robust data are not available to support this. Available data from root deve lopment studies suggest that for plantations, maximum observed rooting de pth for fast growing loblolly pine ( Pinus taeda ) in Spodosols of the Georgia lower Coastal Plain was 85, 85, 85, and 95+ cm at ages 1, 2, 3, and 4 yr, respectively (Adegbidi et al., 2004). Furthe r investigations are needed on this aspect. Conclusions Although silvopasture is practiced in the S outheast and elsewhere in the USA, the biogeochemical consequences of tree integration into pasture system in general and the soil carbon storage and dynamics in particular are little known. This study indicates that tree integration into open pasture systems increased SO C in whole-soil, particularly at lower depths. Increases in aboveand belowground primary production may account for observed increases in SOC in silvopasture portions of the landscape relative to an op en pasture land. Furthermore, results from the soil fractionation revealed that the increase in C in the silvopasture could be due to retention of more SOC of ol der C (by protection in microaggr egates), and association of C with silt + clay and accumulation of more new SO C in macroaggregates fraction in silvopasture. The potential of silvopasture as a strategy for C se questration thus seems clear. However, critical information on several key issues that is need ed for making valid conclusions is still missing. These include determination of NPP in both sil vopasture and open pasture, further isolation of microaggregates and silt + clay fractions fr om the macroaggregates and microaggregates respectively, and determination of microbial ac tivity and biomass in the different fractions.

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58 Research on these areas needs to be intensifie d in order to better unde rstand and exploit this seemingly important environmen tal benefit of silvopasture. 010203040506070 0 5 15 30 50 75125Soil depth, cmSOC, kg m-2 OP SP-A SP-T c a b b a b Figure 4-1 Changes in SOC with depth at three past ure locations [(silvopastu re: the center of the alley (SP-A) and in-between tree rows (SPT); and open pasture (OP)] for whole-soil of Alachua site. Lower case letters next to the error bars i ndicate significant differences in SOC among pastur e locations at a given depth.

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59 0102030405060 0 5 15 30 50 75Soil depth, cmSOC, kg m-2 OP SP-A SP-T a a b b a b b b a b c a b b a Figure 4-2 Changes in SOC with depth at three pasture locations [(silvopasture: the center of the alley (SP-A) and in-between tree rows (SP-T); and open pasture (OP)] for wholesoil of Suwannee site. Lower cas e letters next to the erro r bars indicate significant differences in SOC among pasture locations at a given depth.

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60 020406080100120 05 515 1530 3050 5075 7525Soil depth, cm SOC, kg m-2 OP SP-A SP-T a b b b a a b c b a c bac b a b a Figure 4-3 Changes in SOC in whole-soil with depth at three pa sture locations [(silvopasture: the center of the alley (SP-A) and in-bet ween tree rows (SP-T); and open pasture (OP)] for whole-soil of Hardee site. Lower case letters next to the error bars indicate significant differences in SOC among pa sture locations at a given depth. Bh horizon

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61 0102030405060708090100 0 5 15 30 50 75125Soil depth, cmSOC, kg m-2 OP SP-A SP-T a c b a b a Figure 4-4 Changes in SOC in wh ole-soil with depth at three pa sture locations [(silvopasture: the center of the alley (SP-A) and in-betw een tree rows (SP-T)]; and open pasture (OP) for whole-soil of Osceola site. Lower cas e letters next to the error bars indicate significant differences in SOC among pasture locations at a given depth. Bh horizon

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62 05101520253035404550 05 55 150 300 505 755Soil depth, cm SOC, kg m-2 OP SP-A SP-T ab ab a a b b Figure 4-5 Changes in SOC in macroaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Alachua site. Lower case letters next to the error bars indicate significant differences in SOC among pasture locations at a given depth.

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63 051015202530354045 0 5 15 30 50 75Soil depth, cm SOC, kg m-2 OP SP-A SP-T Figure 4-6 Changes in SOC in macroaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Suwannee site.

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64 010203040506070 0 5 15 30 50 75Soil depth, cmSOC, kg m-2 OP SP-A SP-T a b a a b a c b a Figure 4-7 Changes in SOC in macroaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Hardee site. Lower case letters next to the error bars indicate significant differences in SOC among pasture locations at a given depth Bh horizon

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65 010203040506070 0 5 15 30 50 75Soil depth, cmSOC in macroaggregate, kg m-2 OP SP-A SP-T ab b ab b b a b a a Figure 4-8 Changes in SOC in macroaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Osceola site. Lower case letters next to the error bars indicate significant differences in SOC among pa sture locations at a given depth. Bh horizon

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66 0510152025 05 55 150 300 505 755Soil depth, cmSOC, kg m-2 OP SP-A SP-T Figure 4-9 Changes in SOC in microaggregates with dept h at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Alachua site.

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67 0246810121416 0 5 15 30 50 75Soil depth, cmSOC, kg m-2 OP SP-A SP-TSuwanneb b a Figure 4-10 Changes in SOC in microaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Suwannee site. Lowe r case letters next to the error bars indicate significant differences in SOC among pasture locations at a given depth.

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68 0102030405060 05 515 150 300 505 7525Soil depth, cmSOC, kg m-2 OP SP-A SP-T b a b a b b a b ab a a b Figure 411 Changes in SOC in microaggregat es with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Hardee site. Lower case letters next to the error bars indicate significant differences in SOC among pasture locations at a given depth. Bh horizon

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69 0102030405060 05 515 150 300 505 755Soil depth, cmSOC, kg m-2 OP SP-A SP-T b a aab b b ab b a b ab b b a Figure 412 Changes in SOC in microaggregates with depth at three pa sture locations [(SP-A, SP-T and OP)] for Osceola site. Lower case le tters next to the error bars indicate significant differences in SOC among pa sture locations at a given depth. Bh horizon

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70 012345678 0 515 15 30 50 7525Soil depth, cmSOC, kg m-2 OP SP-A SP-T Figure 4-13 Changes in SOC in silt+clay fractio n with depth at three pasture locations at Alachua site.

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71 00.511.522.533.544.55 05 515 150 300 505 7525Soil depth, cm SOC, kg m-2 OP SP-A SP-T b b a Figure 4-14 Changes in SOC in silt + clay frac tion with depth at three pasture locations for Suwannee site.

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72 01234567 0 515 15 30 50 7525Soil depth, cm SOC, kg m-2 OP SP-A SP-T Figure 4-15. Changes in SOC in silt+clay frac tion with depth at three pasture locations for Hardee site. Bh horizon

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73 0123456789 05 515 150 300 505 7525Soil depth, cmSOC, kg m-2 OP SP-A SP-T b ab a b ab a b b a Figure 4-16 Changes in SOC in silt+clay fr action with depth at three pasture locations [(silvopasture: the center of the alley (SPA) and in-between tree rows (SP-T); and open pasture (OP)] for Osceola site. Bh horizon

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74 100.0 62.5 40.0 22.5 10.0 2.5 100.0 62.5 40.0 22.5 10.0 2.5 60 45 30 15 0 60 45 30 15 0 OP SP-A SP-TSoil dep t h, cmC D B A Figure 4-17 Changes in mean SOC across the Spodosol locations (Hardee and Osceola) in A) whole soil, B) macroaggregates C) microa ggregates, and D) silt + clay fraction down the soil profile depths at three pa sture locations (SP-A, SP-T; and OP). SOC, kg m -2

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75 100.0 62.5 40.0 22.5 10.0 2.5 100.0 62.5 40.0 22.5 10.0 2.5 48 36 24 12 0 48 36 24 12 0 OP SP-A SP-T D C B ASoil depth, cm Figure 4-18 Changes in mean SOC across the U ltisol locations (Alachua and Suwannee) in A) whole soil, B) macroaggregate C) microaggr egates, and D) silt + clay fraction down the soil profile depths at three past ure locations (SP-A, SP-T; and OP). SOC, kg m -2

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76 100.0 62.5 40.0 22.5 10.0 2.5 100.0 62.5 40.0 22.5 10.0 2.5 600 450 300 150 0 600 450 300 150 0 SOC, kg m -2 SP-T SP-A OP A B C DSoil dep t h, cm Figure 4-19 Changes in overall mean SOC across all locations in A) whole soil, B) macroaggregate C) microaggregates, and D) si lt + clay fraction down the soil profile depths at three pasture locations (SP-A, SP-T; and OP). SOC, kg m -2

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77 CHAPTER 5 SOIL CARBON SEQUESTRATION BY TR EES AND GRASS IN SILVOPASTORAL SYSTEMS: EVIDENCE FROM STABLE ISOTOPE ANALYSIS Introduction Soils in agroecosystems can function as both source and a sink for atmospheric C. The direction of this dynamic equilibrium is gene rally determined by the way in which these agroecosystems are managed. Agroforests are planned and managed agroecosystems (Garrett et al., 2000). Increasing the overall productivity and efficiency of the land-use system and its sustainability are major goals of agroforestry (Nair, 2005). The overall goal of combining tr ees and/or shrubs with crops and/or livestock production is to optimize the physical, biological, ecological, ec onomical, and social bene fits resulting from the interactions between or among the components (MacDicken and Vergara, 1990; Garrett et al., 1991; Nair, 1993; Leakey, 1996). Agroforestry and other tree-based systems are believed to enhance C sequestration in soil compared with treeless (agricultural) systems (Montagnini and Nair, 2004). Such claims on C sequestration poten tial are based on the premise that the tree components in agroforestry systems can be signif icant sinks of atmospheric C due to their fast growth, long-term storage of high amounts of C in biomass, and extensive root systems. Silvopasture the integration of trees into forage or/and lives tock has been practiced in the southeastern USA as "tree-pasture" or "p ine-pasture" since the early 1950s (Nowak and Long, 2003). Indeed, silvopasture is the most comm on form of agroforestry in North America (Garrett et al., 2000; Nair et al., 2005). Thes e agroecosystems are usually established by incorporating trees in ex isting, managed pastures of bahiagrass, bermudagrass, or other similar grasses (Nowak and Long, 2003). S ilvopastoral agroforestry systems currently are of great interest due to their potential to improve the envi ronmental quality of land in the southeast as alternative land-use systems to treeless past ures and tree plantations. Recent information

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78 suggests that silvopasture is an ecologically su stainable and environmenta lly desirable approach to mitigating the problem of nutrient pollution resu lting from beef-cattle pastures ( Nair et al., 2007; Michel et al., in press). Information on th e importance of plant components in general and the biogeochemical consequence of tree integr ation into pasture monoculture systems or conversion of pasture-to-silvopas ture on the SOC storages and dyna mics in particular, however, are still not quantified. Available information indicates that functional consequences of integration of trees into grass-dominated vegetation include change s of aboveand belowground productivity, modifications to rooting depth and distribution, and changes in the quantity and quality of litter inputs (Scholes and Hall, 1996; Connin et al., 199 7; Gill and Burke, 1999; Jackson et al., 2000; Jobbgy and Jackson, 2002). These changes in ve getation, litter, and soil characteristics following tree integration into pasture systems modify ecosystem C dynamics and storage and may lead to alterations of local and regional climate systems through feedback interactions (Schlesinger et al., 1990; Ojima et al., 1999). The mechanisms and processes associated with C dynamics and storage in natural tree-based grassland or pasture systems such as silvopasture systems are still poorly unders tood (Jackson et al., 2000, 2002; Archer et al., 2001, 2004; Hudak et al., 2003). Given their great significance, therefore, thoroug h and detailed studies on soil C dynamics and storage in silvopastoral agroforestry systems are crucial. The stable C isotope ratio analysis in SOC studies emerged as a tool to describe the dynamics of C3 and C4 components in vegetatio n (Stout et al., 1981). The relative isotope (12C and 13C isotopes) composition expressed as 13C in plant biomass is rela ted to the photosynthetic pathway (C3, plants average -27 and C4 plants average -12 ; Boutton 1991), and the 13C value of SOC is approximately equal to th e plant C deposited in the upper soil layers

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79 (Nadelhoffer and Fry, 1988; Melillo et al., 1989; Boutton, 1991). Typically, the C3 component is either woody shrubs or trees and the C4 component is grass. The application of stable C isotopic analysis to SOC studies has therefore focuse d largely on descriptions of grass-woody plant dynamics and particularly the dynamics at the savanna-grassland ecotone where the principal question has been the stability of the grassla nd-savanna boundary (Schwartz et al., 1986; Volkoff and Cerri, 1987; Tieszen and Archer, 1990; Ambros e and Sikes, 1991). This author has not come across any previous research using the natural abundance of 13C to study carbon dynamics in pine-based silvopastoral sy stem where C3 and C4 plants are grown simultaneously. The technique requires comparison between a site where the photosynthetic pathway type of dominant vegetation has been changed and re ference site where photosynthetic pathway type of vegetation remains unchanged. The plant community in the common silvopasture systems in southeastern USA comprises slash pine ( Pinus elliottii ) (C3 plants; 13C -29.5 ) and C4 plants dominated by bahiagrass ( Paspalum notatum ) ( 13C -13.3). The 13C value ranges of C3 and C4 plants do not overlap; differences in is otope ratio, therefore, ca n be used to quantify the contribution of plants of each photosynthetic pathway to soil or ganic matter (Balesdent et al., 1988). A shift from open pasture to silvopasture presents a unique opportunity to use stable C isotope methodology to study soil organic matter dynamics following the alteration in vegetation structure due to the integration of trees to open pasture by compari ng with adjacent open pasture. The isotopic difference between the plant community types allows for identifying the contribution of trees in specific fraction size in wh ich SOC is sequestered in silvopasture system. Therefore, the objective of the present study wa s to determine the relative importance of C derived from woody vegetation (C3) vs. grass ve getation (C4) in silvopasture systems where

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80 trees were integrated into open pasture using the natural C isotopic difference between bahiagrass C4 and slash pine C3 plants. Materials and Methods Study Area Soil samples were collected in August 2005 from four sites, located in Alachua (29 N, 82W), Osceola (280 9 N, 810 10 W), Hardee (27 N, 82 W), and Suwannee (30 N, 830 0 W) counties (Figure 3-1) in Florida. Tw o sites represented privately owned farms of Mr. Fred Clark in Alachua and Mr. Harris Hill in Osceola countie s; the other two were at the Florida Sheriff Boys Ranch in Live Oak and the IFAS Range Cattle Research and Education Center, Ona, Florida. For convenience, the site s are designated by county names where the farms are located. Detailed climatic and edaphic characte ristics of the sites are given in chapter 3 Table 3.1. At each site, a silvopasture and an adjacent op en pasture plot were selected from which soil samples were drawn. Slope, aspect, and soil series were uniform across plots in a site, ensuring that land-use systems (pasture vs. silvopasture) was the primary factor influencing the soil C content in plots. Soil Sampling Soil samples were collected from three different sample sets: two locations in a silvopasture that includes locations between trees in a row (SP-T) and at the center of an alley (SP-A); and another set on an ope n pasture (OP). Each of these sa mple sets had stratified grid sampling points made by three rows with four sa mpling points in a row. At each sampling point, soils were collected from six soil depths 0 5, 5 15, 15 30, 30 50, 50 75, and 75 125 cm. While in the field, a composite for each depth interval was prepared by mixing soils of four sampling points in a row, resulting in composite samples of three per set (treatment); the total number of samples was 216 (6 depths 3 replication 3 location 4 sites).

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81 Size Fractionation All field-moist composite samples were air dried and passed through a 2 mm sieve. Soils were physically fractionated by wet-sieving foll owing a procedure modified from Elliott (1986) and Six et al. (1998). The proced ure involved using disruptive forc es of slaking and wet-sieving through a series of two sieve sizes (250 and 53 m) to obtain three aggregate size classes. Briefly, a sub-sample of 100 g of the compos ite soil sample was submerged in deionized water as disruptive forces of slaking for about 5 min prior to placing it on top of 250 m sieve. The sieving was done manually by moving the si eve up and down approximately 50 times in 2 minutes. The fraction remaining on the top of a 250 m sieve was collected in a hard plastic pan and allowed to dry in oven at 65oC and weighed. Water plus soil < 250 m was poured through a 53 m sieve and the same sieving procedure was repeated. The overall wet sieving procedure yielded a water-stable fraction sizes of a macroaggregate-sized fraction 250 2000 m; a microaggregate-sized fraction 53 250 m, and silt +clay fraction size <53 m. The recovery of mass soil fractions after overall we t sieving procedure ranged from 96% to 99% of the initial soil mass. Chemical Analysis For chemical analysis, whole and fractionated soils were oven-dried at 60C for 72 hr, and ground to fine powder using a ball m ill (Cianflone Scientific Instru ments, Pittsburgh, Pa.). Total soil organic C and N concentration was determin ed for whole and fractionated soil samples by dry combustion on an automated FLASH EA 1112 N C elemental analyzer (LECO Corporation) Stable C Isotope Analysis Soil samples were analyzed for C concentrations and for 13C values using a Carlo Erba EA-1108 (CE Elantech, Lakewood, NJ ) interfaced with a Delta Pl us (ThermoFinnigan, San Jose,

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82 CA) isotope ratio mass spectrometer operating in continuous flow mode at the Department of Soil and Water Sciences, University of Florid a. Carbon isotope ratios are presented in -notation: 13C = [(RSAMPLE RSTD)/RSTD] 103 (5-1) Where RSAMPLE is the 13C/12C ratio of the sample, and RSTD is the 13C/12C ratio of the VPDB standard (Coplen, 1996). Precision of duplicate measurements was 0.1. None of the samples contained CaCO3 or other forms of inorganic C. Relative proportions of SOC derived from the bahiagrass, a C4 plant, vs. the slash pine, a C3 plant, was estimated % C4-derived SOC = ( T)/ ( G T) x 100 (5-2) % C3-derived SOC = 100 %C4-derived SOC (5-3) Where is the 13C of a given sample, T a composite sample of the C3 plant and G is a composite sample of pasture grass tissues (C4) Results Changes in the Natural Abundance of 13C SOC Whole Soil Sample The average 13C value of whole soil in open pasture was .8 The value was significantly higher (p< 0.001, not shown) than in SP-A and in SP-T of silvopasture with average 13C value of 22.6 and 22.9 respectively, but there was no difference between SP-A and SP-T locations. In general, whole soil 13C values in silvopasture had strongly C3-dominated signatures ranging 24 to 23 in Alachua, 24 to 23 in Suwannee, 24 to 23 in Hardee and 24 to 23 in Osceola (Table 5-1). At the surface (05 cm), the 13C values of whole soil in open pasture with values 15.9 in Alachua, .9 in Suwannee, .2 in Hardee and .0 in Osceola were the hi ghest top in its respective site.

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83 Fractionated Samples The 13C values of the macroagg regate fraction (250 2000 m, Figure 5-1) showed a similar trend as the whole soil samp le with highest values at the surface of the open pasture in all four sites. At the upper 5 cm, the values in th e open pasture were higher by 38 % in Alachua (Figure 5-1C, p<0.001) 33% in Suwannee (Figure 5-1D, p<0.001) 27% in Hardee (Figure 5-1C, p<0.001) 25% in Osceola sites (Figure 5-1C, p<0.01) than the av erage value of SP-A and SP-T in silvopasture. Except for Hardee site (Figure 5-1A), the open pasture 13C values in macroaggregate fraction were significantly higher th an values on an adjacent silvopasture across all but one depth interval (Figure 5-1B, C, &D). The results on 13C values in microggreg ate size fraction (53 250 m) are shown in Figure 5-2. At any given soil depth interval in the Ultisol sites (Figure 5-2 C & D), the 13C values in open pasture system (average value 21 in Alachua and 25.5 in Suwannee) were consistently higher than the two sampling lo cation in silvopasture, with average values of 24.6 for SP-A and 24.7 for SP-T in Alachua and 25.5 for Sp-A and 25.6 for SP-T in Suwannee. In silt +clay size fraction (<53 m) a clear difference between 13C values on open pasture and silvopasture was observe d in Ultisol sites as opposed to Spodosol sites that showed no difference across depths (Figure 5-3C and D). In the upper 5 cm, for Alachua the values were 17.45 on OP as opposed to 23.8 and 23.5 on SP-A and SP-T respectively. Plant Sources of SOC in Whole Soil Sample In all the sites, the values of SOC percent derived from the C3 in the whole soil sample were consistently higher at th e two silvopasture sampling locati ons than in the open pasture systems. Generally the amount of SOC derived from C3 plants showed an increasing trend with

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84 soil depth. On average, these values range from 16.5% in OP, 55% in SP-A and 65% in SP-T at the surface soil profile to 71% in SP-T, 69% in SPA and 56% on OP at the deepest soil profile. Plant Sources of SOC in 250 to 2000 m Fraction The C3-derived SOC in large (macro) aggreg ate size fraction at both SP-A and SP-T of silvopasture accounted for significantly higher percen tage at any given soil depth interval for all the sites except in Hardee site where no diffe rence between land-uses was observed at depth 15 cm and below (Figures 5-8, 5-9, 5-10 and 5-11). The percent C3-derived SOC consistently increased with an increase in soil depth on both land-use systems for all the sites. For Alachua site the values were 11%, 68% and 75% of SO C in OP, SP-A and SP-T respectively on surface soil (0 5 cm). The corresponding values were 63%, 93% and 82 % in the lowest profile (75 125 cm) in the same site. Plant Sources of SOC in 53 to 250 m Fraction The C3 (slash pine) contributed relatively insignificantly to SOC in surface soils of the open pasture in the microaggregate size (53 250 m ) fraction. The trend was similar to that of the larger fraction in that there were signi ficant difference between open pasture and the silvopasture locations. The highest difference in percent contribution was in the surface in all four sites, ranging from 13 19% in open pasture and 30 76% in the silvopasture in the top 5 cm soil (Figure 5-12, 5-13, 5-14, and 5-15). Plant sources of SOC in <53 m Fraction The C3-derived SOC percent in the silt + cl ay fractions showed no differences among sampling locations on the Spodosol sites except in the upper 15 cm where tree (C3) contribution was higher in the silvopasture. In the S podosol sites at and below spodic horizon (40 50 cm), C3-derived C in the <53 m fraction was relatively high.

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85 The overall mean amount of SOC contribu ted by C3 and C4 plants in whole and fractionated samples at a given soil depth for the Ultisol and Spodosol locations are shown on Figure 5-20 and 5-21. At the surface (0 5 cm), much of the SOC accumulated was C3-derived C in SP-T and SP-A in the whole and the entire fr action sizes in the Ultisol locations. For the Spodosol locations, however, the SOC accumulation at surface had comparable contributions from both C4 and C3 plants in the silvopasture locations.C4-derived SOC in OP remained to be substantially greater in the whole and all three fractionated sizes in both Ultisol and Spodosol. Although total SOC was found to decrease with increasing soil depth, the proportion of C3derived as opposed to C4-derved SOC consistently increased in the whole soil and three fraction sizes in both soil orders. The overa ll means across all locations for C3 and C4derived SOC at a given depth in the whole soil and three fractions are presented in Figure 5-22. It has similar change pattern to that of mean va lues of the separate soil orders. Discussion The main objective of the present study was to substantiate the relative importance of C derived from slash pine (C3) vs. bahiagrass (C4) in the silvopas ture systems compared with an adjacent open pasture using difference in ratio of naturally abundant stable C isotopes ( 13C values) between C4 grasses and C3 woody plants. Fr om this study, it is appa rent that the current land-use significantly affected the 13C values of total SOC in whole soil and soil fractions (Table 5-1, Figures 5-1, 5-2, & 5-3). Generally, the 13C values of terrestrial C3 plants grown under natural conditions are between 22 and 34 (Vogel, 1993). The 13C value of composite sample of plant part or soil underneath for slash pine reported in litera ture are within the a bove expressed range of values. Parasolova et al. (2003) reported 13C range between 25 and 29.5 for slash pine x

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86 Caribbean pine ( Pinus caribaea ) hybrids. Similarly, Mortazavi and Chanton (2002) found value between 27.3 to 28.5, and on average 27.9 for slash pine needles. Although the 13C values measured in the silvopasture soil in this study were (on average 24) within the above expressed range of values for terrestrial C3 plants, they were somewhat high for a typical soil beneath a slash pine stand, s uggesting that th e bahiagrass (C4) in silvopasture had augmented the SOC storage that caused an increase in the 13C values. The most negative values (on average 24) in the silvopasture soils and the least negative values (on average 15) in the open-pasture soil were found on the surface soil profiles. Evidences from radiocar bon studies indicate that the aver age age of SOC increases with depth in the profile (Scharpens eel and Neue, 1984; Balesdent et al., 1990). Thus, the soil C at surface is believed to be composed of recent ac cumulation or young organic C as a case in point a reflection of the current difference in land-use. Given that the a value of 13.3 has been reported for shoots or roots of bahiagrass (Nakano et al., 2001), the current resu lts where the least negative value of 13C was at the surface on the open pasture, confirm that bahiagrass was the main plant source of the SOC in the surface soil. By contrast, the more negative values in the surface silvopasture soils indicate that a significant proportion of the SOC beneath the silvopasture on the SP-T and SP-A locations was deri ved from slash pine litter and fine roots. The calculated means of C3and C4derived SOC at a given depth in th e whole soil and three fractions presented in Figure 5-22 consistently substa ntiate this observation. As a consequence of the varia tions in their C turnover, the 13C values of fraction size separates provide an enhanced view of plant community history. The 13C values of fraction size separates (Figure 5-8 5-16 B& C, Figure 5-20 B &C, Figure 5-21B&C, and Figure 5-22B & C) revealed that the larger (macro) aggregate si ze fraction and micro-size aggregate contributed

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87 most of the C derived from the C3 woody plant in the silvopasture compared to open pasture across the soil depth, but particularly so in the lower profiles. The fact that the soil C in the larger fraction represents C that is larg ely newly incorporated into the soil suggests that most this SOC, including in the lower profile, is contribut ed by the current C3 pl ant component in the silvopasture, i.e. slash pine, that has deeper root systems than th e grass component. When comparing 13C values of functionally distinct SOC fractions within the same soil type, the silt + clay fraction (<53 m) always showed a signifi cantly higher percentage of C3-derived SOC signatures, particularly at the lower soil profile as opposed to larger fraction sizes. However, in the silt + clay fraction no si gnificant difference was observed between silvopasture and open pasture in the lower depths for mo st sites except the Alachua site where the prior land-use history for the current open pasture was agriculture (cor n field). The silt + clay fraction was less enriched with 13C than whole soil and appeared to contai n SOC largely derived from C3. In fact, the C associated with the silt + clay fraction tends to be older C, indicati ng that the prior landuse history of the site has substa ntial effect on the current status of C storage in soil. The study sites in the current study were previously under forest vege tation, popularly called Florida Flatwoods, and it is logical to surm ise that the current C status of the site is a reflection of this. At the Spodosol sites, at and below spodic hor izon, older C3-derived C was relatively high (Figure 5-21). The spodic horizons differ from ot her soil materials due to the prevalence of organically-associated Al and have very high su rface area for retention of older SOC. From an analysis of the depth distribution of 13C in podzols developed under C4 savanna in the Congo, Schwatz and Mariott (1986) showed that the s podic Bh horizons exhibited typical C3 values because the podzolic morphology was inherite d from past forest phases > 30,000 years old.

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88 Change from a C4dominated herbaceous co mmunity (bahiagrass) to a mixed plant community (in the silvopasture) of C3 woody slash pine and bahiagrass in the current study provides a good situation for the use of the natural 13C-leveling technique. The quantification of the progressive incorporation of new C into so il organic fractions pr ovides a powerful means with which to elucidate the pathways of C tr ansformations and stabilizations. However, the 13C method is unable to distinguish between residual primary forest C and new C derived from tree component in the silvopasture. In this regard, the use of 14C and bomb C models will be relevant to date soil fractions and to distinguish newly incorporated C derived from tree component in the silvopasture from old residual C retained from primary forest. Such methods could also reduce uncertainties in turnover rates. Conclusions The results of this study showed an increas e in total SOC pools following tree integration into pasture. This seems to be due to rete ntion of older C3-drive d SOC by protection in microaggregates, retention of C associated with silt + clay, and largely due to accumulation of new C3-derived SOC in macroaggregate fractions. C3 plants seemed to have consistently contributed more C in the silt + clay fraction (<53 m) than C4 plants at all soil depths, particularly in the lower depths, in all sites. Sites where the si lvopasture was older, the impacts of trees on SOC were greater. The results suggest that, in the long term, silvopasture may help sequester more SOC and stabilize C in the soil. These results have promising imp lications in the context of th e Kyoto protocol in view of the evidenced potential of silvopasture to sequester and stabilize C in the soil in the long term. The study also points out the need for intensifying research to address several issues before the suggested potential can be convinc ingly established. A line of res earch of considerable promise is the use of 14C and bomb C models to date soil fractions and to distinguish newly

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89 incorporated C derived from tree component in the sil vopasture from old residual C retained from primary forest.

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90Table 5-1. 13C values of whole soil SOC in different land -use locations at four sites across soil depth. 13C values of SOC () Ultisol sites Spodosol sites Alachua Suwannee Hardee Osceola Soil depth (cm) OP SP-A SP-T OP SP-A SP-T OP SP-A SP-T OP SP-A SP-T 0 5 15.9 a (0.9) 23.6b (0.5) 24.2 b (0.4) 16.9 a (0.3) 23.5 b (0.5) 23.9 b (0.3) 15.2 a (0.4) 20.9 b (0.0) 22.4 c (0.2) 16.0 a (0.1) 17.5 b (0.3) 20.7 c (0.6) 5-15 .6 (1.0) .0 (0.6) .6 (1.1) .2 a (0.4) .6 b (0.1) .2 b (0.3) .1a (0.1) .3ab (0.5) .7 b (0.4) .2 a (0.5) .7 b (0.4) .8 b (0.2) 150 .6 a (0.3) 24.9 b (1.7) 23.2 b (0.1) 20.9 a (0.3) -25.1 b ..(0.9) 24.4 b (0.4) 21.0 (0.2) 21.7 (0.8) 21.2 (0.1) 19.5 (0.4) 20.5 (0.3) 20.0 (0.1) 30 50 19.8 a (0.3) 25.0 b (1.7) 23.2 b (0.3) 21.5 a (0.1) 25.0 b (1.0) 24.7 b (0.4) 22.2 (0.7) 21.0 (1.2) 21.8 (0.4) 20.8 (0.1) 21.2 (0.2) 21.5 (0.5) 50 75 19.5 a (0.6) 25.0 b (1.7) 23.3 b (0.6) 22.0 a (0.3) 23.9 b (0.3) 24.7 b (0.1) 23.7 (0.1) 23.6 (0.7) 24.4 (0.3) 20.9 (0.3) 21.5 (0.5) 21.6 (0.5) 75 125 20.3a (0.9) 23.3 b (0.6) 23.3b (0.5) 22.0a (0.4) 24.3 b (0.2) 25.1 b (0.1) 23.3 a (0.5) 24.7 b (0.2) 24.7 b (0.1) 20.4 (0.7) 21.4 (0.3) 21.4 (0.8) Note: Lower case letters next to the mean values indicate significa nt differences in SOC among pasture locations at a given dep th (Tukey's studentized range test (HSD) performed following ANOVA); the va lues in parenthesis are standard deviations of means.

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91 Figure 5-1. Changes in 13C values of SOC in macroaggrega te fraction in different land-use locations at four sites acro ss soil depth: A) Alachua, B) Suwannee C) Hardee and D) Osceola across soil depth. A C

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92 Figure 5-2. Changes in 13C values in microaggregates size in sites: A) Alac hua, B) Suwannee C) Hardee and D) Osceola across soil depth.

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93 Figure 5-3. Changes in 13C values in silt + clay size fr action for sites: A) Alachua, B) Suwannee C) Hardee and D) Osceola across soil depth.

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94 0102030405060708090100 05 515 150 300 505 7525Soil Depth, cmC3-derived SOC, % OP SP-A SP-T a b ab a b a a b a ab b ab b a b a a a Figure 5-4. Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Alachua site

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95 0102030405060708090100 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-T a a b a a a a b a a b a b a a a b b Figure 5-5. Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Suwannee site.

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96 0102030405060708090 05 55 150 300 505 755Soil depth, cmC3-derive SOC, % OP SP-A SP-T b a c b a a c b a Figure 5-6. Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Hardee site. Bh horizon

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97 010203040506070 05 515 150 300 505 75125Soil depth, cmC3-derived SOC, % OP SP-A SP-T a b c b a a Figure 5-7. Changes in percent of C3-derived C in whole-soil with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Osceola site. Bh horizon

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98 020406080100120 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-T a b c b a a b a a b a a b a a Figure 5-8. Changes in percent of C3-der ived C in 250 2000 m with soil depth on silvopasture (SP-T and SP-A) and adjacen t open pasture (OP) in Alachua site.

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99 020406080100120 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-T b a a a c b a b a a a a b a b a a b Figure 5-9. Changes in percent of C3-derived C in 250 2000 m with soil depth on silvopasture (SP-T and SP-A) and adjacen t open pasture (OP) in Suwannee site.

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100 020406080100120 05 515 150 300 505 755Soil depth, cmC3-derived SOC, % OP SP-A SP-Ta b c Figure 5-10. Changes in percent of C3-d erived C in 250 2000m with soil depth on silvopasture (SP-T and SP-A) and adjacen t open pasture (OP) in Hardee site. Bh horizon

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101 020406080100120 05 515 1530 3050 5075 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tc b a a b a a b a b b a b a a a a b Figure 5-11. Changes in percent of C3-d erived C in 250 2000m with soil depth on silvopasture (SP-T and SP-A) and adjacen t open pasture (OP) in Osceola site. Bh horizon

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102 0102030405060708090 05 515 1530 3050 5075 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-T b a b a a a a a b a a b a a a b b a Figure 5-12. Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Alachua site.

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103 020406080100120 05 515 1530 3050 5075 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tb b a a b a a ab a a b a c b a Figure 5-13. Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Suwannee site.

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104 020406080100120 05 515 15 30 50 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tc a b a b ab a b a a a b b b b Figure 5-14. Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Hardee site. Bh horizon

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105 0102030405060708090 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tb b a b a a c b a Figure 5-15. Changes in percent of C3-derived C in 53 250 m with soil depth on silvopasture (SP-T and SP-A) and adjacent open pasture (OP) in Osceola site. Bh horizon

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106 0102030405060708090100 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-T a b a a b a b a a b a a a a b a a b Figure 5-16. Changes in percent of C3-derived C in <53 m with soil depth on silvopasture (SPT and SP-A) and adjacent open pa sture (OP) in Alachua site.

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107 020406080100120 05 515 15 30 50 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tab a b b a a b a a b a a Figure 5-17. Changes in percent of C3-derived C in <53 m with soil depth on silvopasture (SPT and SP-A) and adjacent open pasture (OP) in Suwannee

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108 site. 0102030405060708090 05 515 1530 3050 5075 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Tb a a a a b Figure 5-18. Changes in percenta ge of C3-derived C in the <53 m fraction with soil depth on silvopasture (SP-T and SP-A) and on an adja cent open pasture (OP) in Hardee site. Bh horizon

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109 0102030405060708090 05 515 150 300 505 7525Soil depth, cmC3-derived SOC, % OP SP-A SP-Ta a b b a a Figure 5-19. Changes in percent of C3-derived C in <53 m with soil depth on silvopasture (SPT and SP-A) and adjacent open pa sture (OP) in Osceola site. Bh horizon

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110 75 50 30 15 5 0 75 50 30 15 5 0 300 240 180 120 60 0 300 240 180 120 60 0 SOC, kg m-2 SP-T SP-A OP Land-useSoil d ep t h, cm D B C A Figure 5-20. Changes in overall mean C3-derived and C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggreg ates, and D) silt + clay fraction down the soil profile depths in Ultisols at thr ee pasture locations (SP-A, SP-T; and OP). Plant source C3-plant C4-plant SOC, kg m -2

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111 100.0 62.5 40.0 22.5 10.5 2.5 100.0 62.5 40.0 22.5 10.5 2.5 400 320 240 160 80 0 400 320 240 160 80 0 SP-T SP-A OP Land-use A B C DSoil depth, cm Figure 5-21. Changes in overall mean C3-derived and C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggreg ates, and D) silt + clay fraction down the soil profile depths in Spodosols at th ree pasture locations (S-A, SP-T; and OP). Plant source C3-plant C4-plant SOC, kg m -2

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112 75 50 30 15 5 0 75 50 30 15 5 0 600 450 300 150 0 600 450 300 150 0 SP-T SP-A OP Land-useSoil d ep t h, cmSOC, kg m -2 Figure 5-22. Changes in overall mean C3-derived and C4-derived SOC across all locations in A) whole soil B) macroaggregate C) microaggr egates, and D) silt + clay fraction down the soil profile depths at three pa sture locations (SP-A, SP-T; and OP). Plant source C3-plant C4-plant SOC, kg m -2

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113 CHAPTER 6 SUMMARY AND CONCLUSIONS Although soil organic carbon (SOC) is a major pool in the terrestrial C cy cle and is directly linked to atmospheric CO2, the idea of using soils in agroecosystems to sequester C and mitigate the risk of accelerated greenhouse eff ect is a relatively new concept. At the same time, the role of silvopasture as an altern ative to intensive open-grass pastur e systems in the Southeastern USA for improving environmental quality has received some attention. An underexplored area of such tree-based systems is their potential for enha ncing C storage and stab ilization in the soils. Current knowledge on the dynamics of soil C st orage following tree integration into pasture systems is scant and largely not quantified. This st udy was undertaken in this scenario, with the objectives of determining the amounts of C stored in the soil, quantifying the C fractions stored within soil profiles, tracing the plant sources of C fractions usi ng stable isotope signatures, and elucidating the mechanisms of physical protect ion and stabilization of SOC. The study was undertaken in slash pine ( Pinus elliottii ) + bahiagrass ( Paspalum notatum ) silvopasture and adjacent open (treeless) pasture systems at four sites on Spodosols and Ultisols in Florida. Soil samples were collected from Alachua (29 N 82W), Osceola (280 9 N, 810 10 W), Hardee (27 N, 82 W), and Suwannee (30 N, 830 0 W) counties in Florida. Each site had a silvopasture system and an adja cent treeless pasture of bahiagrass. Soil samples were collected from three different sample sets: between trees in a row (SP-T) and at the center of an alley (SP-A); and another set on an open pasture (OP). Each of these sample sets had stratified grid sampling points representing three rows with four sampling points in a row. At each sampling point, soils were collected from six depths 0 5, 5 15, 15 30, 30 50, 50 75, and 75 125 cm. Soils were physically fractionate d by wet-sieving through a series of two sieve sizes (250 and 53 m).

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114 In general, soils under silvopasture had lowe r pH and bulk density compared to those under open pasture, especially unde r the old-growth (40 years) of silvopastural stands at Suwannee. This could primarily be due to the re gular application of dolom ite in the open pasture during the past 40 years. Changes in abovea nd belowground productivit y, modification of the rooting depth and distribution, and the input of substant ial quantities of pine litter could also have resulted in lower soil pH under silvopastur e. Total SOC in whole soil was higher under silvopasture, especially at deep er soil depths, by an overall average of 33% in SP-T and 28% in SP-A as compared with adjacent open pastures. Ag ain, this could be a likely consequence of the increases in total biomass productivity abovean d belowground in silvop asture relative to the open pasture land. Soil fractionation studies revealed increas ed retention of older C in SOC in microaggregates (53 250 m), enhanced association of C with silt + clay (<53 m) fractions of soil, and greater accumulation of new SOC in macroaggregate (250 2000 m) fraction under silvopasture compared to open pasture. These fi ndings demonstrate the significant levels of C sequestration that occur in th e tree-based pasture system. Studies on stable C isotopic ratio showed th at the increase in total SOC pools following tree integration into pasture was possibly due to retention of olde r C3-derived SOC by protection in microaggregates, retention of C associated wi th silt + clay, and largely accumulation of new C3-derived SOC in macroaggregate fractions. C3 pl ants seemed to have consistently contributed more C to the silt + clay fraction (<53 m) than C4 plants at all soil depths, particularly in the lower depths, in all sites. Sites where the sil vopasture was older, this impact of trees on SOC was greater. The results show that, in the long term, silvopasture may he lp sequester more SOC and stabilize C in the soil.

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115 Collectively, the results suggest that increases in SOC have occurred primarily due to higher organic matter inputs and slower turnover of the organic matter, possibly due to the poorer quality of pine litter re lative to herbaceous grassland litt er. Protection of organic matter within microaggregates and by finer soil fractions also seems to be an important mechanism contributing to the significant increases in soil C in the silvopasture system as compared to the open pasture lands. Critical information on several ke y issues, however, is needed to make valid conclusions. For example, the increase in total SOC pools followi ng tree integration into pa sture is attributed to possible retention of older C3-derived SOC by protection in microaggr egates, retention of C associated with silt + clay, and accumulation of new C3-derived SOC in macroaggregate fractions. These hypotheses need to be investigated in detail. Fu rther investigations are also needed on the nature of contribution from the d eep rooting systems. Ind eed, the rooting pattern of slash pine and development of its root system with age of trees have not been reported which means that the rate at wh ich slash pine trees of different age groups accumulate or help sequester soil C is also unknown. Rigorous data are also needed on the net primary productivity in both silvopasture and open pasture. Fu rther, isolation of microggregates from macroaggregates and silt+clay fractions from the microaggregates is requ ired. Determination of microbial activity in the different fractions is important to und erstand and explain mechanisms SOC stabilization and dynamics in silvopasture systems. The use of 14C and bomb C models in future studies could be relevant to date soil fractions and to distinguish newly incorporated C derived from the tree component in the silvopasture from old resi dual C retained from primary forest, and to reduce uncerta inties in turnover rates.

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116 It is recognized that, being a pioneering study of this nature on silvopastoral systems, the data presented here may not be of highest level of scientific rigor. Fo r example, the dissimilar site history of the plots and uneven stand characteristics coul d have impacted the results. Nevertheless, as a first approximation and best possible effort under the given conditions, it represents an important contributio n, and these results have promis ing implications in the context of Kyoto Protocol. The potential of silvopasture as a strategy for C sequestration is clear. Silvopasture systems can sequester rapidly and st abilize C in the soil systems in the long term. Besides, the techniques used in the study (SOC fractionation coupled with the use of stable C isotopes) provided a powerful means with which to elucidate the pathways of C transformations and stabilizations. Overall, th is study enhances the understa nding of the effects of tree integration into land uses (liv estock production) and land cover changes (grassland-to-tree-based land conversion) on mechanisms of soil C seque stration in grasslands. Since agroforestry systems where trees are combined in non-fore st land and land-use c onversions are occurring rather extensively in many parts of the world, the processes and mechanisms of soil C storage and dynamics documented here could have signifi cance in understanding th e global C cycles and the earths climate. Expectations might be raised on how these re sults could be used a nd extrapolated to answer the larger question of the role of agroforestry systems in general, and silvopasture in particular, in global C sequestra tion and greenhouse gas mitigati on. Given the preliminary and incomplete nature of the data, any such effort wi ll be largely speculative. Furthermore, lack of authentic data on the extent of area under agrofore stry systems mainly because of the lack of approved methods to express the areas under such integrated systems is a major problem in extrapolating agroforestry research results on a global or even regional scale. Although these

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117 larger and global issues are beyond the scope of th is dissertation study, the author hopes that this study will be useful to the re search community who will take on the challenge of addressing these important issues.

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131 BIOGRAPHICAL SKETCH Solomon Ghebremusssie Haile was born in a small town called Adi-Quala located in Southern Eritrea, North-East Africa. Solomon recei ved his B.Sc. in Forestry from the University of Alemaya, Alemaya, Ethiopia on August 28, 1993. He worked for a year as a research officer at Research Department of Mini stry of Agriculture and another year as graduate assistant at University of Asmara in Eritrea. In A ugust 1995, Solomon was awarded a scholarship at Wagningen University and Research Center, Wagni ngen, the Netherlands, where he received his M.Sc. in Tropical Forestry on January 30, 1997. Solomon was appointed as a lecturer at Department of Plant Sciences, University of Asma ra, Eritrea. He taught forestry and agroforestry courses for five years. In spring 2002, Solom on joined the school of Forest Resource and Conservation at University of Florida for hi s PhD study in agroforestry program under Dr. P.K.R. Nair as his major advisor.