Grass Productivity and Carbon Storage in Relation to Rainfall, Soil Nutrients, and Herbivory in an East African Savanna

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Grass Productivity and Carbon Storage in Relation to Rainfall, Soil Nutrients, and Herbivory in an East African Savanna
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english
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Ngatia, Lucy Wanja
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Doctorate ( Ph.D.)
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University of Florida
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Soil and Water Science
Committee Chair:
Reddy, Konda R
Committee Co-Chair:
Turner, Benjamin
Committee Members:
Nkedi-Kizza, Peter
Nair, Ramachandr P
Palmer, Todd

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Subjects / Keywords:
african -- carbon -- ecosystem -- grass -- herbivory -- nitrogen -- nutrients -- phosphorus -- productivity -- rainfall -- savanna -- storage
Soil and Water Science -- Dissertations, Academic -- UF
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Soil and Water Science thesis, Ph.D.
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Abstract:
Savannas are considered to be carbon (C) neutral. However, climate variability, land use change and land management practices are likely to alter the key ecosystem drivers which will influence the future of the savanna to either a C source or sink. Water, nutrient availability and disturbance (fire, grazing) are considered to be key ecosystem drivers determining the structure and function of savanna ecosystems. Four studies were conducted at Mpala Research Centre, Kenya to determine the effects of rainfall, soil nutrients and herbivory on plant productivity, C and nutrient storage in savanna with two soil types: red sandy loams (Alfisols) and black cotton soils (Vertisols). All studies emphasized on interactions of ecosystem drivers in determining the potential long-term C and nutrient storage in the savanna.  The plant productivity did not follow the rainfall gradient in savanna dominated by Alfisols, but indicated (nitrogen) N limitation to plant productivity. Below average rainfall limited the plant productivity even under nutrient enrichment. Plant litter decomposition was accelerated by rainfall and N enrichment in Alfisols. Nitrogen and P release during litter decomposition was greater in the dry season and did not match the C release, with higher %P release than %N.  Herbivores reduced the aboveground grass biomass in the Vertisols by -45%, but increased the forage quality (+20% foliar P) and soil organic carbon (+4%). Although the savanna dominated by Vertisols indicated N and P (phosphorus) co-limitation to plant productivity, there was luxury uptake of nutrients on nutrient enrichment without increase in biomass suggesting that the plants are adapted to growing under nutrient limiting conditions. In Laikipia district the selected six herbivores produce 5800 kg C km-2, 210 kg N km-2, and 35 kg P km-2 annually. This influence nutrient cycling, plant productivity and C storage.  Future N increase in the Alfisols will favor higher plant productivity and litter decomposition. But in the Vertisols nutrient enrichment improve plant quality, a trade off to C storage. Long term grazing increased SOC and plant quality a trade off to biomass C storage. Future changes in herbivores populations will affect nutrient cycling, indirectly affecting plant production and C storage.
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In the series University of Florida Digital Collections.
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Includes vita.
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by Lucy Wanja Ngatia.
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Thesis (Ph.D.)--University of Florida, 2012.
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Adviser: Reddy, Konda R.
Local:
Co-adviser: Turner, Benjamin.
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1 GRASS PRODUCTIVITY AND CARBON STORAGE IN RELATION TO RAINFALL, SOIL NUTRIENTS, AND HERBIVORY IN AN EAST AFRICAN SAVANNA BY LUCY NGATIA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PART IAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012

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2 2012 Lucy Ngatia

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3 To my family

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4 ACKNOWLEDGMENTS To my advisor, Dr. K. Ramesh Reddy, I would like to convey my gratitude for the guidance and great patience throughout my PhD studies. Thanks go out to Dr. Ben jamin Turner for his enthusiasm, time, and valuable insights on my research. I would also like to thank my committee members: Dr. Peter Nkedi Kizza, Dr. P.K. Nair and D r Todd Palmer and my collaborators: Prof. Truman Young (University of California Davis), Dr. Jacob Goheen (Wyoming University), Dr. Robert Pringle (Princeton University), Dr. Bettina Wolfgramm ( University of Bern and NCCR Switzerland), Dr. Boniface Kiteme (CETRAD Kenya), Dr. Jesee Theuri Njoka (University of Nairobi Kenya ) and Dr. Margaret Kinnaird and staff (Mpala Research Centre) all of whom played a significant role in shaping this research. I would like to th ank Mr Gavin Wilson and Ms. Yu Wang ( We tland Biogeochemistry Laboratory) Tania Romero and staff (Smithsonian Tropical Research Institute Soil Laboratory, Panama) for assistance with the laboratory work and to all my friends and colleagues in the Wetland Biogeochemistry Laboratory. Special thanks go to my fellow students, Dr. Lisa Chambers, Jing Hu, Rupesh Bhomia, Eunice Eshun, Akua Appong Anane and Anna Normand who have been great friends throughout my stay in Gainesville. Most importantly my family Jack son Ngatia, Beatrice Ngatia, Patrick Waweru, Joseph Kariuki, Davidson Ngari, James Mwangi, Beatrice Waweru, Martha Kariuki, P recious Waweru, Destiny Waweru, Shilet Kariuki and the newest family member Jack Ngari for the endless support, encouragement and unfailing love. Thanks go out t o Esther Gachango for always being there through thick and thin, her support and time throughout dissertation writing process was priceless.

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5 Finally, I would like to acknowledge that my Ph.D. was supported by several fundi ng sources, including an assistantship from the University of Florida. Research support was provided by Smithsonian Tropical Research Institute through Levinson Fellowship and by National Centre of Competency in Research Switzerland (North South)

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF ABBREVIATIONS ................................ ................................ ........................... 13 ABSTRACT ................................ ................................ ................................ ................... 14 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 16 Ecosystem Drivers in Tropi cal Savanna That Limit Plant Productivity .................... 16 Effects of Ecosystem Drivers on Plant Productivity and Carbon Storage ............... 16 Dissertati on Overview ................................ ................................ ...................... 19 Objectives and Hypotheses ................................ ................................ .............. 19 Dissertation Organization ................................ ................................ ................. 21 Study Site ................................ ................................ ................................ ............... 21 2 EFFECTS OF HERBIVORY AND NUTRIENT ENRICHMENT ON PLANT PRODUCTIVITY AND CARBON STORAGE ALONG A RAINFALL GRADIENT IN THE ALFISOLS OF EAST AFRICAN SAVANNA ................................ ............... 26 Background ................................ ................................ ................................ ............. 26 Materials and Methods ................................ ................................ ............................ 28 Study Site ................................ ................................ ................................ ......... 28 Experimental Design ................................ ................................ ........................ 29 Grazing experiment ................................ ................................ .................... 29 Nutrient enrichment experiment ................................ ................................ 30 Plant and Soils Chemical and Statistical Analysis ................................ ............ 31 Statistical Analysis ................................ ................................ ............................ 31 Determination of Nutrient Use Efficiency (NUE), Apparent Nutrient Recovery (ANR) and Physiological Nutrient Efficiency (PNE) ....................... 32 Results ................................ ................................ ................................ .................... 32 Baseline Soils and Vegetation Parameters (Before Nutrient Enrichment) ........ 33 Soil Nutrient Concentrations (After Nutrient Enrichment) ................................ 34 Site effects ................................ ................................ ................................ 34 Nutrient enrichment effects ................................ ................................ ........ 35 Site x herbivory and site x nutrient enrichment interaction ......................... 35 Grass Nutrient Concentration (After Nutrient Enrichment) ................................ 36 Site effect ................................ ................................ ................................ ... 36 Nutrient enrichment effects ................................ ................................ ........ 36

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7 Effects of site x herbivory interactions ................................ ........................ 37 Effects of herbivory x treatment intera ctions ................................ .............. 37 Carbon, Nitrogen and Phosphorus Storage in Biomass and Efficiency ............ 37 Site effects ................................ ................................ ................................ 37 Nutrient enrichment effects ................................ ................................ ........ 38 Discussion ................................ ................................ ................................ .............. 39 Rainfall Gradient Influence on Plant Productivity an d Soil Macro Nutrients ..... 39 Effects of Nutrient Enrichment on Biomass Production ................................ .... 40 Response of Foliar Nutrients to Nutrient Enric hment ................................ ....... 41 Effects of Nutrient Enrichment on ANR, NUE and PNE ................................ ... 42 Summary ................................ ................................ ................................ ................ 43 3 SEASONAL PATTERNS IN DECOMPOSITION AND NUTRIENT RELEASE FROM EAST AFRICAN SAVANNA GRASSES GROWN UNDER CONTRASTING NUTRIENT CONDITIONS ................................ ........................... 56 Background ................................ ................................ ................................ ............. 56 Materials and Methods ................................ ................................ ............................ 58 Site Description ................................ ................................ ................................ 58 Experimental Design ................................ ................................ ........................ 59 Experimental Procedure ................................ ................................ ................... 59 Litter Processing and Analysis ................................ ................................ ......... 60 Statistical Analysis ................................ ................................ ............................ 61 Results ................................ ................................ ................................ .................... 61 Pattern of Litter Decomposition ................................ ................................ ........ 61 Trends in Nitrogen and Phosphorus Release from the Grass Litter ................. 63 Discussion ................................ ................................ ................................ .............. 65 Summary ................................ ................................ ................................ ................ 69 4 HERBIVORY AND NUTRIENTS INFLUENCE ON PLANT PRODUCTIVITY AND CARBON STORAGE IN THE VERTISOLS OF THE EAST AFRICAN SAVANNA ................................ ................................ ................................ ............... 77 Background ................................ ................................ ................................ ............. 77 Materials and methods ................................ ................................ ............................ 80 Study Site ................................ ................................ ................................ ......... 80 Experimental Design ................................ ................................ ........................ 81 Grazing experiment ................................ ................................ .................... 81 Nutrient enrichment experiment ................................ ................................ 81 Plant and Soils Chemical and Statistical Analysis ................................ ............ 82 Determination of Nutrient Use Efficiency (NUE), Apparent Nutrient Recovery (ANR) and Physiological Nutrient Efficiency (PNE) ....................... 83 Resul ts ................................ ................................ ................................ .................... 83 Effects of Long Term Grazing on Aboveground Biomass and Soil Nutrients .... 83 Nutrient Enrichment Effects on Soils and Abovegr ound Biomass in the Grazed and Ungrazed Plots ................................ ................................ .......... 84

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8 Apparent Nutrient Recovery (ANP), Nutrient Use Efficiency (NUE) and Physiological Nutrient Efficiency (PNE) ................................ ......................... 86 Discussion ................................ ................................ ................................ .............. 87 Grazing Effects on Soil and Plant Nutrient Concentration and Aboveground Grass Production ................................ ................................ .......................... 87 Effects of Nutrient Enrichment on Soil and Plant Nutrient Concentration and Grass Primary Productivity ................................ ................................ ............ 90 Effects of Nutrient Enrichment on ANR, NUE and PNE ................................ ... 91 Summary ................................ ................................ ................................ ................ 92 5 NUTRIENT PRODUCTION BY SELECTED LARGE HERBIVORES IN LAIKIPIA, KENYA ................................ ................................ ................................ 101 Background ................................ ................................ ................................ ........... 101 Materials and Methods ................................ ................................ .......................... 103 Study Site ................................ ................................ ................................ ....... 103 Manure S ampling and Analysis ................................ ................................ ...... 104 Animal Manure Production in Laikipia ................................ ............................ 105 Chemical and Statistical Analysis ................................ ................................ ... 105 Results ................................ ................................ ................................ .................. 106 Sources of Feed Materials ................................ ................................ .............. 106 Elemental Content of Manure ................................ ................................ ......... 107 Manure Nutrient Production ................................ ................................ ............ 108 Discussion ................................ ................................ ................................ ............ 109 Species Feeding Habits ................................ ................................ .................. 109 Manure Nutrients Concentration ................................ ................................ ..... 110 Annual Nutrients Production by the Herbivores ................................ .............. 113 Summary ................................ ................................ ................................ .............. 114 6 CONCLUSIONS ................................ ................................ ................................ ... 122 Background ................................ ................................ ................................ ........... 122 Obj ective 1: Effects of Herbivory and Nutrient Enrichment on Plant Productivity and C Storage and Nutrient Storage in the Alfisols of East African Savanna ................................ ................................ .......................... 124 Objective 2: Litter Decomposition an d N and P Release in Alfisols Savanna Ecosystem ................................ ................................ ................................ ... 125 Objective 3: Plant Productivity and Carbon Storage as Influenced by Long Term Grazing and Short Term Nutrient Enrichment in the Vertisols of East African Savanna ................................ ................................ .......................... 127 Objective 4: Estimation of Herbivores Ecosystem Services: Carbon and Nutrient Production in Laikipia District, Kenya ................................ ............. 1 29 Synthesis and Future Research ................................ ................................ ............ 131 APPENDIX

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9 A DIAGRAM OF THE STUDY SITE AND HERBIVORY EXPERIMENTAL DESIGN IN THE ALFISOLS ................................ ................................ ................. 137 B DETAILED HERBIVORY EXPERIMENTAL DESIGN IN THE ALFISOLS ............ 138 C NUTRIENT ENRICHMENT EXPERIMENTAL DESIGN IN THE ALFISOLS ......... 139 D KENYA LONG TERM HERBIVORE EXCLUSION EXPERIMENT (KLEE) IN THE VERTISOLS ................................ ................................ ................................ 140 E DIAGRAM OF THE HERBIVORY TREATMENTS IN THE VERTISOLS .............. 141 F NUTRIENT ENRICHMENT EXPERIMENTAL DESIGN IN VERTISOL ................ 142 LIST OF REFERENCES ................................ ................................ ............................. 143 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 154

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10 LIST OF TABLES Table Page 2 1 Baseline soil parameters.. ................................ ................................ .................. 44 2 2 B aseline grass parameters.. ................................ ................................ ............... 45 2 3 Effects of herbivory on s oil C, N and P concentration.. ................................ ....... 46 2 4 Effects of nutrient enrichment on soil C, N and P.. ................................ ............. 47 2 5 Soil NO 3 N (herbivory x site interactions).. ................................ ........................ 48 2 6 Soil NO 3 N (nutrient enrichment x site intera ctions).. ................................ ......... 49 2 7 1 st wet season ANR NUE and PNE. ................................ ................................ ... 50 3 1 Aboveground biomass, biomass C, N and P storage.. ................................ ....... 71 3 2 Intercepts, slopes, spline points and coefficients of determination (R 2 ) of the regression lines for litter decomposition (C) and nitrogen mineralization. ........... 71 4 1 Foliar nutrient concentration under herbivory and nutrient enrichment.. ............. 94 4 2 Grass biomass and biomass nutrients under herbivory. ................................ ..... 94 4 3 Soil parameters under herbivory before nutrient enrichment.. ............................ 95 4 4 Soils parameters after nutrient enrichment.. ................................ ....................... 96 4 5 ANR, NUE and PNE after nutrient enrichment.. ................................ ................. 96 5 1 Laikipia herbivores population and manure production.. ................................ .. 116 5 2 Manure isotopic ratio and mass ratios. ................................ ............................. 117 5 3 Annual nutrients production in Laikipia district.. ................................ ................ 118

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11 LIST OF FIGURES Figure Page 1 1 Simplified diagram highlighting the interactions of ecosystem drivers. ............... 24 1 2 The study site ................................ ................................ ................................ ..... 25 2 1 Monthly rainfall over the study period ................................ ................................ 51 2 2 Soil carbon and nitrogen concentration per site. ................................ ................. 52 2 3 Soil NH4+ N under herbivory and nutrient enrichment.. ................................ ..... 53 2 4 Site effects on grass biomass and nutrient storage. ................................ ........... 54 2 5 Nutrient enrichment effects on grass biomass and nutrients storage. .............. 55 3 1 Rainfall in the study site.. ................................ ................................ .................... 72 3 2 Deco mposition pattern over time.. ................................ ................................ ...... 73 3 3 Percent nitrogen release pattern over time ................................ ......................... 74 3 4 Phosphorus release pattern over time. ................................ ............................... 75 3 5. C:N and C:P ratio over time. ................................ ................................ ................. 76 4 1 Soil nutrients befor e and after nutrient enrichment ................................ ............. 97 4 2 Soil C:N and C:P ra tio after nutrient enrichment. ................................ ................ 98 4 3 Grass biomass and nutrients storage after nutrient enrichment. ........................ 99 4 4 Foliar C:N, C:P and N:P ratio after nutrient enrichment. ................................ ... 100 5 1 15 13 C. ................................ ................................ 119 5 2 Manure nutrient concentration (total C, N, P and extractable K). ...................... 120 5 3 Manure ex tractable nutrients concentration. ................................ ..................... 121 6 1 Conceptual diagram with main effects results in the Alfisols. ........................... 135 6 2 Conceptual diag ram with main effects results in the Vertisols. ......................... 136 A 1 Herbivory experimental design in the Alfisols. ................................ .................. 137 B 1 Detailed layout o f herbivores exclusion treatments in the Alfisols. ................... 138 C 1 Nutrient enrichment experimental design in Alfisols. ................................ ...... 139

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12 D 1 Herbiv ory experimental design in the Vertisols. ................................ ................ 140 E 1 Layout of detailed herbivores exclusio n experiment in the Vertisols. ................ 141 F 1 N utrient enrichment experimental design in the Vertisols. ................................ 142

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13 LIST OF ABBREVIATIONS Al Aluminum Anova Analysis of variance ANR Apparent nutrient recovery C Carbon Ca Calcium CO 2 Carbon dioxide F N Amount of nutrient applied in kg ha 1 HSD Honest significant difference k Decomposition constant K Potassium LOI Loss on ignition Manova Multivariate analysis of variance Mg Magnesium N Nitrogen NCCR National Centre of Competency in Research NH 4 + Ammon ium NO 3 Nitrate NUE Nitrogen use efficiency P Phosphorus PNE Physiological nutrient efficiency SEM Standard error of mean SOC Soil organic carbon SOM Soil organic matter STRI Smithsonian Tropical Research Institute U N nutrient uptake by grass wit h the applied nutrient U O Nutrient uptake by grass without the applied nutrient Y N Grass biomass with the nutrient being tested in kg ha 1 Y O Grass biomass without the nutrient being tested in kg ha 1

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14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy GRASS PRODUCTIVITY AND CARBON STORAGE IN RELATION TO RAINFALL, SOIL NUTRIENTS, AND HERBIVORY IN AN EAST AF RICAN SAVANNA By Lucy Ngatia December 2012 Chair: K. Ramesh Reddy Co chair: Benjamin L.Turner Major: Soil and Water Science Savannas are considered to be carbon (C) neutral. However, climate variability, land use change and land management practices are likely to alter the key ecosystem drivers which will influence the future of the savanna to either a C source or sink. Water, nutrient availability and disturbance (fire, grazing) are considered to be key ecosystem drivers determining the structure an d function of savanna ecosystems. Four studies were conducted at Mpala Research Centre, Kenya to determine the effects of rainfall, soil nutrients and herbivory on plant productivity, C and nutrient storage in savanna with two soil types: red sandy loams ( Alfisols) and black cotton soils (Vertisols). All studies emphasized on interactions of ecosystem drivers in determining the potential long term C and nutrient storage in the savanna The plant productivity did not follow t he rainfall gradient in savanna dominated by Alfisols, but indicated (nitrogen) N limitation to plant productivity. B elow average rainfall limited the plant productivity even under nutrient enrichment. P lant litter decomposition was accelerated by rainfall and N enrichment in Alfisols Nitrogen and P release during

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1 5 litter decomposition was greater in the dry season and did not match the C release, with higher %P release than %N. Herbivores reduce d the aboveground grass biomass in the V ertisols by 45%, but increase d the forage quality (+20% foliar P) and soil organic carbon (+4%). Although the savanna dominated by Vertisols indicated N and P (phosphorus) co limitation to plant productivity, there was luxury uptake of nutrients on nutrient enrichment without increase in biomass suggestin g that the plants are adapted to growing und er nutrient limiting conditions. In Laikipia district the selected six herbivores produce 5800 kg C km 2 210 kg N km 2 and 35 kg P km 2 annually. This influence nutrient cycling, plant p roductivity and C stor age. Future N increase in the Alfisols will favor higher plant productivity and litter decomposition But in the Vertisols nutrient enrichment improve plant quality, a trade off to C storage Long term grazing increase d SOC and plant quality a trade off to biomass C storage. Future changes in herbivores populations will affect nutrient cycling, indirectly affecting plant production and C storage.

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16 CHAPTER 1 INTRODUCTION Ecosystem Drivers in Tropical Savanna That Limit Plant Productivity The savannas cov er one fifth of the earth's land surface and 50% of Africa It is home to a large proportion of the world's human population as well as most of livestock and wild herbivores (Scholes and Archer, 1997, Wang et al. 20 10 ). Savannas produce approximately 29% o f global terrestrial net primary productivity (Grace et al. 2006). The co existence of both trees and grasses in the savanna influence both plant and animal production and impact ecosystem functions including: carbon (C) sequestratio n and nutrient dynamics (Sankaran et al. 2005) as well as co existence of grazers, browsers, and mixed feeders (Scholes and Archer 1997; Goheen et al. 2010). Savannas are currently considered a C neutral system although they have the potential to become either large sinks or sou rces of C, depending on changes in climate, land management, disturbance regime and the timescales under consideration (Williams et al. 2007). Water availability, nutrient availability and disturbance (for example fire, grazing) are considered to be some o f the key ecosystem drivers regulating the structure and function of savanna ecosystems (Sankaran et al. 2004 : Okin et al. 2008 ). The interactions of the ecosystem drivers influence the plant productivity and macro element ( C, N and P ) storage in both soil s and plants. Effects of Ecosystem Drivers on Plant Productivity and Carbon Storage The net primary productivity and decomposition of organic matter regulate the global C budget. The difference between these two opposing processes determines C stocks in sa vannas and other terrestrial ecosystems (Couteaux et al 1995). Decomposition of soil organic matter and litter (or detrital matter) is known to be a

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17 source of carbon dioxide (CO 2 ) to the atmosphere, globally contributing approximately 50% of CO 2 (Couteaux et al 1995). At the same time decomposition of organic matter contributes to the availability of nutrient for plant growth (Aerts et al 1992) through the slow cycle (Coughenour 1991). However, the herbivores can short circuit the slow litter decomposit ion cycle facilitating the faster nutrient cycle through fecal matter deposition. Nutrients are known to limit plant productivity, whereby N and P co limit plant productivity in the savanna (Augustine et al. 2003; Ries and Shugart 2008) and also limit t he litter decomposition. However, nutrient limitation is highly variable spatially depending environmental conditions including precipitation variability, soil types, and vegetation communities. For example Ludwig et al. (2004) reported N limitation in gr asses growing in an open canopy and P limitation in sub canopy of Tanzanian ecosystem indicated luxury uptake of P upon soil P enrichment without increasing plant biomass and su ggested that the plants could have allocated C and nutrients to belowground biomass or they are adapted to growing under low P conditions. In addition the nutrients are reported to increase along the rainfall gradient (Okin et al. 2008). However, Austin a nd Vitousek (1998) found decreasing nutrients amount along a rainfall gradient in Hawaii. In African savanna ecosystems, very little is known about the interactions of nutrient availability and other ecosystem drivers (for example rainfall and herbivory) i nfluence on the plant productivity, litter decomposition and soil C storage. Rainfall uncertainty, scarcity, variability, and patchiness have been reported in African savanna and influence plant productivity (O'Connor 1994). Sankaran et, al.

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18 (2005) sugg ested that in the arid and semi arid savannas receiving mean annual precipitation of less than 650 mm, woody cover is limited by precipitation and the biomass production increased linearly with mean annual precipitation. However, n Southern African savanna could not find a relationship between rainfall gradient and aboveground biomass production, suggesting that ecosystem drivers could be interacting to determine plant productivity. However, very little is known about the effect an d interaction of ecosystem drivers on plant productivity and C storage along a rainfall gradient in East African savanna. Herbivory is a key economic activity in the savanna, with co existence of both wild animals and livestock under shared resources. Her bivores are thought to have both positive and negative effects on the ecosystem. It has been suggested that protection of savanna from herbivores (or a decline in herbivore density) could result in a larger C sink in vegetation (Grace et al. 2006; Tanentza p and Coomes 2012). However, such a suggestion have not considered the ecosystem services rendered by herbivores through short circuiting the litter decomposition slow cycle and accelerating nutrients availability for plant growth via urine and feces (Aug ustine et al. 2003; Stark et.al 2002). The se suggestion s have also not considered the amount of C deposited in form of fecal matter of which a fraction can persist over decades and improve soil C sequestration (Augustine et al. 2003). The black cotton soil (Vertisols) and red sandy loams (Alfisols) in the study site have different properties that are likely to influence plant productivity and C storage. The V ertisols is rich in clay (60%) and contain higher amounts of soil C (14.3 g C kg 1 ; 0 30cm depth) an d N (1.7 g N kg 1 ; 0 10cm depth) (Young et. al 1998) than Alfisols ( 10.9

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19 g C kg 1 1.2 g N kg 1 ; 0 15cm depth). However Alfisols contain higher soil P (150 mg kg 1 ; 0 15cm depth) than the Vertisols (60 mg P kg 1 ; 0 30cm depth). In addition the Alfisols i s reported to have 43% water holding capacity (Augustine et al. 2003). The understory in the Vertisols is characterized by homogeneity (Young et. al 1998) while the Alfisols are patchy, composed of patches of grasses trees and bare ground. Dissertation O verview The savanna ecosystem is considered to be C neutral (Williams et al. 2007). H owever the interactions of ecosystem drivers under changing land management practices and climate variability is likely to influence the stability of the savanna. This wo uld render the savanna ecosystem either C sink or source. The overarching objectives of this dissertation are to determine: (1) the effects of the key ecosystem drivers on plant productivity and C and nutrient storage in the Alfisols of East African savan na; (2) the effect s of opposing processes; plant production and litter decomposition on C and nutrient release in Alfisols of East African savanna; (3) the effects of long term grazing, and nutrient enrichment on plant productivity, C and nutrient storage in the Vertisols of East African savanna; and (4) quantify the role of herbivores on C and nutrient production in savannas. Fig. 1.1 indicates the interactions of ecosystem drivers and their influence on plant production and C storage. Objectives and Hy potheses The first objective of this dissertation was to determine the effects of herbivory N and P soil enrichment on plant productivity, C and nutrient storage along a rainfall gradient in the Alfisols of East African savanna (Chapter 2). I hypothesized that plant biomass, foliar nutrients and C and nutrient storage would increase along the rainfall

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20 gradient, and increase in ungrazed and nutrient enriched conditions, while soil C would increase along the rainfall gradient and under grazed conditions. Th e second objective was to determine the influence of nutrients and seasonality in determining the plant litter decomposition pattern, and nutrient release in the Alfisols of East African savanna (Chapter 3). I hypothesized that the litter decomposition ra te and N and P release would be greater in the wet season than in the dry season. The C, N and P release would follow the same pattern and N and P enrichment would accelerate decomposition rates. The third objective was to determine the effects of long t erm grazing and short term N and P enrichment on the grass aboveground biomass, soils C and nutrients storage in the Vertisols of East African savanna (Chapter 4). The hypothesis for this objective was that grazing reduces grass C and nutrient storage but increases the soil C and nutrients storage. Nitrogen and P enrichment favors grass productivity and biomass nutrient storage in the non grazed plots compared to grazed plots, because natural nutrient cycling has been facilitated through fecal matter deposi tion in grazed plots. The fourth objective (Chapter 5) was to determine the role of herbivores in contributing C and nutrients to savanna ecosystem. This was accomplished by (a) using fecal matter stable isotope signatures to determine the grazers, browse rs and intermediate feeders feed preference, (b) quantifying concentrations of macro elements and secondary nutrients in the manure produced by grazers, browsers and mixed feeders, and (c) estimating annual production of C and nutrients by the herbivores f ecal matter in Laikipia, Kenya. I hypothesized that grazers would graze and both browsers

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21 would have the highest nutrients concentration, mixed feeders would be intermediate a that species population would be the major factor influencing the herbivores fecal matter nutrient and C production in Laikipia district. Dissertation Organization Thi s dissertation is organized into 6 chapters, Chapter 1 (this chapter introduces the concept of major ecosystem drivers in tropical savanna and their effects on plant productivity and carbon storage and nutrient cycling as illustrated by Fig. 1.1. Chapter 2 5 focuses on individual research studies. Chapter 2 focuses on plant productivity and C and nutrient storage along a rainfall gradient under different grazing and nutrient enrichment conditions in Alfisols. Seasonal litter decomposition rates and nutri ents release under nutrient enrichment in Alfisols is the focus of Chapter 3. Chapter 4 presents results of study on plant productivity, C and nutrient storage as influenced by long term grazing and short term nutrients enrichment in Vertisols. Quantificat ion of nutrients and C production by large herbivores in Laikipia district, Kenya is the focus of Chapter 5. The sixth and last chapter provides a synthesis of conclusions drawn from all the dissertation studies on the fate of plant production and C storag e and nutrient cycling as influenced by the key ecosystem drivers. Study Site This dissertation work was conducted at the Mpala Research Centre and associated Mpala Ranch, in Laikipia district, Kenya. This covers approximately 190 km 2 Kenya is located in Sub Saharan Africa specifically in East Africa along the Indian Ocean on the south east (Fig. 1 2). The study site is classified as tropical savanna, with the co existence of both grasses and trees and also co existence of grazers, browsers

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22 and mixed f and Ranch are managed for domestic cattle production and wildlife conservation. The study site has two types of soil which include Vertisols (black cotton soils) and Alfisols (red san dy loams) (Ahn and Geiger 1987). The two soil types have different properties and are dominated by different plant species. Approximately 10% of Mpala is underlain by the V ertisols (Young et al. 1998) with variable rainfall that average 500 600 mm (Veblen 2012). The V ertisols are poorly drained and clay rich (Riginos and Grace 2008) with a p H of 6.2 (measured in water) and bulk density of 1g cm 3 The whistling thorn Vachellia ( Acacia ) drepanolobium comprises 97% of the woody cover, and 90% of the herbaceous cover consist of five species of perennial bunchgrasses (Young et al. 1998) which include Bracharia lachnantha, Pennisetum stramineum, P. mezianum, Themeda triandra and Lintonia nutans. The Alfisols are well drained, moderate to very deep friable sandy lo am soils which are developed from metamorphic basement r ocks (Ahn and Geiger 1987). The woody vegetation is dominated by mainly Senegalia (Acacia) brevispica Vachellia (Acacia) etbaica but also Senegalia ( Acacia) mellifera, Vachellia (Acacia) nilotica an d Vachellia (Acacia) gerrardii Croton dichogamus, Grewia spp and Rhus vulgaris (Young et al 1995). The herbaceous vegetation consists of a discontinuous layer of mostly perennial grasses, which include Pennisetum mezianum, P. stramineum Digitaria milan jiana and Cynodon dactylon At Mpala Research Centre the A lfisols are found along a rainfall gradient with high rainfall in the south, intermediate in the central and lower rainfall in the north. The three years annual mean (2009 2011) indicate 638 mm in south, 583 mm in central and 438 mm in north. The soil pH also vary

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23 between the three sites with 6.4 in the north, 5.8 in the central and 6.3 in the south (measured in water) but the soil bulk density is the same for the three sites (1.4g/ cm 3 ).

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24 Fi gure 1 1. Simplified diagram highlighting the interactions of ecosystem drivers.

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25 Figure 1 2. The study site Maps source: Africa map; Adapted and modified from Boone et al. 2005. Quantifying d eclines in l ivestock d ue to l and s ubdivision (Page 525, Fi gure 1). Rangeland Ecology and Management Journal publication. Kenya map; Adapted from Graham 2006.PhD dissertation, (page 8, Figure 1 llege, University of Cambridge. Laikipia district and Mpala Research Centre map; Mpala Research Centre, unpu blished maps.

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26 CHAPTER 2 EFFECTS OF HERBIVORY AND NUTRIENT ENRICHMENT ON PLANT PRODUCTIVITY AND CARBON STORAGE ALONG A RAINFALL GRADIENT IN THE ALFISOLS OF EAST AFRICAN SAVANNA Background In the tropical savanna, herbivory, soil nutrients and rainfall are key ecosystem drivers that influence plant productivity and carbon storage. The role of soil nutrient (Ries and Shugart 2008 ), rainfall (Sankaran et al. 2005), and herbivory (Pandey and Singh 1992; Wilsey et al. 2002) on plant prod uctivit y have been studied separately. H owever very little is known about the interactions of the three and their mutual influence on plant productivity and carbon storage. Sankaran et al. (2005) indicate that in the arid and semi arid savanna receiving me an annual precipitation of less than 650mm, woody cover is limited by precipitation and the biomass increases linearly with mean annual precipitation. In such ecosystem, water limits woody cover and allows grasses to coexist while fire, herbivory and soil properties are thought to interact to influence the plant cover. Previous studies reported variation in soil and foliar nutrients along a rainfall gradient, where both foliar and soil nutrient decreased with increasing annual pre cipitation (Austin and Vit ousek 1998). Previous studies have indicated that herbivores removal or their population decrease in the savanna would promote plant carbon storage (Grace et al. 2006). Other studies indicate that herbivory increase plant productivity (Pandey and Singh 199 2) while other studies indicate reduced plant productivity (Wilsey et al. 2002) in tropical savanna. It is argued that the capacity by herbivores to increase primary production is due to increased nutrient turnover rates (Holland et al. 1992). While the q uality of the

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27 grass and the intensity of grazing determines the quantity of the standing aboveground biomass (Pandey and Singh 1992). Nitrogen (N) and phosphorus (P) are known to co limit plant productivity in the tropical savanna (Thornley et al.1991 ; Aug ustine et al. 2003; Ries and Shugart 2008 ). However, the limitation of N and P is a function of their availability in the soils and their spatially and temporally in terrest rial ecosystems. The N: P ratio has widely been used as an indicator of nutrient limitations where by the general rule is that N : P ratio > 16 indicates P limitation to plant growth, while an N: P ratio < 14 indicates N limitation. At N: P ratios between 14 a nd 16, either N or P can be limiting or plant growth is co limited by N and P together (Koerselman and Meuleman 1996). Nitrogen is often more limiting in early successional ecosystems before N has accumulated in the soil and P derived from parent materia l i s still available (Crews et al. 1995). In a natural ecosystem the major source of P is the underlying bedrock and hence soil P is usually limiting in highly weathered soils. In addition, P can be added to soils through senescent vegetation, fire and dus t deposition. While N can be fixed by N fixing bacteria through plant roots into plant available form, P cannot and hence P 2010). Ludwig et al. (2001, 2004) reported th at in Tanzanian dry savanna P limited plant productivity in subcanopy grasses, while N limitation was observed in open canopy savanna along a rainfall gradient repor ted luxury uptake of P by the plant without an increase of aboveground biomass. These results suggests that N and P limitation is site

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28 specific and varies spatially and temporally depending on hydrologic conditions, herbivory soil and vegetation types Mos t studies on savanna structure and function are conducted at a single site or without experimental manipulation (Sankaran et al. 2004; Okin et al. 2008; Ries and Shugart 2008). In addition in most cases only a single or two ecosystem drivers are considered plant productivity. For example rainfall (Seagle and McNaughton 1993), herbivory (Pandey and Si ngh, 1992; Wilsey et al. 2002) and nutrients (Ludwig et al. 2001; Ries and Shug art 2008) V enrichment and herbivory on plant productivity along a rainfall gradient in East African savanna. The objective s of this study w ere to determine: (1) the effects of herbivory and nu trient enrichment on soil carbon along a rainfall gradient, (2) the effects of herbivory and nutrients enrichment on grass biomass and biomass nutrients storage along a rainfall gradient. I hypothesized that (1) plant biomass and biomass nutrient increase along the rainfall gradient and under nutrient enrichment, and decrease under herbivory (2) soil C and nutrient concentration increas e along the rainfall gradient and under herbivory. Materials and Methods Study Site The study was conducted in semi arid t ropical savanna at the Mpala Research Centre and C onservancy in Laikipia district, Kenya. Mpala Research Centre and associated Mpala ranch covers 190 k m 2 ( Augustine and McNaughton 2004) It is managed for both livestock production and wildlife conservation Some of the resident

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29 wild large herbivores include elephants ( Loxodonta africana ), hartebeests ( Alcelaphus buselaphus ), giraffes ( Giraffa camelopardalis ), buffaloes ( Syncerus caffer ), g rant gazelles ( Gazella grantii ), zebras ( Equus burchelli ), impala ( Ae pyceros melampus ) and elands ( Taurotragus oryx ). The large livestock herbivores are mainly cattle ( Bos taurus ) and camel ( Camelus dromedaries ) (Young et al. 1998). The study site has red sandy loams (Alfisols) (Ahn and Geiger 1987), the soil bulk density was 1.4g cm 3 for the upper 6cm across the three sites The soil pH was 6.4, 5.9 and 6.3 (measured in water) in north, central and south respectively before nutrient enrichment. The dominant woody vegetation includes Senegalia (Acacia) brevispica Vachellia (Acacia) etbaica Senegalia ( Acacia) mellifera, Vachellia (Acacia) nilotica and Vachellia (Acacia) gerrardii Croton dichogamus, Grewia spp and Rhus vulgaris (Young et al 1995) The herbaceous vegetation consists of a discontinuous layer of mostly peren nial grasses, which include Pennisetum mezianum, P. stramineum Digitaria milanjiana and Cynodon dactylon There is a trimodal rainfall pattern with long rain during April to June and smaller pulses i n August and October (Augustine 2010), with a dry seaso n between January and March (Pringle 2008). Rainfall during the study period is shown in Fig. 2 1. During the study period in the 1 st wet season north received 597 mm, central 599 mm and south 639 mm of rainfall. In the 2 nd season north received 49 mm, cen tral 63 mm and south 82 mm of rainfall. Experimental Design Grazing experiment This study was conducted in conjunction with the Ungulate Herbivory Under Rainfall Uncertainty (UHURU) experiment, established in 2008 at the Mpala Research Centre (Goheen et al unpublished manuscript). The study was conducted at 3 sites

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30 along the rainfall gradient. The sites include north (438 mm rain year 1 ), central (583 mm rain year 1 ) and south (638 mm rain year 1 ) averages from 2009 2011. At each site the herbivore exclu sion experiment was arranged in a three replicates complete block design (Fig. A 1 and B 1) At each site four fertilizer addition plots (16 m 2 each; exclosures (1 ha each) whi ch fenced out all herbivores larger than hares (~2 3 kg) (hereafter ungrazed) and where all herbivores are allowed to graze (grazed/open) (Fig. 1 C) The large herbivores were wild animals only, with no cattle grazing in the control (unfenced) plots. At t he start of the experiment, the grass aboveground biomass in the center 1 m 2 in each 16 m 2 plot was clipped to ground level (Fig. C 1) and the grass dried for chemical analysis. The clipping allowed the grass regrowth to be used for the nutrient enrichment / fertilizer treatment. At the same time, a composite soil sample (0 10 cm depth) was collected within each 16 m 2 plot (four separate cores in each composite) to determine soil carbon and nutrient concentrations. Nutrient enrichment experiment After cli pping and soil sampling, four fertilizer treatments were established: nitrogen alone, phosphorus alone, nitrogen plus phosphorus, and a control (hereafter referred as N, P, NP and control). The four fertilizer treatments were applied in the four 16m 2 that had been established in the grazed and non grazed plots (i.e. each replicate herbivory plot contained one 16 m 2 plot of each treatment). Fertilizer s were applied as urea at 100 kg N ha 1 and/or triple super phosphate at 50 kg P ha 1 for each application ti me. Fertilizer application was done in March 2010, November 2010 and March 2011. Soil was sampled in the 16 m 2 plot to 10cm depth in February 2010 (baseline/pre treatment), May 2010 (to make sure P addition was successful and P was not fixed),

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31 November 201 0 (1 st wet season), May 2011 (2 nd wet season) A boveground biomass in the 1 m 2 subplots was sample d in b aseline (for nutrient concentration determination) and regrowth biomass in the 1 m 2 subplots was harvested in Nov 2010 (1 st wet season). However, in May 2011 there was no grass biomass (regrowth) for harvesting, because the dry season unexpectedly extended to the wet season. Plant and Soils Chemical and Statistical Analysis The grass biomass was dried at 60 o C to a constant weight and then ground. Soil sa mples were air dried for 12 days under room temperature (25 o C). Plant and soil total C and N were determined using Thermo Electron Flash 1112 elemental analyzer. Total P was determined using ignition at 550 o C followed by acid extraction using 1M H 2 SO 4 Di gested solutions were analyzed calorimetrically using a Shimadzu UV visible recording spectrophotometer UV 160.Nutrient ratios were calculated on a mass basis. NH 4 + N and NO 3 N were extracted using 2M KCl. The samples were filtered through a 0.45 m membrane filter (Pall Corporation). The filtrate was analyzed colorimetrically as outlined by (White and Reddy 2000). Resin P was determined as outlined by ( Kouno et al. 1995). Statistical Analysis All statistical analysis was conducted using JMP (versi on 7.02; SAS Institute 2007). Significant differences among the treatments for the variables were determined by Manova for site, herbivory, nutrient enrichment treatment and interactions. Tukey HSD test and t test at 0.05.

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32 Determination of Nutrient Us e Efficiency (NUE), Apparent Nutrient Recovery (ANR) and Physiological Nutrient Efficiency (PNE) Difference method was used to determine apparent nutrient recovery and nutrient use efficiency. Apparent nutrient recovery reflects plant ability to acquire a n applied nutrient from soil (Baligar et.al 2001) and is determined as (U N U O )/F N where U N and U O are the nutrient uptake by grass with and without the applied nutrient, and F N is the amount of nutrient applied (all units are kg ha 1 ). The results are exp ressed as percentage of applied nutrient recovered in plant tissue. Nutrient use efficiency (NUE) is defined as the amount of forage (dry matter) that is produced per each unit of applied N or P (Fageria and Baligar 1999; Zemenchik and Albrecht 2002). Nutr ient use efficiency (NUE) is determined as (Y N Y O )/F N where Y N and Y O are the grass biomass with and without the nutrient being tested all in kg ha 1 and F N is as indicated above (Guillard et al. 1995; Syers et al. 2008). The physiological efficiency o f the applied nutrient is calculated as the kg of grass biomass increase per kg of increase in nutrient taken up, (Y N Y O )/ (U N U O ). In all the above formulas, P can replace N (nutrient) (Syers et al. 2008). Results The results are presented for baseline (p re enriched) and post nutrient enrichment (1 st and 2 nd wet season s ). There are 3 major treatments which include: (1) site and rainfall along a rainfall gradient (south, 638 mm; central 583 mm; and north, 438 mm rainfall ) (2) herbivory; which include graze d and ungrazed and (3) nutrient enrichment; which include N, P, NP application and the control. In addition any interactions between the major treatments are presented. For baseline (pre enriched) results are presented for herbivory and site effects and th ere were no interaction s

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33 between the two. For post enrichment results are presented for site, herbivory, nutrient enrichment and interactions. Baseline Soils and Vegetation Parameters (Before Nutrient Enrichment) In the baseline (pre enriched) soil C ( F= 5.5; P =0.0066) and N (F= 5.5, P =0.0063) were significantly higher in the north and south site s and significantly lower in the central site (Table 2 1 ). Soil total P was significantly h igher in the south site (F= 3.3; P =0.00428) and low in the central wh ile north site did not vary from both central and south (Table 2 1 ). Soil pH was significantly higher in the north and south (F=16.7; P <0.0001) and low in the central (Table 2 1 ). Soil NO 3 N, NH 4 + N and available P did not vary significantly between the 3 sites (Table 2 1 ). Under herbivory only soil NH 4 + N was significantly hig her in the grazed plots (F=21.7; P =0.0432, Table 2 1) but other parameters were not significantly different. There were no significant interactions between herbivory x site in the ba seline data. Site had a significant effect on foliar N, P, biomass P and N: P ratio but had no significant effect on dry matter, biomass C and N in the baseline data (Table 2 2 ). Foliar N was significantly higher in north than central, but south did not dif fer significantly from the other two sites (F=4.9; P =0.0109, Table 2 2 ). Foliar P was significantly higher in north and south but lower in the central site (F=27; P <0.0001, Table 2 2 ). Biomass P was significantly higher in the south, low in the central but north site did not differ significantly from the other two sites (F=3.3; P =0.0438, Table 2 2 ). The N : P ratio was significantly higher in the central site (13.30.7) and significantly lower in the south site (10.2 0.5) but north site (12.20.6) did not dif fer significantly from south or central site (F= 4.4; P = 0.0172)

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34 Herbivory had a significant effect on N : P ratio but did not have a significant effect on any other foliar or biomass parameter in the baseline data (Table 2 2 ). The N: P ratio was significan tly lower in the grazed plots (11.30.5) than in the ungrazed plots (12.50.6) (F=22.8 ; P =0.0412), but it was <14 in the two treatments. Soil Nutrient Concentrations (After Nutrient Enrichment) Site and nutrient enrichment had a significant effect on soil C, N, P, NH 4 + N and NO 3 N. Herbivory had a significant effect on soil NH 4 + N. While, site x herbivory and site x treatment interactions had significant effect on NO 3 N.The details are given below. Site effects The site had a significant effect on soil C and N in the 1 st wet season (Nov 2010) and 2 nd wet season (May 2011) (Fig. 2 2). Both north and south sites had significantly higher soil C than the central site, for 1 st wet season (F=7.4 ; P =0.0015) and 2 nd wet season (F=40.6 ; P <0.0001). Soil N showed a similar trend, with the central site having significantly lower N than north and south sites for 1 st wet season (F=5.7 ; P = 0.0057) but not significantly different for 2 nd wet season (F=2.3 ; P =0.1053). The soil C and N were lowest at the central site which was at the middle of the rainfall gradient, with south and north sites having the highest and lowest rainfall respectively. Site had a significant effect on soil NH 4 + N in the 1 st wet season with north site (driest) having the highest soil NH 4 + N (6.40.9 mg kg 1 ) and south (wettest) having the lowest (4.40.2 mg kg 1 ) while central (5.82 mg kg 1 ) did not significantly differ from north or South (F=5 ; P= 0.0097). In the 2 nd wet season NH 4 + N was not significantly different between the three site (F=2.2 ; P = 0.128) Site had a significant effect on the NO 3 N in the 1 st and 2 nd wet season. However, site x herbivory interaction was

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35 significant in the 1 st wet season and site x nutrient enrichment interaction was significant in the 2 nd wet season which is shown b elow. Nutrient enrichment effects The nutrient enrichment had a significant effect on soil C and N in 1 st wet season (Nov 2010) and 2 nd wet season (May 2011) (Table 2 4 ). The NP treatment had the highest soil C and N, N and P treatment s were intermediate a nd control had the lowest amount of soil C and N (Table 2 4 ). Nutrient enrichment had a significant effect on soil P for the 1 st wet season but not the 2 nd wet season (Table 2 4 ). Results indicate that for the 1 st wet season NP treatment had significantly higher soil P and control plot had the lowest The P treatment did not differ significantly from treatment s NP and N, while For soil available P soil samples collected two months af ter nutrient enrichment experiment indicated significantly increased soil available P under P (24.54.4 mg kg 1 ) and NP (16.12.6 mg kg 1 ) treatment s but significantly lower available P in control (9.71.5 mg kg 1 ) and N (9.00.8 mg kg 1 ) treatment s Nutr ient enrichment significantly increased NH 4 + N in the N and NP treatment in the 2 nd wet season compared to control and P treatment (F=99 ; P <0.0001) but not in the 1 st wet season (Fig.2 3 B ) Site x herbivory and site x nutrient enrichment interaction In the 1 st wet season site x herbivory interaction effect was significant (F=3.4 ; P =0.0401) for NO 3 N. The ungrazed plots in north site had significantly high NO 3 N than in central and south site (Table 2 5 ). However, the grazed plots did not vary signific antly among the three sites. In the 2 nd wet season the site x nutrient enrichment interaction was significant (F=2.6 ; P =0.0254, Table 2 6 ). In the n orth site NP and N

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36 treatments had significantly high NO 3 N and control plot had significantly lower NO 3 N (F=7 ; P =0.0056), while P treatment did not vary significantly from control, N and NP treatment s However for both central (F=10 ; P =0.0011) and south (F=39 ; P <0.0001) N and NP treatment s had significantly high NO 3 N while control and P treatment had signif icantly lower NO 3 N (Table 2 6 ). Grass Nutrient Concentration (After Nutrient Enrichment) Site had a significant effect on foliar N and P. Nutrient enrichment had a significant effect on foliar P but not N. Site x herbivory interaction had a significant e ffect on foliar N, while herbivory x nutrient enrichment interaction had a significant effect on foliar P. Site effect Site had a significant effect on grass P concentration in the 1 st wet season (F=8 ; P = <0.0009). G rass P concentrations were significantly high in the south site (145590 mg P kg 1 ) and significantly lower in both north (119568 mg P kg 1 ) and central sites (1145107 mg P kg 1 ). Nutrient enrichment effect s Nutrient enrichment effect was significant on foliar P (F=5.8 ; P =0.0017), P (1400 mg P kg 1 ) and NP (1390 mg P kg 1 ) treatment s had significantly higher foliar P while N (1075 mg P kg 1 ) treatment had significantly lower foliar P and control (1200 mg P kg 1 ) did not differ from other treatments. However, there was a herbivory x nutrient enri chment interaction as shown below. The foliar N: P ratio did not vary significantly between the nutrient enrichment treatment s; control (11.41), N (13.21.2), P (11.01.5) and NP (10.31.2), after nutrient enrichment all the treatment s N : P ratio s were <14.

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37 Effects of site x herbivory interactions Site herbivory interactions had significant effect on grass N concentration (F=7.5 ; P = 0.002). In the grazed plots after nutrient enrichment (1 st wet season) both north (17.21.2 g kg 1 ) and central (15.71.4 g kg 1 ) had significantly higher grass N concentration than south site (11.71.0 g kg 1 ). In the ungrazed plots grass N concentration was significantly higher in the north (15.51.2 g kg 1 ), lower in the central (12.10.9 g kg 1 ) and south (14.60.9 g kg 1 ) did not differ significantly from both north and south. Effects of herbivory x treatment interactions Herbivory x treatment interaction had a significant effect on foliar P. For the grazed plots grass P concentration was not significantly different betwee n the different nutrient enrichment treatments. However for ungrazed plots P treatment (1783208 mg P kg 1 ) had significantly higher foliar P concentration and N treatment (1037150 mg P kg 1 ) had the lowest while control (1339157 mg P kg 1 ) and NP (1414 110 mg P kg 1 ) treatment did not differ significantly from both N and P treatment s (F=7.2 ; P =0.0207). Carbon, Nitrogen and Phosphorus Storage in Biomass and Efficiency In the 1 st wet season (after nutrient enrichments) both site and nutrient enrichment (fertilizer) had significant effects on grass biomass, biomass C, N and P, but herbivory and interactions had no significant effect s Site effects Site effect s were significant with higher grass biomass (F=10 ; P =0.0002, Fig. 2 4A), biomass C (F=9 ; P =0.00 05, Fig. 2 4B), biomass N (F=6.2 ; P =0.004, Fig. 2 4C), biomass P (F=11 ; P =0.0001, Fig. 2 4D) in south and central site and significantly lower in the north site. The grass biomass, biomass C, and P in the central and south sites

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38 were more than double the a mount in the north. The biomass N in the south and central was almost double the amount in the north. However in the 2 nd wet season there was no grass regrowth in all sites during the sampling period, this was due to the unexpected extension of the dry sea son into the wet season. Nutrient enrichment effects Nutrient enrichment significantly increased grass biomass (F=3 ; P =0.0435, Fig. 2 5A), and biomass C (F=2.9 ; P =0.0453, Fig. 2 5B) in N and NP treatment but not in the control and P treatment. After fert ilization in the 1 st wet season the grass biomass increased by +78, +25, +85% in the N, P and NP treatment respectively, relative to the control and biomass C had a similar trend. The biomass N (F=3.2 ; P =0.031, Fig. 2 5C) and biomass P (F=2.8 ; P =0.05, Fig. 2 5D) were significantly higher in NP treatment and significantly lower in the control; however N and P treatments did not differ significantly from both control and NP treatment. The biomass N increased by +84, +41 and +93% in the N, P and NP treatment s respectively, and biomass P increased by +71, +57 and +130% in the N, P and NP treatment s respectively relative to the control. The apparent nitrogen recovery, nitrogen use efficiency and physiological nitrogen efficiency were the same under both N and NP treatment (Table 2 7 ). However apparent phosphorus recovery was more than twice under NP treatment compared to P treatment, phosphorus use efficiency was more than thrice in the NP treatment compared to the P treatment, while physiological phosphorus effic iency was greater in NP treatment than in P treatment (Table 2 7 ).

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39 Discussion Rainfall Gradient Influence on Plant Productivity and Soil Macro Nutrients The grass biomass results contrasted our hypothesis of increased biomass along the rainfall gradien t (north to south) Our resullts indicate that grass biomass, biomass C,N and P storage were lowest in the driest site (north) but the other two sites (south and central) were not significantly different though they received different amounts of rainfall, with south having more rainfall than central site. These finding s contrasted Sankaran et al. (2005) who indicated that in the African arid and semi arid sava nna with annual precipitation < 650mm the biomass cover increased linearly with increasing precipita grass biomass in the driest site in first year while other sites along the rainfall gradient rted a significantly higher grass biomass in the site at the middle of the rainfall gradient in the second year and suggested that these inconsistencies called for further studies to determine possible interaction of herbivory, nutrient enrichment and fire along the rainfall gradient. However the soil nutrient concentrations were not consistent with grass biomass production. The results indicate soil nutrient concentration differed along the rainfall gradient C, N, P, were lowest at the middle of the rain fall gradient (central site) throughout the study while NH + 4 N and NO 3 N decreased along the rainfall gradient in the first wet season (after nutrient enrichment). Our finding s did not support our hypothesis that soil C and nutrient increases along a rain fall gradient. Also did not support Okin et al. (2008) findings of increased soil nutrient along rainfall gradient in southern Africa savanna. The grass biomass production was similar at south and

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40 central sites and lower in the north site. Although the rai nfall in central site (599 mm rainfall) and north site (597 mm) were the same during the growing period and was higher in the south site (639 mm). In addition after nutrient enrichment there were higher NH 4 + N and NO 3 N in the North site than other sites. Considering that the plant productivity is N limited in th is ecosystem as indicated by N: P<14 ratio (Koerselman and Meuleman 1996) and positive plant response to N enrichment. This study indicates that there could be other factors in the north site that l imit plant productivity. Such factors could include water holding capacity. The plant biomass increased in N and NP treatment in the 1 st wet season when rainfall was above average and there was no biomass production in the 2 nd wet season when rainfall was below average. This indicate s that when rainfall is above average plant productivity is limited by N and when rainfall is below average plant productivity is limited by rainfall even when nutrients are available. Similarly Walker and Knoop (1987) reported that below average rainfall, could negatively affect plant productivity under nutrient enrichment. Effects of Nutrient Enrichment on Biomass Production The baseline grass N: P ratio was <14 and this ratio persisted even after nutrient enrichment. The gras s biomass significantly increased under N and NP treatment s but not P treatment, supporting that the plant productivity in this ecosystem is N limited (Koerselman and Meuleman 1996 ). These findings agreed with Ludwig et al. (2004) study in Tanzanian savann a who reported N limitation to plant productivity in open canopy. Other studies by Walker and Knoop (1987) in a Burkea Africana savanna and Augustine et al. (2003) in Mpala Kenya agreed with our findings that addition of N alone increased the plant product ivity but addition of P alone had no effect. However our

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41 findings at the same time contrasted Augustine e t al. (2003) who reported an N: P ratio between 3.5 11.5 (mass basis) and indicated that in the N treatment the biomass increased approximately by 30% a nd in NP treatment the biomass nearly doubled and hence suggested that N and P co limited the plant productivity. In our study both the N and NP treatments nearly doubled the grass biomass and were not significantly different indicating that N limit plant productivity. Although our study and Augustine et al. (2003) study were conducted in the same ecosystem we had different findings. This could have been because Augustine et al. (2003) set the experiment on a sward dominated by Cynodon plectostachyus Portu laca oleracea Tribbalus terrestris and Sporobolus pellucid. The site had homogenous species composition and was close to settlement which is likely to have altered soil nutrient concentration. Unlike in our study where plots were randomly selected and gr ass species composition was dominated by Pennisetum stramineum cynodon plectostachyus and Cynodon dactylon Hence there is a likelyhood that different grass species composition in the two sites could be having different physiological responses to resource s availability and nutrient use eff iciency (Zemenchik and Albrecht 2002). In our study, NP, N and P treatment increased grass biomass by 85, 78, and 25% respectively compared to the control, which was low compared to 220, 120, 55% respectively, increase of the same in (Ries and Shugart 2008) study in Botswana savanna. However the grass species between the two study sites were different. Response of Foliar Nutrients to Nutrient Enrichment The grass N concentration was higher in the north (driest site) for bo th grazed and ungrazed plots. This contrasted the biomass response where biomass was lowest in the north ( driest site ) Our study agreed with Ries and Shugart (2008), observation of

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42 increased foliar P for grasses with increased soil P, no increase in folia r N concentration for grasses with increased soil N. Suggesting that the luxury P uptake was a trade off to plant productivity under P additions or the plants were adapted to growing under low soil P (Berendse and Aerts 1987). Our study indicated herbivor y by treatment interactions effects on foliar P, where grazed grasses foliar P was not significantly different between nutrient addition treatments and ungrazed grasses foliar P positively responded to P additions by increasing foliar P. Our findings contr that in the grazed plots both P and NP treatments increased the foliar P in Southern Africa savanna. Despite the increased foliar P in the ungrazed grasses under P additions this was not reflected in the g rass biomass production and both grazed and ungrazed plots were not significantly different in biomass production. This could be due to luxury uptake of nutrients in ungrazed plots or allocation of resources to belowground biomass (O' Halloran et al. 2010) For an efficient plant, increased nutrient uptake should translate to increased biomass production, otherwise indicates plant inefficiency and a possibility that the plant has slow growth r ate (Baligar and Bennett 1986). Effects of Nutrient Enrichment on ANR, NUE and PNE Our study indicates similarity in apparent N recovery, N use efficiency and physiological N efficiency between N and NP treatment s While P recovery, P use efficiency and physiological P efficiency was much lower in P treatment compared t o NP treatment. This further supported that in this ecosystem N limited the plant productivity. Overall absorption efficiency of applied fertilizer has been reported to be about or lower than 50% for N and less than 10% for P (Baligar et.al 2001). In contr ast to our finding of similar N use efficiency between N and NP treatment Snyman (2002) reported higher N

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43 use efficiency in the N treatment than NP treatment in South Africa savanna. Since NUE depend on the (1) uptake efficiency (acquiring nutrient from so il), (2) incorporation efficiency (transport to shoots and leaves) and (3) utilization efficiency (Baligar et.al 2001). The increased foliar P and unmatched primary production after fertilization indicate that the species in our study were inefficient in u tilizing the added P or were adapted to P limiting soil conditions. While the increased biomass under N and NP nutrient addition suggests better utilization of N. Summary The soil C and nutrient storage w ere lowest at the centre of the rainfall gradient su ggesting that rainfall is not the dominant driver of C storage and other factors for example litter decomposition could be more prominent. Nitrogen limits plant productivity when rainfall is above average. While rainfall limit productivity when rainfall is below average even under N availability. The species in this study were adapted to growing under P limiting conditions as indicated by luxury P uptake. In the north site plant productivity seems to be limited by other factors apart from rainfall and N. Th e future increase in N deposition from the atmosphere ( Lamarque et al. 2005) or other sources for example fertilizer pollution or plant root fixation manure, coupled with projected CO 2 increase will have a great impact on plant production in this ecosystem if other plant productivity drivers will be above the threshold level, with a great impact on the carbon storage. However, short term grazing do es not have an effect on both soil C and plant productivity.

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44 Table 2 1. Ba seline soil parameters. Data presented for baseline (Feb 2010) for site and herbivory treatment. The data indicate mean SEM, the site mean is from 24 replicates. Sites vary in mean annual precipitation (north=438 mm, central= 583, south= 638, 3 year mean ). Herbivory include grazed and ungrazed, the means is from 36 replicates. Different letters indicate Tukey HSD significant difference between means for both site and herbivory at P<0.05. C N P Available P NH 4 + N NO 3 N pH g kg 1 mg kg 1 Site North 9.51.0 a 1.10.1 a 20812 ab 6.41.4 a 6.30.7 a 3.13.2 a 6.40.1 a Central 6.40.7 b 0.80.1 b 18712 b 8.40.8 a 5.40.4 a 1.13.4 a 5.90.1 b South 9.20.7 a 1.10.1 a 22913 a 10.31.7 a 6.10.4 a 2.23.2 a 6.30.1 a F ratio 5.5 5.5 3.3 2 .5 1.3 0.02 16.7 P Value 0.0066 0.0063 0.0428 0.0894 0.2748 0.9797 <.0001 Herbivory Grazed 8.30.8 a 1.00.1 a 19710 a 7.60.7 a 6.50.4 a 3.91.8 a 6.20.1 a Ungrazed 8.20.6 a 1.00.1 a 21811 a 8.91.4 a 5.40.4 b 2.81.9 a 6. 10.1 a F ratio 0.06 6.3 2.7 0.8 21.7 1.2 0.5919 P Value 0.8251 0.1287 0.2442 0.4694 0.0432 0.3924 0.5221

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45 Table 2 2. Baseline grass parameters. Data presented for baseline (Feb 2010) for site and herbivory treatment. The data indicate mean SEM the site mean is from 24 replicates. Sites vary in mean annual precipitation (north=438 mm, central=583, south=638, 3 years mean). Herbivory include grazed and ungrazed, the means are from 36 replicates. Different letters indicate Tukey HSD significant d ifference between means for both site and herbivory at P<0.05. Foliar concentration Grass biomass C N P Dry matter C N P g kg 1 mg kg 1 k g ha 1 Site North 3982 a 14.80.6 a 121269 a 737132 a 29352 a 111 a 0.90.2 ab Central 39 82 a 11.60.7 b 87156 b 860166 a 34267 a 102 a 0.70.1 b South 36016 a 14.00.7 ab 136555 a 929184 a 33578 a 133 a 1.30.2 a F ratio 2.3 4.9 27 0.8 0.4 0.8 3.3 P value 0.1087 0.0109 <.0001 0.4723 0.6944 0.4363 0.0438 Herbivory Grazed 37210 a 14.40.6 a 127951 a 650109 a 24243 a 92 a 0.80.2 a Ungrazed 3993 a 12.40.5 a 99760 a 1080141 a 43158 a 132 a 1.10.2 a F ratio 9 3.0 9 6.4 6.7 2.1 1.0 P value 0.0951 0.2242 0.0978 0.12 68 0.1225 0.2841 0.4299

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46 Table 2 3. Effects of herbivory on soil C, N and P concentration. Data presented for Nov 2010 (1st wet season) and May 2011 (2nd wet season).The data indicate mean SEM, the mean is from 36 replicates. Herbivory include grazing and no grazing. Different letters indicate Tukey HSD significant difference between grazing treatments (means) at P<0.05. Nov 10 May 11 Soil C (g kg 1 ) Grazed 9.41 a 9.80.5 a Ungrazed 8.81 a 9.50.4 a F ratio 0.68 0.37 P value 0.4148 0.5458 Soil N (g kg 1 ) Grazed 1.20.1 a 1.20.1 a Ungrazed 1.10.1 a 1.20.1 a F ratio 0.19 0.64 P value 0.6679 0.4267 Soil P ( m g kg 1 ) Grazed 2656 a 24210 a Ungrazed 2807 a 22510 a F ratio 0.44 0.32 P value 0.577 0.6286

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47 Table 2 4. Effects of nutrie nt enrichment on soil C, N and P. Data presented for Nov 2010 (1st wet season) and May 2010 (2nd wet season). The data indicate mean SEM. Nutrient enrichment treatment includes added N, added P, added N+P and control (where no nutrients were added) the m ean is from 18 replicates. Different letters indicate Tukey HSD significant difference between means at P<0.05. Nov 2010 May 2011 Soil C (g kg 1 ) Control 7.30.8 b 8.40.6 b N 9.50.9 ab 9.90.8 ab P 9.60.9 ab 9.60.6 ab NP 10.30.7 a 10.70.5 a F ratio 3. 3 3.5 P value 0.0276 0.0221 Soil N (g kg 1 ) Control 0.970.10 b 1.070.06 c N 1.220.09 ab 1.310.07 ab P 1.150.09 ab 1.140.05 bc NP 1.290.07 a 1.370.05 a F ratio 3.0 7.3 P value 0.0396 0.0003 Soil P ( m g kg 1 ) Control 22512 c 24016 a N 25311 bc 200 8 a P 29713 ab 21314 a NP 32414 a 29016 a F ratio 8.5 2 P value 0.0001 0.1252

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48 Table 2 5. Soil NO 3 N (herbivory x site interactions). Data presented for Nov 2010 (1st wet season) and May 2011 (2nd wet season). Herbivory include grazed and ungrazed. Sites vary in mean annual precipitation (north=438 mm, central= 583, south= 638, 3 year mean) the mean is from 12 replicates. Different letters indicate Tukey HSD significant difference between means at P<0.05. NO 3 N ( mg kg 1 ) Nov 10 May 11 Grazed North 5.41.1 a 16.53.8 a Central 5.52.5 a 15.35.2 a South 3.30.4 a 16.510 a F ratio 2.5 0.02 P value 0.0978 0.9761 Ungrazed North 5.71.6 a 15.02.6 a Central 2.40.5 b 7.51.9 a South 3.30.5a b 15.54.9 a F ratio 5.7 3.0 P value 0.0076 0.061

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49 Table 2 6. Soil NO 3 N (nutrient enrichment x site interactions). Data presented for Nov 2010 (1st wet season) and May 2011 (2nd wet season). The mean is from 6 replicates. Nutrient enrichment treatment includes added N, added P, added N+P and control (where no nutrients were added). Sites vary in mean annual precipitation (north=438 mm, central=583, south=638, 3 years mean).Different letters indicate Tukey HSD significant difference between nutrient enrichment means at P<0.05. Treatment Fertilization Control N P NP F ratio P value mg kg 1 North site Nov 2010 4.31.8 a 5.41 a 5.41 a 7.63 a 0.6 0.6229 May 2011 8.64 b 23.25.4 a 13.13.5 ab 23.62.2 a 7 0.0056 Central site Nov 2010 2.71 a 4.21.1 a 3.82 a 4.15 a 0.4 0.7699 May 20 11 6.52.6 b 21.79.2 a 5.81.2 b 16.21.5 a 10.6 0.0011 South site Nov 2010 2.50.4 a 2.90.6 a 3.91 a 4.20.3 a 0.001 0.9942 May 2011 5.31.5 b 41.313 a 6.83 b 43.88 a 39 <0.0001

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50 Table 2 7. 1st wet season ANR NUE and PNE calculated from the means using formula given in the method section. Treatment ANR (%) NUE PNE Nitrogen N 7.1 4.8 68 NP 8 5.3 67 Phosphorus P 0.7 3 460 NP 1.7 10.6 624

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51 Figure 2 1. Monthly rainfall over the study period, the study period was from F ebruary 2010 April 2011. S ampling was done on early May 2011

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52 Figure 2 2 Soil carbon and nitrogen concentration per site. A) Soil carbon concentration. B) Soil nitrogen concentration. Data presented for baseline (Feb 2010), Nov 2010 (1st wet season) and May 2011 (2nd wet season). The bars indicate mean, error bars indicate SEM, and the mean is from 24 replicates. Sites vary in mean annual precipitation (north=438 mm, central= 583, south= 638, 3 years mean). Different letters on top of the bar indicat e Tukey HSD significant difference between sites (means) for different seasons at P<0.05.

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53 Figure 2 3. Soil NH 4 + N under herbivory and nutrient enrichment. A) Herbivory treatment. B) Nutrient enrichment treatment. Data presented for baseline (Feb 2010), Nov 2010 (1st wet season) and May 2011 (2nd wet season). The bars indicate mean and error bars indicate SEM. Herbivory include grazed and ungrazed and the mean is from 36 replicates. Nutrient enrichment treatment include added N, added P, added N+P and control ( where no nutrients were added) and is a mean of 18 replicates. Different letters on top of the bar indicate Tukey HSD significant difference between (A) herbivory means and (B) fertilization means for different seasons at P<0.05.

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54 Figu re 2 4 Site effect s on grass biomass and nutrient storage. A) Grass biomass. B) Biomass C. C) Biomass N. D) Biomass P. Data presented for 1st wet season (Nov 2010). The bars indicate mean and error bars indicate SEM, the mean is from 24 replicates. Site s vary in mean annual precipitation (north=438 mm, central= 583, south= 638, 3 years mean). Different letters on top of the data indicate Tukey HSD significant difference at P<0.05.

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55 Figure 2 5. Nutrient enrichment effect s on grass biomass and nutrients storage. A ) Grass biomass B) B iomass C C) B iomass N D) B iomass P. Data presented for 1st wet season (Nov 2010). The bars indicate mean and error bars indicate SEM, the mean is from 18 replicates. Nutrient enrichment treatment includes added N, added P, added N+P and control (where no nutrients were added). Different letters on top of the bar indicate Tukey HSD significant difference between nutrient enrichment treatments at P<0.05.

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56 CHAPTER 3 SEASONAL PATTERNS IN DECOMPOSITION AND NUTRIENT RELEASE FROM EAST AFRICAN SAVANNA GRASSES GROWN UNDER CONTRASTING NUTRIENT CONDITIONS Background The balance between net primary productivity and decomposition of organic matter is a key process regulating the global C budget. The difference between these two opposing processes determines C stocks in savannas and other terrestrial ecosystems (Couteaux et al. 1995). The amount of soil organic matter (SOM) is highly susceptible to climate variability, including changes in temperature and precipitation (Austin 2002; Rey e t al. 2005). Moreover, decomposition of plant litter is an important source of CO 2 release to the atmosphere. G lobally, it is estimated that litter decomposition contributes approximately half of the CO 2 released from soils (Couteaux et al. 1995). The dec omposition of plant litter and soil organic matter also contributes to availability of plant nutrients (Aerts et al. 1992) and the regional balance of greenhouse gases (Gill and Burke 2002). Accumulated litter mass may be decomposed through photo degradati on and microbial breakdown (Facelli and Pickett 1991). Previous studies have suggested that the decomposition process is primarily controlled by extrinsic drivers such as climate (Austin and Vitousek 2000; Gill and Burke 2002; Aerts 2006) and soil conditio ns (Gill and Burke 2002). Intrinsic drivers may include plant quality (Mugendi and Nair 1997; Gindaba et al. 2004; Aerts 2006), macro and microfauna (Couteaux et al. 1995; Aerts 2006), and litter physical properties (Meentemeyer 1978). The relative impor tance of these drivers varies depending on region, ecological zone, type of plant, and plant part undergoing decomposition. Warmer regions typically have

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57 greater organic matter decomposition rates than colder regions (Epstein et al. 2002). Climate is consi dered to be a dominant factor in areas with colder weather conditions, while litter quality dominates in areas under warmer weather conditions (Couteaux et al. 1995). In African savannas, most litter fall s occurs in the dry season, yet most decomposition studies have been initiated at the onset of the wet season ( Deshmukh 1985; Jama and Nair 1996; Mafongoya et al. 1997; Mugendi and Nair 1997; Mugendi et al. 1999 ) with few in the dry season ( Fornara and Du Toit 2008). Moreover, most of these studies involve d leguminous forbs and trees (Jama and Nair 1996; Mafongoya et al. 1997; Mugendi and Nair 1997; Mugendi et al. 1999; Fosu et al. 2007; Fornara and Du Toit 2008; Oladoye et al. 2008 ), and few included grasses (Ohiagu andWood 1979; Deshmukh 1985) Atmospheri c N and P deposition has been reported by different authors (Herut et al. 1999; Tamatamah et al. 2005) and there are projections of future increased depositions (Phoenix et al. 2006) However, little is known about the influence of N and P enrichment on pr imary productivity and decomposition in East African savannas. The objectives of our study were to determine: (1) the influence of nutrients in determining the decomposition pattern of a mixture of nitrogen and/or phosphorus pre enriched grasses in an une nriched site; (2) patterns of nitrogen and phosphorus release during decomposition of organic matter in both wet and dry seasons. We hypothesized that the decomposition rate and release of both nitrogen and phosphorus would be greater in the wet season tha n in the dry season, and that N and P enrichment would accelerate decomposition rates.

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58 Materials and Methods Site Description The study was conducted in 2010 and 2011 at Mpala Research Centre, Kenya, which encompasses 190 km 2 of semiarid savanna within the Laikipia County of the Rift Valley Province (37 0 0 livestock co exist and share resources. The dominant woody vegetation includes Senegalia (Acacia) brevispica Vachellia (Acacia) etbaica Senegal ia ( Acacia) mellifera, Vachellia (Acacia) nilotica and Vachellia (Acacia) gerrardii Croton dichogamus, Grewia spp and Rhus vulgaris (Young et al 1995) The herbaceous vegetation consists of a discontinuous layer of mostly perennial grasses, which includ e Pennisetum mezianum, P. stramineum Digitaria milanjiana and Cynodon dactylon The soils are red sandy loams Alfisols (Ahn and Geiger 1987) derived from metamorphic basement rock (Pringle 2008). Prior to fertilization, soil pH was 6.3 (measured in wat er) and the soil total P and available P content was 230 mg kg 1 and 11 mg kg 1 respectively. The soil C and nitrogen contents were 10 g kg 1 and 1.1 g kg 1 respectively (Goheen et al. unpublished manuscript). The mean annual rainfall is approximately 5 00 mm in Laikipia County (Riginos et al. 2009). The rainfall follows a trimodal pattern with long rains during April to June, and smaller pulses in August and October (Augustine 2010), and a dry season between January and March (Pringle 2008). The rainfall long term average from 1999 2011 is as shown in (Fig. 3 1A). Monthly maximum temperatures range from 25 to 33 o C and minimum temperatures from 12 to 17 o C (Young et al. 1998).

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59 Experimental Design This study was conducted in conjunction with the Ungulate H erbivory Under Rainfall Uncertainty (UHURU) experiment, established in 2008 at Mpala Research Centre (Goheen et al unpublished manuscript). The study was conducted in the southern (wettest) site of the UHURU experiment, which received an average of 63 8 mm rain year 1 from 2009 to 2011. Four fertilizer addition plots (16 m 2 each; hereafter each) which fenced out all herbivores larger than hares (~2 3 kg). Thus, there were a total of 12 plots arranged across the three exclosures. Within each of the 12 plots, grass was clipped to gro und level in 1 m 2 patch and discarded and fertilizer s were applied to the entire 16m 2 plot on the onset of rainfall (March 2010), this allowed re growth from 1m 2 plot to be used for the decomposition experiment. The fertilizer treatments included: (1) nitrogen only, (2) phosphorus only, (3) a mixture of nitrogen and phosphorus, and (4) unfertilized control (hereafter referred as N, P, NP and control ). The fertilizer application included consisted of N (urea) at 100 kg ha 1 and/or P (triple super phosphate) at 50 kg ha 1 All treatments were arranged in a randomized complete block design within each of the exclosures. In November/December 2010, grass regrowth from the 1 m 2 fertilizer enriched patches was clipped for the decomposition study. Experimental Procedure Decomposition and nutrient (N and P) disappearance rates of litter from the different fertilization treatments were estimated using litter ba gs technique (Dubeux et al. 2006). After fertilization aboveground regrowth was clipped from each plot and air dried to a constant weight. The litter bag s (15 x 20 cm) were made from 2 mm mesh

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60 polyester to allow entry of micro and mesofauna. Three grams o f a mixture of grasses from each fertilizer enriched plot was put into each of 10 litter bags, which were heat sealed and placed in the thatch layer in a common un enriched decomposition area away from the plots of origin. After placement on the ground, th e bags were lightly covered with litter from the common unenriched plot. This experiment was maintained from 22 December 2010 to 11 May 2011. Two litter bags from each treatment were retrieved after 0, 4, 8, 12, 16, and 20 weeks. The bags and their conten ts were air dried to a constant weight. Prior to weighing the content of each bag, soil particles and any other extraneous matter were carefully removed. The content from the duplicate litter bags in each treatment was composited for chemical analysis afte r weighing. Litter samples were then ground to pass through a 1 mm screen and analyzed for total C, N, P and lignin concentrations. The N and P concentration and remaining litter biomass at any given time were used to estimate the N and P release. The N o r P release was the balance between the original N or P mass and the mass at any given time. Litter Processing and Analysis Total C and N were determined using Costech Model 4010 Elemental Analyzer (Costech Analytical Industries, Inc., Valencia, CA) coupl ed to a Finnigan MAT Deltaplus XL mass Spectrometer (CF IRMS, Thermo Finnigan) via a Finnigan Conflo II interface. Total P was determined by ignition at 550 o C followed by extraction in 1 M H 2 SO 4 acid and detection by automated molybdate colorimetry. Digest ed solutions were analyzed colorimetrically using Shimadzu UV visible recording spectrophotometer UV 160. Lignin was determined by a modified sequential fiber extraction method (Ankom Technology, Fairport, NY) modified from the feed and forage analysis by Van Soest (1970).

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61 Statistical Analysis Statistical differences in treatments were determined using analysis of variance (ANOVA) for a randomized complete block design using Tukey HSD test at 0.05. Sidak adjustment for multiple comparisons test was used to determine the significant difference for the multiple phase regression model at 0.05. A nonlinear procedure of SAS (SAS 9.2) for multiphase regression models was fitted for C and a sing le exponential model was fitted for N release. The results indicated that the multiphase regression model was applicable for loss of litter C during decomposition as the response variable but was not applicable to describe nitrogen release. Hence for N we used the single exponential model Y t =Y o x e kt (where Y o is the original amount of material incubated and Y t the proportion of the initial N remaining after a period of time t, (weeks) and k is the decomposition rate constant. The multiphase regression model was applied for C loss during decomposition; parameters include intercept and slope of each phase, and the spline. The slope of the linear regression represents the decomposition rate constant (k). The spline point is the end of phase 1 and beginnin g of phase 2 ( Jama and Nair 1996) For N release half life was determined by natural log of 2 divided by decomposition constant. This is because N release followed a single exponential model. For C release half life was calculated from multiple regression model by determining the time (x) that half of the original C mass had been released. Results Pattern of Litter Decomposition Fertilizer enriched plots stored more C as aboveground biomass compared to the control (Table 3 1) with the NP enriched treatment having more than twice C storage compared to the control. Despite the Tukey HSD test indicating no significant difference

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62 between the treatments. However, the initial aboveground biomass nutrient concentration was not influenced by addition of soil nutri ents (Table 3 1). The decomposition pattern followed a biphasic pattern, with an initial phase of slow decomposition rates followed by faster rates (Fig 3 2 A B C D ; Table 3 2) after the onset of the long rains in April. Within nutrient enrichment treatm ents, the decomposition constants (k1 and k2) for the two phases differed significantly between wet and dry season (The phase one and two are hereafter referred to as dry and wet phase s respectively). Sidak adjustment for multiple comparisons test indicate that decomposition constant was higher in the wet phase than the dry phase for all treatments, at P < 0.0001 for the control and N enrichment treatment at P = 0.0003 for the P enrichment treatment and at P = 0.04 for the NP enrichment treatment (Fig 3 2). The decomposition constant for the dry phase did not differ significantly between N, P, and NP enriched grass, and was two times greater for the enriched treatments than for the control (Table 3 2). For the wet phase, the decomposition constant was significantly higher for N enriched grasses (11% wk 1 ; P = 0.0008), whereas P, NP enriched grass and control grass did not differ significantly from one another (Fig. 3 2 A B C D ; Table 3 2). The decomposition rate of grass from the control treatment wa s 10 g kg 1 week 1 and 70 g kg 1 week 1 in the dry and the wet phases respectively (Fig 3 2A). This corresponds to a half life of 38.6 weeks The wet phase of decomposition started at 12.9 weeks at 88% initial C mass. At the close of the study (after 20 we eks), 50% of the total C mass remained. The decomposition rate of N only enriched grass was 20 g kg 1 week 1 and 110 g kg 1 week 1 in the dry and wet phases, respectively, (Fig. 3

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63 2 B) Corresponding to a half life of 35 weeks. The wet phase of litter decomp osition commenced on week 14.1 with 75% of the initial C biomass remaining. At the end of the study, 35% of the initial C mass remained. The decomposition rate of the P only enriched grass was 20 g kg 1 week 1 in the dry phase, and 90 g kg 1 week 1 in the wet phase (Fig. 3 2 C). With a half life of 40 weeks. The wet phase of decomposition began at 15.5 weeks with 73% of the initial C mass remaining. At the end of the study, 45% of the initial C mass remained. The NP enriched grass decomposition rate was 20 g kg 1 week 1 and 80 g kg 1 week 1 in the dry and wet phases, respectively (Fig. 3 2 D ) with a half life of 42 weeks Wet phase decomposition started at 15.8 weeks (Fig. 3 2 D ) with 73% of the initial C mass remaining. At the end of the study 50% of the ini tial C mass remained. Although there were no significant differences between the initial nutrient concentrations of the enriched grasses (Table 3 1), Sidak adjustment for multiple comparisons test indicate that N enriched grasses had higher decomposition rates in the second phase ( P =0.0008; Fig. 3 2 B ) compared to other treatments, with the N enriched grasses having the least amount of C mass remaining at the end of the study. The seasonal rainfall positively correlated with decomposition rate (R 2 = 0.80; P <0.0001). Trends in Nitrogen and Phosphorus Release from the Grass Litter The single exponential model equations of time against log nitrogen mass of the grass indicated that the rate of nitrogen release in the control treatment was 50 g kg 1 week 1 (Table 3 2) with a half life of 14 weeks Cumulatively a t the end of the first 8 weeks of the study which were during the dry season 450 g kg 1 of the initial nitrogen had been released (Fig. 3 3 A ). The rate of nitrogen release from the N enrichment

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64 treatment w as 60 g kg 1 week 1 (Table 3 2) with a half life of 11.5 weeks Cumulatively a fter the first 8 weeks of the study, which were in the dry season, 480 g kg 1 (Fig. 3 3 B ) of N had been released. For the P enriched grass, nitrogen release was 55 g kg 1 week 1 (Table 3 2) with a half life of 12.6 weeks, cumulatively 410 g kg 1 (Fig. 3 3 C ) of the initial N mass had been released after the first 8 weeks of the study. For the NP enriched grass, the N release rate was 54 g kg 1 week 1 (Table 3 2) with a half life of 12.8 weeks. After the first 8 weeks of the decomposition study, cumulatively 550 g kg 1 (Fig. 3 3 D ) of the initial N had been released. By the end of the study (20 weeks) the N, P and NP treatments had each released 75% of initial N mass, while the contr ol released 68% of initial N mass (Fig. 3 3 A B C D ). Although the C:N ratio of the litter was more than 25 throughout the study, except for the N and P treatment in the initial litter (Fig. 3 5 A ), the C:N ratio increased throughout the study despite th e expected immobilization trend. Phosphorus release was rapid in the initial 8 weeks (dry season) of the study. In this period, the control, N, P, and NP treatments released 490 g kg 1 610 g kg 1 580 g kg 1 and 700 g kg 1 of the initial P mass, respect ively. By the end of the study (20 weeks) these four treatments had released 73%, 80%, 77%, and 83% of initial P mass, respectively (Fig. 3 4 A B C D ) For all treatmen ts, the C: P ratio was > 300 throughout the decomposition study, apart from the initia l litter from the N enrichment treatment (which was 275; Fig. 3 5 B ). However, there was a continuous increase in C : P ratio throughout the study period although immobilization was expected to take place

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65 Discussion Our study assessed litter decomposition in the dry and wet seasons of a semi arid African savanna by emulating the timing of processes involved in natural decomposition. Our results are consistent with previous work initiated in a wet season (Jama and Nair 1996; Hamadi et al. 2000; Dubeux et al. 2 006 ) reporting a biphasic litter decomposition pattern, but in our study initiated in the dry season the order was reversed, with first slow decomposition rate in the dry season, followed by a faster decomposition rate on the onset of rainfall. The initia l slow phase of decomposition in the dry season, followed by a faster second phase of decomposition in the wet season, is consistent with moisture being the main factor influencing decomposition rates (Vitousek et al 1994; Mugendi and Nair 1997; Austin an d Vitousek 2000; Epstein et al. 2002). This interpretation is further supported by the positive correlation observed between the decomposition constant ( k ) and rainfall. Notably, however, C and N in our study did not mineralize simultaneously as observed b y several previous studies in which patterns of both C loss and N release followed a multiphase regression model (Jama and Nair 1996; Hamadi et al. 2000 ). In our study, C decomposition followed a multiphase regression model, whereas N release followed a si ngle exponential model; this was similar to the findings of Dubeux et al. (2006) in sub tropical Florida USA Minimal C was lost in the dry season; in the first four weeks, only 4 9% C was lost, whereas 33 47% N was released in the same period. This contr asts with the findings of Austin and Vitousek (2000), who reported that proportional release of C biomass was faster than nutrient release, which is expected in a decomposition process Our study suggests that, although moisture limits the decomposition pr ocess in the dry season there is a process driving nutrient release in

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66 this environment, we suspect dew could be driving the nutrients leaching ( see also Tukey Jr.1970; Leonard and Fluckiger 1989) This possibility need s further investigation. As indicated by Hamadi et al. (2000) that decrease in litter nutrients after field incubation could be due to leaching to deeper soil layers or due to uptake by primary producers. D ue to greater than 25 and 300 C:N and C: P ratio, respectively, in the initial litter ob served in this study immobilization was expected to take place, as suggested by previous authors (Swift et al. 1979; Palm and Sanchez 1990), and both C : P and C : N ratio would have decreased as the nitrogen and phosphorus content of the decomposing material would have exceeded 100% of the original content. On the contrary, the C : N and C : P ratio continuously increased, suggesting that microbial decomposition was not the dominant processes leading to N and P release, and further suggesting that N and P leachin g could have occurred and a possibility of dew driving this process need further investigation, especially considering that P is not volatilized. Dubeux et al. (2006) suggested that higher tissue C : N ratios indicate lower nitrogen concentration in tissues rather than changes in C concentrations. The decrease in litter N over the study period contrasted with the findings of several previous studies (Deshmukh 1985; Hamadi et al. 2000; Dubeux et al. 2006; Lie et al. 2011), which reported increased nitrogen mas ses over the study period. The 680 750 g N kg 1 and 730 830 g P kg 1 released by the end of our 20 week study were higher than the 200 300 g N kg 1 and 600 g P kg 1 mineralization reported by Dubeux et al. (2006) for 18 week incubation in sub tropical Flor ida, USA. Considering that the initial grass biomass had 40 41% C and that 21 25% of the C was lignin, the initial slow rate of decomposition in the dry season likely results from

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67 the loss of very soluble C compounds that can decompose even under minimal moisture. The subsequent phase of higher decomposition rates with the onset of rain suggests that the more recalcitrant compounds need more moisture for breakdown to be accomplished (Meentemeyer 1978). Overall disappearance of C biomass was greater in the wet season than the dry season. These findings contrast with Ohiagu and Wood (1979), who reported 69% of grass litter disappearance over a four month dry season in Nigerian savanna, and indicated this loss was due to consumption by fungus growing termites Termites were excluded in our study, which might explain this discrepancy. were absent and the decomposition study was initiated in the wet season, only 50% of the grass litter was lost over a 23 month study period in spite of the use of litter bags with larger aperture mesh (5mm) that would have allowed in arthropods and higher rainfall amounts (Deshmukh 1985). In the current study, 50% of C biomass in the control treatment was lost in only 20 weeks. This suggest s the influence of factors other than just rainfall and the absence of termites on the decomposition process, such as the mixture of grass species used in the study. In our study site, the grass mixture was dominated by Pennisetum mezianum, P. stramineum, Digitaria milanjiana and Cynodon dactylon while the study site of Deshmukh (1985) was dominated by Themeda triandra and Setaria phleoides which might be responding differently to decomposition. Likewise, rates of C los s in our study were higher than the 50% Zoysia japonica grass litter mass loss reported by Nakagami et al. (2010) in Japan over a one year incubation period.

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68 Nevertheless, our findings of 50 65% C biomass loss were comparable to the findings of Dubeux et al. (2006), who reported 40 60% biomass loss of Paspalum notatum Fluegg (Bahia grass) in Florida in a study period of the same length. The findings were also comparable to 36 55% Cynodon dactylon (L.) Pers. ( Bermuda grass) organic matter loss reported by Liu (2011) within 18 weeks of incubation in Florida. We acknowledge that the use of the 2 mm litter bags in our study could alter the litter micro climate (Witkamp and Olson 1963; Meentemeyer 1978) and exclude the larger macrofauna (Elkins and Whitford 19 82) thus reducing the fragmentation of the leaves and causing the bagged leaves to decompose at a different rate compared to non confined leaves. Our study assumes that these effects did not mask the differences resulting from climatic conditions and litte r quality. Nutrient enrichment increased grass biomass production. However, there was no significant difference in nutrient concentration between initial incubated litters from different treatments, the N enriched litter lost the highest amount of biomass C (65%) and had significantly higher rate of decomposition in the second (wet) decomposition phase (11%). These findings agree with previous observations ( Lupwayi and Haque 1999; Liu et al. 2011) that nitrogen fertilization increases biomass decomposition, but suggest that, apart from litter N concentration there are other unidentified factors that accelerate decomposition in N enriched litter. Exa mples of such factors could include increases in C lability resulting from nitrogen enrichment, a possibility t hat requires further investigation. With increasing global atmospheric N deposition that is expected to more than double by 2050 (Phoenix et al. 2006), it is of global concern that N enrichment could translate to greater CO 2 emissions during decomposition On the other

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69 hand, a combination of N and P enrichment favored primary productivity while keeping the decomposition low and promoting C storage. The high N and P release in the dry period compared to the wetter period contrasted with the suggestion by (Swift et al. 1979) that precipitation can control the physical process of leaching through accelerated breakdown of surface litter by increased rainfall. After the first 8 weeks of the incubation period, more than 40 50% and 50 70% of the total N and P ha d been released, respectively. This occurred in a dry season; we suspect that dew might have influenced the leaching process and suggest that further investigation is needed. The rapid N and P release that did not match the C biomass loss suggest that in t he long term, both N and P are likely to limit the microbial decomposition process unless there is an external source of nutrients to drive the decomposition process or photodegradation would prevail and short cir cuits the decomposition process leading to direct C loss to the atmosphere (Austin and Vivanco 2006). On the other hand, Couteaux et al. (1995) suggested that decrease in litter quality could lead to increased production of ligninolytic enzymes, leading to increased degradation of recalcitrant comp ounds. However, the extent to which lowering the nitrogen level would influence lignin degradation is still unknown. Summary In conclusion, it has been argued that future elevation of atmospheric CO 2 would lead to plant N and P dilution, leading to N and P limiting plant productivity in the long term (Gifford et al 2000). However, our findings suggest that under projected increases in atmospheric nutrient deposition in this ecosystem where N and P limit productivity, plants are likely to partially overcome the dilution, with N and P enrichment having the potential to retain the greatest soil C biomass through increased primary productivity.

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70 Our results also indicate that factors other than litter N concentration increase decomposition under N enrichment; fu ture studies should address this possibility. With the highest N and P release occurring in the dry season, with a greater percent P than N released, and considering that P is not volatilized and that there was minimal microbial decomposition taking place coupled with increasing C : P and C : N ratio where immobilization was expected indicate that N and P leaching was dominant in the dry season and a possibility of dew driving the leaching process need further investi gation.

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71 Table 3 1. Aboveground biomass, b iomass C, N and P storage. Chemical composition of the initial grasses biomass after fertilization and ratios of initial aboveground biomass. The values are means SEM of 3 composite replicates. Treatment Control N P NP Aboveground Storage (kg ha 1 ) Ab oveground biomass 1155187 a 1494407 a 1602490 a 2411735 a Carbon 47277 a 620170 a 663193 a 974313 a Nitrogen 14.72.7 a 25.63.6 a 27.35.2 a 33.417.6 a Phosphorus 1.40.2 a 1.90.6 a 2.00.4 a 4.10.9 a Chemical composition of the initial aboveground biomass (g kg 1 ) Carbon 4045 a 4157 a 4146 a 4049 a Nitrogen 13 1 a 172 a 172 a 142 a Phosphorus 1.30.1 a 1.50.3 a 1.40.1 a 1.20.2 a Lignin 25030 a 21025 a 2505 a 23013 a Ratios of the initial aboveground biomass Lignin: N 190.8 a 133.1 a 151.8 a 173 a C :N 323 a 253 a 253 a 303.6 a C:P 32623 a 27649 a 30022 a 33546 a Table 3 2. Intercepts, slopes, spline points and coefficients of determination (R2) of the regression lines for litter decomposition (C) and nitrogen mineralization. Decomposition ra te constant, k, (% week 1 ) Intercept Phase one Phase two Spline point (weeks) R 2 Carbon decomposition Control 6 1 7 12.9 0.93 N 6 2 11 14.1 0.97 P 6 2 9 15.5 0.75 NP 6 2 8 15.8 0.61 Nitrogen mineralization Control 2.5 5 0.67 N 2.8 6 0.68 P 2.8 5.5 0.74 NP 2.4 5.4 0.60

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72 Figure 3 1 Rainfall in the study site. A) Long te rm rainfall average (1999 2011). B) Rainfall during the study period from 12/22/2010 5/11/2011, presented in four weeks interval to emulate the litter incub ation period.

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73 Figure 3 2. Decomposition pattern over time A) Control B) Nitrogen C) Phosphorus D) Nitrogen+ phosphorus enriched aboveground biomass placed on the unenriched soil surface. The means are replicates of 3 composite samples.

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74 Fig ure 3 3. Percent n itrogen release pattern over time A) Control B) Nitrogen C) Phosphorus D) Nitrogen + Phosphorus enriched grasses. The means are replicates of 3 composite samples; error bars indicate SEM. Different letters along the line graph indica te significant difference between the means at P<0.05.

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75 Figure 3 4. Phosphorus release pattern over time. A) Control. B) Nitrogen. C) Phosphorus. D) Nitrogen + Phosphorus enriched grasses. The means are replicates of 3 composite samples; error bars indicate SEM. Different letters along the line graph indicate significant difference between means.

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76 Figure 3 5. C:N and C: P ratio over time. A) Carbon/ nitrogen ratio. B) Carbon/ phosphorus ratio of the litter over time. The litter is from fertilized plots and decomposed in a common unfertilized site. The means are replicates of 3 composite samples.

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77 CHAPTER 4 HERBIVORY AND NUTRIENTS INFLUENCE ON PLANT PRODUCTIVITY AND CARBON STORAGE IN THE VERTISOLS OF THE EAST AFRICAN SAVANNA Background Herbivory a nd soil nutrients are among the major determinants of tropical savanna functioning, influencing both plant primary productivity and carbon storage. Nitrogen (N) and phosphorus (P) are known to limit plant primary productivity in the tropical savanna (Thorn ley et al.1991; Augustine et al. 2003). The limitation to plant productivity by a specific nutrient is diagnosed when the addition of the given nutrient results in an increase in net primary production where no other factor limits plant productivity (Leba uer 2008). Large mammalian herbivores could have positive, neutral or negative effects on annual net aboveground plant production in different ecosystems depending on their direct effects on availability of key nutrients (Bagchi and Ritchie 2010). Previou s studies indicate that grazing increased primary productivity in some tropical areas (Pandey and Singh 1992) while decreasing in others (Wilsey et al. 2002). Similar findings were reported in the temperate areas, with decrease reported by (Coughenour 1991 ; Pucheta 1998; Singer 2003) and increase by (Coughenour 1991; Frank and McNaughton 1993; Pandey and Singh 1992). Holland et al. (1992) argued that the capacity by herbivores to increase primary production is due to increased nutrient turnover rates. In na tural grasslands, productivity and soil fertility is mainly maintained by recycling of nutrients through herbivores fecal matter and plant litter decomposition (Grant et al. 1995). In addition N can also be added to the soil through atmospheric deposition (Herut et al. 1999; Galy Lacaux 2003), agricultural pollution and also be fixed

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78 by N fixing bacteria through plant roots into plant available form ( McCulley et al. 2007) Nitrogen is often more limiting in early succession ecosystems before N has accumulat ed in the soil and while P derived from parent material is still available (Crews et al. 1995). On the other hand P cannot be sequestered from a gaseous state hence P 20 10). In a natural ecosystem the major source of P is the underlying bedrock and hence soil P is usually limiting in highly weathered soils. In addition P can be added to the soils through senescent vegetation, fire and dust deposition (Tamatamah et al. 200 The response of net primary production to nutrient enrichment will depend on the extent to which a particular nutrient limits the net primary production (Lebauer 2008). Grasses are more tolerant of soils lo w in P than legumes, and divide their P between shoot and root in the same way irrespective of the soil P level (Caradus 1980). Phosphorus deficiency is more critical in highly weathered soils as well as in calcareous and alkaline soils (Shenoy and Kalagud i 2005). However, in the tropical savanna the plant productivity will be determined by a number of factors for example moisture availability and nutrient use efficiency. Nutrient deposition rate, grass species, and soil type may affect the efficiency of nu trient use (Zemenchik and Albrecht 2002). Nutrient limitation to plant productivity varies spatially across the African savanna. The foliar N:P ratio has widely been used as an indicator of nutrient limitations where by the general rule is that N: P ratio > 16 indicates P limitati on to plant growth, while an N: P ratio < 1 4 indicates N limitation. At N: P ratios between 14 and 16, either N or P can be limiting or plant growth is co limited by N and P together (Koerselman and

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79 Meuleman 1996). Ludwig et al. (200 1) indicated that N limited plant productivity under the open canopy and P under the sub canopy in Tanzanian savanna. Moreover, Ries and Shugart (2008) indicated N and P co limitation in African woodland savanna in indicated luxury uptake of nutrients on nutrient enrichment in Southern Africa savanna. Previous studies have focused on nutrients (Ludwig et al. 20 01: Ries and Shugart 2008 ) or herbivory (Grace et al. 2006) as major factors influencing plant productivi ty and C storage but few studies have considered the interactions of the two factors in East African savanna. It is important to understand the interactions between nutrient and herbivory effects because CO 2 is increasing (Stokes et al. 2005). The herbivor e population is also in the rise especially domestic animals and some wild animals (Kinnaird et al. 2010; Thornton 2010). The interactions of rising CO 2 herbivory and availability of nutrients is likely to affect the plant productivity and C storage. The objective s of this study were to; (1) determine the effects of long term grazing on the grasses aboveground biomass and soils carbon and nutrients storage, (2) to determine grazed and ungrazed plots, grass aboveground primary productivity, biomass nutr ients storage and soils response as influence by N and P enrichment. We hypothesized that; (1) grazing reduced grass carbon and nutrient storage but increased the soil organic carbon (SOC) and nutrients storage, (2) N and P enrichment favored grass product ivity in the non grazed plots compared to grazed plots, because natural nutrient cycling has been facilitated through fecal matter deposition in grazed plots.

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80 Materials and methods Study Site The study site is located on a semi arid tropical savanna at M pala Research Centre and C onservancy in Laikipia district, Kenya (37 o E, 8 o N; 1800 m elevation). Mpala Research Centre and associated Mpala ranch covers 190 Km 2 ( Augustine and McNaughton 2004) The mean annual rainfall is approximately 500 mm in Laikipia County (Riginos et al. 2009). Mpala Research Centre is managed for both livestock production and wildlife conservation. Some of the resident wild large herbivores include elephants ( Loxodonta africana ), hartebeests ( Alcelaphus buselaphus ), giraffes ( Giraff a camelopardalis ), buffaloes ( Syncerus caffer ), Grant gazelles ( Gazella grantii ), zebras ( Equus burchelli ), impala ( Aepyceros melampus ) and elands ( Taurotragus oryx ). The large livestock herbivores are mainly cattle ( Bos taurus ) and camel ( Camelus dromedar ies ) (Young et al. 1998). The study site has black cotton soils (Vertisols) with a pH of 6.2 (measured in water) before nutrient enrichment and bulk density of 1g cm 3 Black cotton soils cover approximately 43% of the Laikipia ecosystem (Young et al. 1998 ). The whistling thorn Vachellia (Acacia) drepanolobium is the dominant tree species on black cotton soils accounting for 97% of the overstory, while more than 90% of the understory constitute of five grass species and two forb species, the details can b e found in (Young et al. 1998). Augustine and McNaughton (2004) indicate that Mpala Research Centre landscape contain a two phase vegetation mosaic, whereby 1% of the landscape is covered by short grass glades which has no woody vegetation, and is embedded within the background that is dominated by Acacia community. There is a trimodal rainfall

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81 pattern with long rain during April to June and smaller pulses in August and October (Augustine 2010), with a dry season between January and March (Pringle 2008). E xperimental Design Grazing experiment This study was conducted in the Kenya Long term Exclusion Experiment (KLEE). KLEE is a herbivore exclusion experiment that uses semi permeable barriers to exclude different category of animals according to their sizes, which was established in 1995 (Fig D 1 and E 1) It is arranged in a three replicates complete block design (Young et al. 1998). In October 2010 four 16m 2 plot (Fig. F 1) were established in each of the three blocks, within exclosures that exclude all l arge herbivores (complete exclosure) and where all herbivores are allowed to graze (open) (Fig. E 1) The large herbivores included both wild and domestic animals. At the start of the experiment, the center 1 m 2 in each 16 m 2 plot (Fig. F 1) was clipped to ground level, and the grass dried and weighed. This provided an estimate of the effect of 17 years of herbivore exclosure on standing grass biomass. At the same time, a composite soil sample (0 10 cm depth) was collected within each 16 m 2 plot (four sep arate cores in each composite) to estimate soil carbon and nutrient concentrations. Nutrient enrichment experiment After clipping and soil sampling, four fertilizer treatments were established: nitrogen alone, phosphorus alone, a mixture of nitrogen and p hosphorus, and a control (hereafter referred to as N, P, NP and control). The four fertilizer treatments were applied in the four 16m 2 that had been established in the grazed and non grazed plots (i.e. each replicate herbivory plot contained one 16 m 2 plot of each treatment). After fertilizer application the 1m 2 in both grazed and ungrazed plots were caged to keep off

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82 all herbivores. The fertilizer application included N (urea) at 100 kg N ha 1 and/or P (triple super phosphate) at 50 kg P ha 1 applied in late October 2010 and mid March 2011. Soil was sampled in the 16m 2 plot and aboveground regrowth biomass in the 1 m 2 subplots was harvested in early May 2011. The grass biomass was dried at 60 o C (Wrench et al. 1996) until constant weight and then ground. S oil samples were air dried for 12 days at 25 o C (Wrench et al. 1996). All samples were analyzed in the University of Florida Biogeochemistry Laboratory. Plant and Soils Chemical and Statistical Analysis Plant and soil total C and N were determined using Th ermo Electron Flash 1112 elemental analyzer. Organic matter was measured by loss on ignition from 0.3 and 0.5 g samples of dried and ground plant material and soils. The samples were placed in a muffle furnace and brought to 250 o C for 30 min. The furnace temperature was then increased to 550 o C for 4 h (Anderson 1976) Organic matter content was calculated as the mass loss on ignition on a dry weight basis (Luczak et al. 1997). Total P was determined using ignition at 550 o C followed by acid extraction usin g 1M H 2 SO 4 acid Digested solutions were analyzed colorimetrically using Shimadzu UV visible recording spectrophotometer UV 160. Extractable P, potassium, calcium, magnesium, iron and aluminum were determined using Mehlich 1 method as outlined by (Kuo 1996 ). Nutrient ratios were calculated on a mass basis. NH + 4 N and NO 3 N were extracted using 2M KCl. The samples were filtered through a 0.45 m membrane filter (Pall Corporation). The filtrate was analyzed colorimetrically as outlined by (White and Reddy 20 00). All statistical analysis was conducted using JMP (version 7.02; SAS Institute 2007). Significant differences among the treatments for the variables were determined by one way analysis of variance for grazing versus non grazed treatment before nutri ent

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83 enrichment and two way Anova after nutrient enrichment using Tu key HSD test and t test at 0.05. Determination of Nutrient Use Efficiency (NUE), Apparent Nutrient Recovery (ANR) and Physiological Nutrient Efficiency (PNE) Difference method was used to determine apparent nutrient recovery and nutrient use efficiency. Apparent nutrient recover y reflects plant ability to acquire applied nutrient from soil (Baligar et.al 2001) is determined as (U N U O )/F N where U N and U O are the nutrient uptake by grass with and without the applied nutrient, and F N is the amount of nutrient applied all in kg ha 1 the results are expressed as percentage. Nutrient use efficiency ( NUE ) is defined as the amount of forage (dry matter) that is produced for each unit of applied N or P (Fageria and Baligar 1999; Zemenchik and Albrecht 2002). Nutrient use efficiency (NUE) is determined as (Y N Y O )/F N where Y N and Y O are the grass biomass with and without the nutrient being tested all in kg ha 1 and F N is as indicated above (Guillard et al. 1995; Syers et al. 2008). The physiological efficiency of the applied nutrient is calculated as the kilograms of grass increase per kilogram of increase in nutrient taken up, (Y N Y O )/ (U N U O ). In all the above, P can replace N (nutrient) in the formulae (Syers et al. 2008). Results Effects of Long Term Grazing on Aboveground Biomass an d Soil Nutrients Analysis of the grass aboveground biomass after 17 years of grazing treatment indicated a significant grazing effect on grass P concentration (Table 4 1). The t test results suggest that the concentration of P was significantly higher in the grazed plot aboveground biomass ( t=5864; P = 0.0002) compared to the ungrazed plots before nutrient enrichment. However, N concentration was not significantly different but had an

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84 increasing trend in the grazed plots (Table 4 1). The study findings indi cate that grazing reduced the grass aboveground biomass, after 17 years of herbivore exclusion. Grazed plots had significantly lower aboveground grass biomass ( t=0.03; P =0.03; 45%) and biomass C ( t=21; P = 0.04; 45%) than the non grazed plots (Table 4 2). The grazed plots had approximately 55% aboveground biomass and biomass C of the non grazed plots. Both grass aboveground biomass N ( t=31; P = 0.03) and P ( t=22; P = 0.04) were significantly higher in the non grazed plots than in the grazed plots. With gra zed plots having 54% biomass N and 60% biomass P of the ungrazed p lots (Table 4 2). The grass C:N, C:P and N: P ratio s did not vary significantly between the grazed and ungrazed plots. The C:N and C:P were greater than 25 a nd 300 respectively while the N: P ratio was 17 18 ( Table 4 2). However, contrary to our hypothesis the soil parameters total C, N, P, extractable P, K, Ca, Mg, Al NO 3 N and NH 4 + N were not significantly different between grazed and ungrazed plots (Table 4 3 ), only soil organic C (SOC) as indicated by loss on ignition (LOI) and Fe concentration differed significantly between grazed and ungrazed treatments. S oil organic C was significantly higher ( t=83; P = 0.012; +4%) in the grazed plots than ungrazed plot (Table 4 3 ), while Fe was signi ficantly higher in the ungrazed plot ( t= 59; P = 0.017; +38%) than grazed plots (Table 4 3 ). Nutrient Enrichment Effects on Soils and Aboveground Biomass in the Grazed and Ungrazed Plots A two way Anova indicated that grazed and ungrazed plots aboveground b iomass, nutrient concentration, biomass C, N and P and soils parameters were not significantly different after nutrient enrichment. There were no significant interactions between nutrient enrichment and grazing. Hence data from above parameters from

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85 graze d and ungrazed plots were combined when conducting the statistical analysis. Therefore the post nutrient enrichment results only reflect the effects of fertilization. After nutrient enrichment soil total P, available P, NH + 4 N, NO 3 N and Mg changed signif icantly. However, there were no significant changes in soil C, total N, K, Ca, Fe and Al (Table 4 4 ). Soil total P was significantly higher under P and NP treatment s ( t= 37; P <0.0001) than in the control and N treatment (Fig.4 1A). After nutrient enrichme nt total soil phosphorus in P and NP treatment was 225 % and 230% respectively, while for control and N treatment it was 123% and 105% of the amount before nutrient enrichment. The available P was also significantly higher under P and NP treatment ( F= 101; P <0.0001) than in control and N treatment (Fig.4 1B). The control, N, P and NP treatment s had 180%, 170%, 1960%, 2200% available P of the amount before nutrient enrichment. The NH + 4 N( F=109; P <0.0001) and NO 3 N ( F=64; P <0.0001) were significantly high er in N and NP treatments compared to control and P treatment. After nutrient enrichment the control, N, P and NP treatment s had 142%, 2348%, 127% and 2044% NH 4 + N and 125%, 845%, 142% and 530% NO 3 N, respectively, of the amount before nutrient enrichment (Fig.4 1 C, D). After nutrient enrichment the soil C : N was significantly lower in N treatment ( F=7; P = 0.0061) compared to other treatments (Fig. 4 2A). The soil C : P ratio was significantly lower in P and NP treatment s ( F=30; P =0.0001) than other treatmen ts (Fig.4 2B). After nutrient enrichment the two way Anova indicated that the nutrient enrichment had a significant effect on the grass nutrient concentration, N ( F= 46; P <0.0001) and P ( F=55; P <0.0001) (Table 4 1).The NP, N and P fertilized aboveground gr ass N concentration increased by +50%, +35% and 8% respectively, and P

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86 concentration increased by +110%, +0% and +150% respectively compared to the control plot grass. The application of the N and/or P fertilizer did not have a significant effect on both grass biomass and biomass C, but had significant effect on biomass N and P. There was a relative increase in grass biomass and biomass C, whereby NP, N and P application increased the aboveground biomass by +42%, +14% and 0% and biomass C by +48%, +13% and +1% respectively, relative to the control (Fig. 4 3 A ,B). Grass biomass N was significantly higher in the N and NP treatment s ( F=3.6; P = 0.0472) than in control and P treatment. The biomass N increased by +121%, +53% and 10% in the NP, N and P treatme nts respectively, compared to the control (Fig. 4 3C). The grass biomass P was significantly higher in the NP treatment compared to other treatments ( F=5.2; P =0.0153; Fig. 4 3D). The grass biomass P increased by +200%, +12% and +146% in NP, N and P treatm ent s compared to the control. The grass C : N, C : P and N : P ratio s were significantly different across the treatments after nutrient enrichment (Fig.4 4). The grass C : N ratio was significantly lower in the N and NP treatment s (<20) than in the control and P t reatments (>20; F=45; P <0.0001; Fig. 4 4A ). The grass C : P ratio was lower in the P and NP treatment s (<200) compared to control and N treatment (>300; F=58; P <0.0001; Fig. 4 4B). The N : P ratio was 18, 25, 7 and 14 for control, N, P and NP treatment respect ively ( F= 51; P <0.0001; Fig. 4 4C). Apparent Nutrient Recovery (ANP), Nutrient Use Efficiency (NUE) and Physiological Nutrient Efficiency (PNE) After nutrient enrichment apparent nitrogen recovery more than double d in the NP treatment compared to the N tre atment. While apparent P recovery of P treatment is 75% of the recovery by the NP treatment (Table 4 5 ). Nitrogen use efficiency was

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87 approximately 3 times higher under NP treatment compared to N treatment, while P use efficiency was negative in the P treat ment and positive (3.5 kg of aboveground biomass per kg of P applied) in NP treatment (Table 4 5 ). The Phosphorus physiological efficiency indicates that application of P alone would lead to reduction of aboveground biomass by 4.1 kg for every unit (kg) in crease of P uptake, an increase of 219 kg of aboveground biomass is expected in NP treatment after a unit (kg) uptake of P. Discussion Grazing Effects on Soil and Plant Nutrient Concentration and Aboveground Grass Production Our observation of decreased a boveground grass biomass (45%) after 17 year of grazing was consistent with (Mbatha and Ward 2010) observation in South Africa semi arid savanna. They were also similar to Singer (2003) study in Rocky Mountain National Park, Colorado and Pucheta (1998) who reported 33% decrease of standing biomass und er grazing in Central Argentina. This was compared with non grazed area that had been excluded from grazing for 2 years. However our findings contrasted the findings by Frank and McNaughton (1993) who reporte d 47% increase in aboveground grass biomass in grazed plots in Yellowstone, National Park Wyoming USA Note that this is a temperate area and grazing only occurred over winter season (7 months). Unlike in our study in savanna which indicated lower grass bi omass production in the grazed plots and grazing occurred throughout the year. Our study show that the grazed plots had 55% aboveground biomass of the biomass in the ungrazed plots; this was much higher co mpared to the Cui et al (2005) data indicating that grazed plots had 21 35% aboveground biomass of the ungrazed plots after 25 years of herbivore exclusion in Mongolia. However, in a degraded/overgrazed site where herbivores had been

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88 excluded for 10 years Cui reported that the grazed site biomass was 68 8 2% of the ungrazed site. This suggests that the period of herbivore exclusion, quality of the grass and the intensity of grazing determine the quantity of the standing aboveground biomass (Pandey and Singh 1992). Our findings of decreased aboveground biom ass N in the grazed plots was similar to the findings in Rocky Mountain National Park Colorado but contrasted the findings in Yellowstone National Park in Wyoming which indicated increased aboveground biomass N in grazed plots (Singer 2003), suggesting tha t the herbivores densities and rates of plant consumption determine whether there will be accelerating or decelerating nutrient cycling. However, the findings by Singer (2003) of increased soil N in Yellowstone National Park Wyoming and a decline of the sa me in Rocky Mountain National Park contrasted our findings of no significant difference between the grazed and ungrazed plot soil N. Our study findings indicated no significant difference in foliar N concentration between grazed and ungrazed plots which c ontrasted (Coughenour 1991; Singer 2003; Turner et al. 1993) findings. However, in our study there was a trend of increasing foliar N concentration in the grazed grass. Grazing increase soil N through N rich urine and feces (Augustine et al. 2003; Stark et .al 2002) in a more easily decomposable form, which bypass the slow litter decomposition (Coughenour 1991). Hence increasing nutrient uptake by plants (Ruess 1984) and increasing the shoot nitrogen content (Knapp et al. 1999). However, in our study site it is possible that the grazers are mining the nutrients from the grazed plot grasses and depositing them elsewhere, as indicated by a trend of lower soil N concentration in the grazed plots. The grazers feed from one

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89 area and defecate in a different area, mostly in the glades (former cattle kraals) where they spend a lot of time. Hence there is a flow of nutrients from the nutrient poor bushland to nutrient rich glades (Augustine et al. 2003). This flow we believe is from the grazed plot soils to the grass to the grazers and finally most of the nutrients are deposited in the glades. Our observation of higher foliar P in grazed plots was consistent with Turner et al. (1993) in Kansas where herbivores were excluded for 10 years. Our study further indicated h igher SOC in grazed plots, as demonstrated by higher LOI, which suggest that although most of the fecal matter is deposited elsewhere, especially in the glades after feeding, the fecal matter the herbivores deposit s in the process of feeding impacts the SO C storage positively. Our study findings of an increased SOC in the grazed plots compared to ungrazed plots contrasted Thornley et al. (1991) findings that in semi arid tropical savanna grazing decreases soil organic matter levels. Cui et al. (2005) also indicated that in non degraded site where herbivores were excluded for 25 year SOC was not significantly different between grazed and ungrazed plots, however Cui further indicated that in the degraded/overgrazed site where herbivores had been excluded for 10 years SOC was lower in the grazed plots than in the ungrazed plots. The assumption by Grace et al. (2006) that exclusion of herbivores from tropical savanna could increase the carbon storage, need to put into consideration the possibility of long term S OC storage enhanced by the herbivores.

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90 Effects of Nutrient Enrichment on Soil and Plant Nutrient Concentration and Grass Primary Productivity It was evident that the grazed and ungrazed plots soil and biomass parameters were not significantly different aft er nutrient enrichment. This was due to similarity in soil parameters before nutrients enrichment and also the aboveground biomass was clipped to ground level before nutrient enrichment. The foliar N:P ratio suggested that N and P co limit plant productivi ty. But there was a significant increase in foliar N and foliar P with increased soil N and P without significant increase in grass biomas s or biomass C which indicated luxury uptake of nutri ents (O' Halloran et al. 2010). The grasses growing in this ecosy stem could be adapted to growing in nutrients limited enviro nment (Berendse and Aerts 1987) Baligar and Bennett (1986) indicated that for an efficient plant, increased nutrient uptake should translate to increased biomass production, otherwise indicates pl ant inefficiency and a possibility that the plant has slow growth rate. Our observation of increased foliar P with increasing soil P concurred with (Ludwig et al. 2001; Ries and Shugart 2008) observations, while our observation of increased foliar N concen tration with increasing soil N was similar to Ludwig et al. (2001) in Tanzanian savanna but contrasted Ries and Shugart (2008) findings in African woodland savanna in Botswana. Although the NP treatment in our study relatively increased aboveground biomass by +42%, this increase was low compared to Ries and Shugart (2008) +220% aboveground biomass increase in Botswana savanna after applying similar quantities of N and P as in our study, However Ries and Shugart reported that the foliar N concentration in th eir study did not differ significantly from the control, while in our study

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91 there was luxury uptake of both N and P after nutrient enrichment supporting that the luxury uptake of both N and P in our study was a trade off to aboveground biomass production. Our findings of reduced C : N ratio after soil N enrichment contrasted Ries and Shugart (2008) in Botswana savanna. However it is important to note that Ries and Shugart study site was dominated by grass Panicum maximum and had higher rainfall (698mm). While in our study site had lower rainfall and the dominant grasses were Pennisetum stramineum, Lintonia nutans, Themeda triandra, P. mezianum and Brachiaria lachnantha The different grass species in the two sites could be having different physiological respon ses to resources availability and nutrient use efficiency (Zemenchik and Albrecht 2002). The N: P ratio has widely been used as an indicator of nutrient limitations where by the gener al rule is that N: P ratio > 16 indicates P limitati on to plant growth, whi le an N: P ratio < 1 4 indicates N limitation. At N: P ratios between 14 and 16, either N or P can be limiting or plant growth is co limited by N and P together (Koerselman and Meuleman 1996). Our st udy findings of 17 18 foliar N: P ratio before nutrient enric hment coupled with relatively higher grass biomass increase in NP treatment (+42%) compared to N (+14%) and P (0%) treatments suggested N and P co limitation to primary productivity. This contrasted Ludwig et al. (2001) observation in Tanzanian tropical dr y savanna that primary production is N limited under open canopy. Effects of Nutrient Enrichment on ANR, NUE and PNE Overall nutrient uptake efficiency has been reported to be about or lower than 50% for N and less than 10% for P (Baligar et.al 2001). Sny man (2002) findings that N use efficiency was higher in the N fertilization than NP fertilization in South Africa savanna contrasted our study finding of higher N use efficiency in NP than N treatment.

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92 In our study we broadcasted the fertilizer without man ually mixing the fertilizer with the soils. Hence because the root occupies around 1 2% of the soil surface volume and the amount and proportion of added nutrients that reach roots determines the efficiency of nutrient uptake (Baligar and Bennett 1986). Th ere is a possibility that this application method limited the apparent nutrient recovery. Baligar and Bennett (1986) further indicated that nutrient uptake capacity is determined by the ability of the soils to supply and the capacity of plant to uptake, th is is highly influenced by genetic makeup of the plant and interactions with environmental factors example rainfall, solar radiation and temperature as a result influencing the NUE (Baligar et.al 2001). Since NUE depend on the (1) uptake efficiency (acquir ing nutrient from soil), (2) incorporation efficiency (transport to shoots and leaves) and (3) utilization efficiency (Baligar et.al 2001). The increased foliar N and P and unmatched primary production after nutrient enrichment indicate that the species in our study were inefficient in utilizing the added and absorbed nutrients. Summary Grazing improves the quality while reducing the quantity of the aboveground biomass in this ecosystem. However the increased SOC suggests that, in the long term grazing imp roves SOC storage possibly resulting from fecal matter deposition which is likely to partly compensate for the C lost through herbivores consumption of the aboveground biomass. Referencing to our 17 years grazing exclusion experiment, our study indicate th at both grazed and ungrazed plots aboveground biomass, foliar nutrients and soil nutrients will respond similarly upon N and/or P addition (this is based on the assumption of biomass clipped to ground level before addition of the nutrients in grazed and un grazed plots). The luxury uptake of both N and P is a tradeoff to

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93 increasing plant productivity which is likely to compromise plant C storage upon nutrients availability especially under increasing CO 2 It would be paramount for future research to focus on the interactions of the belowground biomass and aboveground biomass under herbivory and nutrient enrichment.

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94 Table 4 1. Foliar nutrient concentration under herbivory and nutrient enrichment. Means of the C, N and P concentration between grazed and ungraz ed plots before and after fertilization. Before fertilization the grazed and ungrazed plot means were replicates of twelve (n=12). The fertilization includes addition of N only, P only, N+P and control (where no nutrients were added). After fertilization the grazed and ungrazed plots were not significantly different and hence results were combined for statistical analysis, the means were from 6 replicates (n=6). The data indicate mean SEM. The different letters indicate significant difference between tre atments means at P<0.05. C N P g kg 1 Herbivory (Before fertilization) Grazed 4023.7 a 9.70.5 a 0.570.01 a Ungrazed 3936 a 9.40.3 a 0.520.02 b t ratio 1.2 0.54 5864 P value 0.4 0.54 0.0002 After fertilization Control 3963.4 a 18.51 b 10.03 b N 3941.2 a 251 a 10.05 b P 3923.7 a 170.6 b 2.50.3 a NP 3982.7 a 280.8 a 2.10.1 a F ratio 1 46 55 P value 0.425 <0.0001 <0.0001 Table 4 2. Grass biomass and biomass nutrients under herbivory. Means of grass biomass, biomass nutrients and ratios between grazed and ungrazed plots before fertilization. The mean were from 12 replicates, the data indicate mean SEM. The different letters indicate significant difference between treatments means at P<0.05. Aboveground C P N C : N C : P N : P Bi omass kg ha 1 Ratio Grazed 2100212 a 85084 a 1.20.1 a 253 a 422 a 70010 a 170.83 a Ungrazed 3810417 b 1500148 b 2.00.3 b 465 b 421 a 75038 a 180.84 a t ratio 31 21 22 31 0.04 14 0.9 P value 0.03 0.04 0.04 0.03 0.85 0.06 0.39

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95 T a ble 4 3. Soil parameters under herbivory before nutrient enrichment. Means of the soil parameters between grazed and ungrazed plots before fertilization. There were 12 replicates for each treatment mean. The data indicate mean SEM. The differen t letters indicate significant difference between treatments means at P <0.05 Extractable LOI C N P K P NH 4 + N % g kg 1 mg kg 1 Grazed 15.60.3 a 211 a 2.00.1 a 1385 a 83250 a 5.81 a 5.90.3 a Ungrazed 150.2 b 241 a 2.20.1 a 1422 a 86834 a 6.00.4 a 6.10.3 a t ratio 83 1.5 1.5 0.2 2.6 0.4 0.2 P value 0.012 0.35 0.34 0.70 0.25 0.61 0.68 NO 3 N Ca Mg Fe Al Extractable ( m g kg 1 ) Grazed 5.31 a 4085200 a 87925 a 293 a 47221 a Ungrazed 5.50.7 a 3955190 a 89217 a 404 b 53017 a t ratio 0.3 0.34 0.6 59 5.6 P value 0.8 0.61 0.52 0.017 0.14

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96 Table 4 4. Soils parameters after nutrient enrichment. Both grazed and ungrazed were statistically analyzed together because they were not significantly different. There were 6 replicates for each treatment mean. The data indicate mean SEM. The dif ferent letters indicate significant difference between treatments means at P <0.05 Table 4 5. ANR, NUE and PNE after nutrient enrichment. Appa rent nutrient recovery (ANR) (%), Nutrient use efficiency (NUE), (kg of grass biomass produced for each unit of applied N or P), Physiological nutrient efficiency (PNE) (kilograms of grass increase per kilogram of increase in nutrient taken up). The values were calculated from the means as indicated in the method section C N K Ca Mg Fe Al g kg 1 mg kg 1 Control 222 a 2.10.2 a 78848 a 3509287 a 80429 ab 284 a 42811 a N 191 a 2.30.1 a 71434 a 3160386 a 74445 b 332 a 43545 a P 231 a 2 .20.1 a 82864 a 3801193 a 81346 ab 303 a 47335 a NP 212 a 2.250.1 a 88332 a 4185215 a 85828 a 255 a 50818 a F ratio 1.5 0.1 2 3.12 3.6 1.49 2 P value 0.27 0.77 0.16 0.06 0.045 0.27 0.156 Treatment ANR (%) NUE PE Nitrogen N 3.8 0.6 14.5 NP 8.9 1.7 20 Phosphorus P 1.2 0.05 4.1 NP 1.6 3.5 219

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97 Figure 4 1. Soil nutrients before and after nutrient e nrichment. A) Total P. B) Available P. C) Nitrate N. D) Ammonium N before and after fertilization. Bars represent mean and error bars represent SEM. Means are from six replications. The different letters indicate significant difference between treatments f or before and after fertilization means at P<0.05.

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98 Figure 4 2. Soil C:N and C:P ratio after nutrient enrichment. A) Soil C:N ratio B) Soil C:P ratio. Bars represent mean values and error bars represent SEM from six replications. The different lette rs indicate significant difference between treatments means at P<0.05.

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99 Figure 4 3. Grass biomass and nutrient s storage after nutrient enrichment. A) Above ground biomass. B) Biomass C. C) Biomass N. D ) Biomass P in a hectare of land after fertiliz ation. Bars represent mean and error bars represent standard error of mean from six replications. The different letters indicate significant difference between treatments means at P <0.05

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100 Figure 4 4. Foliar C:N, C:P and N:P ratio after nutrient enrichm ent. A) Carbon/Nitrogen ratio. B) Carbon/Phosphorus ratio. C) Nitrogen/Phosphorus ratio after fertilization. Bars represent the mean and error bars represent SEM from six replications. The different letters indicate significant difference between treatmen ts means at P<0.05.

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101 CHAPTER 5 NUTRIENT PRODUCTION BY SELECTED LARGE HERBIVORES IN LAIKIPIA, KENYA Background In Africa tropical savanna, manure deposition by large herbivores influences carbon dynamics and soil nutrient cycling (McNaughton et al. 1997). Plant primary productivity is dependent on soil nutrient availability, which in turn influences the availability and quality of forage for herbivores. It has been suggested that protection of savanna from herbivores (or a decline in herbivore density) cou ld result in a larger carbon sink in vegetation (Grace et al. 2006; Tanentzap and Coomes 2012). However, such suggestions do not consider the contribution made to carbon and nutrient cycling by large herbivores. Africa has relatively intact communities o f large herbivores compared to other continents, the majority of which are found in savanna ecosystems (Fritz and Duncan 1994). In many savanna regions, natural herbivores co exist with low intensity cattle grazing. Within the natural herbivore community, tree grass mixtures in savanna ecosystems allow the co existence of grazers, browsers, and mixed feeders (Scholes and Archer 1997; Goheen et al. 2010). Large herbivores have different feeding habits, allowing the partitioning of feed resources. For examp le cattle and common zebras are classified as grazers (Grunow 1980; Moehlman et al. 2008), elephants and impala as mixed feeders (Copeland et al. 2009; Codron et al. 2010), and camels and giraffes as browsers (Codron et al. 2006; Infonet biovision 2010). T he one humped camel is common in the African savanna and is useful for meat and milk production as well as transportation (Coughenour et al. 1985; Reid et al. 2004). Mixed feeders predominantly

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102 graze in the wet season and browse in the dry season (Codron e t al. 2010; Kos et al. 2011). Factors that influence the herbivores feeding habits and or quality and quantity of manure produced include body size (Edwards 1991; Liebenberg 2010; Kleynhans et al. 2011), digestive system efficiency (Edwards 1991), gut fee d retention time (Bell 1971), condition and availability of pasture (Edwards 1991) and season of the year (Liebenberg et al. 2010). Elephant and zebra are non ruminants, monogastric species and hindgut fermenters and their manure has a coarse texture (Bell 1971; Paetel 2001). Giraffe, cattle, camel and impala are ruminants and their manure has a fine dough like texture that can be a pat or pellet (Paetel 2001). The gut capacity of mammalian herbivores increases in proportion to body weight. Ruminants have a more efficient digestion system (Demment and Van Soest 1985) compared to non ruminants (Edwards 1991) of the same size, with the capacity to digest cellulose and chemically protected proteins (Demment and Van Soest 1985). Ruminants have a slower passage o f feed through the gut and select for high protein plant components to ensure efficiency of extraction and utilization of protein. In contrast, non ruminants have a strategy of high rates of food intake and processing to compensate for their less efficient digestion process (Bell 1971). In the savanna, herbivore grazing accelerates nutrient cycling and promotes soil fertility and plant productivity (Grant et al. 1995) compared to the relatively slow recycling of nutrients through litter deposition (McNaught on et al. 1988; Bardgett et al. 1998). Differences in nutrient concentrations in manure from different animals, coupled

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103 with the roaming nature of the animals in the savanna, have the potential to influence nutrient redistribution in the landscape (Augusti ne 2003). Previous studies have reported concentrations of major plant nutrients (especially nitrogen and phosphorus) in manure from large herbivores in savanna ecosystems of South Africa, using stable isotopes to identify dietary preferences of herbivore species (Botha and Stock 2005; Codron D. and Codron J. 2009). However, limited information is available on potential turnover of macro elements (C, N, P and K) and secondary nutrients from large herbivores to savanna ecosystems in East African savannas. T he objectives of this study were (1) to use stable isotope ratios to determine the feed preferences of large herbivores in the savanna ecosystem of Laikipia, Kenya (2) quantify concentrations of macro elements and secondary nutrients in manure produced by grazers, browsers, and mixed feeders, and (3) estimate annual nutrient inputs from large herbivore manure for the entire Laikipia district. Materials and Methods Study Site The study site is located on a semi arid savanna at Mpala Research Centre and Cons ervancy covers 190 km 2 of savanna in the Laikipia district of Kenya (37 o E, 8 o N; 1800 m elevation). protected areas (Young et al. 1998). Mpala Research Centre is managed for both live stock production and wildlife conservation. The wild animals and livestock co exist and share resources. Some of the resident wild large herbivores include elephants, hartebeests ( Alcelaphus buselaphus ), giraffes, buffaloes ( Syncerus caffer ), Grant gazell es ( Gazella grantii ), zebras, impala and elands ( Taurotragus oryx ) (Young et al.

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104 1998). The large livestock herbivores are mainly cattle ( Bos taurus ) and camel ( Camelus dromedaries ). Soils in the study area include black cotton soils (Vertisols) and red s andy loams (Alfisols ) (Ahn and Geiger 1987). Black cotton soils cover approximately 43% of the Laikipia ecosystem (Young et al. 1998). At Mpala Research Centre whistling thorn Vachellia (Acacia) drepanolobium is the dominant tree species on black cotton soils, accounting for 97% of the overstory, while > 90% of the understory consists of five grass species and two forb species (Young et al. 1998). Vachellia (Acacia) etb aica and Senegalia (Acacia) mellifera are the dominant trees in the red sandy loams (Pringle 2008). Augustine and McNaughton (2004) indicate that Mpala Research Centre landscape contains a two phase vegetation mosaic, whereby 1% of the landscape is covered by short grass glades with no woody vegetation, embedded within a background that is dominated by the Acacia community. There is a trimodal rainfall pattern, with a l ong rainy period between April and June and smaller pulses of rain in August and October (Augustine 2010), with a predictable dry season between January and March (Pringle 2008). Most of the wild animals roam along the Laikipia and the larger Ewaso Nyiro e cosystem tracking the forage at different times of the year. Manure Sampling and Analysis Manure sampling was conducted in February 2011. The study was carried out in a dry season (February 2011) to allow conservative estimation of nutrients production (M igingo Bake and Hansen 1987; Grant et al. 1995). Security guards at Mpala Research Centre were involved on the identification of the animal species location at a given time, which helped to avoid repeated sampling; a unique structure/feature of the herd wa s also recorded to help differentiate the herds. Manure was sampled from

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105 adult/weaned animals to ensure exclusion of suckling juveniles (Wrench et al. 1996). This entailed following the selected large herbivores in the field as they grazed/browsed and col lecting fresh manure. Manure was collected from 10 animals from each species and composited. A total of six herds were sampled for each species. For elephant and giraffe herds with less than ten animals, manure was sampled from at least 5 or 6 animals. The high cattle population allowed sampling from at least 20 animals per composite sample. Each composite manure sample was collected from animals in the same herd and on the same day. Soils or foreign plant fragments from manure were removed by hand. Twelve grass composite samples and nine Acacia leaf composite samples were also collected for determination of isotope signatures. Manure and vegetation samples were oven dried at 60 o C to a constant weight. Samples were then ground and shipped to the University of Florida, Wetland Biogeochemistry Laboratory for chemical analysis. Animal Manure Production in Laikipia Daily manure production by each animal species (Table 5 1) was determined from the literature. A recent animal census in the Laikipia district (Table 5 1), a conservative census (Kinnaird et al. 2010), was used to estimate nutrient production in the region. The total Laikipia area (9600 km 2 ) was used to estimate the annual nutrient production by each herbivore species per square kilometer. The estimate is therefore conservative, considering that the herbivore habitat does not cover the entire Laikipia district. Chemical and Statistical Analysis 13 15 N were determined using a Costech Model 4010 Elemental Analyzer (Costech Analy tical Industries, Inc., Valencia, CA) coupled to an

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106 MAT Deltaplus XL mass Spectrometer via a Conflo II interface (CF IRMS; Thermo Finnigan). Total P was determined by ignition at 550 o C followed by extraction in 1 M H 2 SO 4 acid and detection by automated mol ybdate colorimetry. Digested solutions were analyzed colorimetrically using Shimadzu UV visible recording spectrophotometer UV 160. Extractable calcium, potassium, and magnesium were determined using the Mehlich 1 method (Kuo 1996). Both C : N and C : P ratio were determined on a mass basis. All statistical analysis was conducted using JMP (version 7.02; SAS Institute 2007). Significant differences among the herbivores for all the variables were determined by one way analysis of variance and Tukey HSD test a t 0.05. Results Sources of Feed Materials 13 C signature of Acacia ( F= 2573; P <0.0001) from that of grass leaves ( 15 N signature did not differ significantly between acacia leaves 13 C signature of manure samples differed markedly between browsers and grazers ( F= 158; P <0.0001; Fig. 5 1; Table 5 2). Of the mixed feeders, impala manure was distinct from both graz ers and browsers, with an 13 C signature of 13 C signature of 25.2 impala predominantly grazed, while elephan ts predominantly browsed (Fig. 5 1). 15 N was highest for impala (9.3 0.7 1 2), but differences among species showed no overall trend.

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107 Elemental Content of Manure Manure produced by browsers contained high er carbon and nitrogen conc entrations compared to manure from grazers and mixed feeders (Fig. 5 2A,B ), while manure from grazers contained low carbon and nitrogen concentrations compared to mixed feeders. Manure from impala (mixed feeder) and browsers (camel and giraffe) contained t he highest P concentration ( Fig.5 2C ), while manure from elephant (mixed feeder) and grazers (cattle and zebra) contained the lowest P concentration. Impala, a ruminant species that predominantly grazed in this study, had the highest manure P concentration of any species, (6.6 g P kg 1 ), while cattle, also a ruminant and an all season grazer, had the lowest P concentration (Fig. 5 2C ). The high P concentration in impala manure was striking compared to the low P concentration in elephant manure, considering that both are mixed feeders, although the stable isotope values indicated that impala predominantly grazed while elephant predominantly browsed (see above). Manure from browsers and impala contained significantly lower C : N ratios (< 25) than manure from gr azers and elephants (> 25) ( F= 24; P <0.0001; Table 5 2). Manure from all species contained C : P ratios < 200. Impala and camel manure had significantly lower C : P ratios than other species ( F= 13; P <0.0001). Manure from cattle had the greatest C : P ratio, whi le giraffe, elephant and zebra manures were intermediate (Table 5 2). Impala manure had significantly lower N : P ratio ( P =0.0013), elephant and giraffe had the highest N : P ratio while zebra and camel were intermediate (Table 5 2). Though non ruminants (elep hant and zebra) foraged on different vegetation types as indicated by stable isotope data (Fig.5 1; Table 5 2) their manure had the greatest extractable potassium concentration ( F= 8.4; P <0.0001), while ruminant manure had significantly lower potassium con centrations (Fig. 5 2D ). Manure from

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108 ruminants contained the highest calcium concentrations ( F=16; P <0.0 0 01), while manure from non ruminants contained the lowest calcium concentration (Fig. 5 3A ). Camel manure contained the highest magnesium concentration (6.6 g Mg kg 1 ; P <0.0 0 01), which was more than double that in manure from other species. Manure from grazers contained the lowest magnesium concentrations, while manure from giraffe and mixed feeders contained intermediate magnesium concentrations (Fig. 5 3B ). Manure Nutrient Production The high density of cattle in the Laikipia district resulted in the greatest amount of major elements (C, N, P) and extractable cations (Ca, K, and Mg) cycling through this herbivore (Table 5 3). Cattle, zebra and elephant accounted for significantly higher manure nutrient production than impala, giraffe and camel. On average, all six large herbivores populations were significantly different in production of C ( F= 8445; P <0.0001; Table 5 3), N ( F= 746; P <0.0001; Table 5 3), P ( F= 133; P < 0.0001; Table 5 3) and K ( F=212; P <0.0001; Table 5 3). Approximately 5800 kg C km 2 yr 1 was produced, of which 55% was produced by cattle, followed by 24% by elephant, 18% by zebra, 2% by camel, 1% by giraffe, and 0.8% by impala (Table 5 3) Nitrogen production followed a similar pattern, with approximately 210 kg N km 2 yr 1 of total nitrogen in manure (Table 5 3). The total P and extractable K production was approximately 35 kg P km 2 yr 1 and 180 kg K km 2 yr 1 Approximately 93 and 99% t otal P and extractable K production resulted from cattle, elephant and zebra respectively (Table 5 3). There was a significant difference in production of Ca ( P <0.0001) and Mg ( P <0.0001) (Table 5 3) by the six herbivores populations. Approximately 95% Ca a nd 92% Mg were produced

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109 by cattle, elephant and zebra. Thus, cattle produced approximately half of the total annual manure nutrients from the six herbivore populations in Laikipia. Discussion Species Feeding Habits 13 C data supported the expect ation that cattle and zebra grazed, 13 13 C value grazers and range/variation of ( 2 to 4) suggested by Codron D. and Codron J (2009). However, both the impala and elephant the mixed feeders were expected to predominantly browse in a dry season (Codron et al. 2006; Codron et al. 2010; Kos et al. 2011) when the grass quality is low, and there is minimal forage (Augustine et al., 2003) and browse maintains it nutritive value (Grant et al. 1995). Elephant feeding was consistent with this expectation and is consistent with Ihwagi et al. (2011) observation of a negative correlation between rainfall and elephant debarking indices that elephant debarking ranged from no debarking in wet season to complete stem girding in the dry season. But impala remained a mixed feeder over the dry season possibly grazing more than browsing suggesting that impala being a selective feeder (Edwards 1991; Grant et al. 1995) could meet a proportion of its nutrition needs from selectively feeding on grasses on the glades and supplementing this with the browse. Augustine et al. (2003) observed that impala used the glades (abandoned cattle kraals) in the dry se ason when forage availability was at its minimum, bedding on the glade at night and browsing the surrounding shrubs during the day time. 15 N findings concurred with Codron D. and Codron J. (2009) conclusion 15 N is difficult to interpret and also 15 N in the manure of

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110 herbivores is influenced by other parameters apart from isotopic composition of the herbivores diet. However in the contrary Codron D. and Codron J. (2009) suggestions 15 15 N than 15 N in our study. Manure Nutrients Concentration There were marked differences in manure nutrient concentration even between animals with simi lar feeding preferences; except in a few cases. Suggesting that other factors other than feeding preferences influenced the manure nutrient concentration, such factors include digestive system efficiency (ruminant versus non ruminants) and rate of feed pas sage through the gut. Our results suggest that the high C concentration in the browsers manure is due to feeding on woody species (Codron et al. 2006) with high lignin content and other complex carbon compounds, however the elephant a non ruminant and a m ixed feeder, which predominantly browsed in our study was an exceptional, with lower C concentration and contrasted (Anderson and Coe 1974) findings in Tsavo National Park who reported 498 g C kg 1 supporting that, the amount of C produced depended on qua lity of forage consumed, on the other hand though the browsers and elephant predominantly browsed in this study, there is a likelihood that browsers selected for plant parts high in C, N and P while elephant was a non selective feeder. Our findings concu rred with Grant et al. (1995) observation that browsers had the highest manure N, mixed feeders were intermediate and grazers were the lowest. However, although camel, giraffe and elephant browsed in our study elephant produced significant lower N, this co uld be due to its inefficient digestive system (non ruminat)

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111 and non selective feeding habit. But impala which was a mixed feeder and predominatly grazed in our study had higher N production compared to elephant which browsed, indicating that impala select ive feeding trait (Edwards (1991) facilitated the high N concentration. Kleynhans et al. (2011) suggest that large herbivores (example elephant) utilize abundant low quality feed while small herbivores (example impala) utilize the scarce high quality feed. This suggests that the quality of feed consumed and digestive system efficiency influenced the quality of manure produced. This was supported by (Bell 1971) who indicated that ruminants (e.g. impala, camel and giraffe) select for high protein plant compo nents to ensure efficiency of extraction and utilization of protein instead of high rate of intake and processing of food, like in the case of elephant (non ruminant). The less than 14 g N kg 1 in grazers manure suggested N deficiency (Wrench et al. 1997). The giraffe manure N in our study (29.4 g N kg 1 ) was much higher than the 20.3 g N kg 1 value reported by Grant et al (1995) and 20.6 g N kg 1 by Codron D, and Codron J, (2009) in Kruger National Park, South Africa. The elephant N production in our study (16 g N kg 1 ) was higher than Anderson and Coe (1974) estimated 13.9 g N kg 1 in Tsavo National Park, Kenya. While Sponheimer et al. (2003) and Codron D, and Codron J, (2009) reported impala manure N at 21 and 20 g N kg 1 respectively, in Kruger National Park, South Africa which is consistent with 22.10.9 g N kg 1 reported in our study, but Botha and Stock (2005) reported impala manure N at 18 g N kg 1 in Hluhluwe Umfolozi Park, South Africa, and Grant et al. (1995) reported 18.7 g N kg 1 in Kruger Nat ional Park, South Africa in a drought year. The variability of quantity of N in a unit of manure of the same herbivore species in different locale indicates that the quality of feed consumed is a major determinant of the quantity of N in the manure. While the

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112 lower N concentration in elephant manure compared to other species that browsed in our study suggested an effect of digestive system efficiency or feeding selectivity. Impala manure had the highest concentration of P in spite of its mixed feeding habit ; this can be attributed to its highly selective feeding nature and ability to obtain a diet adequate in quality and quantity even under adverse condition (Grant et al. 1995). Schryver et al. (1983) findings indicate that both ruminants and non ruminants f eeding on the same type and quality of forage excrete similar amount of P. Manure P is directly proportional to forage P (Dou et al. 2002), these findings supported our study that P excretion is more dependent on quality of forage consumed than the digesti ve system efficiency of the animal. This is supported by higher elephant manure P concentration (3.9 g P Kg 1 ) in the Tsavo National park (Anderson and Coe 1974) compared to our study site (2.5 g P Kg 1 ). Cattle manure contained less than 2 g P kg 1 sugges ting P deficiency in it feeds (Wrench et al. 1997) Browsers and impala has a C : N ratio below 25 which suggest that mineralization of the manure is likely to take place, while the grazers have a C : N ratio above 25 suggesting that immobilization is likely t o take place (Scott et al.1996). However (Anderson and Coe 1974) reported 36.1 C : N ratio for elephant manure which is greater : N ratio, this ratio was influenced by the high elephant manure C (50%) reported by (Anderson and Coe 1974) The C : P ratio for all the species manure was below 200, suggesting that P is not likely to limit mineralization ( Dubeux et al. 2006) The high K production in non ruminants which fed on different vegetation type and are known to have less thorough di gestive system (Edwards 1991), suggest that

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113 because non ruminants utilize faster passage of food through the gut (Bell 1971) which is likely to limit time available for K absorption and hence most of the K is deposited in manure. The non ruminants (zebra and elephant) are also monogastric species and hindgut fermenters (Paetel 2001) which contribute to limited amount of time for K Calcium was significantly higher in the rumin feeding habits; this was consistent with Schryver et al. (1983) findings that non ruminant ungulates absorb a larger portion of forage calcium compared to ruminants. Although camel, giraffe and elephant browsed in our study, camel manure Mg was more than double compared to the other two suggesting that either camel selected for vegetation high in Mg or was inefficient in Mg absorption. Khorasani et al. 1997 suggested that source of the forage can be influential on the site, extent of absorption, manure output and digestibility of nutrients. In this study the animal digestive system efficiency, rate of feed passage through the gut and in some cases quality of feed consumed determined the nutrient concentration in manure. Annual Nutrients Production by the Herbivores Although some animals have high/low concentration of nutrients in manures, the overall impact on nutrient production depends on the abundance of the animals in an ecosystem and the quantity of daily manure pr oduced by the species. We acknowledge estimates and animal population in Laikipia lead to conservative estimation of nutrient production in Laikipia. In addition use of the t otal amount of land in Laikipia district (9600km 2

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114 nutrients production per kilometer squared of land further leads to conservative estimation of nutrient production. Cattle and zebra had the highest population while elephant had lower population but high daily manure production. The three animals had the greatest influence on nutrient production in Laikipia, despite the low nutrient concentration in their manure. Cattle had the greatest contribution in all nutrients production (C, N, P, K, Ca and Mg) in Laikipia district due to the high population, however though elephant population in Laikipia was only 2% of the cattle population; its high daily manure production makes its K and Mg production not significantly different from that of the cattle. The approximately 5800 kg C km 2 yr 1 great impact on the carbon budget in this ecosystem; however this is dependent on the fate of the C after deposition. The production of 210 kg N km 2 yr 1 and 35 kg P km 2 yr 1 by the six herbivores in Laikipia ecosystem are important, because both N and P limit plant productivity in this ecosystem (Augustine et al. 2003). The lower giraffe and camel population and lower impala daily manure production put the three in the lower end of nutrients production in Laikipia district. Our findings agreed with (Augustine 2003) that herbivores influence the distribution of nutrients, in an eco system. Summary The widespread assumption that mixed feeders predominantly browse in a dry season was not supported in our study, the impala remained a mixed feeder in the dry season and grazing more than browsing. This could have been due to availability of relatively more graze in dry season that could meet most of its nutritive requirements especially in the glades. Our study indicate that the feeding habits (grazers, browsers and mixed feeders) is not the only factor influencing the concentration of the nutrients in

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115 browse but had different manure nutrients concentration. Hence feeding habits coupled with digestive system efficiency (ruminant versus non ruminant) and selectiv e feeding trait influenced the manure nutrient concentration. For manure nutrient concentration; browsers manure had the greatest concentration of C and N, browsers and impala P, ruminants Ca, non ruminants K and camel Mg. Large herbivores are important cyclers of carbon and other nutrients in an ecosystem. Despite the concentration of the nutrients in animals manure, the animal population and the quantity of daily manure production are the overall determinants of tem. In Laikipia cattle, elephant and zebra are the key animals influencing the nutrients re cycling. Between 1981 to 2010 elephant population has been increasing by 5% yr 1 zebra population has been increasing by 2% yr 1 and livestock have continuously b een increasing (Kinnaird et al. 2010). If this trend continues in the future this would translate to accelerated nutrients cycling with feedbacks to plant productivity. The future management decision influencing the trend of the both domestic and wild anim nutrients production trend and plant productivity. The manure carbon sequestration and nutrients availability to plants will depend on the fate of the manure after deposition. Hence more research is needed to study the fate of the manure carbon and nutrients after their deposition.

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116 Table 5 1. Laikipia herbivores population and manure production. Means of the selected herbivores population and daily manure production Mean SEM are given. Species Populati on a Manure production Remarks Reference kg animal 1 day 1 Camel 2870 840 2.17 kg Mean weight 527 73 kg Seboussi et al. (2009) Giraffe 1430 270 2.5 kg Calculated from wild giraffe (22 yrs old) in captivity with no supplementation Kearne y (2005) Elephant 2603 452 35 kg Average Owen Smith (1988) Impala 5560 1180 0.54 kg Calculated from dry season feed intake (900 g d 1 ) and 40% digestibility Pietersen et al. (1993) (Cattle 120,000 6000 2.1 kg Average for a 250 kg cow in the dry season ILCA (1993) Zebra 19,780 2290 4.1 kg Abaturov et al. (1995) a from (Kinnaird et al. 2010)

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117 Table 5 2. Manure i 13 C 15 C:N, C: P and N : P ratio. The m eans are from composite samples collected on February 2011 with six replicates (n=6). Mean SEM are given. For each ratio, different letters along the column indicate that, those categories differ significantly at the P<0.05. 13 C 15 N C: N C: P N: P Brows ers Camel 26.30.3 c 5.80.2 b 17.10.5 b 815 c 4.70.2 ab Giraffe 27.10.4 c 6.80.8 ab 16.90.9 b 10911 bc 6.60.9 a Mixed feeders Elephant 25.20.5 c 5.60.4 b 25.50.8 a 16311 ab 6.40.3 a Impala 18.70.8 b 9.30.7 a 17.50.9 b 6211 c 3.50.6 b Grazers Cattle 15.20.2 a 5.70.5 b 29.60.4 a 1856 a 6.30.2 a Zebra 14.90.2 a 7.01 ab 27.12.3 a 14825 ab 5.20.6 ab F 158 4.7 24 13 5 P <0.0001 0.0027 <0.0001 <0.0001 0.0013

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118 Table 5 3. Annual nutrients production in Laikipia district. The means annual nutrients production by each species per km 2 in Laikipia district. Total C, N and P and extractable K, Ca and Mg. The means are from six replicates (n=6). Mean SEM are given. For each nutrient, different letters along the column indicate that, those cate gories differ significantly at P<0.05. The mean annual nutrient production for each species and each nutrient are calculated using the mean of the species population in Laikipia multiplied by the mean of species manure production and the species fecal matt er nutrient concentration. Then the statistical analysis is done to obtain the reported mean. Species Manure Population C N P K Ca Mg kg animal 1 day 1 (mean) Head (mean) kg km 2 yr 1 Camel 2.17 2870 1081 d 6.30.2 d 1.30.1 c 1.10.2 c 30 .4 d 20.1 c Giraffe 2.5 1430 670.7 e 40.2 e 0.60.1 d 0.80.2 c 20.3 e 0.40.1 d Elephant 35 2603 138815 b 551.5 b 90.6 b 513.4 ab 212.3 b 90.5 a Impala 0.54 5560 441.3 f 30.1 f 0.80.1 d 0.60.1 c 10.2 e 0.40.04 d Cattle 2.1 120000 320456 a 1082 a 1 70.6 a 866 a 791.7 a 151.4 a Zebra 4.1 19780 106432 c 405.2 c 82.4 b 374 b 111.1 c 51.1 b F 8445 746 133 212 99 114 P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

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119 Figure 5 1. 15 13 C. The replicates are c omposited from 10 20 samples (n=6) apart from giraffe and elephant, the composite was from atleast 5 6 samples. The following markers indicate the following herbivore species camel, X cattle, Y elephant, + impala, Z zebra.

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120 Figure 5 2. Manure nutrient concentration (total C, N, P a nd extractable K).The means of A) T otal carbon B ) Total nitrogen. C ) Total phosphorus. D) E xtractable potassium; in a kilogram of fresh dried manure. The means are from composite sample s collected on February 2011 with six replicates (n=6) and the error bars represent SEM. For each nutrient different letters above the species category indicate those categories differ significantly at P<0.05. The following letters represent the selected l arge herbivores; Cm camel, Gi giraffe, El elephant, Im Impala, Ct cattle, Ze zebra.

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121 Figure 5 3. Manure extractable nutrients concentration. The means of extractable A) Calcium concentration. B) Magnesium concentration in fresh dried manure. The mea ns are from composite samples collected on February 2011, with six replicates (n=6) and error bars show represent SEM. For each nutrient different letters above the species category indicate those categories differ significantly at the P<0.05. The followin g letters represent the selected large herbivores; Cm camel, Gi giraffe, El elephant, Im Impala, Ct cattle, Ze zebra.

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122 CHAPTER 6 CONCLUSIONS Background African savanna sequesters and stores a substantial amount of carbon (C). Climate change/ variabil ity, land use change, and land management practices have influenced the key ecosystem drivers, and are likely to influence the capacity of C storage by savanna ecosystems. The key ecosystem drivers include: rainfall, herbivory, and soil nutrients. In rec ent years, rainfall variability has increased with extreme cases being reported in different parts of the continent. Nitrogen (N) and phosphorus (P) deposition has been reported from different sources including atmospheric deposition, agricultural pollutio n and animal manure deposition. Major livestock species populations have also been reported to increase (Thornt on 2010) while some wild animal population s are increasing and others decreasing (Kinnaird et al. 2010). All these changes on ecosystem drivers a re likely to impact plant productivity and the capacity of the savanna to sequester and store C as well as cycle nutrients. This dissertation evaluated the effects of rainfall, herbivory and nutrient enrichment on plant productivity nutrient and C storage in East African savanna ecosystems dominated two soil types, Alfisols (red sandy loams ) and Vertisols (black cotton soils). The first objective of this dissertation was to determine the effects of the key ecosystem drivers on plant productivity and carb on storage in the Alfisols of East African savanna. This was achieved through setting up an insitu experiment along a rainfall gradient, to incorporate varying soil moisture regimes, under different herbivory and nutrient enrichment conditions to simulate possible future changes and project the

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123 effects on plant productivity and C storage (Chapter 2). Measurements were taken for plant productivity and carbon and nutrient storage both in plants and soils (Chapter 2). The second objective was to determine the effect of opposing processes; plant production and litter decomposition on carbon storage and N and P release. This was disentangled by conducting nutrient enrichment experiment and litter decomposition subsequently and considering the seasonality and nutr ient enrichment effects on decomposition process. This involved sequential litter biomass and nutrient concentration determination (Chapter 3). The third objective was to determine the plant productivity, C and nutrient storage as influenced by long term gr azing and N and P enrichment in the Vertisols of East African savanna An insitu long term herbivore exclusion experiment was used to determine the herbivory effects on both soils and plant C and nutrients storage. This was followed by N and P enrichment experiment to determine any interaction s between herbivory and increased nutrient availability and their influence on plant productivity and C storage (Chapter 4). carbon and nutri ent production. This was achieved through determination of six herbivores feeding habits, determination of fresh fecal matter nutrient concentration, and use of Laikipia district species population and daily fecal matter production to quantify the C and nu trient production from the fecal matter (Chapter 5). In the subsequent section, each research objective presented in the introduction (Chapter 1) is re evaluated in the context of the new knowledge gained from this research and the four over arching goals are further discussed.

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124 Objective 1: Effects of Herbivory and Nutrient Enrichment on Plant Productivity and C Storage and Nutrient Storage in the Alfisols of East African Savanna In the savanna most of the research on plant productivity and soil C ha ve bee n conducted under herbivory (Thornley et al. 1991: Cui et al. 2005) and a few along a rainfall gradient (Feral et al. 2003) without consideration of interactions among key ecosystem drivers. Therefore, the goal of studying the plant productivity and C stor age along a rainfall gradient as influenced by herbivory and nutrient enrichment was to determine the interactions of the key ecosystem drivers (Chapter 2). This was accomplished through conducting nutrient enrichment experiment Through establishment of p lots with the following treatments: nitrogen only (N), phosphorus only (P), nitrogen + phosphorus (NP) and control (no fertilizer added). This was set up in a herbivore exclusion experiment (grazed and ungrazed) along a rainfall gradient, north (638 mm rai n year 1 ) central (58 3 mm rain year 1 ) south (43 8 mm rain year 1 ) three years average. The herbivore exclosures had been in place for 2 years. Grass biomass and soils were sampled seasonally in all treatments and carbon and nutrient content determined. Results indicated that soil C content and nutrients concentration were not influenced by the current rainfall gradient. Values were lowest at the middle of the rainfall gradient (central site) indicating that soil C and nutrients are not mainly influenced on soil C and nutrients in this study. The results further indicate d that after nutrient enrichment (1 st wet season) the grass biomass was lowest in the north site (510111 k g ha 1 ), while south (1190177 kg ha 1) and central (1140240 kg ha 1 ), had significantly higher grass biomass with a similar trend for biomass C, N and P. Despite, the fact that south had higher rainfall than central site and central had similar rainfall amount s with

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125 north, in the 1 st wet season After nutrient enrichment there was a significant effect of herbivory by site interaction on foliar N concentration. Foliar N for grazed plots was lowest in wettest site (south) while ungrazed plots had lower foli ar N in the middle of rainfall gradient (central). Nutrient enrichment treatment significantly increased grass biomass in N (1100218 kg ha 1 ) and NP (1150303 kg ha 1 ) but not in P (780198 kg ha 1 ) and control (620130 kg ha 1 ) treatment, though soil ava ilable P more than doubled in P and NP treatments on nutrien t enrichment. The foliar N: P ratio was < 14 in all treatments and all seasons. Herbivory by nutrient enrichment interaction effects on foliar P concentration indicated in ungrazed plots, P treatme nt had a significant effect on foliar P. However there was no increased foliar N with increased soil N. But in the second wet season (May 2011) despite nutrient enrichment there was no plant biomass response amidst insufficient rainfall in all sites. Plant productivity did not follow rainfall gradient but increased under N and NP enriched conditions in equal quantities. This suggests that N limit plant productivity in Alfisols when rainfall is above average. However, in the 2 nd wet season when rainfall was extremely low plant productivity was limited in all sites despite nutrient enrichment. Nitrogen enrichment had a positive effect on biomass production, indicating that future increase in soil N will have a great impact on plant productivity in this ecosyst em if no other factor limits plant productivity. Objective 2: Litter Decomposition and N and P Release in Alfisols Savanna Ecosystem Grass litter decomposition is a key component of the savanna C budget (Couteaux et al 1995) and a source of soil nutrient s (Aerts et al. 1992). Savanna plant productivity is co limited by nitrogen (N) and phosphorus (P) (Augustine et al. 2003;

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126 Ries and Shugart 2008).But little is known about N and P influence on litter decomposition. Most of the decomposition studies in Afri can savanna ecosystem are mainly on legumes forbs and trees ( Fosu et al. 2007; Fornara and Du Toit 2008; Oladoye et al. 2008) and a few on grasses ( Ohiagu and Wood 1979; Deshmukh 1985). Although the litter fall occurs in the dry season, many of the decompo sition studies were initiated in a wet season ( Mugendi and Nair 1997; Mugendi et al. 1999 ). The literature data primarily represents wet season effects on decomposition and nutrient release, thus limiting interpretation of the data on annual basis. Ve ry little is known about the influence of N and P enrichment on grass primary productivity and decomposition patterns in Alfisol savannas of Africa. The goal of this study was to determine the influence of rainfall and nutrients (N addition, P addition, an d NP addition) on plant productivity and litter decomposition. This was achieved through conducting an insitu litter decomposition study in exclosure s that excluded all herbivores larger than hare (~2 3kg) (Chapter 3). The decomposition study was conducted for a period of 20 weeks (December 22/2010 May 11/2011). The experiment was initiated in a dry season to simulate natural litter fall. A biphasic decomposition pattern was evident; slow decomposition rates were typical in dry season (1 to 2% wk 1 ) and w ere significantly faster in the wet season (7 to 11% wk 1 ). Higher decomposition rates occurred for N enriched grasses (11% wk 1 ) in the wet season, although initial N concentration was not significantly different between treatments, indicating that factor s other than litter N concentration influenced decomposition rates. The decomposition constant was positively correlated with rainfall (r 2 = 0.8). In addition the results indicate that N and P release were greatest during the dry season and did not match th e C loss,

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127 suggesting that both N and P release were not mainly influenced by a microbial process In the first two weeks of the decomposition study which was in a dry season the control, N, P and NP treatments had released 450 g N kg 1 480 g N kg 1 410 g N kg 1 ,550 g N kg 1 respectively and 490 g P kg 1 610 g P kg 1 580 g P kg 1 700 g P kg 1 .The C:N and C: P ratios increased over time where immobilization was expected, suggesting that N and P release was not mainly influenced by a microbial process Re sults indicate that 68 75% of N and 73 83% of P biomass had been released by the end of the 20 week study. Nitrogen enrichment and seasonal rains accelerated litter decomposition rates. N itrogen and P release was more prevalent in the dry season, with more % P released than % N. Objective 3: P lant Productivity and Carbon Storage as Influenced by Long Term Grazing and Short Term Nutrient Enrichment in the Vertisols of East African Savanna Herbivory and soil nutrients are major ecosystem drivers in the Afric an tropical savanna with major influence on the C storage. The influence of herbivory on C storage in aboveground biomass and soil is region/ecosystem specific and also influenced by management practices, with different authors reporting positive, negative or no impacts for example (Thornley et al. 1991; Pandey and Singh, 1992; Wilsey et al. 2002; Bagchi and Ritchie, 2010). It has been suggested that protection of savanna from herbivores could result in a larger C sink in vegetation (Grace et al. 2006; Tane ntzap and Coomes 2012). However, such suggestions have not considered the effects of herbivory on soil organic carbon (SOC). On the other hand plant productivity is N and P co limited (Augustine et al. 2003; Ries and Shugart 2008). The availability of limi ting nutrients is expected to increase plant productivity, if no other factor limits plant productivity (Lebauer 2008). However, very little is known about interactive effects of nutrients

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128 availability and herbivory on plant productivity. There were two sp ecific objectives for this study The first objective was (a) to determine the effects of long term grazing enrichment on the grasses aboveground biomass and soils carbon and nutrients storage. This was achieved through using a long term herbivore exclusion experiment in Vertisols (Chapter 4). The experiment had excluded the herbivores for 17 years. Only exclosures excluding all herbivores were used (ungrazed) and the plots where all herbivores were allowed in (grazed). The soils and grass aboveground biomas s w ere sampled and nutrients concentration and biomass weights determined. Grazing caused a marked reduction in aboveground biomass (45%) and associated tissue nutrients compared to ungrazed plots, although foliar P was 20% greater in grazed plots. In addi tion, grazing increased SOC by 4% compared to ungrazed plots. Overall grazing reduced aboveground C storage, but improved soil C stocks and forage quality. The second specific objective was to (b) determine the interactive effects of long term grazing and short term N and P enrichment on plant productivity. This was achieved through using a 17 years old herbivore exclusion experiment. Exclosures excluding all herbivores (ungrazed) were used and the plots allowing all herbivores to graze (grazed). The graze d and ungrazed plots were replicated three times in form of complete blocked design. In each block four 16m 2 nutrient enrichment sub plots were established in grazed and ungrazed plots. Nutrient enrichment inv olved N addition, P addition, NP addition and c ontrol (where no fertilizer was applied) (Chapter 4).The 1m 2 plots within a larger nutrient enrichment (16m 2 ) were clipped to ground level in grazed and ungrazed plots to ensure grass regrowth was harvested after nutrient enrichment. Caging excluded all he rbivores in both grazed and ungrazed plots for the study period.

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129 Results indicated that there was no significant difference in grazed and ungrazed plots response to nutrient enrichment and N and P co limited plant productivity. There was a significant incr ease in biomass N and P after nutrient enrichment without significant increase in grass biomass or biomass C. This indicated luxury uptake of nutrient by the grasses suggesting that these grasses are adapted to growing in nutrient limited conditions or the y could be allocating biomass to belowground biomass. Objective 4: Estimation o f Herbivores Ecosystem Services: C arbon a nd Nutrient Production i n Laikipia District, Kenya An outstanding feature of the tropical savanna is the co existence of trees and gra sses ( Sankaran et al. 2005) with seemingly non successional persistence; this is a the co existence of grazers, browsers and mixed feeders (Scholes and Archer 1997; Goh een et al. 2010). It is indicated that grazers and browsers, graze and browse all the time respectively, (Codron et al. 2006; Moehlman et al. 2008) However mixed feeders are thought to be intermediate feeders in the wet season and browse in the dry season due to scarcity of pasture and the low quality feed (Codron et al. 2010; Kos et al. 2011) This has widely been determined in the South African savanna with limited work done in East African savanna. In African savanna a considerable amount of nutrients c ycle through large herbivores. With a distinct feed partitioning between grazers, browsers and mixed feeders (Codron et al. 2006; Moehlman et al. 2008; Codron et al 2010). Yet little is known about the influence of feeding habit on manure nutrient concentr ation in East African savanna. Furthermore, previous studies have indicated that herbivores removal or their population decrease in the savanna would promote plant carbon storage (Grace et al. 2006; Tanentzap and Coomes 2012). However such suggestions

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130 have not considered the ecosystem services provided by the herbivores in terms of nutrients and carbon production in their manure. Part of this carbon can persist in soil for decades (Augustine et al. 2003). There were three specific objectives for this study. The first specific objective was to (a) use 13 15 N) to confirm the herbivores dietary preference in East African savanna. This was addressed through collecting fresh fecal matter from six selected large herbivores which included grazers (cattle, Bos indicus ; zebra, Equu s burchellii ), mixed feeders (elephant, Loxodonta Africana ; impala, Aepyceros melampus ) and browsers (giraffe, Giraffa c amelopardalis ; camel Camelus dromedaries ) ( C 13 C 15 N. The results revealed that of th e mixed feeders, elephant mainly browsed, while impala mainly grazed in a dry season while for grazers and browsers as expected grazed and browsed respectively. Suggesting that the selective feeding nature of the impala and its small body size, allow it t o obtain adequate feeds through grazing in the dry season. The second objective was to (b) determine the manure nutrient concentration as in fluenced by the feeding habits. N u trient concentrations in manure from the six selected large herbivores were measu red There were marked differences in manure nutrient concentrations among animals, despite sometimes similar feed preferences. For example, manure from browsers contained higher concentrations of total carbon (450 490 g C kg 1 ) and total nitrogen (26 29 g N kg 1 ) than grazers (330 340 g C kg 1 and 11 13 g N kg 1 ) or mixed feeders (380 400 g C kg 1 and 16 22 g N kg 1 ). In contrast, total phosphorus was highest in manure from impala and browsers (4.7 6.6 g P kg 1 ) than from grazers and elephants (1.8 2.6 g P kg 1 ). For extractable base cations,

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131 potassium concentrations were higher in manure from non ruminants (12 14 g K kg 1 ) compared to ruminants (5 9 g K kg 1 ), while calcium concentrations were higher in manure from ruminants (8 13 g Ca kg 1 ) than non rumin ants (3.6 6 g Ca kg 1 ). Overall manure nutrient concentrations are not wholly dependent on herbivore feeding habits. They are also influenced by other factors for example digestive system efficiency, selective feeding and rate of feed passage through the g ut. The third specific objective was to (c) quantify the amount of C and nutrients produced by the six selected large herbivores in Laikipia. This was achieved through measuring nutrient concentrations in manures from large herbivores in Laikipia, Kenya. T he animal densities in Laikipia and daily fecal matter were used to estimate the amounts of manure nutrients produced by the selected six herbivores in an annual cycle (Chapter 5). Herbivores included grazers (cattle and zebra), mixed feeders (elephant and impala) and browsers (giraffe and camel ) Results indicated that when calculated across the entire 9600 km 2 Laikipia region, large herbivores produced 5800 kg C km 2 210 kg N km 2 and 35 kg P km 2 annually. Changes in animal populations could have impor tant consequences on nutrient cycling in savanna ecosystems. Further research is required to determine the fate of manure C and nutrients after deposition. Synthesis and Future Research This body of research addressed the objectives presented in the introd uction. The key ecosystem drivers were identified to be important in influencing plant productivity and C storage. However, there was no direct relationship between long term rainfall gradient and soil C. There was also no direct relationship between rain fall gradient and plant productivity during the study period in the 1 st wet season in the A lfisols (chapter 2; Fig. 6.1). But, N enrichment increased plant biomass in the 1 st wet

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132 season and no grass regrowth in the 2 nd wet season when there was insufficien t rainfall. This suggests that above average rainfall grass biomass productio n is limited by N. While, below average rainfall grass production is limited by rainfall even under nutrient enrichment. Though central and north received the same amount of rainf all in the 1 st season it was unclear why central site produced twice the amount of biomass in the north. Considering that there were no interactions between site and herbivory or nutrient enrichment. This suggests there could be other factors at play influ encing plant productivity in this ecosystem ; such factors could include water holding capacity The grass response in both Alfisol s and Vertisols suggest that nutrients limitation vary spatially and the response to nutrients availability is species specifi c. The grasses growing in A lfisols indicated N limitation to plant productivity and both N and NP enrichment increased plant production in similar quantities while P enrichment had no effect but indicated luxury uptake of P (Chapter 2). The grasses growing in V ertisols indicated N and P co limitation, with increased foliar N and P upon nutrient enrichment without significant increase in grass biomass or biomass C (Chapter 4; Fig. 6.1). The luxury uptake of N and P in V ertisols and P in Alfisols is a trade o ff to plant biomass production. The two soil types supported different plant species, and indicated that the species growing in the A lfisols respond positively to the limiting N but the species growing in the V ertisols are adapted to growing in N and P lim iting conditions or could be allocating biomass to below ground biomass. The plant productivity and decomposition processes that oppose each other are key determinants of carbon storage in the savanna ecosystem (Chapter 3; Fig. 6.1). The C storage is depen dent on rainfall and limiting nutrients availability. Nitrogen

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133 promoted plant production and also accelerated litter decomposition rates Hence the future nutrients deposition, may it be from atmospheric deposition, agricultural pollution, fires or herbivo res deposition will have a great impact on savanna C storage. However, though N enriched grasses had higher decomposition rate than other treatments, there was no significant difference in N concentration across the treatment, suggesting that other than N concentration there were other factors driving decomposition rate under N fertilization that need further investigation. This research also filled some research gaps, for example the litter decomposition study that simulated the natural litter fall in a d ry season indicated that most of the N and P release occurs in the dry season, a nd more %P than %N is released. M ore research is required to determine the factors driving the N and P release in a dry season. Herbivores ecosystem services have both direct and indirect influence on carbon storage. Although long term grazing reduces the plant biomass by 45% in the V ertisols, grazing improved the SOC which could partially compensate for the lost plant carbon through herbivory ( C hapter 4; Fig 6.2). In addition herbivore fecal matter deposition is a source of nutrients that is likely to influen ce both plant productivity and organic matter decomposition with indirect influences on C storage. Herbivore fecal matter is also a direct source of soil C, of which a frac tion of the C can persist in the soil for decades (Chapter 4 and 5: Fig 6 1, 2). The herbivores fecal matter nutrients is not d ependent on the feeding habits but are influenced by other factors which include digestive system efficiency, selective feeding, and rate of feed passage through the gut. T he changes in the species population in the savanna will have a great impact on nutrients cycling and

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134 C storage. However, since this dissertation determined the production of nutrients and C in fresh fecal matter, further research is needed to determine the fate of the nutrients and C after deposition. I suggest the next biogeochemical research in savanna should focus on: (1) the inter linkages between the aboveground biomass and belowground biomass and the respon se of the belowground biomass to herbivory, nutrient enrichment and rainfall gradient; (2) the role of photo degradation on litter decomposition, given than African savanna experience long dry and hot season with limited rainfall, (3) determination of th e fate of both manure C and nutrients in the long term after deposition, to determine the C sequestration potential and the nutrients availability, and (4) determination of the processes involved in the release of the litter nutrients in the dry season whe n there is minimal rainfall.

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135 Figure 6 1. Conceptual diagram with main effects results in the A lfisols.

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136 Figure 6 2. Conceptual diagram with main effects results in the Vertisols.

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137 APPENDIX A DIAGRAM OF THE STUDY SITE AND HERBIVORY EXPERIMENTA L DESIGN IN THE ALFISOLS Figure A 1. Herbivory experimental design in the Alfisols. The sites along the rainfall gradient included north (438 mm rainfall), central (538 mm rainfall), and south (638 mm rainfall), three years mean. The herbivory treatme nts included LMH; all herbivores greater than ~2 3 kg excluded, MESO; herbivores greater than dik dik excluded, MEGA; giraffe and elephant excluded and CNT; all wild herbivores allowed in. This study was conducted in LMH (ungrazed) and CNT (grazed) (Diagra m; by Pringle et al unpublished).

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138 APPENDIX B DETAILED HERBIVORY EXPERIMENTAL DESIGN IN THE ALFISOLS Figure B 1. Detailed layout of herbivores exclusion treatments in the Alfisols. Herbivory treatments; include (A) LMH; all herbivores greater than ~2 3 kg excluded, (B) MESO; herbivores larger than dik dik excluded, (C) MEGA; giraffe and elephant excluded and (D) CNT; all wild herbivores allowed in. This study was conducted in LMH (ungrazed) and CNT (grazed). There were three replications in form of blo cks.

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139 APPENDIX C NUTRIENT ENRICHMENT EXPERIMENTAL DESIGN IN THE ALFISOLS Figure C 1. Nutrient enrichment e xperimental design in Alfisols. Each of the grazed and ungrazed plots was 1ha.The nutrient enrichment experiment was set up on the periphery of the 1ha plot, on the outer 20m. Each nutrient enrichment plot was 16m 2 ; the nutrient enrichment treatments entailed adding N only, P only, N+P and the control (where no nutrient was added). At the middle of the 16m 2 a 1m2 sub plot was used for aboveground biomass sampling. In both grazed and ungrazed plots nutrient enrichment was replicated three times in form of complete randomized blocks.

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140 APPENDIX D: KENYA LONG TERM HERBIVORE EXCLUSION EXPERIMENT (KLEE) IN THE VERTISOLS Figure D 1. Herbivory experi mental design in the Vertisols. The herbivore exclusion experiment was set up in 1995 and excluded herbivores in six different categories. This was replicated in form of three blocks and each herbivory treatment was 4 ha. The experiment included (1) Contro l; all large herbivores excluded, (2) C; cattle allowed in (3) W; elephant, giraffe and cattle excluded, (4) MW; cattle excluded (5) WC; giraffe and elephant excluded (6) MWC; all herbivores allowed (Site map by; Young et al. unpublished ).

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141 APPENDIX E DIAGRAM OF THE HERBIVORY TREATMENT S IN THE VERTISOLS Figure E 1. Layout of detailed herbivores exclusion experiment in the Vertisols. The experiment included (1) Control; all large herbivores excluded, (2) C; cattle allowed in (3) W; elephant, giraffe and cattle excluded, (4) MW; cattle excluded (5) WC; giraffe and elephant excluded (6) MWC; all herbivores allowed. The study was conducted in the control (ungrazed) and MWC (grazed) where all herbivores were allowed in.

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142 APPENDIX F: NUTRIENT ENRICHMEN T EXPERIMENTAL DESIGN IN VERTISOL Figure F 1. Nutrient enrichment experimental design in the Vertisols. Each of the grazed and ungrazed plots was 4ha.The nutrient enrichment experiment was set up on the periphery of the 4ha plot, on the outer 20m and i n the same 0.25ha grid which was selected randomly. Each nutrient enrichment plot was 16m 2 ; the nutrient enrichment treatments entailed adding N only, P only, N+P and the control (where no nutrient was added). At the middle of each of the 16m 2 a 1m 2 sub pl ot was used for aboveground biomass sampling.

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153 Vitousek PM, Turner DR, Parton WJ, Sanford RL (1994) Litter decomposition on the Mauna Loa environmental matrix, Hawaii patterns, mechanisms, and models. Ecology 75 : 418 429 Walker BH Knoop WT (1987) The response of the herbaceous layer in a dystrophic Burkea Africana savana to increased levels of nitrogen, phosphate and potassium. Journal of the Grassland Society of Southern Africa 4: 31 34 oran LR, Caylor K Macko S (2010 ) Combined effect of soil moisture and nitrogen availability variations on grass productivity in African savannas: The case of the Kalahari Transect. Plant and Soil 328: 95 108 White JR, Reddy KR (2000) Influence of phosphorus loading on or ganic nitrogen mineralization of Everglades soils. Soil Science Society of America Journal 64:1525 1534 Williams CA, Hanan NP, Neff JC, Scholes RJ, Berry JA, Denning AS, Baker DF (2007) Africa and the global carbon cycle. Carbon Balance Management 2:3. do i:10.1186/1750 0680 2 3 Wilsey BJ, Parent G, Roulet NT, Moore TR, Potvin CP (2002) Tropical pasture carbon cycling: relationships between C source/sink strength, above ground biomass and grazing. Ecology Letters 5: 367 376 Witkamp M, Olson JS (1963) Break down of confined and nonconfined oak litter. Oikos 14 :138 147 Wrench JM, Meissner HH, Grant CC, Casey NH (1996) Environmental factors that affect the concentration of P and N in faecal samples collected for the determination of nutritional status. Koedoe 39:1 6 Wrench JM, Meissner HH, Grant CC (1997) Assessing diet quality of African ungulates from faecal analyses: The effect of forage quality, intake and herbivore species. African protected area conservation and science 40:125 136 Young TP, Okello B, Kiny ua D, Palmer TM (1998) KLEE: A long term multi species herbivore exclusion experiment in Laikipia, Kenya. African Journal of Range and Forage Science14:92 104 Young TP, Patridge N, Macrae A (1995) Long term glades in acacia bushland and their edge effects in Laikipia, Kenya. Ecological Applications 5: 97 108 Zemenchik RA, Albrecht KA (2002) Nitrogen use efficiency and apparent nitrogen recovery of Kentucky bluegrass, smooth bromegrass, and orchard grass. Agronomy Journal 94:421 428

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154 BIOGRAPHICAL SKETCH Lucy Ngatia grew up in Kenya with her parents Jackson Ngatia and Beatrice Ngatia and her brothers Patrick Ngatia, Joseph Ngatia, Davidson Ngatia and James Ngatia. When growing up she observed the dramatic environmental degradation that occurred in her home country. This triggered the desire in her to focus on environmental protection and conservation as a career. In pursuit of her passion in environmental protection and conservation she enrolled at the University of Nairobi. She pursued her e in Range Management and Master in Range Management (Ecology Option) under the mentorship of Dr. Jesee Theuri Njoka who further gave her good degree she was awarded a Smiths onian Tropical Research Institute Fellowship from She developed a keen interest on climate science, climate change mitigation and e she founded the Nanyuki Dairy lighting as a way of mitigating climate change. She was later offered an assistantship position at the University of Florida to pursue a doct orate in soil science under the mentorship of Dr. Ramesh Reddy and Dr. Benjamin Turner. Following her passion for climate science, climate change mitigation and adaptation, her PhD dissertation focused on grass productivity, carbon and nutrient storage as influenced by nutrients, rainfall and herbivory in East African savanna. This research was externally funded by both Smithsonian Tropical Research Institute and National Centre of Competency in Research North South, Switzerland. She hopes to carry on this line of research in future. During her field work she gave talks to NGOs and

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155 schools motivating students and teaching the community on climate change mitigation. She received her PhD from the University of Florida in the fall of 2012. d her career to studying and researching on climate science, climate change mitigation and adaptation. Working closely with the community, students, managers and policy makers to facilitate the best management practices implementation.