SAVANNA VEGETATION DYNAMICS AND THEIR INFLUENCE ON LANDSCAPE-SCALE C, N, AND P BIOGEOCHEMISTRY A Dissertation by YONG ZHOU Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Thomas W. Boutton Co-Chair of Committee, X. Ben Wu Committee Members, Jason B. West Frank M. Hons Head of Department, Kathleen Kavanagh May 2018 Major Subject: Ecosystem Science and Management Copyright 2018 Yong Zhou ABSTRACT Recent global trends of woody encroachment into grass-dominated ecosystems have substantially altered soil biogeochemical cycles. However, previous studies were mostly conducted at the ecosystem level and results were not spatially-explicit. Meanwhile, most of these studies considered only surface soils, and did not assess the extent to which biogeochemical cycles in subsurface soils may be altered by woody encroachment. In this dissertation, spatially-specific soil samples were taken to a depth of 1.2 m across a 160 m × 100 m subtropical savanna landscape which has undergone the encroachment by Prosopis glandulosa and other woody species in southern Texas, USA. Soil samples were analyzed for root biomass, soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), soil δ13C, and soil δ15N. 13 Soil δ C values throughout the soil profile indicate that this landscape was once primarily dominated by C4 grasses, and woody encroachment has occurred within the past century. Subsurface non-argillic inclusions across this landscape favor the establishment and persistence of large woody patches by enabling root penetration deeper into the profile, providing greater access to water and soil nutrients and thereby regulating vegetation distribution. Woody encroachment increased SOC, TN, and TP throughout the entire 1.2 m soil profile, albeit at different rates. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns following woody encroachment, while TP increased ii slower than SOC and TN in surface soils but faster in subsurface soils. Spatial patterns of soil C: P and N: P ratios were similar throughout the soil profile, but differed from those of soil C: N ratio. Soil δ15N increased with depth, reached the maximum at an intermediate depth, and then decreased in the deepest portions of the profile. Woody encroachment decreased soil δ15N, creating spatial patterns of soil δ15N resembling the spatial distribution of woody patches throughout the soil profile. These results highlight the difference of soil C, N, and P dynamics in response to woody encroachment and the change of mechanistic controls throughout the soil profile, providing valuable insights necessary to developing integrative climate-biogeochemical models that could represent changes in soil biogeochemical cycles following vegetation change. iii DEDICATION To my parents For their unfailing support iv ACKNOWLEDGEMENTS Special thanks to Thomas Boutton for his generous assistance through this Ph. D. program. Despite the passage of time, I can still vividly remember Tom picked me up from the George Bush Intercontinental Airport when I first came to this land on the 6th of August 2013. On the way back to College Station, we had a really nice conversation from ecosystems in Texas to cultures in America. For the past four years, I cannot thank him enough for making me feel so welcome and included here at Texas A&M. He has spent countless hours helping me through this program and offered me on life advice. Thanks for his patience, encouragement, and belief in me. I would also like to thank Ben Wu for serving as co-chair and for his encouragement, provision of resources and wisdom, and ongoing motivation. I appreciate Frank Hons (Department of Soil and Crop Sciences) and Jason West for serving on my Ph. D. program committee. Thanks are extended to my colleagues in the Stable Isotopes for Biosphere Science Laboratory: Ryan Mushinski, Ayumi Hyodo, Yang Zhang, Elizabeth Wilson, Darcy Moreland, Anais Dion, Rachel Adams, Matthew Smith, and Saadat Malghani for their generous help on field/lab work and on my writing and presentations. Thank you to my friends in the Department of Ecosystem Science and Management, to name a few, Michele Clark, Xiangmin Sun, and Cynthia Wright for fun activities and their dependable support. v I am sincerely grateful to David McKown and his wife Stacy for their ongoing hospitality at the Texas A&M AgriLife La Copita Research Area in southern Texas. Thanks for teaching me how to shoot and inviting me to their bay house for fishing. I enjoyed my stay there, even in the face of a damn hot Texan summer. Finally, thanks to my parents and siblings for their unfailing love and support during my stay at Texas A&M. vi CONTRIBUTORS AND FUNDING SOURCES Contributors This work was supervised by a dissertation committee consisting of Dr. Thomas W. Boutton (advisor), Dr. X. Ben Wu (co-advisor), and Dr. Jason B. West of the Department of Ecosystem Science and Management and Dr. Frank M. Hons of the Department of Soil and Crop Sciences. Yong Zhou, Dr. Thomas W. Boutton, and Dr. X. Ben Wu designed this work. Yong Zhou conducted the field work, lab and data analyses, interpreted the data and wrote this dissertation with advice from Dr. Thomas W. Boutton, Dr. X. Ben Wu, Dr. Jason B. West, and Dr. Frank M. Hons. Funding Sources Yong Zhou was supported by a Sid Kyle Graduate Merit Assistantship (2013- 2017) from the Department of Ecosystem Science and Management and a Tom Slick Graduate Research Fellowship (2017-2018) from the College of Agriculture and Life Sciences, Texas A&M University. This work was supported by a Doctoral Dissertation Improvement Grant from the U.S. National Science Foundation (DEB/DDIG1600790), USDA/NIFA Hatch Project (1003961), an Exploration Fund Grant from the Explorers Club, a Howard McCarley Student Research Award from the Southwestern Association of Naturalists, and a Graduate Student Research Mini-Grant from Department of Ecosystem Science and Management, Texas A&M University. vii TABLE OF CONTENTS Page ABSTRACT .............................................................................................................. ii DEDICATION .......................................................................................................... iv ACKNOWLEDGEMENTS ...................................................................................... v CONTRIBUTORS AND FUNDING SOURCES ..................................................... vii TABLE OF CONTENTS .......................................................................................... viii LIST OF FIGURES ................................................................................................... xi LIST OF TABLES .................................................................................................... xvi CHAPTER I INTRODUCTION .......................................................................... 1 CHAPTER II VEGETATION CHANGE AND SOIL CARBON DYNAMICS IN A SUBTROPICAL SAVANNA: A LANDSCAPE-SCALE ASSESSMENT BASED ON SOIL δ13C ............................................................................................. 6 Synopsis .............................................................................................. 6 Introduction ........................................................................................ 7 Materials and Methods ......................................................................... 10 Results .................................................................................................. 17 Discussion ............................................................................................ 27 Conclusions ......................................................................................... 33 CHAPTER III SPATIAL HETEROGENEITY OF SUBSURFACE SOIL TEXTURE DRIVES LANDSCAPE-SCALE PATTERNS OF WOODY PATCHES IN A SUBTROPICAL SAVANNA ....................................................... 34 Synopsis .............................................................................................. 34 Introduction ........................................................................................ 35 Materials and Methods ......................................................................... 40 Results .................................................................................................. 44 Discussion ............................................................................................ 55 Conclusions ......................................................................................... 62 viii Page CHAPTER IV ROOTING STRATEGIES IN A SUBTROPICAL SAVANNA: A LANDSCAPE-SCALE THREE-DIMENSIONAL ASSESSMENT ........................ 64 Synopsis .............................................................................................. 64 Introduction ........................................................................................ 65 Materials and Methods ......................................................................... 67 Results .................................................................................................. 72 Discussion ............................................................................................ 78 Conclusions ......................................................................................... 83 CHAPTER V SOIL CARBON RESPONSE TO WOODY PLANT ENCROACHMENT: IMPORTANCE OF SPATIAL HETEROGENEITY