Zhou, Yong (2018-05). Savanna Vegetation Dynamics and Their Influence on Landscape-Scale C, N, and P Biogeochemistry. Doctoral Dissertation.
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 x 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 ?13C, and soil ?15N. Soil ?13C 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 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 ?15N increased with depth, reached the maximum at an intermediate depth, and then decreased in the deepest portions of the profile. Woody encroachment decreased soil ?15N, creating spatial patterns of soil ?15N 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.