CHANGES TO SOIL ORGANIC MATTER TURNOVER AND SOURCE WITH ENCROACHMENT OF EASTERN RED CEDAR INTO A CENTRAL OKLAHOMA DEGRADED GRASSLAND

Abstract

Woody plant encroachment (WPE) is a global land change phenomenon that is caused by a combination of drivers such as a decrease in native grazers, suppression of wildfires, land disturbance from overgrazing, and increases in atmospheric CO2. As grasslands are an important source of biodiversity and are large terrestrial carbon sinks, there is concern on how WPE will affect global carbon cycling. In the Southern Great Plains, the impacts of WPE by trees such as honey mesquite (Prosopis glandulosa), ashe juniper (Juniperus ashei), and eastern red cedar (Juniperus virginiana) on grassland biodiversity, above- and below-ground litter inputs, and soil organic carbon (SOC) stocks have been studied extensively. However, the impact of WPE on degraded grassland soils is not well understood, and the impacts of WPE on SOC are not generalizable as SOC storage is affected by climate, litter quality, microbial community composition, and soil texture. This thesis utilizes stable carbon isotopes to track changes in soil physical and chemical properties, litter inputs, SOC, and SOC turnover after 25 years of eastern red cedar (ERC) encroachment in a Central Oklahoma degraded grassland ecosystem. We hypothesized that there would be a complete replacement of remnant grassland (RG) above- and below-ground litter and that there would be an increase in SOC in the upper 5 cm of the soil profile with decreasing influence with depth. Secondly, we explored how using different endmembers for a two end-member isotope mixing model influences the fraction of remaining grassland carbon that is reported. To test our hypotheses, we estalished 15 paired ERC-RG plots at the Kessler Atmospheric and Ecological Field Station (KAEFS) in McClain County, OK. Soil cores and litter samples were collected from each paired ERC-RG plot, and stable isotope methodologies were used to analyze litter and soil OC contents. A two-endmember mixing model was also used to estimate how much RG litter, roots, and SOC was remaining beneath the ERC canopies. We found a 42% increase in fine root carbon stock and a 556% increase in above-ground litter stock. Using stable carbon isotope modeling, we found that only 11-15% of the RG C4 roots were remaining beneath the ERC canopies and 68-77% RG carbon was remaining in the upper 5 cm of the soil after ~25 years of ERC. Despite these findings, there was no significant increase in SOC stock at any depth interval as soil bulk density decreased significantly in the surface soil under ERC. Under some of the ERC trees, carbon stable isotope modeling indicated that RG carbon is lost at the same rate as ERC carbon is being incorporated with not net change in carbon content and that ERC establishment had yet to influence the accumulation or replacement of SOC below 15 cm depth. Lastly, we found that the choice of ERC soil carbon input to the RG, e.g ERC needles, root, or needle litter, could result in dramatic difference in isotope mixing model results given variations in d13C values among plant components. Overall, we identified a significant shift to greater above and below ground litter input, driving changes in SOC source and content in the near surface soil horizon during this early stage in ecosystem change.