Soil Carbon and Nitrogen Dynamics in Abandoned Agricultural Lands

Abandoned agricultural lands (old fields) have the potential to sequester C and mitigate atmospheric CO2. Currently, factors controlling the soil C and N dynamics after agricultural cessation are poorly understood, especially the long-term effect of burrowing animal soil disturbance, although they are ubiquitous in abandoned agricultural lands. Also, most C sequestration research only focuses on the top 30 cm, and we lack a clear understanding of C dynamics below 30 cm. In this dissertation, I first developed a process model to simulate the effect of pocket gopher disturbance on soil C accumulation in the old field ecosystems (Chapter 2). I found pocket gopher disturbance reduced soil C accumulation rates, and vegetation recolonization on gopher mounds was the critical factor that determines the impact that gophers have on the soil C pool. Next, I examined the changes in C and N pools in both surface and subsurface soil at 21 old fields. I found the C accumulation in the surface soil was offset by the losses in the subsurface soil. By quantifing the inputs and outputs at different depths in these old fields, I found that roots are the primary input of C to subsurface soil. However, 85% of roots are located in the top 20 cm of soil. Thus, subsurface soil receives little organic matter inputs from roots, which leads to losses of C. Finally, in Chapter 4, I analyzed part of a soil C survey to the depth of 100 cm across the conterminous United States. I compared the soil organic C (SOC) stocks between the croplands and Conservation Reserve Program (CRP) lands at 0-5, 5-30, and 30-100 cm. I found CRP lands, on average, did not accumulate a significant amount of SOC after agricultural cessation, which is likely due to a reduction of SOC stock in the subsurface, despite SOC accumulated in the surface. My dissertation research highlights the essential roles of burrowing animals and subsurface soil C dynamics in C sequestration efforts after agricultural abandonment. My results suggest active land management strategies should be applied to achieve CO2 mitigation goals in abandoned agricultural lands