Surface soil organic carbon sequestration under post agricultural grasslands offset by net loss at depth

Abstract

Post agricultural grasslands are thought to accumulate soil organic carbon (SOC) after cultivation cessation. The Conservation Reserve Program (CRP) in the U.S. is a wide-scale, covering approximately 8.9 Mha as of 2020, example of row-crop to grassland conversion. To date, changes in SOC stock in CRP lands have mostly been evaluated at local scales and focused on the surface 20–30 cm of the soil profile. Thus, we lack knowledge of SOC dynamics in CRP lands on a continental scale, especially in the subsurface soil, after agricultural cessation. The Rapid Carbon Assessment (RaCA) project is the most recent effort by the United States Department of Agriculture (USDA) to systematically quantify C stock in the 0–100 cm soil profile across the conterminous US. Here we analyzed data from RaCA to evaluate the SOC stocks of both surface and subsurface soil of the CRP on a continental scale. We found there was no difference in SOC stock between croplands and CRP lands when comparing the 0–100 cm soil profiles, which indicates that the C sequestration in CRP lands is insignificant overall. We did find that CRP lands have higher SOC stocks in the surface soil (0–5 cm). However, such higher SOC levels in surface (0–5 cm) soil were offset by the lower SOC stock in the subsurface (30–100 cm) of the CRP. We also found that CRP lands in humid and warm regions may have net soil C sequestration because they have much more SOC in the surface as compared with croplands in the same regions. Whether the lower SOC in the subsurface of CRP lands is caused by legacy effects or is a result of C losses needs to be verified by long-term repeated sampling in both surface and subsurface soil. This analysis highlights the importance of examining C dynamics in subsurface soil after agricultural cessation to accurately measure and improve C sequestration rates in CRP lands.