Factors controlling soil organic carbon with depth at the basin scale

Soil organic carbon (SOC) plays a vital role in mitigating climate change and ensuring food security. In this study, a legacy database containing analytical data for 216 soil profiles collated from the most recent soil series survey in China was used to clarify the spatial distribution and storage of soil organic carbon in the Dongting Lake basin. The results showed that the SOC contents decreased with increasing depth of the soil. The spatial distributions of SOC contents decreased from the central to the northeast part of the basin, and the spatial variation trend of each layer was consistent. Elevation, normalized difference vegetation index, topographic position index, and land use intensity were the key factors affecting SOC contents. Natural forests contributed more to SOC sequestration than plantations did in medium slope areas (6°–25°). For the 0–100 cm soil layer, the average density and total storage of SOC were 12.71 kg C m−2 and 3.32 Pg, respectively. The study suggested that an improved algorithm for SOC estimation and effective ecological restoration policy is needed in the future.