Responses of Soil Organic Carbon and Its Physical Fractions to Ecological Restoration in the Karst Area of Southwest China

ABSTRACT Understanding soil quality changes and environmentally sustainable development requires a knowledge of how ecological restoration affects the contents of soil organic carbon (SOC), particulate organic carbon (POC), and mineral‐associated organic carbon (MAOC). However, understanding of how ecological restoration affects SOC and its physical fractions of POC and MAOC—particularly in karst regions—remains limited. Soil samples were collected from secondary forest (SEC), osmanthus forest (OSM), paddy field (PAD), and dryland (DRY) in a karst valley in southwest China. Contents of total organic carbon, labile carbon fraction, and recalcitrant carbon fraction in bulk soil, particulate fraction, and mineral fraction were measured using the potassium dichromate oxidation method followed by ferrous sulfate titration. Ecological restoration significantly increased SOC content by enhancing aboveground and belowground biomass, reducing wind and water erosion, and improving soil structure. Compared to DRY, contents of labile and recalcitrant carbon in PAD increased by 19.49% and 89.14%, respectively, for POC, and by 38.68% and 57.52%, respectively, for MAOC. The carbon stability index of MAOC was 27.61% higher than that of POC, indicating greater carbon stability in MAOC. Compared to DRY, the contribution of the mineral fraction to SOC in PAD and SEC decreased by 65.27% and 15.84%, respectively, while the contribution of the particulate fraction increased by 70.00% and 26.45%, respectively. Ecological restoration affected SOC content but not SOC stability, and PAD was the best land use option for both economic benefits and ecological environmental improvement in southwest China's karst region.