Rural Depopulation and Increased Soil Organic Carbon Storage in the South China Karst: The Roles of Cropland Abandonment and Vegetation Restoration

Vegetation restoration following cropland abandonment is commonly accompanied by soil organic carbon accumulation. However, the relationships among rural depopulation, land-use transition, ecological restoration, and regional soil carbon dynamics remain insufficiently quantified. Focusing on the South China Karst, this study integrated long-term remote-sensing data, the Revised Universal Soil Loss Equation, structural equation modelling, and random forest analysis to evaluate the statistical relationships and predictive importance linking rural depopulation, cropland abandonment, vegetation restoration, and surface processes to changes in soil organic carbon storage (SOCS) in the upper 0–20 cm of soil. During the study period, the rural population share declined from 82.89% to 48.91%, while cumulative net cropland abandonment during 1991–2019 reached 4.45 × 104 km2. Over the same period, forest aboveground biomass and tree cover on abandoned cropland increased at rates of 1.79 Mg ha−1 yr−1 and 1.27% yr−1, respectively. Structural equation modelling showed that rural depopulation and the abandonment of sloping cropland were associated with vegetation restoration. Vegetation restoration was, in turn, associated with reduced soil erosion, changes in soil sand content, and increased net primary productivity (NPP), which were further associated with increased SOCS. Random forest analysis identified NPP and tree cover as the principal predictors of SOCS change, with the strongest responses occurring on moderate and steep slopes above 15°. These findings indicate that rural depopulation and the accompanying withdrawal of marginal sloping cropland and vegetation restoration were closely associated with increased SOCS on karst slopes, providing statistical evidence for understanding regional soil carbon dynamics in the context of rural demographic transition.