The karst inorganic carbon sink is crucial for carbon neutrality, but its trends and drivers in subtropical humid zones remain unclear. This study selected subtropical humid zones in China with significant forest greening, quantified the carbonate rock weathering carbon sink (CCS) using a thermodynamic dissolution model, and explored the effects of climate, vegetation, hydrology, and radiation energy on CCS through importance analysis. The results showed that from 1982 to 2020, the CCS flux was 12.40 t C km⁻² yr⁻¹, and the total carbon sink was 1188.54 × 10⁴ t C yr⁻¹. Normalized difference vegetation index, leaf area index, and CCS exhibited an increasing trend, with growth rates of 0.002, 0.01 m² m⁻², and 0.05 t C km⁻² yr⁻¹, respectively. Surface available water, precipitation, and evapotranspiration were the dominant factors affecting CCS. This study found that forest greening caused precipitation to increase faster than evapotranspiration, driving an increase in available surface water and ultimately promoting the karst carbon sink in subtropical humid zones. Our findings highlight forest greening as a vital strategy for carbon neutrality.

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.