Decipher soil organic carbon density dynamics, driving forces and future stability across Southwest China

An accurate understanding of the spatiotemporal distribution, drivers, and stability of soil organic carbon density (SOCD) is essential to improve regional carbon (C) sequestration and mitigate climate change. In Southwest China, where complex terrain and ecological engineering-induced surface disturbances have led to high SOCD variability, such assessments remain limited because of the scarcity of organized soil profile datasets. This study compiled 2883 recent SOCD field observations and associated driver variables (climate, soil, terrain, vegetation, and environmental changes). An ensemble machine learning approach was employed to map the baseline SOCD (2000–2020) at a 1-km resolution, quantify its spatiotemporal variability and associated drivers, and project decadal changes (2021–2060) under three CMIP6-based scenarios (SSP1-1.9, SSP2-4.5, and SSP5-8.5), incorporating the effects of climate change and CO2 fertilization. Key findings are as follows. (1) Historical SOCD declined from 5.46 to 5.29 kgC·m−2 between the 2000 s and 2010 s, with approximately 25 % of the region exhibiting significant losses (P < 0.05). (2) Environmental variation (−3.48 %) and climate change (−0.84 %) were dominant drivers of SOCD loss, whereas vegetation greenness contributed to modest gains (0.04 % yr−1). (3) Pronounced spatial heterogeneity in SOCD changes was observed across multiple restoration portfolios, where strong environmental variations and soil moisture thresholds offset vegetation gains and induced net C losses. (4) Future projections suggest persistent SOCD losses under warming, with potential gains possible through increased precipitation or CO2 fertilization. These results underscore the spatial heterogeneity of SOCD dynamics and the importance of spatially explicit strategies to guide regional C management and restoration.