Historical and future dynamics of soil organic carbon and driving mechanisms in mountainous soils of China

Mountain ecosystems exhibit unique microclimate conditions and high plant diversity, resulting in heterogeneous patterns and dynamics of soil organic carbon (SOC). Climate change strongly impacts the spatial and temporal dynamics of SOC, yet long-term spatiotemporal variations of SOC stocks in mountainous soils and their responses to climate change are not well understood. In this study, we employed machine learning to comprehensively investigate the spatiotemporal distribution patterns of SOC and their drivers in the Qinling Mountains from 2006 to 2022, and further projected future SOC trajectories under different climate scenarios. Results showed that the SOC pools within the top 20 cm were 1.20 Pg C. Forest ecosystems accounted for the largest proportion (74 %), followed by cropland (18 %), grassland (7 %), and shrub ecosystems (1 %). Overall, SOC in the Qinling Mountains significantly increased from 2006 to 2022. Nevertheless, the SOC in forest ecosystems of high-altitude regions exhibited a declining trend, suggesting that SOC in high-altitude forests is more sensitive to climate change and more likely to be lost. A structural equation model revealed that climate drivers (mean annual temperature and aridity index) negatively affected SOC through both direct and indirect pathways, which indicates the risk of soil carbon losses in mountains due to warming and drought. In contrast, gross primary productivity positively impacted SOC, underscoring the decisive role of plant carbon inputs in SOC accumulation in mountain ecosystems. Comparatively, soil characteristics and topographical features had little effect on SOC. Our projections further indicated an increase in SOC under the low-emission scenario (SSP1-1.9), while SOC would decrease under medium (SSP2-4.5) and high-emission (SSP5-8.5) scenarios. This study suggests that future global warming will lead to the loss of SOC in mountainous soils. Therefore, ecosystem protection, particularly for high-altitude forests, could effectively maintain SOC sequestration capacity and mitigate the negative impacts of climate change.