Mechanisms of soil organic carbon accumulation driven by karst vegetation restoration

With the support of ecological restoration projects, the karst region of Southwest China is now recognized as a hotspot for "Greening Earth" and a major contributor to vegetation carbon (C) sinks in China. However, the function of soil C sequestration in the region and its driving mechanisms are not well understood. To address this gap, we first selected 362 fixed locations within a typical karst catchment to examine alterations in soil organic C (SOC) content and density before and after cropland restoration. We found that both natural (39 g C m–2 yr–1) and managed vegetation restoration (18–27 g C m–2 yr–1) significantly increased SOC accumulation rates. Surprisingly, bedrock exposure ratio, traditionally viewed as a limiting factor for soil C sequestration, indirectly contributes to SOC accumulation by influencing initial soil C pools, external nitrogen inputs, and soil calcium content. Furthermore, we extended our analysis to scrutinize SOC fractions and their determinants in karst (carbonate rocks) and non-karst (clastic rocks) forest soils across a climatic gradient in Southwest China. For comparative purposes, adjacent croplands with a history of long-term soil tillage were included. We found that both SOC and mineral-associated organic C (MAOC) contents, as well as microbial necromass C content, were higher in karst forest soils compared to non-karst. Notably, MAOC in karst soils displayed lower sensitivity to warming than non-karst soils. Our results revealed that karst vegetation restoration, by augmenting mineral protection mediated by calcium and modulating microbial life history strategies (shifting from r-strategy to K-strategy), fosters the accrual of microbial necromass C, thereby facilitating SOC accumulation. In conclusion, our findings underscore the substantial potential of karst vegetation restoration-driven soil C sinks and emphasize the pivotal role of rapid weathering of carbonate rock in soil C accumulation.