Slope position regulates soil organic carbon sequestration across particulate and mineral-associated fractions in karst soils

The sources and stability of particulate organic matter (POM) and mineral-associated organic matter (MAOM) fundamentally regulate soil carbon dynamics, but their characteristics in karst ecosystems remain poorly understood. This study focused on the typical peakless basin system in Guangxi, and soil samples were collected across slope positions. Using physical fractionation, 13 C nuclear magnetic resonance spectroscopy, and biomarker analysis, our study characterized the sources and stability of POM and MAOM. Our results showed that with slope position, the contents of dissolved organic carbon, exchangeable magnesium, and exchangeable sodium in both POM and MAOM decreased significantly, whereas exchangeable calcium and iron increased. The alkyl C/O-alkyl C ratio in POM generally decreased along downslope, whereas the ratio in MAOM significantly increased at WD. Biomarker analyses indicated that microbial necromass carbon (MNC), primarily of fungal origin, was the dominant source of soil organic carbon (SOC) in POM and MAOM, while total lignin phenols (TLP) was significantly higher in POM than that in MAOM. Random forest analysis and structural equation modeling revealed divergent accumulation pathways. Specifically, Fe exc was identified as the primary predictor for POM, with TLP directly driving SOC sequestration. Conversely, microbial richness indirectly promoted SOC sequestration by enhancing MNC formation, which served as the primary mediator. By differentiating soil carbon fractions, this study reveals the distinct mechanisms of organic matter accumulation in POM and MAOM across karst slope positions. Consequently, these findings advance the mechanistic understanding of the spatial heterogeneity of soil carbon sinks and their underlying biogeochemical drivers in karst ecosystems.