Land Use and Lithology Drive Divergent Vertical Distribution and Stabilization of Soil Organic Carbon Fractions in Karst Agroecosystems

In karst agroecosystems, the stability of soil organic carbon (SOC), which is essential for sustainable land management, is strongly influenced by the carbonate parent material. However, how land use and lithology interactively affect the vertical distribution and stabilization of SOC fractions remains unknown. This study investigated the vertical distribution (0–70 cm) of SOC fractions (particulate, POC; mineral–associated, MOC; recalcitrant, ROC; microbial biomass, MBC; dissolved, DOC; permanganate–oxidizable, LOC) in paddy and upland systems overlying limestone and dolomite. Results showed that land use and lithology jointly determined depth–dependent patterns of SOC fractions. The key finding was a depth–dependent reversal of land use effects: paddy soils had higher concentrations of SOC fractions (especially MBC and LOC) in the top 0–20 cm, whereas upland soils had higher concentrations of DOC, ROC, and POC below 50 cm. This pattern underscores an overlooked deep carbon sequestration potential in aerated uplands. Furthermore, lithology exerted fundamental control on SOC stability, with limestone soils exhibiting higher MOC/SOC and MOC/POC ratios than dolomite soils, indicating stronger organo–mineral association. Random Forest and Structural Equation Modeling revealed that soil aggregate stability is a key factor influencing the stability of SOC. These findings demonstrate that effective SOC management in karst agroecosystems must consider both surface land use decisions and deep lithological constraints. Specifically, promoting aggregate stability offers a more targeted pathway for long–term carbon stabilization in these ecologically fragile agricultural systems.