Natural vegetation regeneration facilitated soil organic carbon sequestration and microbial community stability in the degraded karst ecosystem

Vegetation restoration is an effective strategy for sequestering soil organic carbon (SOC) and restoring degraded ecosystems. However, the effects of different restoration strategies on SOC quantity and quality and the underlying microbial response mechanisms in a karst region of southwest China remain unclear. This study quantified soil physicochemical properties, soil labile SOC fractions (dissolved organic carbon (DOC), microbial biomass carbon (MBC), light fraction organic carbon (LFOC), coarse particulate organic carbon (CPOC), and fine particulate organic carbon (FPOC)), and the abundances, α-diversities, and compositions of bacterial and fungal communities under afforestation (Macadamia ternifolia F. Muell. and Mangifera indica L.) and natural regeneration (grassland and secondary forest), using maize field as the reference. Compared to afforestation, under natural regeneration the SOC and soil moisture content (SMC) increased by 80.3% and 21.7%, respectively, and pH decreased by 0.35, and labile SOC fractions (DOC, MBC, LFOC, CPOC, and FPOC) increased by 72.2%–172.0%, indicating that natural regeneration facilitated SOC sequestration, especially for labile SOC fractions. Phospholipid fatty acid analysis indicated that natural regeneration and afforestation significantly (p < 0.05) increased bacterial and fungal abundances and altered community compositions. Moreover, the natural regeneration co-occurrence network was more complex and stable than that of afforestation. Redundancy analysis and structural equation modeling indicated that variations in bacterial and fungal composition were primarily driven by soil C/N, available nutrient, pH, particulate organic carbon, and MBC. Overall, these findings suggest that natural regeneration is a promising restoration strategy for sequestering labile SOC and maintaining bacterial and fungal community stability. Additionally, soil properties and labile SOC fractions may coregulate microbial community changes following vegetation restoration in this karst region.