Microbial necromass carbon dominates the accumulation of soil organic carbon in bedrock-exposed areas of karst regions

• Lignin phenols and MNC show non-linear correlations with bedrock exposure rates. • Fungal-dominated MNC constitutes the primary source of SOC. • Lignin phenol accumulation is regulated by humification, soil minerals, and nutrient balance. • MNC accumulation is regulated by microbial traits and soil minerals. Bedrock exposure alters the stability of soil organic carbon (SOC) in karst ecosystems; however, the responses of plant- and microbial-derived carbon—key indicators for assessing SOC stability—to bedrock exposure rates remain unclear. To address this, we employed lignin phenols and amino sugars as biomarkers to track plant-derived and microbial necromass carbon (MNC), respectively, across a bedrock exposure gradient (0 %–70 %). Our findings revealed that MNC contributed substantially more to SOC (30.37 %–40.83 %) than plant-derived lignin phenols (1.06 %–3.52 %), with fungal necromass identified as the dominant component of MNC. Both plant- and microbial-derived carbon exhibited significant nonlinear relationships with the bedrock exposure rate. Along the bedrock exposure gradient, mineral (Ca) provided a stabilization foundation for both carbon fractions, with lignin phenol accumulation further enhanced on humification and soil nutrient balance, and soil nutrient balance, and MNC accumulation further enhanced by microbial metabolic activity. This study elucidates the distinct stabilization mechanisms between plant- and microbial-derived carbon in karst soils, providing crucial mechanistic insight for predicting the persistence of soil carbon in rocky desertification environments.