Vegetation restoration is associated with soil microbial community reassembly and necromass carbon accumulation via calcium mediation in a degraded karst ecosystem

The karst region in Southwest China has experienced severe degradation due to intensified agricultural activities. Although artificial and natural vegetation restoration represent key strategies, their effects on soil microbial communities and necromass carbon (C) accumulation remain unclear. This study compared the impacts of 21 years of artificial restoration (forage grassland [FG] and plantation forest [PF]) and natural restoration (naturally restored shrubland [SL]) on soil microbial community structure and necromass C accumulation, using cropland (CL) as the pre-restoration baseline and mature forest (MF) as the successional reference. The results indicated that bacterial diversity (Shannon index) significantly increased in SL and PF, accompanied by higher relative abundances of Proteobacteria and lower abundances of Acidobacteriota and Crenarchaeota , indicating a shift toward copiotrophic taxa. Fungal diversity exhibited a slight but nonsignificant upward trend, while Basidiomycota abundance markedly increased under restoration. SL and PF significantly enhanced bacterial network complexity, but fungal networks changed little. All restoration types increased microbial necromass C, with PF showing the highest bacterial and fungal necromass accumulation (78% and 64% higher than CL, respectively). Fungal necromass consistently contributed more to SOC than bacterial necromass under all treatments. In addition, SOC, TN, available substrates, and exchangeable Ca 2+ were identified as the main determinants of microbial community structure and necromass accumulation. Exchangeable Ca 2+ was statistically associated with necromass C accumulation through both direct and indirect pathways in the SEM, suggesting a potential dual role. It may contribute to necromass C accumulation potentially through physicochemical interactions (e.g., cation bridging) and indirectly through its associations with SOC, TN, and microbial community composition. The PF exhibited the greatest potential for soil C sequestration and microbial-driven ecosystem recovery, while SL contributed substantially to microbial community reassembly. By integrating long-term restored sites with microbial and biogeochemical indicators, this study provides a mechanistic framework for understanding calcium-mediated microbial carbon accumulation in degraded karst ecosystems.