Macroaggregate disintegration mediates forest gap impacts on MAOC decrease in karst ecosystems

• The study combined aggregate fractionation with biomarker analysis to trace C sources and stability. • Physical breakdown of macroaggregates is the primary mechanism of MAOC loss. • Microbial necromass depletion, not plant inputs, is the critical driver of this C loss. • Protecting soil structure is key to mitigating C loss under increasing forest disturbances. Forest gap disturbances, which are projected to intensify with climate change, threaten the stability of mineral-associated organic carbon (MAOC) in karst soils. We sampled soils under forest gaps (FG), the forest extended gap edge (FE), and closed forest canopies (FC) in the Maolan National Nature Reserve, China, where gaps typically form and persist for approximately 15 years. We combined soil aggregate fractionation with carbon component analysis to quantify how forest gaps altered three key drivers of MAOC persistence: (1) the allocation of plant-derived versus microbial necromass carbon, (2) physical protection via macroaggregate stability, and (3) organic-mineral bonding capacity. Forest gaps significantly disrupted soil aggregate structure, decreasing aggregate stability by 36.66–49.29%. This disruption led to a 53.25% reduction in microbial necromass carbon within large macroaggregates (LMA)—likely due to enhanced mineralization or leaching—and consequently hindered MAOC accumulation by reducing the contribution of LMA by 56.60%. Furthermore, Ca-bound and Fe-bound organic carbon (Ca-OC, Fe-OC) within small macroaggregates (SMA) also decreased under forest gap conditions, indicating a synergistic failure of both physical and chemical protection mechanisms. Correlation and random forest analyses demonstrated that Microbial necromass carbon influenced MAOC accumulation more significantly than plant-derived carbon, underscoring the dominance of physical protection. Our findings suggest that forest gaps in karst soils primarily drive MAOC loss by disrupting large macroaggregates and depleting their associated microbial necromass carbon.