<p>Land-use patterns critically regulate soil organic carbon (SOC) dynamics through differential impacts on particulate (POC) and mineral-associated organic carbon (MAOC) fractions, thereby shaping terrestrial carbon sequestration potential. This study aimed to elucidate how contrasting land-use systems influence SOC stabilization mechanisms in fragile karst ecosystems. SOC fractions and microbial necromass carbon (MNC) were quantified across two restored systems (forestland and grassland) and three intensively managed croplands (sugarcane, maize, and banana) in a subtropical karst region. Agricultural systems experienced substantial SOC losses (18.4–57.0%) compared with restored ecosystems, possibly due to reductions in POC derived from plant residues. Forestland exhibited the highest MAOC content, indicating greater long-term SOC stabilization. The restored systems also showed significantly higher MNC accumulation, likely driven by greater root biomass and minimal soil disturbance. Fungal necromass carbon (FNC) dominated MNC pools (61.6–72.9%) and contributed 18.5–36.1% to total SOC, significantly exceeding bacterial necromass carbon (BNC). Redundancy analysis revealed that soil nutrient stoichiometry, particularly the dissolved organic C: N ratio and N: P ratio, was closely linked to MNC dynamics across land-use types. Comparisons between wet and dry seasons revealed that most land-use types exhibited higher FNC and MNC accumulation during dry seasons, whereas banana fields experienced carbon losses, likely due to tillage disturbance after harvesting and accelerated mineralization of SOC. Intensive agriculture disrupts SOC stabilization in karst ecosystems, partly by reducing microbial necromass accumulation and limiting MAOC formation relative to restored systems, highlighting the critical need for conservation-oriented land management to preserve soil carbon stocks and enhance carbon sequestration in fragile karst ecosystems.</p>