Fungal necromass dominates soil organic carbon accrual after decadal active restoration of alpine grassland.

Active (seeding native grasses) and passive (natural recovery via degradation factor removal) restoration practices regulate soil organic carbon (SOC) recovery in degraded grasslands through distinct mechanisms. We quantified microbial- (amino sugars) and plant-derived (lignin phenols) C contributions to SOC after a 10-year restoration of degraded grasslands on the Qinghai-Tibetan Plateau. Active restoration increased SOC to 17.8 and 18.1 g·kg-1 in the topsoil and subsoil, respectively, whereas passive restoration maintained SOC at degraded levels (9.3 and 3.5 g·kg-1). Microbial-derived C contributed 31-37%, while plant-derived C contributed only 1-2% of the SOC pool under active restoration. Active restoration accelerated lignin oxidation while driving an influx of plant-derived phenols into the subsoil, which stimulated microbial turnover and fueled the microbial carbon pump. Microbial necromass accumulation outpaced the direct preservation of recalcitrant plant residues, with fungal necromass dominating the stable microbial-derived C pool. This disproportionate accumulation of biochemically persistent fungal necromass drives long-term SOC sequestration. Land managers should prioritize active grass seeding to maximize root inputs, stimulate fungal dominance, and secure long-term grassland carbon sinks.