Aims <p>Erosion and deposition critically influence soil organic carbon mobilization and stabilization, yet how microbial necromass contributes to soil organic carbon under these processes remains unclear. This study investigated the contribution of microbial necromass to soil organic carbon accumulation and stability across land-use types in a dam-controlled watershed on the Chinese Loess Plateau.</p> Methods <p>Soil samples (0–20 cm) were collected from erosional (sloping farmland, woodland, grassland, and shrubland) and depositional (dammed land) zones. Soil properties and soil organic carbon mineralization were analyzed, and the contributions of bacterial and fungal necromass carbon to soil organic carbon were quantified. Key drivers were identified via redundancy analysis.</p> Results <p>Soil organic carbon content and stability—characterized by a higher proportion of stabilized carbon pools and longer mean residence time—were higher in erosional soils. The proportion of microbial necromass carbon in soil organic carbon ranged from 20.97% to 62.25% in erosional zones and from 32.27% to 42.97% in depositional zones. Fungal necromass carbon contributed 4.28 times more to soil organic carbon than bacterial necromass carbon in erosional zones, whereas this ratio decreased to 2.05 in depositional zones. Ammonium nitrogen and clay content governed soil organic carbon and microbial necromass dynamics in erosional zones, while available phosphorus and soil pH were primary regulators in depositional zones.</p> Conclusions <p>Erosion–deposition processes reshape soil organic carbon stabilization by altering necromass distribution and soil biogeochemistry, with land-use modulating necromass accumulation via vegetation inputs. Fungal necromass carbon plays a key role in enhancing soil organic carbon persistence in erosional landscapes.</p>