Effects of Grazing Exclusion on Soil Microbial Necromass Carbon in a Tibetan Alpine Grassland

Grazing exclusion via fencing is widely acknowledged as an effective strategy for restoring soil organic carbon (SOC) levels in degraded grasslands worldwide. However, the contribution of microbial necromass to SOC dynamics following grazing exclusion remains inadequately addressed, particularly in alpine grasslands. In this study, we quantified soil microbial necromass C (MNC) and its proportional contribution to SOC across three alpine grasslands subjected to 0, 4, and 12 years of grazing exclusion on the eastern Tibetan Plateau. Four amino sugars, including glucosamine (GluN), galactosamine (GalN), muramic acid (MurA), and mannosamine (ManN) were employed as biomarkers to estimate MNC. The results showed that soil total organic C (TOC) increased by 58%–82% following grazing exclusion, with significantly higher TOC content in grassland restored for 12 years (GE12) than in unrestored grassland (GE0). Grazing exclusion did not alter the contents of GluN, GalN, or ManN. In contrast, MurA content in GE12 was significantly higher than that in grassland restored for 4 years (GE4). Across all three grasslands, fungal necromass C (FNC) represented an average of 84% of MNC, indicating that fungi represent the dominant source of microbially derived C. Although grazing exclusion induced increases of 21%–51% in MNC and 25%–49% in FNC, these shifts were not statistically significant. Bacterial necromass C (BNC) remained unchanged during the first 4 years of exclusion but increased significantly with continued exclusion. Soil physical properties and microbial biomass C were identified as key factors modulating MNC dynamics during grazing exclusion. The proportions of MNC, FNC, and BNC relative to TOC in GE4 and GE12 were generally lower compared to those in GE0, suggesting that grazing exclusion diminished the role of microbial necromass in SOC sequestration. The negative correlation between MNC/TOC and plant biomass further implied that plant‐derived C may be more crucial for SOC accumulation than microbially derived C during grazing exclusion. In conclusion, our findings demonstrate a limited role of microbial necromass in SOC sequestration over the 12‐year grazing exclusion period in Tibetan alpine grasslands.