Contrasting responses of particulate and mineral-associated organic carbon stocks to grazing exclusion in an alpine meadow

Grazing exclusion is widely used as an effective soil restoration strategy in degraded grasslands to enhance soil organic carbon (SOC) stocks. However, the magnitude and direction of the effects of grazing exclusion on SOC remain unclear, due to limited understanding of changes in SOC constituent pools. Here, we investigated the impacts of 6-year (GE6) and 14-year (GE14) grazing exclusion on particulate (POC) and mineral-associated organic carbon (MAOC) stocks in an alpine meadow on the Qinghai-Tibet Plateau. Compared to grazing treatments, GE6 and GE14 reduced POC stocks by 41 % and 68 %, respectively, across the 0–30 cm soil depth. In contrast, GE6 increased MAOC stocks by 49 %, whereas GE14 had no significant effect on MAOC stocks. The decline in POC likely resulted from reduced litter incorporation into soils, together with enhanced decomposition driven by elevated oxidative enzyme activities. Changes in MAOC stocks were linked to increased above- and belowground plant biomass, which promoted microbial necromass C production. However, the absence of MAOC gains under GE14 limited overall SOC accumulation, underscoring the need to optimize grazing exclusion duration. Collectively, these results show that the responses of SOC stocks to grazing exclusion depend largely on microbial transformation of plant-derived C rather than on increased plant biomass alone, highlighting the pivotal role of microbes in regulating SOC sequestration in alpine grasslands.