Spring rest-grazing time affected soil organic carbon stability and storage in subalpine meadows of Carex

Scientific grazing management is essential to maintain soil organic carbon (SOC) stability in subalpine meadows on the Qinghai-Tibet Plateau (QTP). We studied the stability of SOC in Carex subalpine meadows in QTP with five rest-grazing treatments. (ST1: soil surfaces started thawing; ST2: soil thawing depth > 10 cm; RG1: re-greening coverage reached 30–40%; RG2: re-greening coverage reached 80%; CK: local traditional rest-grazing time). The results showed that rest-grazing at different times in spring affected the active SOC composition by affecting the stability of physical structure. The underground organic carbon storage was dominated by the root carbon storage, while the active organic carbon component had a significant correlation with the litter content. Compared with the traditional rest-grazing, ST1, ST2, RG1 and RG2 increased the topsoil SOC storage by 5.87%, 7.24%, 13.55%, 2.17%, and the topsoil root carbon storage by 32.77%, 36.25%, 9.91%, −0.79%. Rest-grazing at ST1 promoted the production of soil macro-aggregates, increased the hydrolase activity and active SOC contents in topsoil, and promoted the soil nutrient cycling. Continuing grazing after the plant re-green increased the physical structure stability and oxidase activity but decreased the active SOC and SOC storage. Although a late rest-grazing in spring has increased the SOC stability, it reduced soil carbon use efficiency and sequestration. It is necessary to rest-grazing subalpine QTP Carex meadows before soil thaw, which can promote SOC accumulation and nutrient cycling, and transport plant carbon into the deep soil through roots, which is beneficial for grassland productivity and grassland carbon sequestration.