The Dynamics of Soil Organic Carbon in Alpine Grasslands Affected by Livestock Grazing Intensity

As one of the most widespread anthropogenic activities in grasslands, livestock grazing exerts profound and complex influences on soil organic carbon (SOC) dynamics. However, the responses of SOC dynamics to grazing intensity in alpine grassland are still unclear. We manipulated a grazing intensity (including no grazing, CK; moderate grazing, MG; and heavy grazing, HG) experiment from 2019 to 2024 on the Tibetan Plateau. Plant community characteristics, i.e., above-ground biomass (AGB), cover, and height, as well as SOC and soil nutrient characteristics at topsoil (0-10 cm) and subsoil (10-20 cm) layers, were systematically measured. Climate variables from 2019 to 2024 were derived from the fifth-generation global climate atmospheric reanalysis data. We found that SOC content declined temporally in both soil layers. The temporal stability of SOC under MG was lowest in the topsoil compared with CK and HG. Under MG, plants positively promote SOC accumulation through biomass and coverage; however, under HG, this synergy is disrupted, and plant height becomes the core proxy indicator for characterizing the negative effects of grazing. Precipitation and soil available nutrients have a stronger direct positive effect on topsoil SOC, while the negative impact of grazing on subsoil SOC is mainly achieved through the indirect path of suppressing plant height. Concluding, grazing intensity drives SOC dynamics by altering plant community traits, with the net effect contingent on soil depth and coupled with climate and soil nutrient availability. These findings can provide scientific data support for the sustainable management of alpine grasslands and soil carbon sinks.