Effects of grazing exclusion on soil carbon dynamics in alpine grasslands of the Tibetan Plateau

Globally, excessive grazing is identified as one of the key disturbances leading to grassland degradation and soil carbon (C) loss. Grazing exclusion has been proposed as an effective practice to restore degraded grasslands and to promote C sequestration. However, there is still little knowledge about how soil C changes with grazing exclusion in high-altitude alpine ecosystems with very cold climates. We synthesized data from 63 sites in the literature and 15 sites in a field sampling and investigated the dynamics of soil C stocks following grazing exclusion in alpine grasslands of the Tibetan Plateau. The results showed that the soil C stock increased with grazing exclusion at most sites, with average C sequestration rates of 0.84, 0.58, and 0.49 Mg ha(-1) yr(-1) in the soil layers of 0-10, 10-20, and 20-30 cm, respectively. Based on these results, if 60 million ha of the grasslands on Tibetan Plateau were excluded from grazing livestock by 2020 according to the national plan, then approximately 0.11 Pg C yr(-1) would be sequestered in the soil which equates to about 4.4% of fossil fuel and cement CO2 emissions in China in 2013. Generally, the rates of soil C increase exhibited a declining pattern with increasing years of grazing exclusion, with a significant decrease occurring after ten years of grazing exclusion. Of the factors examined, the rates of absolute and relative soil C change were both positively related to mean annual precipitation but negatively related to the year of grazing exclusion and initial soil C stock, respectively. The rates of soil C changes increased linearly with those of N change, and no matter how soil C changed (whether it increased or decreased), soil C:N ratios remained stable over the years of grazing exclusion. Our results implied that grazing exclusion is beneficial for soil C sequestration in degraded alpine grassland, especially in humid areas. Moreover, the intrinsic increase in N could keep up with the pace of soil C changes and would sustain soil C sequestration during the recovery process.