No abstract

Alpine meadows are robust nitrous oxide (N2O) sources that continually experience overgrazing on the Tibetan Plateau. However, the mechanisms underlying N2O fluxes are poorly understood. The effects of grazing activity on grassland N2O emission rates, soil and plant characteristics were investigated using a meta-analysis approach. This study revealed that the effect size of grazing was 0.31 ± 0.08 on N2O emission rates (p < 0.0001), and N2O fluxes increased by 36.27% than control. Light, moderate, and high grazing increased N2O emission rates by 34.62, 19.48, and 62.16%, respectively. The effect size of moderate grazing was significantly lower than that of high grazing (p < 0.05). The effect size of grazing on pH was significant (p < 0.05), and pH increased by 6.51% compared with control. Both soil ammonia and nitrate levels increased by 12.24 and 8.60%, respectively. However, grazing decreased soil total carbon, total phosphorus, and available phosphorus by 14.4, 10.25, and 10.15%, respectively. Grazing significantly decreased plant diversity (p < 0.05), richness, and aboveground biomass by 15.16, 23.7, and 30.7% (p < 0.01), respectively (p < 0.01). Aboveground biomass significantly influenced effect size on N2O emissions, explaining 13.36% of the variations. The direct coefficient of aboveground biomass on effect sizes was −0.631 based on the structural equation model. Although grazing significantly decreased aboveground biomass and diversity, moderate grazing is optimal for mitigating N2O emissions on the Tibetan Plateau.

Ecological security barriers on the Tibetan Plateau are threatened by climate change and human activities, such as grassland degradation and reduction of carbon fixation capacity. Understanding the influence of grazing on soil organic carbon (SOC) content and its regulating factors is important for improving the ecological barrier function of alpine grasslands. In this meta-analysis, we analysed the effects of grazing on SOC. The results indicate that grazing disturbance significantly reduced the SOC content by 13.93%, with an effect size of −0.15 ± 0.04 (p < 0.001). The effect of light grazing was not significant. The reduction range gradually increased with increasing grazing intensity. The effect of grazing activity on SOC content was driven mainly by pH and total nitrogen through a structural equation model. Future nitrogen deposition scenarios would significantly increase alpine meadow SOC on the Tibetan Plateau.