What influences does grazing bring about to stream nutrient fluxes in alpine meadows?

Non-point source pollution is a serious environmental issue and has attracted a lot of scientific attention. Currently, the influences of grazing on the water and nutrient cycles in alpine meadows, as well as corresponding underlying mechanisms, still remain unclear. The quantitative contributions of influence variables to the nutrient loading variations in rivers and streams in alpine-meadow watersheds need to be elucidated. In the study, the Soil and Water Assessment Tool (SWAT) was applied to a typical alpine-meadow watershed (the Buha River Basin, BRB) on the Tibetan Plateau, where non-intensive grazing and farming were developed, to reproduce the water and nutrient cycles. Results showed that SWAT model performed well in simulating monthly streamflow, TN, and TP fluxes from 2016 to 2022 (NSE > 0.6). Grazing slightly increased the streamflow by 4.27% because it reduced aboveground biomass and leaf area index of vegetation, which subsequently reduced the ET of grasslands. Grazing increased the annual NH4-N fluxes by 0.50% due to livestock manure inputs to the basin; in contrast, grazing reduced the annual fluxes of TN and TP of Buha River by 3.36% and 0.48%, respectively, during 2016 – 2022, which were driven by the increased plant uptake of NO3-N/soluble P from soil layer due to grazing. As a result, non-intensive grazing was beneficial for the water management and nutrient control in the studied alpine-meadow watershed. The hydrological, water-quality, and meteorological variables accounted for 30.78%, 28.27%, and 21.23% of the variations in N fluxes in Buha River, respectively. The dissolved oxygen (10.68%), soluble phosphorus (10.51%), precipitation (6.27%), overland flow (5.60%), and water temperature (5.53%) each explained more than 5% of the N flux variations. The present study quantified the effects of grazing and farming on stream nutrient fluxes based on mathematical modeling and statistical methods, and the findings could provide scientific supports for the managements of alpine-meadow watersheds.