Abstract
Study region: Alpine meadows in the northeastern Qinghai-Tibet Plateau (Shaliu River Basin), a permafrost-affected cold region sensitive to nitrogen deposition and climate change. Study focus: Nitrogen addition can alter soil structure and hydrological processes, yet its impacts on soil-water dynamics and source partitioning in alpine meadows remain unclear. A two-year field experiment (2023–2024) with four nitrogen levels (0, 5, 10, 15 g m⁻² yr⁻¹) was conducted. Stable isotopes (δ²H, δ¹⁸O), LC-excess, and End-Member Mixing Analysis (EMMA) were used to trace soil-water origin, infiltration pathways, and source contributions under varying nitrogen inputs. New hydrological insights for the region: Nitrogen addition significantly shifted infiltration regimes from preferential flow under control conditions to piston-flow dominance under N5 (May and July-September) and sustained piston flow under N10-N15 (June-September). (2) Precipitation supplied 34–46% of soil water, while underground ice and supra-permafrost water each contributed 26–33%, with seasonal switching from precipitation dominance (May-July) to increased frozen-ground inputs (August-September). (3) Higher nitrogen enhanced late-season supra-permafrost contributions and altered evapotranspiration partitioning. (4) Moderate nitrogen input (5 g m⁻² yr⁻¹) most effectively maintained piston-flow-dominated infiltration, enhanced deep percolation, and stabilized soil-water interactions, whereas excessive nitrogen reduced water-use efficiency and altered source contributions.