Tracing Nitrate Source and Transformation in Glacier Runoffs on the Tibetan Plateau Using Triple Nitrate Isotopes

Glaciers are the largest reservoir of fresh water on earth. Global warming accelerates glacier retreat and enhances glacier runoff, which heavily impacts the geochemical cycling in downstream ecosystems. Nitrate is the main form of nitrogen in glacier runoff, while little is known about its source in high-altitude regions. Here, we measured triple nitrate isotopes (δ 15 N, δ 18 O, and Δ 17 O) in ten geographically separated glacier runoffs on the Tibetan Plateau with the altitudes ranged from 2,409 to 5,564 m. The isotope results were combined with hydrogeochemical indicators to assess the primary sources and transformation processes of nitrate. Results showed that all the glacier runoffs exhibited low nitrate concentration, which ranged from 0.3 ± 0.0 to 1.5 ± 0.8 mg·L -1 . The SIAR model results indicated that soil organic nitrogen and atmospherically oxidized nitrate accounted for 67% of nitrate in the runoffs, whereas human-related source (i.e., chemical fertilizers) contributed 33% on average. Significant geospatial differences in the contribution of chemical fertilizers were identified, with their contributions being significantly higher in the monsoon domain (43% ± 3%) than that in the westerlies domain (22% ± 8%). Furthermore, δ 18 O of nitrate ranged from -15‰ to 15‰, this suggested that the nitrification process is responsible for the nitrogen transformation in Tibetan glacier ecosystems. Our study reveals the source of nitrogen in Tibetan glaciers and highlights the substantial impact of human activity on glacier nutrient cycling. Our work may shed light on the understanding of nitrogen cycling in downstream ecosystems.