Accelerated glacier melt driven by global warming is increasing nitrate (NO 3 − ) fluxes to downstream ecosystems, thereby attracting wide attention to nitrogen cycling in glacial-fed regions. Proglacial lakes are closely connected to glaciers, and glacier-derived NO 3 − plays a critical role in regulating lake water NO 3 − concentrations. However, the relative contributions of glacier runoff (GR) versus in-lake biogeochemical processes to lake water NO 3 − remain poorly understood. This study uses measurements of NO 3 − concentrations and isotopic compositions (δ15O-NO 3 − , δ18O-NO 3 − , and Δ17O-NO 3 − ) from the Qiangyong Glacier watershed on the Tibetan Plateau, combined with the MixSIAR model, to quantify the relative contributions of different NO 3 − sources. The results showed that GR was the dominant source to lake water NO 3 − during the melting season, accounting for 83%±5%, followed by in-lake microbial nitrification (MN) contributing 15%±4% and direct atmospheric deposition contributing 2%±2%. Further analysis revealed that both GR input and internal lake processes controlled the seasonal variations of NO 3 − concentrations. During the early melt season, GR and enhanced in-lake nitrification increase NO 3 − concentrations, while sediment-mediated physical adsorption may contribute to their subsequent decrease. In contrast, during the non-melt season, NO 3 − concentrations gradually declined through microbial removal processes, such as denitrification and assimilation. This study quantified the relative contributions of GR and in-lake MN to lake NO 3 − , highlighting that proglacial lakes are hotspots for nitrogen transformation.
