Seasonal variations of nitrate concentrations and isotopic compositions in a Tibetan Plateau proglacial lake

Accelerated glacier melt driven by global warming is increasing nitrate (NO3−) 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 NO3− plays a critical role in regulating lake water NO3− concentrations. However, the relative contributions of glacier runoff (GR) versus in-lake biogeochemical processes to lake water NO3− remain poorly understood. This study uses measurements of NO3− concentrations and isotopic compositions (δ15O-NO3−, δ18O-NO3−, and ∆17O-NO3−) from the Qiangyong Glacier watershed on the Tibetan Plateau, combined with the MixSIAR model, to quantify the relative contributions of different NO3− sources. The results showed that GR was the dominant source to lake water NO3− 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 NO3− concentrations. During the early melt season, GR and enhanced in-lake nitrification increase NO3− concentrations, while sediment-mediated physical adsorption may contribute to their subsequent decrease. In contrast, during the non-melt season, NO3− 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 NO3−, highlighting that proglacial lakes are hotspots for nitrogen transformation.