Mixed nitrogen inputs affected nitrate distribution and biogeochemical processes during ice-covered and ice-free periods in a boreal eutrophic steppe lake basin

Nitrogen enrichment from nitrate is a significant contributor to water quality eutrophication in various freshwater basins. However, the input pathways and transformation processes of nitrate remain unclear, particularly during different hydrological processes in boreal eutrophic steppe lake basins that are susceptible to human activity and climate change stressors. In this study, we experimentally investigate the nitrogen sources, transport and transformation in the high-latitude Hulun Lake basin during both ice-covered and ice-free periods using the dual stable isotopes of nitrate (δ¹⁵N–NO₃^- and δ¹⁸O–NO₃^-) with the Stable Isotope Analysis in R (SIAR) model. Our results indicate that nitrate input contributed to water body eutrophication in the Hulun Lake basin, with varying inputs during different hydrological processes. Our quantitative analysis reveals that during the ice-covered period, manure/sewage water (MS) was the dominant nitrate input pathway, accounting for 55.3%, while atmospheric precipitation (AP) contributed the most at 49.8% during the ice-free period. The primary transport and transformation process of nitrate was further identified as nitrification instead of denitrification, as evidenced by the Cl⁻ and NO₃⁻/Cl⁻ levels and redundancy analysis. This study highlights the complexity of nitrate input pathways and transformation processes under different hydrological conditions and emphasizes their significant impact on the nitrogen cycle and ecology of water environments in boreal steppe lake basins.