Quantifying the environmental fate and source of nitrate contamination using dual-isotope tracing coupled with nitrogen cascade model on the basin scale

Nitrate (NO₃⁻) contamination in riverine networks has threatened the environment and human health. Clarifying the NO₃⁻ source and environmental fate within the basin under different underlying surfaces is essential for water body protection, especially China's two mother rivers. A series of combination methods were established i.e., field survey, index measurements, isotope-tracing techniques, and material flow analysis in four typical basins to investigate the spatiotemporal variation and source of NO₃⁻ pollution and nitrogen cascade characteristics. The dual-isotope coupled with MixSIAR model revealed that manure and sewage were the major NO₃⁻ source in the irrigation basin (WY, 76.7%), hilly mountainous basin (YC, 52.3%), and plateau lake basin (DC, 48.7%). However, for the plain-river network basin (CZ), soil leachate was the main source (55.5%). In terms of the N losses to water within agri-environment system, livestock-breeding system in three basins made the biggest contribution among the systems, WY (77.3%), YC (47.3%), and DC (41.8%). While in CZ, about 34.4% of N was delivered from the crop-production system. The N cascade model verified the results of isotope-tracing techniques for each basin. The study provides new insight into NO₃⁻-tracing combining hydrogeochemical indicators, isotopic-tracing techniques, and material flow analysis and guides strategies for mitigating the negative impacts of NO₃⁻ pollution on aquatic environments on basin scale.