Identifying nitrate sources and transformations in the Yellow River: Insights from dual isotopes and long-term...

Anthropogenic activities pose increasingly significant nitrate (NO₃-N) pollution to surface water. The NO₃-N source tracking is key to effective pollutant control and policy development. Nitrate dual isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-) and long-term NO₃-N concentrations in the Yellow River were used to identify NO₃-N sources, transformations, and driving factors. The NO₃-N concentrations and proportional contributions from different sources varied considerably from upper reaches to lower reaches in the high flow season. The NO₃-N concentrations were high in the reach downstream of Lanzhou relative to the headwater area, driven by intensive agricultural and urban inputs; soil organic nitrogen dominated in the upper (54.8%) and lower (35.2%) reaches, whereas manure and sewage constituted the primary source in the middle reaches (43.3%). The NO₃-N concentrations along the Yellow River decreased over a decade of environmental regulations, but key transformations remained unchanged, with nitrification still being the dominant process. The NO₃-N and dissolved inorganic nitrogen (DIN) concentrations in the lower Yellow River significantly increased from 1981 to 2005 but decreased from 2005 to 2021. The increases in NO₃-N and DIN concentrations were mainly attributed to agricultural activities and sewage discharge, while the reduction in fertilizer application accounted for approximately 62% of the DIN decrease. The study reveals that continued fertilizer management, strict wastewater discharge control, and sub-regional cooperation in the Yellow River are crucial for reducing nitrate, which has implications for effective pollutant control of nitrate pollution.