Resolving Nitrate Sources in Rivers Through Dual Isotope Analysis of δ¹⁵N and δ¹⁸O

Nitrate (NO₃⁻) pollution in rivers within agricultural regions has become a global issue that cannot be ignored. Identifying the sources and transformation processes of NO₃⁻ is crucial for safeguarding water quality in agricultural catchment areas. This study traces the sources and transformation processes of NO₃⁻ in the Songhua River basin of Northeast China by analysing hydrochemical parameters and NO₃⁻ dual isotopes (δ¹⁵N and δ¹⁸O) in river water. It estimates the proportional contributions of NO₃⁻ sources using Bayesian modelling via the MixSIAR package （3.1.12）in the R programming language. (1) The relatively low NO₃⁻/Cl⁻ ratio and high chloride concentrations in the upstream section indicate that the primary sources of NO₃⁻ in this area are manure and sewage (M&S). (2) Dual isotope analysis of NO₃⁻ indicates that the primary sources of NO₃⁻ in the Songhua River basin are M&S, soil nitrogen (SN), and chemical fertilizers (CF). (3) Nitrification occurs throughout the entire watershed. (4) Model estimation results indicate that SN constitutes the primary source of NO₃⁻ throughout the entire watershed (48%), with no significant variation observed across the studied river sections. However, other major NO₃⁻ sources exhibit spatially significant differences, primarily manifested as follows: M&S constitute the primary upstream source of NO₃⁻ (39%), whilst downstream contributions are predominantly attributable to CF (20%). Intermediate regions experience combined impacts from both MS and CF sources.