Utilizing Hydrochemistry and Multiple Isotopes to Identify the Accumulation Mechanism of Nitrate in the Yangtze River Basin

The Yangtze River, the largest river system in Asia, continues to receive substantial nitrogen loads despite the implementation of management measures. Within this vast and complex system, the spatial patterns and drivers of key nitrogen transformation processes, such as nitrification and denitrification, remain poorly constrained. In particular, systematic isotopic evidence from studies spanning the entire upstream–midstream–downstream continuum remains scarce. This study integrates multiple isotopes (δ15N-NO3−, δ18O-NO3−, δ15N-NH4+) with hydrochemical techniques to elucidate the dominant controls on nitrogen transport and transformation and their spatial heterogeneity across the Yangtze River Basin. Results indicate that dissolved inorganic nitrogen (DIN) is the dominant form of nitrogen pollution in the basin. NO3− concentrations exhibited significant spatial variability, following the pattern downstream (2.86 mg/L) > upstream (1.83 mg/L) > midstream (1.75 mg/L). Isotopic signatures revealed that nitrification is the dominant process controlling the formation and transformation of NO3− throughout the basin. Most δ18O-NO3− values (−5.20‰ to +12.78‰) fell within or close to the theoretical range for nitrification, and a strong positive correlation was observed between δ15N-NO3− and δ15N-NH4+ (R2 = 0.72, p < 0.01), collectively confirming that the conversion of NH4+ to NO3− is the primary pathway. Conversely, denitrification was significantly suppressed under the prevailing high dissolved oxygen conditions (mean 9.78 ± 2.46 mg/L), as further evidenced by the lack of a significant correlation between δ15N-NO3− and ln(NO3−). Furthermore, preferential assimilation of NH4+ by phytoplankton reduced the efficiency of nitrate removal via biological assimilation and influenced isotopic composition. These findings provide a scientific basis for identifying priority nitrogen sources and optimizing targeted nitrogen management strategies in the Yangtze River Basin.