[Tracking Riverine Nitrate Sources and Transformations in the Yiluo River Basin by Nitrogen and Oxygen Isotopes]

The impacts of natural processes and anthropogenic input on riverine nitrate (NO^-₃) could be identified by NO^-₃ concentrations and nitrogen and oxygen isotope ratios (δ¹⁵N-NO^-₃ and δ¹⁸O-NO^-₃); however, the effects of variable land use on riverine NO^-₃ sources and transformations remain unclear. In particular, the human impacts on riverine NO^-₃ in mountain areas are still unknown. The Yihe River and Luohe River were used to elucidate this question due to their spatially heterogeneous land use. Hydrochemical compositions, water isotope ratios (δD-H₂O and δ¹⁸O-H₂O), and δ¹⁵N-NO^-₃ and δ¹⁸O-NO^-₃ values were utilized to constrain the NO^-₃ sources and transformations affected by different land use types. The results indicated that ① the mean nitrate concentrations in the Yihe River and Luohe River waters were 6.57 and 9.29 mg·L^-1, the mean values of δ¹⁵N-NO^-₃ were 9.6‰ and 10.4‰, and the average δ¹⁸O-NO^-₃ values were -2.2‰ and -2.7‰, respectively. Based on the analysis of δ¹⁵N-NO^-₃ and δ¹⁸O-NO^-₃ values, the NO^-₃ in the Yihe and Luohe Rivers were derived from multiple sources, and nitrogen removal existed in the Luohe River, but the biological removal in the Yihe River was weak. ② The contributions of different nitrate sources were calculated using a Bayesian isotope mixing model (BIMM) based on δ¹⁵N-NO^-₃ and δ¹⁸O-NO^-₃ values of river water in the mainstream and tributaries with spatial locations. The results revealed that sewage and manure had major impacts on riverine nitrate in the upper reaches of both the Luohe River and Yihe River, where forest vegetation was widely distributed. However, the contributions from soil organic nitrogen and chemical fertilizer were higher in the upper reaches than in downstream ones. The contributions of sewage and manure still increased in the downstream reaches. Our results confirmed the primary impacts of point sources, e.g., sewage and manure, on riverine nitrate in the studied area, and the contributions of nonpoint sources, e.g., chemical fertilizer, had not increased as the agricultural activities elevated the downstream. Therefore, more attention should be paid to point source pollution treatment, and the high-quality development of ecological civilization in the Yellow River Basin should be maintained.