Tracing nitrate sources and transformations using △17O, δ15N, and δ18O-NO3− in a coastal plain river network of eastern China.

[Display omitted] • △17O-NO 3 − was firstly used to trace riverine nitrate sources in eastern China. • Nitrification was identified as dominant nitrogen-cycling process by δ 15N/ δ 18O-NO 3 −. • δ 15N/△17O-NO 3 − and SIAR modeling determined MS as the primary nitrate source. • Uncertainties of source contributions decreased in the order: SN > MS > NF > AD. Identifying the sources and transformations of riverine nitrate plays a critical role in mitigating nitrogen enrichment of river networks. Several previous studies have used δ 18O-NO 3 − to quantitatively assess riverine nitrate contributed by atmospheric nitrate and terrestrial sources, but their results have great uncertainty due to the wide range of δ 18O-NO 3 − values and isotopic fractionation during nitrogen-cycling processes in terrestrial environment. The nitrate 17O anomaly (△17O-NO 3 −), as an unambiguous tracer of atmospheric nitrate, is a promising tool to effectively separate atmospheric nitrate from microbially produced nitrate. However, to our knowledge, △17O-NO 3 − approach has not been previously applied to identify nitrate pollution sources in plain river networks of eastern China. In this study, we used a multiple isotope approach (δ D/ δ 18O-H 2 O and δ 15N/ δ 18O/△17O-NO 3 −) for the first time to quantitatively identify sources and transformations of riverine nitrate in a hypereutrophic coastal plain river network namely Wen-Rui Tang River located in eastern China, which is a region receiving high inputs of atmospheric nitrogen deposition. The △17O-NO 3 − values in precipitation and river water during the study period (April–June of 2021) varied from 14.83‰ to 31.39‰ and from − 2.82‰ to 9.66‰, respectively. The δ D/ δ 18O-H 2 O values revealed that river water mainly originated from recent precipitation with little evaporation. Moreover, the δ 15N/ δ 18O-NO 3 − values indicated that microbial nitrification, not denitrification, was the predominant nitrogen-cycling process in the watershed. Based on a Bayesian mixing model (Stable Isotope Analysis in R, SIAR) using δ 15N/△17O-NO 3 −, municipal sewage was identified as the dominant nitrate source (50.5 ± 11.7%), followed by soil nitrogen (23.8 ± 13.7%), atmospheric nitrate deposition (14.3 ± 2.9%), and nitrogen fertilizer (11.4 ± 8.7%). Finally, an uncertainty analysis for nitrate source apportionment demonstrated that the greatest uncertainty was associated with soil nitrogen, followed by municipal sewage, nitrogen fertilizer, and atmospheric nitrate deposition. This study provides important scientific information on riverine nitrate source apportionment to guide pollution control/remediation strategies and highlights the benefits of utilizing △17O-NO 3 − to enhance nitrate source apportionment. [ABSTRACT FROM AUTHOR]