Research papers Tracing nitrate sources and transformations using Δ17O, d15N, and d18O-NO3- in a coastal plain river network of eastern China

Identifying the sources and transformations of riverine nitrate plays a critical role in mitigating nitrogen enrichment of river networks. Several previous studies have used delta O-18-NO3- to quantitatively assess riverine nitrate contributed by atmospheric nitrate and terrestrial sources, but their results have great uncertainty due to the wide range of delta O-18-NO3- values and isotopic fractionation during nitrogen-cycling processes in terrestrial environment. The nitrate O-17 anomaly (Delta O-17-NO3-), as an unambiguous tracer of atmospheric nitrate, is a promising tool to effectively separate atmospheric nitrate from microbially produced nitrate. However, to our knowledge, Delta O-17-NO3- 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 (delta D/delta O-18-H2O and delta N-15/ delta O-18/Delta O-17-NO3-) 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 Delta O-17-NO3- values in precipitation and river water during the study period (April-June of 2021) varied from 14.83%o to 31.39%o and from -2.82%o to 9.66%o, respectively. The delta D/delta O-18-H2O values revealed that river water mainly originated from recent precipitation with little evaporation. Moreover, the delta N-15/delta O-18-NO3- 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 delta N-15/Delta O-17-NO3-, 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 Delta O-17-NO3- to enhance nitrate source apportionment.

Automated summary

This study used a multiple isotope approach to quantitatively identify sources and transformations of riverine nitrate in a wetland plain river network in eastern China. The results showed that municipal sewage was the dominant nitrate source, followed by soil nitrogen, atmospheric nitrate deposition, and nitrogen fertilizer. Additionally, the study highlighted the benefits of using Delta O-17-NO3- to enhance nitrate source apportionment.