Sources and transformations of riverine nitrogen across a coastal-plain river network of eastern China: New insights from multiple stable isotopes

Wenli Chen; Xiaohan Zhang; Nianting Wu; Can Yuan; Yinli Liu; Yue Yang; Zheng Chen; Randy A Dahlgren; Minghua Zhang; Xiaoliang Ji

doi:10.1016/j.scitotenv.2024.171671

Sources and transformations of riverine nitrogen across a coastal-plain river network of eastern China: New insights from multiple stable isotopes

Sci Total Environ. 2024 May 10:924:171671. doi: 10.1016/j.scitotenv.2024.171671. Epub 2024 Mar 11.

Authors

Wenli Chen¹, Xiaohan Zhang², Nianting Wu², Can Yuan², Yinli Liu¹, Yue Yang³, Zheng Chen¹, Randy A Dahlgren⁴, Minghua Zhang⁵, Xiaoliang Ji⁶

Affiliations

¹ Key Laboratory of Watershed Science and Health of Zhejiang Province, School of Public Health and Management, Wenzhou Medical University, Wenzhou 325035, China.
² Zhejiang Provincial Key Laboratory for Water Environment and Marine Biological Resources Protection, College of Life and Environmental Science, Wenzhou University, Wenzhou 325035, China.
³ Zhejiang Provincial Key Laboratory for Water Environment and Marine Biological Resources Protection, College of Life and Environmental Science, Wenzhou University, Wenzhou 325035, China; Southern Zhejiang Water Research Institute, Wenzhou 325035, China.
⁴ Department of Land, Air and Water Resources, University of California, Davis, California 95616, USA.
⁵ Key Laboratory of Watershed Science and Health of Zhejiang Province, School of Public Health and Management, Wenzhou Medical University, Wenzhou 325035, China; Southern Zhejiang Water Research Institute, Wenzhou 325035, China; Department of Land, Air and Water Resources, University of California, Davis, California 95616, USA.
⁶ Key Laboratory of Watershed Science and Health of Zhejiang Province, School of Public Health and Management, Wenzhou Medical University, Wenzhou 325035, China. Electronic address: jixiao556677@wmu.edu.cn.

PMID: 38479520
DOI: 10.1016/j.scitotenv.2024.171671

Abstract

Riverine nitrogen pollution is ubiquitous and attracts considerable global attention. Nitrate is commonly the dominant total nitrogen (TN) constituent in surface and ground waters; thus, stable isotopes of nitrate (δ¹⁵N/δ¹⁸O-NO₃^-) are widely used to differentiate nitrate sources. However, δ¹⁵N/δ¹⁸O-NO₃^- approach fails to present a holistic perspective of nitrogen pollution for many coastal-plain river networks because diverse nitrogen species contribute to high TN loads. In this study, multiple isotopes, namely, δ¹⁵N/δ¹⁸O-NO₃^-, δ¹⁸O-H₂O, δ¹⁵N-NH₄⁺, δ¹⁵N-PN, and δ¹⁵N^bulk/δ¹⁸O/SP-N₂O in the Wen-Rui Tang River, a typical coastal-plain river network of Eastern China, were investigated to identify transformation processes and sources of nitrogen. Then, a stable isotope analysis in R (SIAR) model-TN source apportionment method was developed to quantify the contributions of different nitrogen sources to riverine TN loads. Results showed that nitrogen pollution in the river network was serious with TN concentrations ranging from 1.71 to 8.09 mg/L (mean ± SD: 3.77 ± 1.39 mg/L). Ammonium, nitrate, and suspended particulate nitrogen were the most prominent nitrogen components during the study period, constituting 45.4 %, 28.9 %, and 19.9 % of TN, respectively. Multiple hydrochemical and isotopic analysis identified nitrification as the dominant N cycling process. Biological assimilation and denitrification were minor N cycling processes, whereas ammonia volatilization was deemed negligible. Isotopic evidence and SIAR modeling revealed municipal sewage was the dominant contributor to nitrogen pollution. Based on quantitative estimates from the SIAR model, nitrogen source contributions to the Wen-Rui Tang River watershed followed: municipal sewage (40.6 %) ≈ soil nitrogen (39.5 %) > nitrogen fertilizer (9.7 %) > atmospheric deposition (2.8 %) during wet season; and municipal sewage (59.1 %) > soil nitrogen (30.4 %) > nitrogen fertilizer (4.1 %) > atmospheric deposition (1.0 %) during dry season. This study provides a deeper understanding of nitrogen dynamics in eutrophic coastal-plain river networks, which informs strategies for efficient control and remediation of riverine nitrogen pollution.

Keywords: Coastal-plain rivers; Multiple stable isotopes; Nitrogen biogeochemistry; Nitrogen pollution; SIAR model.