Isotopic and hydrochemical analyses reveal nitrogen source variation and enhanced nitrification in a managed peri-urban watershed

Abstract

Watershed management practices (WMPs) alter the sources and transformation of reactive nitrogen (N) in peri-urban watersheds, but a precise description of how WMPs impact N cycling is still lacking. In this study, four sampling campaigns were conducted in the wet and dry seasons of 2019 (before WMPs) and 2020 (after WMPs) to determine the spatiotemporal variations in nitrate isotopes (¹⁵N-${NO}_3^{-}$ and ¹⁸O-${NO}_3^{-}$) and hydrochemical compositions in the Muli River watershed. The results showed that the WMPs could significantly reduce the N load in the middle and lower reaches, but substantial improvements were not observed in 2020. Manure and sewage (M&S, 36.2 ± 15.8–55.0 ± 19.4%) was the major source of nitrate (${NO}_3^{-}$) in the stream water, followed by smaller-scale wastewater treatment plants (WWTPs, 14.0 ± 10.9–25.6 ± 11.5%). The WMPs were effective in controlling M&S, resulting in an approximately 16.7% (p < 0.01) lower M&S contribution during the dry season in 2020 compared to that in 2019. However, the smaller-scale WWTP input increased by approximately 5.4% (p < 0.01) after the WMPs. During the study period, the assimilation of ${NO}_3^{-}$ by phytoplankton was important for ${NO}_3^{-}$ loss, but the WMPs promoted nitrification in the watershed because of the elevated redox potential (Eh). Overall, the present study provides a better estimate of the variations in nitrogen sources and transformation in a peri-urban watershed after WMPs and provides an approach for developing timely nitrogen management solutions.