Quantification of Nitrate Sources and Its Spatial Heterogeneity by Dual Isotopes

Abstract

Nitrate source allocation is important for watershed environmental management. In this paper, 59 surface water samples were collected in late July 2022 in the Erhai Basin of the Yunnan–Guizhou Plateau in southwestern China, where the source of nitrate pollution in surface water is still unknown. The potential sources of nitrate were identified and estimated using hydrochemistry indicators, the δ 15 N / δ 18 O − NO 3 − and the Stable Isotope Analysis in R (SIAR) model. The results showed that the various forms of nitrogen compounds, transport and transformation processes, and their sources in the Erhai Basin were characterized by obvious spatial scale differentiation. First, the mean values of NO 3 − , NO 2 − , and NH 4 + in the watershed were 1.14 ± 1.49 mg/l, 0.11 ± 0.06 mg/l, and 0.14 ± 0.09 mg/l, respectively, while the differences among the three in the secondary zoning scale were mainly reflected in the ES region. Second, both δ 15 N − NO 3 − (−0.64‰ to +17.67‰) and δ 18 O − NO 3 − (−1.87‰ to +24.42‰) had some differences in primary and secondary zoning. Third, nitrification was the dominant process of nitrate transport and transformation in the watershed, but it was also characterized by spatial differentiation. Fourth, the output of the SIAR model showed that the proportion of nitrate pollution sources in the basin was soil nitrogen (37.16%), domestic sewage (36.06%), livestock manure (13.49%), and chemical fertilizer (13.29%), and there was also obvious spatial differentiation. This study provides a new perspective for the spatial heterogeneity analysis of nitrate pollution sources and the corresponding pollution control.