[Identifying Seasonal Variation of Nitrate Sources in Mountainous Rivers at the Qiandao Lake Basin Based on Nitrogen and Oxygen Isotopes].

Qiandao Lake is an important source of drinking water in the Yangtze River Delta, and its ecological environment is of great strategic significance to the surrounding areas. To identify the sources and spatial distribution characteristics of nitrate （NO₃^--N） pollution in the Qiandao Lake, we conducted, for the first time, the collection of water samples from four typical mountainous inlet river basins in the Qiandao Lake Basin, analyzed the concentrations of NO₃^--N, and resolved different sources and their contribution in each water system by combining the δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- dual stable isotope analysis in R （SIAR） model. The results showed that： ① Nitrogen concentrations in the different watersheds were relatively low, with mean total nitrogen （TN） levels ranging from 0.99 to 4.31 mg·L^-1. NO₃^--N emerged as the main nitrogen source, and conspicuous disparities were observed in NO₃^--N concentrations across the four rivers, consistently demonstrating a pattern of spring > winter > summer > autumn, of which the NO₃^--N concentration during spring could be up to 3.2 times of that observed during autumn. ② The values of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in each watershed ranged from 1.52‰ to 14.29‰ and from -2.76‰ to 10.13‰, respectively. ③ All four rivers showed a greater proportion of fertilizer and soil nitrogen during spring and summer, which accounted for approximately 25% to 51% and 23% to 39%, respectively, and a greater proportion of domestic sewage during autumn and winter, which accounted for approximately 26% to 67%. The study showed that the main source of NO₃^--N pollution in the Qiandao Lake Basin was agricultural non-point source pollution, and some variabilities were also observed in NO₃^--N pollution in different land-use type areas. NO₃^--N pollution contributions remained relatively stable across the larger basin area, while exhibiting significant fluctuations in the smaller basin area. This work analyzed the main sources of NO₃^--N in the Qiandao Lake Basin, providing a basis for water quality management and pollution source control in this area.