[Identify the Nitrate Sources in Different Land Use Areas Based on Multiple Isotopes]

Different land uses have different impacts on the water quality of the region. Multiple isotopes (δD-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-) and the SIAR (stable isotope analysis in R) model were applied to identify the nitrate sources and estimate the proportional contributions of multiple nitrate sources in a river in a typical urban area (the Grand Canal, Hangzhou) and a river in a typical forest and agricultural area (Yuying Riveri). The results indicated that there were different degrees of nitrogen pollution in the Grand Canal and Yuying River; NO₃^--N and NH₄⁺-N are the predominant forms of nitrogen in the Grand Canal, and the primary form of nitrogen in Yuying River was NO₃^--N. There was an obvious linear relationship between the hydrogen and oxygen isotopes (R²=0.78). The δD-H₂O and δ¹⁸O-H₂O values for the Grand Canal and Yuying River were distributed along the local meteoric waterline, indicating that precipitation served as the primary water source in these rivers. All of the δ¹⁸O-NO₃^- values of the Grand Canal and Yuying River were lower than 15 ‰. It was revealed that nitrification, rather than denitrification, was the primary N cycling process in the two rivers. The δ¹⁵N-NO₃^-/δ¹⁸O-NO₃^- ratios of some of the samples from the Grand Canal ranged from 1.3 to 2.1, accompanied by low concentrations of DO and NO₂^-, indicating that denitrification existed in some sections of the Grand Canal. The δ¹⁵N-NO₃^- values of the samples from the Grand Canal (average:6.1‰) were higher than those from the Yuying River (average:2.3‰). The NO₃^- source contributions differed significantly between the Grand Canal and Yuying River. The contributions of NO₃^- sources in the Grand Canal were sewage/manure (37.0%) > soil nitrogen (35.7%) > chemical fertilizer (19.1%) > precipitation (8.2%), and those in the Yuying River were chemical fertilizer (46.1%) > soil nitrogen (22.8%) > precipitation (17.3%) > sewage/manure (13.8%). The contribution of the sewage/manure was substantially increased in the Grand Canal in the urban area with stronger human activities primarily due to the sporadic discharge of domestic sewage and urban runoff. Chemical fertilizer is the main NO₃^- source in the Yuying River near the forest and agricultural area, suggesting that the nitrogen pollution caused by agricultural non-point sources was extremely serious. The contribution of precipitation decreased in the areas of substantial human activities. The isotopic fractionation produced by denitrification was affected by the contributions of the NO₃^- sources, which were calculated by SIAR model. Sewage/manure and chemical fertilizer produced significant impacts, followed by soil nitrogen and precipitation.