[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.

[Contribution of Nitrogen Sources in Water Sources by Combining Nitrogen and Oxygen Isotopes and SIAR].

It is very important to identify nitrate sources in reservoirs that serve as high quality water sources to control its eutrophication. Stable isotopes (δ15 N and δ18O) and a Bayesian model (stable isotope analysis in R, SIAR) were applied to identify nitrate sources and estimate the proportional contributions of multiple nitrate sources in four reservoirs (Qingshan reservoir, Duihekou reservoir, Siling reservoir, and Lifan reservoir) that serve as sources of drinking water in the Hangjiahu area, one of the most densely populated and most quickly developing areas in East China. It was shown that nitrogen pollution, which was dominated by nitrate (NO3-), existed in the four reservoirs. Greater human activities caused more nitrogen pollution (average NO3- concentration 0.21 mmol ·L-1) in the Qingshan reservoir. A significant positive correlation (P<0.01) was observed between Cl- and NO3-. The analysis of the water in the Duihekou reservoir, Siling reservoir, and Lifan reservoir, with lower Cl- concentrations and higher NO3-/Cl- ratios, suggested that chemical fertilizer was the main source, while the analysis of the water in the Qingshan reservoir, with medium Cl- concentrations and NO3-/Cl- ratios, indicated a mixture of NO3- sources. The δ15 N ranged from 0.9‰ to 7.2‰, and the δ18O ranged from 2.8‰ to 14.1‰ in the four reservoirs. The δ18O values in more than 86% of the water samples were less than 10‰, and the δ15 N/δ18O values in 93% of the water samples were less than 1.3. It was identified that nitrification rather than denitrification acted as the primary N cycling process in the four reservoirs. SIAR was used to estimate the proportional contribution of five NO3- sources (industrial wastewater, sewage/manure, chemical fertilizer, soil nitrogen, and precipitation) in the Qingshan reservoir and of three NO3- sources (chemical fertilizer, soil nitrogen, and precipitation) in the Duihekou reservoir, Siling reservoir, and Lifan reservoir. The source apportionment results showed that chemical fertilizers and soil nitrogen were the dominant nitrate sources and their contributions were 75%-82%. It was revealed that nitrogen pollution in the water source reservoir caused by cropping non-point source pollution was very serious. Nitrate source contributions in Qingshan reservoir also included sewage/manure (25%), soil nitrogen (7%), and precipitation (6%), indicating that nitrogen pollution by sewage/manure should not be ignored in the higher human activity areas. The nitrate source in the Duihekou reservoir, Siling reservoir, and Lifan reservoir also included precipitation, with the nitrate contribution from precipitation at 21%, 24%, and 15%, respectively. It was suggested that precipitation contributed more nitrate to the water in areas with less human activity.