[Analysis of Nitrate Sources in Different Rivers Flowing into the Sea Based on Nitrogen and Oxygen Isotopes]

The nitrogen flux from terrestrial input is the primary source of nitrogen load in estuarine areas, leading to ecological issues, such as coastal eutrophication and hypoxia. Systematically identifying the nitrogen sources from different rivers draining into the sea is a prerequisite for controlling the nitrogen pollution in estuarine areas. Based on the nitrogen and oxygen isotopes of 179 sampling sites across 12 rivers in Liaoning and Hebei Provinces, the nitrate sources of different rivers were systematically identified. Moreover, the stable isotope analysis in the R （SIAR） model was employed to quantitatively calculate the contribution rates of various pollution sources to river nitrate. The results indicated that nitrification was the primary process of nitrate transformation. For all 12 rivers, the contribution rate of animal manure and sewage ranged from 13.9% to 90.1% and that of soil nitrogen ranged from 4.3% to 54.0%. Additionally, for the Liaojiawa River, Xuanhui River, and Nanpai River in Cangzhou City, agricultural fertilizers were observed to have relatively high contribution rates, ranging from 25.2% to 64.4%. Overall, the nitrogen loads in these 12 rivers were highly disturbed by anthropogenic activities and the primary sources of nitrate varied among different rivers. Therefore implementing targeted and differentiated control measures in watershed nitrogen control and management is important.