Tracing sources of nitrate using water chemistry, land use and nitrogen isotopes in the Ganjiang River, China

In this work, we traced sources of nitrate in the Ganjiang River, a major tributary of Yangtze River, China, by analysing the water chemistry, nitrogen isotopes and land use. Water samples from 20 sites in the main stream and tributaries were collected in the dry and wet seasons. The δ15N- NO3− ranged from 0.97 to 8.60 ‰, and was significantly higher in the wet season than in the dry season, and significantly higher in tributaries than in the main stream. In the dry season, NO3− concentrations and δ15N- NO3− were significantly negatively correlated with forest and grassland areas, and positively correlated with paddy field and residential area. However, most of the correlations were not significant in the wet season. The results showed that fertilizer was the main source of nitrate in the Ganjiang River, and domestic sewage was important in the dry season, but its contribution was lower than that in other rivers in the Yangtze Basin. In the wet season, the intensified nitrogen cycle caused by high temperature and the mixing effect caused by rainfall made it difficult to trace nitrate sources using δ15N- NO3− and land use.