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.

本研究以中国长江主要支流赣江为研究对象，通过分析水体化学特征、氮同位素组成与土地利用类型，对其硝酸盐来源开展溯源分析。研究于枯水期与丰水期采集了赣江干流及支流共20个监测点位的水样。赣江水体的δ15N-NO3−范围为0.97‰~8.60‰，且丰水期该比值显著高于枯水期，支流点位的δ15N-NO3−显著高于干流点位。枯水期时，水体硝酸根浓度与δ15N-NO3−均与林地、草地面积呈显著负相关，与稻田面积及居民区用地面积呈显著正相关；但在丰水期，多数相关关系并不显著。研究结果表明，化肥是赣江硝酸盐的主要来源；枯水期生活污水虽为重要贡献源，但其贡献占比低于长江流域其他河流。丰水期时，高温引发的氮循环强化效应与降雨带来的混合作用，使得通过δ15N-NO3−与土地利用类型开展硝酸盐溯源分析的难度大幅提升。