Spatial-temporal distribution characteristics of nitrogen in the Jingjiang reach of the Yangtze River affected by the operation of Three Gorges Reservoir

The regulation of water storage in the Three Gorges Reservoir (TGR) has altered the migration and transformation of nutrients in the middle and lower reaches of the Yangtze River. In this study, long-term hydrological and water quality monitoring data (before and after the operation of the TGR) for Yichang, Zhicheng, Shashi, and Chenglingji along the Jingjiang Reach in the middle-stream of Yangtze River were analyzed, and the distribution characteristics of nitrogen (N) in overlying water were quantified. The results shows that the annual average total nitrogen (TN) concentration in the Jingjiang Reach increased from 1992 to 2015, while the ammonium nitrogen (NH4+-N) concentrations showed a downward trend. The variation trends of both N forms shifted around 2003 when TGR begins to storage water. Seasonal variations were observed, with the highest concentrations of TN and nitrate nitrogen (NO3–N) occurring during the normal water period (March-May). During this period, the average TN concentration across the four monitoring sites was 33% and 19.8% higher than those during the dry and wet periods, respectively. In contrast, the peak concentrations of NH4+-N was observed during the dry period (October-February). Additionally, correlation analysis and Redundancy Analysis (RDA) revealed that Chemical Oxygen Demand (COD) and the distance from the TGR were the most influential factors on N concentrations in the Jingjiang Reach, with corresponding contribution rates of 36.8% and 32.5%, and the distance from TGR exerted a significant positive effect on NH4+-N concentration, while COD showed a significant negative effect on TN concentration. The results suggested that the impoundment of the TGR has had a significant impact on the N content in the Jingjiang Reach, providing a theoretical support for nutrient management in the Yangtze River.