As one of the most widespread elements, nitrogen has been broadly concerned in water bodies. Understanding variations in nitrogen is of paramount importance to ecosystem stability and human health. The spatiotemporal variations in total nitrogen (TN) and ammonia in the Middle Route of the South-to-North Water Diversion Project (MRP) during the period from 2015 to 2019 were evaluated. The correlation between anthropogenic activities based on quantitative land use cover and nitrogen concentration was addressed. The results indicated that TN increased by 0.072 mg/L from south to north over the period (p < 0.05), but ammonia decreased by 0.018 mg/L (p < 0.05), notably, in five years. In addition, Chl a had the highest concentration in autumn, showing seasonal variation. The linear regression showed that ammonia concentration was significantly negatively correlated with Chl a (p < 0.1). Furthermore, as human activities’ intensity increased by 6‰ from 2015 to 2019, TN increased and ammonia decreased. The rhythm of meteorological conditions could also result in the variation in nitrogen, which affected N concentration in the MRP. The increase in construction land and agricultural land led to TN increase, and algae absorption was one of the reasons leading to the decrease in ammonia. It could be concluded that climatic changes and anthropogenic activities were the driving forces of nitrogen changes in the MRP. Thus, land use changes around the MRP should be the focus of attention to reduce the nitrogen concentration. This study is the first report on the nitrogen distribution pattern in the MRP. It could be useful to authorities for the control and management of nitrogen pollution and better protection of water quality.
High-frequency NO<sub>3</sub><sup>−</sup> isotope (δ<sup>15</sup>N, δ<sup>18</sup>O) patterns in groundwater recharge reveal that short-term land use and climatic changes influence nitrate contamination trends