Abstract: In recent years, nitrogen pollution in rivers within China's agricultural and urbanized regions has become increasingly severe. Excessive nitrogen input has led to significant ecological and environmental issues, such as river eutrophication, while the complex sources and transformation processes of nitrogen pose new challenges for watershed management and pollution control. Field sampling was conducted during the dry season and rainfall periods of the wet season in 2023. Water samples were collected from the upstream, midstream, and downstream sections of the mainstream and tributaries of the Yinma River in Qinhuangdao City. The spatiotemporal variation characteristics of different forms of nitrogen were analyzed and, by using nitrogen and oxygen isotope techniques combined with the MixSIAR model, the contribution rates of different pollution sources to nitrate in the river were evaluated. The results showed the concentrations of TN and \mathrmNO_3^- -N in the study area showed the characteristics of the tributaries < upstream < downstream < midstream, and the concentrations of \mathrmNH_4^+ -N were relatively low. Nitrogen concentrations were influenced by both the flushing and dilution effects of rainfall. The sources of nitrate in the water primarily included atmospheric deposition, manure and sewage, soil nitrogen, and agricultural fertilizers. The river showed no significant evidence of denitrification or ammonia volatilization. During the dry season, nitrification was the dominant nitrogen transformation process, whereas in the wet season, nitrification was relatively insignificant. Before rainfall in both the dry and wet seasons, the primary source of nitrate in the water was urban sources (manure and sewage). After rainfall in the wet season, the contribution rate of agricultural sources (agricultural fertilizers and soil nitrogen) increased significantly. It was recommended to enhance the management of domestic wastewater during the dry season, and rationalize the application of agricultural fertilizers during the wet season to reduce the impact of nitrogen pollution on the aquatic environment. This study reveals the spatiotemporal characteristics and driving mechanisms of river nitrogen under different environmental conditions, and provides a scientific basis for understanding the mechanism of nitrogen migration and transformation in rivers.