Nitrogen pollution and source identification in the Haicheng River basin in Northeast China

► Nitrogen pollution showed significant seasonal variations (p < 0.01 or p < 0.05), but little spatial differences. ► Nitrogen pollution in the river was driven by nitrification and denitrification processes during the sampling periods. ► The sampling sites were divided into three clusters, representing low, moderate and high nitrogen pollution, respectively. ► The pollution sources were mainly from nitrogen fertilizers, animal wastes, domestic sewage, and industrial wastewater. ► The river provided about 700 tons of the annual TN flux, contributing to the eutrophication of the Liaodong Bay.