Nitrogen Pollution in the Water Source Region of the Middle Route of China’s South-to-North Water Diversion Project: A Review of Research Progress and Challenges

The Danjiangkou Reservoir and the upper reaches of the Han River constitute the primary water source region of the Middle Route of the South-to-North Water Diversion Project, which supplies freshwater to major cities in northern China, including Beijing and Tianjin. Despite substantial investments in pollution control in this region, N pollution persists, posing a high risk of eutrophication and harmful algal blooms and threatening water quality, ecosystem stability, and regional water security. Systematic reviews summarizing key advances in N pollution research in the study area are also limited. In this review, we synthesize recent advances in N pollution research in this water source region, focusing on major N sources, and analyze the spatial heterogeneity and temporal dynamics of the N pollution sources under hydrological regulation and climate variability. We also elucidate key N transformation pathways within the water–soil–biota continuum, highlighting the coupled roles of microbial and plant communities. Additionally, we evaluate the effectiveness and limitations of existing control strategies, including watershed management, ecological restoration, constructed wetlands, and policy interventions, and propose future research priorities and integrated management approaches to support targeted N mitigation and long-term water quality protection.