The study systematically reviews the intrinsic mechanisms and reservoir dynamics of watershed-scale nitrogen retention, summarizing its spatiotemporal distribution characteristics, key influencing factors, as well as the current research and application status of numerical models. Research has found that there are bottlenecks in understanding the coupling mechanism of multimedia interfaces, quantifying the lag effect of nitrogen retention, and uncertainty in cross scale models, leading to spatial and temporal mismatches between management policies and water quality responses. Future research needs to deepen the understanding of the surface water-sediment-groundwater continuum coupling mechanism, promote the deep integration of artificial intelligence and mechanistic models, and innovate adaptive management paradigms based on nature-based solutions.