High spatiotemporal resolution process-based model revealed an aggravated nitrogen legacy-related water pollution risk in a large basin: spatio-seasonal patterns, drivers, and management opportunities

Legacy nitrogen, i.e., long-term nitrogen storage in landscapes, poses a formidable water environmental management challenge due to its ongoing threats to surface water quality, considerable magnitude, and dispersed spatial distribution. A comprehensive understanding of legacy nitrogen and its impacts on water quality at broader, finer spatiotemporal scales is crucial for legacy-related water pollution control and eco-environment sustainable development. Here, we developed a process-based model system with high spatiotemporal resolution to generate seasonal maps of legacy nitrogen in soils ( N leg _ soi ) and groundwater ( N leg _ gr ) for the Songhuajiang River basin during 1978–2020. The model revealed an upswing in seasonal legacy nitrogen storage, with increases of approximately 1.4- to 3.6-fold. Legacy hotspots covering 10% of the basin area contributed approximately 38–67% of total legacy nitrogen storage, implying markedly uneven spatial distribution of legacy nitrogen. N leg _ soi showed higher sensitivity to anthropic factors, while N leg _ gr was more sensitive to natural factors. With increasing legacy effects over time, legacy nitrogen was responsible for approximately 48–73% of seasonal riverine nitrogen loading. A combination of livestock-crop-human coupled system, fossil fuel use reduction, and soil and manure management can effectively abate legacy-related water pollution.