Abstract
The Yangtze River Basin has experienced rapid population growth, agricultural intensification, urbanization and industrial development since the 1970s, leading to eutrophication problems. However, there is large spatial heterogeneity of development trajectories and associated changes in nutrient sources and fates within the Yangtze River basin. To capture this heterogeneity, here we applied the Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model (IMAGE-DGNM) to quantify nutrient dynamics across 12 subbasins in the Yangtze River Basin during 1970–2019. Results show that nutrient sources, delivery, retention, export, and ratios and their changes substantially differed across the subbasins. Agricultural groundwater runoff from middle-reach subbasins contributed most to the basin-wide total nitrogen (TN) delivery, while agricultural surface runoff from upstream subbasins and point sources from middle- and lower-reach subbasins had the largest contribution to the basin-wide total phosphorus (TP) delivery. The Middle Mainstream Subbasin (including Three Gorges Reservoir) accounts for <10% of the total basin area but contributed over one-fourth of basin-wide TN and TP retention, primarily due to its constructed dams and reservoirs. Molar N:P ratios of both nutrient delivery and export increased across all subbasins, shifting from predominantly below the Redfield ratio in 1970 to widespread P limitation in 2019. Nutrient management controlling local sources alone could only mitigate 8–32% TN loads and 10–33% TP loads in the four mainstream subbasins.