Legacies in Flux: How Nutrient Histories, Land Use, and Climate Change Reshape Riverine N:P Ratios

Long-term dynamics of riverine nitrogen-to-phosphorus (N:P) ratios remain inadequately understood due to interactions among climate, land use, and legacy nutrients. Based on a 40-year (1980–2020) data set from the Yongan Watershed in eastern China, this study quantified watershed N and P inputs and riverine N/P ratio dynamics, disentangled the effects of legacy nutrients, land use, and climate variability on river N/P ratios, and projected their future trends under alternative scenarios of land use and climate change. Our results revealed nonlinear and asynchronous relationships between watershed inputs and riverine exports, driven by differential legacy responses of N and P. A 14-year lag provided the best fit between watershed input ratios and riverine export ratios (R2 = 0.93). Riverine N/P ratios were positively correlated with agricultural land and grassland cover but negatively correlated with urban land and temperature. Our scenario-based projections indicate that future climate warming, urban expansion, and grassland loss will further decrease riverine N/P ratios and heighten the risk of eutrophication. Therefore, river management in the Yongan Watershed should consider both N and P together in a coordinated manner with particular attention to legacy nutrient pools and urban P runoff.