Lowland artificial watersheds with unique nutrient transport: Response to natural and anthropogenic drivers

Excess nitrogen (N) and phosphorus (P) from watersheds is a key factor causing eutrophication and water quality deterioration in downstream lakes/rivers. However, tracking N & P loss and transport at a watershed scale is challenging especially in lowland artificial watersheds (polders) with complex hydrological processes. To address this challenge, this study selected 171 polders in lower reach of Yangtze River in China as the study area, and investigated the response of N & P loss to both natural and anthropogenic drivers using two process-based models (PDP & NDP) specifically developed for polders. Our modelling practices revealed that these polders had a high N & P loss intensity of 38.37 and 1.75 kg/ha/yr, respectively. Rainy-season precipitation and TN & TP concentrations in connected-rivers were characterized as sensitive factors determining polder N & P loss with a threshold value of 950 mm/yr, 0.2 and 0.02 mg/L, respectively. Compared with annual precipitation, rainy-season precipitation more highly (P < 0.001, R2 >0.8) determined polder N & P loss, with a loss-intensity increase of 6.05 and 0.28 kg/ha/yr for 100 mm increase of rainy-season precipitation. Polder N & P loss was highly related (P < 0.001) to the area of farmland and surface water within these polders. This study can potentially improve our understanding of N & P cycling for lowland polders, and can thus support N & P control in water management practices.