Modeling nitrogen export from 2539 lowland artificial watersheds in Lake Taihu Basin, China: Insights from process-based modeling

Excess nitrogen (N) poses a risk of aquatic eutrophication and ecosystem degradation to downstream areas. However, it is poorly investigated in lowland artificial watersheds (polders) due to their complex hydrological processes. This study investigated the N export and retention at all 2539 polders in Lake Taihu Basin, China using a nitrogen dynamic model (NDP), specially developed for polders. The response of N export to global climate change (air temperature and precipitation) and human activities (N fertilization) in the future (2014-2049) were quantified through scenario simulations. The simulation results revealed that these polders have a larger N export coefficient (a spatially-averaged value of 15.3 kg N/ha/yr) than that of non-polder areas (similar to 6.7 kg N/ha/yr) mainly due to their intensive agricultural activities and high population density. However, these polders also showed considerable N retention with estimated retention rates of 52.7-54.6%. In model results from the future (2014-2049), human activities (N fertilization) determined the magnitude of polder N export, while global climate change determined the fluctuation of polder N export. The estimation of polder N sources, sinks and retention capacity at a watershed scale can be used to identify the hotspot of N loss, and thus guide decision-making to control N loss.