Release Experiment Reveals Eutrophication Risk of using Polders to Impound Flood Water in the Yangtze River

In the riparian zone of the Yangtze River, using polders to impound flood water with high sediment content probably brings eutrophication. A nutrient (nitrogen and phosphorus) release experiment was conducted to reveal the risk. Bare soil and arable soil were used as underlying soils. Overlying water with two sediment concentrations (formed 1 cm and 4 cm thickness layer after deposited, respectively) was used to simulate reserved floods. Higher nutrient content in arable soil led to faster nutrient release into overlying water (16–21 and 0.32–1.32 mg/d·cm² for TN and TP, respectively) relative to bare soil (5–13.5 and 0.25–1.26 mg/d·cm² for TN and TP, respectively), suggesting that bare land with lower nutrient content is better than arable land for storing flood water. Nutrient content in sediment deposit layer relative to in original underlying soil determined whether deposited sediment accelerated or mitigated the release of nutrients into overlying water. Because sediment has higher nutrient content than bare soil, the preferred time to impound flood is at the end of flood seasons when sediment load steeply decreases. It is more likely to suffer eutrophication in the initial stage of flood impounding, when overlying water was abundant in both nitrogen and phosphorus and N/P ratio (10–23) was fit for phytoplankton growth. This study helps to identify the risk of eutrophication due to reserving turbid flood water in polders.