Effects of Drying-Flooding Alternation on Sediment–Water Nitrogen Fluxes in Hydro-fluctuation Belt of the Danjiangkou Reservoir

Danjiangkou reservoir is the water intake of the north-to-south water transfer middle route project and the largest freshwater lake in Asia. According to the operation mode of the reservoir, a large area is formed in the alternating state of drying-flooding annually and this may cause eutrophication in the aquatic ecosystem. The effects of drying-flooding alteration on nitrogen fluxes in the hydro-fluctuation belt of the Danjiangkou reservoir were investigated by conducting three experimental treatments of long-term (LFD), short-term drying-flooding alternating frequency (SFD), and continuous flooding (CF) with indoor incubation experiment. The results showed that the periodic drying-rewetting state is adverse to the ammonium release. The average exchange fluxes of NH4+-N in treatment LFD, SFD, and CF were -10.27, 11.79, and 48.12 mg m−2 d−1, respectively. The varying frequency of periodic drying-flooding had little influence on the exchange fluxes of TDN (FTDN-LFD = -50.52 mg m−2 d−1, FTDN-SFD = -51.60 mg m−2 d−1), but it affected the exchange fluxes significantly for different forms of nitrogenous compounds; the exchange fluxes of ammonium and nitrate in SFD treatment (11.79 and 3.77 mg m−2 d−1) were higher than those in LFD treatment (-10.27 and 0.43 mg m−2 d−1). Ammonium is the main nitrogen form in the bodies of water with long-term inundation; by extending the drying period of the sediment, the ammonium concentrations decreased while the nitrate concentrations increased.