| Estuaries are the main channel linking both land and coastal seas. With increasing human activities in recent decades, large quantities of reactive nitrogen derived from the upstream watershed have been transported into estuarine and coastal areas via runoff, atmospheric deposition, and sewage discharge. The enrichment of reactive nitrogen in estuarine and coastal ecosystems has already caused a series of environmental problems including eutrophication, harmful algal blooms, and seasonal hypoxia in these aquatic ecosystems. Therefore, it is of significance to carry out the study on the transportation and transformation of nitrogen in estuarine and coastal environments.Denitrification and anaerobic ammonium oxidation (anammox) are the most important pathways of removing reactive nitrogen in estuarine wetlands via effectively converting the reactive nitrogen to nitrogen gas. In the present study, the eastern tidal flat of Chongming Island as a typical study area was selected to quantify the fluxes of inorganic nitrogen (NH4+-N plus NO3--N) across the sediment-water interface and the denitrification and anammox rates. We also explored the function of sediment as inorganic N sink/source and the contibutions of denitrification and anammox to the removal of nitrogen at the study area. The main findings of this work are as follows:(1) The flux of NH4+-N ranged from-5.91 to 14.44 mmol m-2 d-1, with an average of 2.74 mmol m-2 d-1. The flux of NO3--N ranged from-20.09 to 8.72 mmol m-2 d-1, with an average of-2.06 mmol m-2 d-1. Of all the detected environmental variables, temperature, salinity, NH4+-N, NO3--N and total organic carbon (TOC) significantly affected the fluxes of inorganic nitrogen between the sediment-water interface. On an annual basis, there were about 2101 tons of NH4+-N released from the sediment to the overlying water, and about 1577 tons of NO3--N in the overlying water diffused into the sediment in the eastern tidal flat of Chongming Island. The flux of inorganic nitrogen across the sediment-water interface showed that the sediment played an important role in adjusting inorganic nitrogen levels in the overlying water.(2) The denitrification rates ranged from 0.01 to 40.08 μmol N m-2 h-1, with an average of 11.18μmol Nm-2 h-1. The rates of denitrification were generally higher in the warm seasons than in cool seasons. At the same time, the denitrification rates were significant higher in salt-water controlling section than in freshwater controlling section. Also, relatively higher rates of denitrification were observed in the high tidal flat than in the middle and low tidal flats. It was observed that the denitrification rates were significantly and positively related to temperature, TOC, TN and exchangeable Mn, while they were negatively related to sediment Eh. The denitrification process was estimated to remove 202.89 t N/a, which contributed 61.37%～99.64% to the total nitrogen loss. Therefore, denitrification was the main nitrogen removal pathway at the study area.(3) The rates of anammox ranged from 0.0001 to 7.71 μmol N m-2 h-1, with an average of 0.85 μmol N m-2 h-1. It was found that the anammox rates were relatively lower in cold seasons than in warm seasons. Meanwhile, the rates of anammox were significantly higher in freshwater controlling section than in seawater controlling section significantly. Also, higher anammox rates were detected in the high tidal flat than in the middle and low tidal flats. Statistical analyses showed that the rates of anammox were positively related to temperature, while they are negatively related to water content and salinity. The anammox process was estimated to remove 15.37 t N /a, which contributed 0.34%-38.63% to the total nitrogen loss. |
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