Inorganic nitrogen fluxes at the sediment-water interface in Tidal flats of the Yangtze estuary

Inorganic nitrogen fluxes at the sediment-water interface of seven typical sites in tidal flats of the Yangtze estuary had been determined seasonally during more than three years. The results revealed that the sediment-water exchanges of inorganic nitrogen appeared to be of complicated spatial differences and seasonal variations. The measured fluxes of N0₃^- -N and NH₄⁺ -N in the Yangtze estuary were higher than those in other estuaries, varying respectively from -32.82 to 24.13 mmol ·m^-2 ·d^-1 and from -18.45 to 10.65 mmol ·m^-2 ·d^-1. However, the fluxes of NO₂^- -N were very low, and only varied from -1.15 to 2.82 mmol ·m^-2· d^-1. The spatial and seasonal differences between the upper and lower estuary for the NO₃^- -N fluxes were observed clearly, but the NH₄⁺ -N fluxes exhibited the spatial and seasonal differences between the south and north bank. It had been recognized that the NH₄⁺ -N exchange behavior was mainly controlled by the salinity, whereas the NO₃^- -N exchange behavior was influenced by the different combinations of the sediment grain size, nitrate concentration in overlying water, sediment organic matter content, water temperature and dissolved oxygen concentration.