Dissolved inorganic nitrogen fluxes at sediment-water interface in Yangtze estuarine tidal flat

Yangtze estuary data, collected over three years, indicates that the temporal and spatial distributions of the environmental gradients reflect complicated seasonal changes and spatial differences in the exchange flux of the dissolved inorganic nitrogen (DIN= NH₄ ⁺+ NO₃ ^-+ NO₂ ^-) across the sediment-water interface. Overall in northern sites of Yangtze estuary, sediment was a source of ammonium (NH₄ ⁺) (-3.67~10.65 mmol·m^-2·d^-1) probably because of higher salinities. Sediment was a sink for NH4 + in southern sites (-18.45~3.33 mmol·m^-2·d^-1) during most years. The exchange behavior of nitrate (NO₃ ^-) showed temporal and spatial variation from the upper to lower estuary and ranged from -32.8 mmol·m^-2·d^-1 to 35.8 mmol·m^-2·d^-1. The interface exchange direction of ammonium was affected by NH₄ ⁺ concentration, but the relationship between NO₃ ^- concentration and the direction of flux was not obvious. The concentration of nitrite (NO₂ ^-) was very low and its interface flux was not related to DIN concentration. Overall, the sediment of Yangtze Estuarine tidal flat was a source of DIN to overlying water in the spring, but a sink for DIN during the other three seasons of the year.