Inorganic nitrogen exchange across the sediment–water interface in the eastern Chongming tidal flat of the Yangtze Estuary

This study investigated environmental influences on inorganic nitrogen exchange at the sediment–water interface and quantified the function of sediment as inorganic nitrogen sink/source in the eastern Chongming tidal flat of the Yangtze Estuary. Intact sediment cores (20–25 cm deep) were collected from the high, middle and low tidal flats of the study area every 2 months from July 2012 to May 2013, and incubated to measure the fluxes of ammonium and nitrate (plus nitrite) across the sediment–water interface via continuous-flow experiments. The incubation experimental results showed that ammonium was mainly released from the sediment into the overlying water, with a flux of −5.91 to 14.44 mmol m⁻² day⁻¹. In contrast, nitrate was primarily influxed from the overlying water into the sediment, with a flux of −20.09 to 8.72 mmol m⁻² day⁻¹. In this work, temperature, salinity, sedimentary ammonium and nitrate, and total organic carbon were observed to significantly affect the dynamic exchange of inorganic nitrogen across the sediment–water interface of the study area, via controlling nitrogen transformation processes in sediments. It was also estimated that ca. 2101 t N year⁻¹ for ammonium was totally released from the sediment to overlying water while ca. 1577 t N year⁻¹ for nitrate was removed from the water to sediment. The dynamic exchanges of inorganic nitrogen at the sediment–water interface implied that the sediment has important influence on adjusting the levels of ammonium and nitrate in tidal water of the study area.