Dissimilatory nitrate reduction processes and associated contribution to nitrogen removal in subtidal sediments of the Yangtze Estuary

Abstract:

Hou Lijun, Liu Min, Deng Fengyu

State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China

Dissimilatory nitrate reduction processes, including denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) can determine nitrate dynamics and fate in estuarine and coastal environments. However, factors controlling the denitrification, anammox and DNRA processes and their respective contributions to the nitrogen removal remain unclear for specific aquatic environments. In this study, we investigate potential rates of denitrification, anammox and DNRA in the subtidal sediments of the Yangtze Estuary via slurry incubation experiments combined with isotope-tracing techniques to reveal their respective contributions to total nitrate reduction in this hypereutrophic estuarine ecosystem. Measured rates of denitrification, anammox and DNRA ranged from 0.06 to 4.51 μmol N kg^-1 h^-1, 0.01 to 0.52 μmol N kg^-1 h^-1, and 0.03 to 0.89 μmol N kg^-1 h^-1, respectively. These potential dissimilatory nitrate reduction process rates correlated significantly with salinity, sulfide, organic carbon and nitrogen. Denitrification contributed 38 – 96 % total nitrate reduction in the Yangtze Estuary, as compared to 3 – 45 % for DNRA and 1 – 36 % for anammox. In total, the denitrification and anammox processes removed approximately 25% of the external inorganic nitrogen transported annually into the estuary. In contrast, most external inorganic nitrogen was retained in the estuary and contributes substantially to the severe eutrophication of the Yangtze Estuary.

 Keywords: Nitrogen; Ammonium; Nitrate; Denitrification; Anaerobic ammonium oxidation; Dissimilatory nitrate reduction to ammonium; Sediment