Transformation and fate of nitrate near the sediment–water interface of Copano Bay

This study investigated potential transformation processes and fates of nitrate at the sediment–water interface of Copano Bay during a period of drought by conducting continuous-flow and slurry experiments combined with a 15NO3− addition technique. Rates of 15NO3−-based denitrification, anaerobic ammonium oxidation (ANAMMOX) and potential dissimilatory nitrate reduction to ammonium (DNRA) were in the range of 27.7–40.1, 0.26–1.6 and 1.4–3.8 μmol 15N m–2 h–1, respectively. Compared with the total 15NO3−fluxes into sediments, dissimilatory processes contributed 29–49% to loss of the spiked 15NO3−. Based on the mass balance of 15NO3−, microbial assimilation was estimated to consume about 50–70% of the added 15NO3−, indicating that most of nitrate was incorporated by microorganisms in this N-limiting system. In addition, significant correlations of nitrate transformation rates with sediment characteristics reflect that the depth related behaviors of nitrate transformations in core sediments were coupled strongly to organic matter, iron (Fe) and sulfur (S) cycles.

► Assimilation was the main pathway of nitrate reduction in a N-limited coastal system.
► Dissimilatory processes competed with assimilation in controlling the fate of nitrate.
► Nitrate transformation processes is coupled to organic matter, Fe and sulfur cycles.