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 ¹⁵NO ₃ ^- addition technique. Rates of ¹⁵NO ₃ ^--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 1.4-3.8μmol ¹⁵Nm ^-2h ^-1, respectively. Compared with the total ¹⁵NO ₃ ^-fluxes into sediments, dissimilatory processes contributed 29-49% to loss of the spiked ¹⁵NO ₃ ^-. Based on the mass balance of ¹⁵NO ₃ ^-, microbial assimilation was estimated to consume about 50-70% of the added ¹⁵NO ₃ ^-, 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.