Vertical activity distribution of dissimilatory nitrate reduction in coastal marine sediments

Abstract. The relative importance of two dissimilatory nitrate reduction pathways, denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA), was investigated in intact sediment cores from five different coastal marine field sites (Dorum, Aarhus Bight, Mississippi Delta, Limfjord and Janssand). The vertical distribution of DEN activity was examined using the acetylene inhibition technique combined with N₂O microsensor measurements, whereas NH₄⁺ production via DNRA was measured with a recently developed gel probe-stable isotope technique. At all field sites, dissimilatory nitrate reduction was clearly dominated by DEN (59–131% of the total NO₃⁻ reduced) rather than by DNRA, irrespective of the sedimentary inventories of electron donors such as organic carbon, sulfide, and iron. Highest ammonium production via DNRA, accounting for up to 8.9% of the total NO₃⁻ reduced, was found at a site with very high concentrations of total sulfide and NH₄⁺ within and below the layer in which NO₃⁻ reduction occurred. Sediment from two field sites, one with low and one with high DNRA activity in the core incubations, was also used for slurry incubations. Now, in both sediments high DNRA activity was detected accounting for 37–77% of the total NO₃⁻ reduced. These contradictory results might be explained by enhanced NO₃⁻ availability for DNRA bacteria in the sediment slurries compared to the core-incubated sediments in which diffusion of NO₃⁻ from the water column may only reach DEN bacteria, but not DNRA bacteria. The true partitioning of dissimilatory nitrate reduction between DNRA and DEN may thus lie in between the values found in whole core (underestimation of DNRA) and slurry incubations (overestimation of DNRA).