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 N 2 O microsensor measurements, whereas NH + 4 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 NO − 3 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 NO − 3 reduced, was found at a site with very high concentrations of total sulfide and NH + 4 within and below the layer in which NO − 3 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 NO − 3 reduced. These contradictory results might be explained by enhanced NO − 3 availability for DNRA bacteria in the sediment slurries compared to the core-incubated sediments in which diffusion of NO − 3 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).

Dissimilatory NO 3؊ reduction in sediments is often measured in bulk incubations that destroy in situ gradients of controlling factors such as sulfide and oxygen. Additionally, the use of unnaturally high NO 3 ؊ concentrations yields potential rather than actual activities of dissimilatory NO 3 ؊ reduction. We developed a technique to determine the vertical distribution of the net rates of dissimilatory nitrate reduction to ammonium (DNRA) with minimal physical disturbance in intact sediment cores at millimeter-level resolution. This allows DNRA activity to be directly linked to the microenvironmental conditions in the layer of NO 3 ؊ consumption. The water column of the sediment core is amended with 15 NO 3 ؊ at the in situ 14 NO 3 ؊ concentration. A gel probe is deployed in the sediment and is retrieved after complete diffusive equilibration between the gel and the sediment pore water. The gel is then sliced and the NH 4 ؉ dissolved in the gel slices is chemically converted by hypobromite to N 2 in reaction vials. The isotopic composition of N 2 is determined by mass spectrometry. We used the combined gel probe and isotopic labeling technique with freshwater and marine sediment cores and with sterile quartz sand with artificial gradients of 15 NH 4 ؉ . The results were compared to the NH 4 ؉ microsensor profiles measured in freshwater sediment and quartz sand and to the N 2 O microsensor profiles measured in acetylene-amended sediments to trace denitrification.Nitrate accounts for the eutrophication of many humanaffected aquatic ecosystems (19,21). Sediment bacteria may mitigate NO 3 Ϫ pollution by denitrification and anaerobic ammonium oxidation (anammox), which produce N 2 (13, 18). However, inorganic nitrogen is retained in aquatic ecosystems when sediment bacteria reduce NO 3 Ϫ to NH 4 ϩ by dissimilatory nitrate reduction to ammonium (DNRA) (5,12,16,39). Hence, DNRA contributes to rather than counteracts eutrophication (23). DNRA may be the dominant pathway of dissimilatory NO 3 Ϫ reduction in sediments that are rich in electron donors, such as labile organic carbon and sulfide (4,8,17,38,55). High rates of DNRA are thus found in sediments affected by coastal aquaculture (8, 36) and settling algal blooms (16).DNRA, denitrification, and the chemical factors that control the partitioning between them (e.g., sulfide) should ideally be investigated in undisturbed sediments. The redox stratification of sediments involves vertical concentration gradients of pore water solutes. These gradients are often very steep, and their measurement requires high-resolution techniques, such as microsensors (26, 42) and gel probes (9, 54). If, for instance, the influence of sulfide on DNRA and denitrification is to be investigated, one wants to know exactly the sulfide concentration in the layers of DNRA and denitrification activity, as well as the flux of sulfide into these layers. This information can easily be obtained using H 2 S and pH microsensors (22,43). It is less trivial to determine the vertical distribution of DNRA...

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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. 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% of the total NO₃⁻ reduced) rather than by DNRA, irrespective of the sedimentary inventories of electron donors such as organic carbon, sulfide, and iron. Ammonium production via DNRA (8.9% of the total NO₃⁻ reduced) was exclusively found at one site with very high concentrations of total sulfide and NH₄⁺ in the layer of NO₃⁻ reduction and below. Sediment from two field sites, one with and one without 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 can be explained by enhanced NO₃⁻ availability for DNRA bacteria in the sediment slurries compared to the core-incubated sediments.

It can be argued that the gel probe technique gives more realistic estimates of DNRA activity in diffusion-dominated sediments, while slurry incubations are more suitable for advection-dominated sediments