Combined gel probe and isotope labeling technique for measuring dissimilatory nitrate reduction to ammonium in sediments at millimeter-level resolution.

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.