Combined Gel Probe and Isotope Labeling Technique for Measuring Dissimilatory Nitrate Reduction to Ammonium in Sediments at Millimeter-Level Resolution

Dissimilatory NO3- 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 NO3- concentrations yields potential rather than actual activities of dissimilatory NO3- 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 NO3- consumption. The water column of the sediment core is amended with (NO3-)-N-15 at the in situ (NO3-)-N-14 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 NH4+ 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 (NH4+)-N-15. The results were compared to the NH4+ microsensor profiles measured in freshwater sediment and quartz sand and to the N2O microsensor profiles measured in acetylene-amended sediments to trace denitrification.