Estimation of Nitrification and Denitrification from Microprofiles of Oxygen and Nitrate in Model Sediment Systems

The coupling between nitrification and denitrification and the regulation of these processes by oxygen were studied in freshwater sediment microcosms with O₂ and NO₃^- microsensors. Depth profiles of nitrification (indicated as NO₃^- production), denitrification (indicated as NO₃^- consumption), and O₂ consumption activities within the sediment were calculated from the measured concentration profiles. From the concentration profiles, it was furthermore possible to distinguish between the rate of denitrification based on the diffusional supply of NO₃^- from the overlying water and the rate based on NO₃^- supplied by benthic nitrification (D_w and D_n, respectively). An increase in O₂ concentration caused a deeper O₂ penetration while a decrease in D_w and an increase in D_n were observed. The relative importance for total denitrification of NO₃^- produced by nitrification thus increased compared with NO₃^- supplied from the water phase. The decrease in D_w at high oxygen was due to an increase in diffusion path for NO₃^- from the overlying water to the denitrifying layers in the anoxic sediment. At high O₂ concentrations, nitrifying activity was restricted to the lower part of the oxic zone where there was a continuous diffusional supply of NH₄⁺ from deeper mineralization processes, and the long diffusion path from the nitrification zone to the overlying water compared with the path to the denitrifying layers led to a stimulation in D_n.