Estimation of nitrification and denitrification from microprofiles of oxygen and nitrate in model sediment systems

Abstract

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).