Nitrification and denitrification as sources of sediment nitrous oxide production: A microsensor approach

Estuarine sediments contribute substantially to global N2O emission originating from marine environments, and it is often speculated whether nitrification or denitrification is the most significant process contributing to N2O production in soils and sediments. These processes are driven by different precursor compounds, biotic and abiotic variables, and improved causal understanding is required for minimising N2O emission rates. In the present study, we used microsensors to measure porewater profiles of NOx−, NO2−, O2 and N2O to determine how N2O is produced under high nutrient conditions in a mangrove sediment. These microsensors allow measurement of the rates and vertical location of nitrification and denitrification (NOx− reduction) at sub-millimeter scale. Production of N2O can then be associated with each process. Sediment cores from the fringe of a subtropical mangrove forest were analysed in the laboratory to a depth of 10 mm. Production of N2O under low nutrient conditions was below the detection limit of the sensor, but simulating eutrophication scenarios by adding NH4+ or NO3−, led to N2O production from both nitrification and denitrification. Under NH4+-rich conditions, N2O production contributed to 5.4 and 18% of the N turnover of from nitrification and denitrification, respectively. Although denitrification produced more N2O, nitrification was the more important process for sediment N2O emission. Nitrous oxide originating from denitrification was produced in deeper sediment layers, and mostly consumed within the sediment, whereas N2O originating from nitrification was produced close to the sediment surface, allowing N2O to diffuse to the overlying water and the atmosphere. This result stresses the importance of considering both actual production rates from each process, and their vertical position in the sediment with regard to diffusive transport to the sediment surface.