Estuarine sediments contribute substantially to global N 2 O emission originating from marine environments, and it is often speculated whether nitrification or denitrification is the most significant process contributing to N 2 O 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 N 2 O emission rates. In the present study, we used microsensors to measure porewater profiles of NO x -, NO 2 -, O 2 and N 2 O to determine how N 2 O is produced under high nutrient conditions in a mangrove sediment. These microsensors allow measurement of the rates and vertical location of nitrification and denitrification (NO x -reduction) at sub-millimeter scale. Production of N 2 O 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 N 2 O under low nutrient conditions was below the detection limit of the sensor, but simulating eutrophication scenarios by adding NH 4 + or NO 3 -, led to N 2 O production from both nitrification and denitrification. Under NH 4 + -rich conditions, N 2 O production contributed to 5.4 and 18% of the N turnover of from nitrification and denitrification, respectively. Although denitrification produced more N 2 O, nitrification was the more important process for sediment N 2 O emission. Nitrous oxide originating from denitrification was produced in deeper sediment layers, and mostly consumed within the sediment, whereas N 2 O originating from nitrification was produced close to the sediment surface, allowing N 2 O 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.