High-frequency measurement of N2O emissions from a full-scale vertical subsurface flow constructed wetland

In the last few years, the emission of nitrous oxide (N2O) from constructed wetlands has become a topic of increased interest. Most N2O emission measurements made so far are based on limited grab-samples, leading to sparse temporal resolution in the data. Using a high sampling frequency strategy, gaseous and dissolved N2O emissions from a two-stage vertical subsurface flow constructed wetland, treating domestic raw wastewater, were investigated. The monitoring period that lasted 3 weeks corresponded to an entire operation cycle of the filters including the feeding and rest periods. Gaseous N2O fluxes exhibited remarkable temporal fluctuations with contrasted emission rates between the feeding and rest periods, as well as high variability within a given period. Total potential emission rates (gaseous + dissolved N2O fluxes) were found to be similar in both stages, with a mean value over the entire operation cycle of 1.50 and 1.78 mgN/m(2)/h in Stage 1 and Stage 2, respectively. The dynamic behavior of the emissions was found to be correlated to the changes in the biogeochemical conditions within the filters. A positive relationship between gaseous N2O emissions and oxygen concentrations in filter media was observed during conditions favoring nitrification and denitrification, suggesting that N2O can be emitted from both processes.