Nitrous oxide exchanges with the atmosphere of a constructed wetland treating wastewater

Abstract

Static chamber measurements of N₂O fluxes were taken during the 1998 and 1999 growth seasons in a Swedish constructed wetland receiving wastewater. The dominating plant species in different parts of the wetland were Lemna minor L., Typha latifolia L., Spirogyra sp. and Glyceria maxima (Hartm.) and Phalaris arundinacea (L.), respectively. There were large temporal and spatial variations in N₂O fluxes, which ranged from consumption at –350 to emissions at 1791 μg N₂O m⁻² h⁻¹. The largest positive flux occurred in October 1999 and the lowest in the middle of July 1999. The average N₂O flux for the two years was 130 μg N₂O m⁻² h⁻¹ (SD = 220). No significant differences in N₂O fluxes were found between the years, even though the two growing seasons differed considerably with respect to both air temperature and precipitation. 15% of the fluxes were negative, showing a consumption of N₂O. Consumption occurred on a few occasions at most measurement sites and ranged from 1–350 μg N₂O m⁻² h⁻¹. 13–43% of the variation in N₂O fluxes was explained by multiple linear regression analysis including principal components. Emission factors were calculated according to IPCC methods from the N₂O fluxes in the constructed wetland. The calculated emission factors were always lower (0.02–0.27%) compared to the default factor provided by the IPCC (0.75%). Thus, direct application of the IPCC default factor may lead to overestimation of N₂O fluxes from constructed wastewater-treating wetlands.