N2O, N2 and CH4 fluxes were measured from a horizontal subsurface flow (HSSF) constructed wetland (CW) for wastewater treatment in Estonia. The closed chamber method was used in the field and the He-O method (intact soil core analyses) in the lab throughout the period from October 2001 to June 2002. The average flux of N2O-N, N2-N and CH4-C from various microsites ranged from 0.1 to 59, 4.1 to 1,458 and -0.04 to 2,094 mg m-2 d-1, respectively. A significantly higher flux of N2O was found in chambers installed above the inlet pipes, while the methane flux was higher in the inlet part of the bed with wetter conditions. The groundwater table significantly correlates with gas emission rates of all the gases studied; N2 emission was enhanced by higher temperature of wastewater. PO43- and NH4+ content significantly enhanced, and NO2- and NO3- content inhibited, both N2O and CH4 fluxes. NH4+ showed a negative correlation with N2 flux. Nitrification and denitrification are the main processes of the N removal in the CW covering 42.9%. The specific global warming potential was highest in the wet bed and lowest in the dry bed with lowered water table (32 and 9 g CO2 pe-1 d-1, respectively).

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