Nitrous oxide, dinitrogen and methane emission in a subsurface flow constructed wetland

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. PO₄^3- 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).