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The fluxes of greenhouse gases (methane and nitrous oxide) emission from a constructed wetland in the East China as municipal sewage treatment were measured from June 1999 to August 2000 by the closed chamber method. The constructed wetland for municipal sewage treatment is a significant source of methane, up to 976.6 x 106 gCH4/a, which was emitted from the constructed wetland with the area of 495000 m2 and wastewater loading rate of 12000 m3/d. Its daily mean methane flux reached 5.22 gCH4/(m2*d),250 times as much as that in natural wetland in the same latitude region. 227.8 mgCH4 was produced from the treatment of 1 liter wastewater, up to 700-1000 times as much as that in the secondary treatment. The emission of nitrous oxide from the constructed wetland is not higher than that from secondary treatment of wastewater, only 0.07 mgN2O/L.