Emissions of nitrous oxide from a constructed wetland using a groundfilterand macrophytes in waste-water purification of a dairy farm

In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragm ites australis) are increasingly used to purify waste water. Particularly i n the rhizosphere of the reed, nitrification and denitrification processes take place varying locally and temporally, and the question arises to what extent this type of waste-water treatment plant may contribute to the relea se of N2O. In situ N2O measurements were carried out in the two reed beds o f the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste wa ter from a cheese dairy and 70 local inhabitants (12 m(3) waste water or 6 kg BOD5 or 11 kg chemical O-2 demand (CODMn) day(-1)). During November 1995 to March 1996, the release of N2O was measured weekly at Im distances usin g eight open chambers and molecular-sieve traps to collect and absorb the e mitted N2O. Simultanously, the N2O trapped in the soil, the soil temperatur e, as well as the concentrations of NH4+-N, NO3--N, NO2--N, water-soluble C and the pH were determined at depths of 0-20, 20-40 and 40-60 cm. In the w aste water from the in- and outflow the concentrations of CODMn, BOD5, NH4-N, NO3--N, NO2--N, as well as the pH, were determined weekly. Highly varyi ng amounts of N2O were emitted at all measuring dates during the winter. Ev en at soil temperatures of -1.5 degrees C in 10 cm depth of soil or 2 degre es C at a depth of 50 cm, N2O was released. The highest organic matter and N transformation rates were recorded in the upper 20 cm of soil and in the region closest to the outflow of the constructed wetland. Not until a freez ing period of several weeks did the N2O emissions drop drastically. During the period of decreasing temperatures less NO3--N was formed in the soil, b ut the NH4+-N concentrations increased. On average the constructed wetlands of Friedelhausen emitted about 15 mg N2O-N inhabitant equivalent(-1) day(- 1) during the winter period. Nitrification-denitrification processes rather than heterotrophic denitrification are assumed to be responsible for the N 2O production.