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

Abstract

 In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragmites australis) are increasingly used to purify waste water. Particularly in 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 release of N₂O. In situ N₂O measurements were carried out in the two reed beds of the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste water from a cheese dairy and 70 local inhabitants (12 m³ waste water or 6 kg BOD₅ or 11 kg chemical O₂ demand (COD_Mn) day^–1). During November 1995 to March 1996, the release of N₂O was measured weekly at 1 m distances using eight open chambers and molecular-sieve traps to collect and absorb the emitted N₂O. Simultanously, the N₂O trapped in the soil, the soil temperature, 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 waste water from the in- and outflow the concentrations of COD_Mn, BOD₅, NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, as well as the pH, were determined weekly. Highly varying amounts of N₂O were emitted at all measuring dates during the winter. Even at soil temperatures of –1.5  °C in 10 cm depth of soil or 2  °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 freezing period of several weeks did the N₂O emissions drop drastically. During the period of decreasing temperatures less NO₃ ^–-N was formed in the soil, but 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 N₂O production.