Temporal export of nitrogen from a constructed wetland: influence of hydrology and senescing submerged plants

Abstract

Nitrogen export was measured during monthly monitoring in an 18-ha constructed wetland in southeast Sweden 1998–2001. To investigate the influence of increased water flow on wetland nitrogen export, we performed intensive sampling (eight samples per day) of total-N, urea-N, NH 4 +-N, NO 3 −-N in January–March 2001. Investigations of wetland plant distribution 1997–2001 and submerged plant biomass were combined with investigations of plant nitrogen content 1998–2001, to test the hypothesis that nitrogen export was linked to wetland plant decomposition. Nitrogen was exported from the wetland (3.3 kg N ha −1 d −1), during the end of the intensive sampling period (19 February–12 March), coinciding with increased water flow (from 0.2 to 0.6 m 3 s −1). Plant cover expanded rapidly and nitrogen assimilated in the submerged plant community was estimated to be 39 kg ha −1 in July 2001. After senescence in April the following year, plant biomass was reduced by 75%. We concluded that a significant part (40%) of this wetland nitrogen export might be associated with the release of organic and inorganic nitrogen from the senesced submerged plant community. Moreover, we hypothesize that nitrogen removal in wetlands in cold temperate climate may be limited by temporal nitrogen export induced by high water flow velocity when wetland plants are decomposing.