Enhancing wetland nitrogen removal through macrophyte harvest and installation of woodchips-based floating beds

• Constructed wetlands (CWs) remove nitrogen (N) more efficiently as wetlands age and vegetation establishes. However, when CWs become heavily overgrown and filled with plant litter and root mats, channels form and N removal declines. Here, management may maintain high N removal. We tested two CW management options 1) restoration through macrophyte harvesting including root mat removal, and 2) subsequent installation of woodchips-based floating beds (WFBs) to compensate macrophyte loss. In a field experiment, using 16 heavily overgrown experimental CWs, we applied four treatments: i) macrophyte harvesting, ii) macrophyte harvesting and 5% of the wetland surface covered with WFBs, iii) macrophyte harvesting and 20% WFB cover, and iv) a control treatment (heavily overgrown). WFBs were planted with Glyceria maxima and Filipendula ulmaria before installation. N removal efficiency, removal rate and removal rate coefficient ka were estimated on nine occasions. After the experiment, WFBs were removed and plant biomass accrual, N assimilation, and denitrification gene (nirS, nirK, nosZI and nosZII) abundance on plant roots and woodchips were studied.Macrophyte harvesting significantly improved N removal of heavily overgrown CWs, whereas WFBs installation only improved N removal in harvested treatments on some occasions. Both G. maxima and F. ulmaria grew well on WFBs. Relative biomass production, root length and root surface area were higher for G. maxima than for F. ulmaria whereas biomass N assimilation was higher for F. ulmaria. Denitrification gene abundance was higher on plant roots than on woodchips while G. maxima hosted higher root denitrification gene abundance than F. ulmaria. We conclude that macrophyte harvesting improves N removal in heavily overgrown CWs. Further long-term field studies are needed to precisely evaluate the contribution of WFBs to N removal in CWs.

Ämnesord

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Nyckelord

Aquatic plants

floating wetlands

floating treatment

nitrogen uptake

denitrification potential

wetland restoration

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