Nitrogen Removal in Created Wetlands : Considerations – Challenges – Possibilities

Abstract: Created wetlands in agricultural landscapes deliver a multitude of ecosystem services, one of which is the removal of nitrogen (N) from water to reduce eutrophication. Wetland N removal, primarily through denitrification, is influenced by various factors. For instance, macrophytes support denitrifying microorganisms and thus N removal, and the extent of N removal varies both spatially and temporally. The overall aim of this thesis was to provide a broadened understanding of considerations, challenges, and possibilities associated with achieving high N removal in created wetlands. To fulfil this aim, and thereby address knowledge gaps concerning wetland N removal, this thesis evaluates how N removal is affected by wetland placement and design, planting and harvesting of vegetation, installation of floating wetlands, changing climatic conditions, and interactions with other ecosystem services. These assessments were done using a combination of experimental wetland studies, a field study of created wetlands, and a literature review, all presented in the five included papers. The results highlight the rapid achievement of high N removal in wetlands planted with emergent vegetation, but also the diminishing effects of initial planting as wetland ecosystems approach maturity. Further, N removal is promoted in wetlands placed downstream of fertilised arable land to intercept as much runoff as possible, and in wetlands of elongated shape with maximised distance between inlet and outlet. Through such placement and design, created wetlands can maintain efficient N removal in spite of the anticipated increase in summer droughts. Additionally, multiple wetlands can jointly enhance landscape multifunctionality despite trade-offs between ecosystem services in individual wetlands. Even within a wetland, ecosystem services can synergistically interact. Although dependent on design, created wetlands can efficiently remove N and attenuate floods, without elevating greenhouse gas emissions. Moreover, specific wetland properties promote ecosystem services additional to N removal, making it possible to increase N removal by creating wetlands with other primary objectives. Lastly, N removal in heavily overgrown wetlands can be enhanced through macrophyte harvest, and floating wetlands offer further possibilities of improved N removal. In conclusion, with an improved understanding of wetland N removal, future creation and restoration of wetlands in agricultural areas hold the potential to further contribute to mitigating eutrophication and its detrimental consequences.