Nitrogen removal by wetlands in a cold climate

Nitrogen (N), a fundamental component of living organisms, has become one of the main global concerns for human society due to the myriad of negative effects of excessive N on ecosystems. Anthropogenic activities such as agriculture, industrial production, urbanisation and mining are major sources of N to freshwaters. Semi-natural and constructed wetlands planted with macrophytes are now widely used in many parts of the world to remove N from water. However, the potential of constructed wetlands for N removal under cold climatic conditions is still not well studied. We also have limited understanding of how macrophyte species and growth form richness as well as functional trait diversity affect N-cycling in constructed wetlands by influencing plant N accumulation, plant associated denitrification and abundance of denitrifying bacterial communities.In mesocosm experiments and in situ studies, I investigated how species and growth form (emerging and submerged macrophytes and bryophytes) richness as well as plant functional trait diversity of macrophytes affect N-cycling in wetlands. Moreover, I tested the applicability of constructed floating wetlands for improved N removal at the local scale in a cold climate.My results highlight that macrophytes are important for both main N removal pathways, viz. uptake and denitrification. Moreover, bacterial denitrification gene abundance on roots and shoots of macrophytes were an important predictor of the denitrification potential of macrophytes. Species and growth form richness of macrophytes enhanced N removal in wetlands. Moreover, I identified complementarity and a selection effect as important diversity related mechanisms, explaining the total N removal from water including plant N accumulation. Functional traits of macrophytes affected N-cycling in wetlands through direct and indirect pathways. The application of constructed floating wetlands at the local scale is feasible in a cold climate with denitrification as the main N removal pathway in these wetland type. Further in situ studies with high numbers of species and growth forms are needed to generalize my findings. Future studies should also consider plant secondary metabolites to better understand the function of the macrophyte-denitrifier interplay.