Nutrient kinetics in submerged plant beds: A mesocosm study simulating constructed drainage wetlands

Constructed wetlands have become a widespread mean to reduce nutrient loading from tile drained agricultural areas. As a supplement to emergent plants usually present in these wetlands, submerged plants may, if present, enhance nutrient retention by occupying the deeper zones of the wetland basins. The nutrient retention efficiency may, however, vary among submerged species and also vary between multi-species communities compared to single-species communities. In this study we performed a mesocosm experiment to quantify the inorganic nitrogen (NH4-N and NO3-N) and phosphorus (PO4-P) uptake kinetic parameters (Vmax and Cmin) in constructed wetlands (1) in habitats with and without plants; and (2) in multi-species communities and single-species communities using four submerged plant species relevant for use in these wetlands. We found that uptake rates of PO4-P and NH4-N was three and five times higher, respectively, in habitats with plants compared to habitats without plants, whereas the uptake rates of NO3-N was similar. Multi-species communities were not more efficient in nutrient retention than single-species communities, although a residual analysis indicated that multi-species communities might be better in taking up and depleting NH4-N but not PO4-P and NO3-N. Overall, our study shows that submerged plants in deeper waters of drainage wetlands can be an important nutrient retaining component, and that a high biomass of one efficient plant species (e.g. R. aquatilis) is working similarly well as multi-species communities in this context.