Observations of nitrogen and phosphorus biogeochemistry in a surface flow constructed wetland

Free surface water constructed wetlands (CWs) provide a buffer between domestic wastewater treatment plants and natural waterways. Understanding the biogeochemical processes in CWs is crucial to improve their performance. In this study we measured a range of water and sediment parameters, and biogeochemical processes, in an effort to describe the processing of nutrients within two wetland cells in series. As a whole the studied CW effectively absorbed both nitrogen (N) and phosphorus (P) emanating from the waste treatment plant. However the two individual cells showed marked differences related to the availability of oxygen within the water column and the sediments. In one cell we speculated that the prevalence of surface plant species reduced its ability to function as a net nutrient sink. Here we observed a build-up of sediment organic matter, sediment anoxia, a decoupling of nitrification–denitrification, and a flux of N and P out of the sediments to the overlying water. The availability of DO in the surface sediments of the second studied cell led to improved coupling between nitrification–denitrification and a net uptake of both NH₄⁺ and PO₄³⁻. We hypothesise that the dominance of deeply rooted macrophytes in the second cell was responsible for the improved sediment quality.