Potential of constructed wetlands to reduce nitrate pollution in agricultural catchments

Nitrate (NO3-) concentrations and loads are still above ecologically acceptable levels in many open waters, including coastal seas. Agricultural measures aimed at reducing the nitrogen (N) surplus in fields usually only have an effect in the long term. A short-term reduction of N pollution can be achieved through technical solutions such as surface-flow constructed wetlands (SFWs). We analyzed the effect of SFWs to reduce nitrate-nitrogen (NO3--N) loads in the tile-drained agricultural Warnow river basin (approx. 3000 km(2)) in northeastern Germany. The aim of the study was to transfer the results from individual SFWs to the scale of the river basin. For this purpose, an eco-hydrological model (Soil and Water Assessment Tool) was used to reproduce streamflow and NO3--N loads on a monthly basis for the basin outlet and six subbasins. Measured water and compound fluxes depicted well with the model, so that we could conduct a scenario analysis. 97 locations for the establishment of SFWs in tile-drained areas throughout the river basin were identified and integrated into the model environment. The results indicate that the NO3--N loads for the entire basin could be reduced from 902,200 kg yr(-1) to 842,900 kg yr(-1), which corresponds to a NO3--N reduction of 6.6%. NO3--N removal rates varied widely be-tween the subbasins, ranging from 5 to 86 g m(-2) yr(-1), and were positively correlated with NO3--N input loads. We conclude that SFWs, if installed area-wide, can help to significantly reduce NO3--N loads in a river basin and can be an important component of a comprehensive management strategy to reduce N loads in surface waters to acceptable levels. The installation of SFWs should be prioritized in hot-spot areas where the greatest benefit can be expected.