Modelling nitrogen transformations in freshwater wetlands. Estimating nitrogen retention and removal in natural wetlands in relation to their hydrology and nutrient loadings

The agricultural utilization of the transition zone between the terrestrial and the aquatic system has strongly reduced these important buffer zones in the last 30 years. The reestablishment of wetlands in relation to the aquatic environment is getting more and more in focus in the debate on eutrophication. A general simulation model has been developed for freshwater wetlands to determine the retention and removal of nitrogen in wetlands as water flows from intensively cultivated farm land through wetlands and into the aquatic system. The model consists of a simple hydrological submodel and a more complex biological submodel including heterotrophic nitrogen dynamics and plant uptake. The whole biogeochemical pathway from mineralization of organic matter to ammonia and further to nitrate in the oxic microzone by nitrifiers, before denitrification, is explicitly modelled. The model has been calibrated with field data from three wetlands with different levels of NO 3 −-loading (587–1502 kg NO 3 −-N/ha · y) and vegetation. The calculated N-retention varies from 0 to 107 kg N/ha · y and the denitrification from 199 to 743 kg NO 3 −-N/ha · y with the lowest value in a Sphagnum-dominated wetland and the highest in a reed swamp. The wetland model can be applied to a model system describing the nitrogen turnover and transport from agricultural fertilization through soil and groundwater processes to the final washout into the aquatic environment. Moreover, the model can be used as a prognostic tool for an assessment of the potential effects on the aquatic ecosystem if relevant wetlands were reestablished.