The goal of this project is to determine the role of tidal freshwater marshes in the nitrogen cycle of the Scheldt Estuary. It is generally thought that fringing marshes act as a filter for the estuarine water by removing inorganic and organic forms of nitrogen from the floodwaters or by changing the nitrogen speciation. The Scheldt Estuary, with its very densely populated watershed, is a typical example of an ecosystem receiving high loads of nitrogen and where important surfaces of tidal freshwater marshes could represent important potential N-sinks. Also, in the near future, it is planned by the regional authorities to lay out new controlled flooding areas, thereby increasing the possible filter capacity of tidal freshwater marshes.An integrative, in-situ study on the nitrogen exchange and cycling in tidal marshes will allow us not only to evaluate the role of these marshes in the estuarine nitrogen-budget, but also to identify and quantify the most important processes in the marsh that underlie this role. Classically, main approaches for studying the relationships between estuarine waters and fringing marshes suffer from inaccuracies in for instance the water budget, track only net changes, do not identify the processes and marsh compartments involved in the removal of nitrogen and are often difficult to extrapolate to a conclusive N-budget of whole marshes (Nixon 1980, Howarth 1993, Smith & Hollibaugh 1997). Recent advances in stable isotope analysis make it possible to do whole system labeling studies (Holmes et al. 2000, Middelburg et al. 2000), in which the stable isotope 15N can be used as a sensitive tracer of nitrogen cycling. We suggest to combine a whole-ecosystem labeling study with several additional studies directed at understanding the nitrogen cycling in important marsh compartments like the floodwater, sediments and macrophytes.