NITREX is a consortium of European experiments in which nitrogen deposition is drastically changed for whole catchments or large forest stands at eight sites spanning the present-day gradient of nitrogen deposition across Europe. NITREX focuses on the impact of nitrogen deposition on forest ecosystems, in particular the factors and processes affecting nitrogen saturation . A large number of ecosystem processes and components are investigated as part of NITREX to identify and quantify the factors which lead to changes in nitrogen cycling and outputs due to experimental changes in nitrogen inputs. The data are used to develop and test several types of nitrogen models. NITREX is linked to several other European research projects, as well as to similar large-scale ecosystem experiments in North America.