This paper aims to set the scene for the session by elaborating on the challenges of modelling issues of integrated nitrogen management. Reactive nitrogen (Nr) plays a vital role in different areas, such as air pollution, climate change, eutrophication and acidification of soils, ecosystems and surface waters and such like. It inherently presents a multi-pollutant, multi-effect and multi-media problem. \ud Historically, many of these individual problems have been addressed by models developed for a specific purpose, limited to single pollutants, effects or environmental compartments and media. In addition to that, relevant datasets and model approaches have been developed within different scientific and science-policy communities and hence their integration is facing not only technical and methodological problems, but as well needs communication and collaboration across disciplines, in many areas outside the comfort zones of modellers and decision makers alike. \ud While climate change and carbon have been receiving a significant amount of attention in science, policy and the general public in the course of the last decade, the changes of the global nitrogen cycle and its implications have emerged to – potentially – become an even greater challenge for science An ever ongoing increase in vehicular traffic, energy use, industry, and animal husbandry are the principal causes of the increased emissions of oxidized and reduced forms of nitrogen. The negative impacts of these emissions include: (i) air pollution, such as increased concentrations of nitrogen oxides (NOx), ozone and fine particles, leading to effects on humans and vegetation and influencing the earth’s radiation balance, (ii) elevated emissions of nitrous oxide (N2O), being one of the most important greenhouse gases, thus affecting climate change and to climate change, (iii) eutrophication and acidification of terrestrial and aquatic ecosystems, with related impacts on plant species and faunal species diversity, (iv) surface water pollution with nitrate (NO3) and ammonium (NH4), including damage to fisheries in coastal ecosystems and (v) drinking water (ground water) pollution by nitrate pollution (e.g. Vitousek et al., 1997, Galloway and Cowling, 2002; Galloway et al., 2003; Matson et al., 2002;) The undesirable “cascading effects” of nitrogen, as Galloway et al. (2003) call them, thus affect different environmental media, across different time spans and different nitrogen species contribute to most contemporary environmental pressures. It is thus a formidable example for taking stock of the methods and approaches which form the state-of-the-art in environmental modelling and how they may be integrated and combined to tackle the nitrogen problem. \ud The relevance of nitrogen and the need for an integrated approach to address the complex issues of managing the nitrogen cycle have recently been highlighted at the 3rd of a series of workshops organised by the Swedish ASTA programme (http://asta.ivl.se/Saltsjobaden3.htm). It is anticipated, that under the UNECE Convention on Long Range Transboundary Air Pollution (CLRTAP) the development of strategies to address the nitrogen challenge will have a prominent role in the coming years. \ud The paper will discuss some approaches currently taken in research projects in Europe, e.g. the NitroEurope IP (http://www.nitroeurope.eu, in particular the work on the INTEGRATOR model), the IIASA RAINS/GAINS model (http://www.iiasa.ac.at/rains/gains/) and link to discussions and developments on concepts for model integration and coupling such as OpenMI (http://www.openmi.org/). It will present an overview and aims to inspire the discussion within the session and beyond.\u
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