The impact of airborne nitrogen on sensitive ecosystems has already been addressed in a series of previous workshops held under the Convention on Long-range Transboundary Air Pollution covering the region of the United Nations Economic Commission for Europe (UNECE). Critical loads for nitrogen, derived from the scientific knowledge available at that time, were part of the effect-orientated approach chosen for the development of the Protocol on the Abatement of Acidification, Eutrophication and Ground-level Ozone, signed at Gothenburg (Sweden) in 1999. With this protocol, nitrogen measures were strengthened compared with earlier protocols and a multi-pollutant/multi-effect approach was adopted addressing simultaneously several pollutants and environmental effects in an integrated way. This was a substantial progress. But we all knew that the Gothenburg Protocol was an interim step towards a sustainable air pollution control policy. There is significant potential and interest to further improve our approaches and to prepare a sound scientific and technical basis for the coming revision of the Gothenburg agreement. This workshop is part of the preparatory pahse of planned revision of the Gothenburg protocol. The effect-oriented analysis of the Protocol has clearly shown that more efforts are needed to address - inter alia - the eutrophication problem. The scientific literature as well as international workshops and conferences on nitrogen have always highlighted that nitrogen is a very complex topic and that a single reactive nitrogen molecule can cascade through a wide variety of environmental systems and contribute to multiple sequential effects. Scientific experimental and field work can contribute to a better understanding of these processes and to improving our ability to assess how much a sensitive ecosystem can tolerate in the long-term. In this sense the empirical work, and the process understanding derived from it, are essential prerequisites for the development of process-oriented and dynamic ecosystem modelling. The proceedings reflect the comprehensive assessment and update of the scientific knowledge on empirical critical loads for nitrogen prepared before the workshop and the results of the discussions carried out during the meeting. We are aware that there will always be gaps in knowledge hindering us to be final in our assessment. But since the work under the Convention is an iterative process, this should not hinder us from setting from time to time certain milestones. Such milestones are important for the political discussion on the need to further control emissions of air pollutants from all sources contributing to impacts on the environment over large areas. In addition, they can serve as a stimulus to further research.
