This background paper summarizes established and new science on the effects of nitrogen (N)\ud deposition on ecosystems and considers the potential for improved assessment of N deposition\ud impacts on Natura 2000 sites. The key aspects covered are N deposition effects on biodiversity and\ud on biogeochemistry, links to ecosystem services, the importance of N form, N deposition indicators,\ud management practices and ecosystem reversibility following decreased N input.\ud The paper shows that:\ud • Evidence of N impacts on different vegetation types in Europe exists, but that it is important\ud that it is translated meaningfully to the target habitats listed under the Habitats Directive.\ud Evidence for N deposition effects to important ecosystem services, such as carbon\ud sequestration, also exists but the cause and effect relationships underlying them are often\ud complex and not sufficiently understood.\ud • Chemical N form can influence both the rate of ecosystem change, and the extent of impacts\ud on the short and long-term. Evidence is presented for ammonia causing detrimental plant\ud physiological effects, probably on the majority of species, whilst ammonium and nitrate\ud effects will depend on plant species present.\ud • Plant species composition as well as plant biochemical parameters may be useful\ud bioindicators for assessment of N deposition effects in Natura 2000 sites, however\ud “baseline” data are mostly not available for rare species. Ecosystem specific indicators,\ud that are predictive of further damage, rather than a consequence of already existing adverse\ud effects (i.e., early warning indicators) are needed.\ud • Site level management practices can be useful to reduce the impact if N deposition but they\ud will certainly not be able to mitigate all the impacts of enhanced N deposition and enhances N concentration on Natura 2000 habitats. More knowledge is needed to better understand\ud where and if management intervention is appropriate to mitigate N effects.\ud • Studies on the reversibility of N impacts show that some ecosystem parameters may revert\ud quickly, while other components may show strong inertia. In some cases reversion to the\ud original state may however be impossible.\ud • Climatic factors interfere with ecosystem effects of N deposition. It is clear that climate\ud both can emphasize and mitigate effects of N deposition. Current climate and expected\ud climatic changes must be included in assessments and predictions of N deposition effects\ud on ecosystems.\ud The aim is to provide a broad picture of scientific advancement within the field of N deposition\ud research, and to provide a starting point for workshop discussions. Workshop discussions addressed\ud the relevance of new science in assessing N deposition impacts on Natura 2000 sites, and identified\ud when there is potential to make management adjustments to mitigate such effects.\u
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