New science on the effects of nitrogen deposition and concentrations on Natura 2000 sites (theme 3): background documement

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