Nature of the problemBiodiversity is the variability among living organisms, from genes to the biosphere. Th e value of biodiversity is multifold, from preserving • the integrity of the biosphere as a whole, to providing food and medicines, to spiritual and aesthetic well-being. One of the major drivers of biodiversity loss in Europe is atmospheric deposition of reactive nitrogen (N • r ). ApproachesTh is chapter focuses on N • r impacts on European plant species diversity; in particular, the number and abundance of diff erent species in a given area, and the presence of characteristic species of sensitive ecosystems. We summarise both the scientifi c and the policy aspects of N • r impacts on diversity and identify, using a range of evidence, the most vulnerable ecosystems and regions in Europe. Key fi ndings / state of knowledgeReactive nitrogen impacts vegetation diversity through direct foliar damage, eutrophication, acidifi cation, and susceptibility to secondary • stress. Species and communities most sensitive to chronically elevated N • r deposition are those that are adapted to low nutrient levels, or are poorly buff ered against acidifi cation. Grassland, heathland, peatland, forest, and arctic/montane ecosystems are recognised as vulnerable habitats in Europe; other habitats may be vulnerable but are still poorly studied. It is not yet clear if diff erent wet-deposited forms of N • r (e.g. nitrate, NO 3 − versus ammonium, NH 4 + ) have diff erent eff ects on biodiversity. However, gaseous ammonia (NH 3 ) can be particularly harmful to vegetation, especially lower plants, through direct foliar damage. Th ere are some clear examples of reductions in faunal diversity that can be linked to N • r deposition, but overall, our knowledge of faunal eff ects is still limited. Changes to above-ground faunal communities probably occur primarily through changes in vegetation diversity, composition or structure. Evidence is strong that ecological communities respond to the accumulated pool of plant-available N in the soil. Th us the • cumulative load of enhanced N r impacting an ecosystem is probably highly important. Because of this response to cumulative inputs, it is likely that biodiversity has been in decline in Europe for many decades due to enhanced • N r deposition. Equally, full recovery in response to reduced N r deposition is likely to be slow, especially in highly impacted ecosystems. In some cases recovery may require management intervention. Exceedence of critical loads for nutrient nitrogen is linked to reduced plant species richness in a broad range of European ecosystems. • Major uncertainties/challengesIt is very likely that N • r deposition acts synergistically with other stressors, in particular climate change, acid deposition, and ground-level ozone; these synergies are poorly understood. Th e nature and rate of recovery of biodiversity from nitrogen pollution is not well understood. Th e optimal strategy to restore a habitat, • and exactly what this 'restored' habitat constitutes, are both hard ...

During recent years, it has become clear that ammonia is an important gas in relation to di!erent environmental issues, such as acidi"cation, eutrophication, human health and climate change (through particle formation). Therefore, there is a growing need to develop and apply instrumentation suitable for research into emission, dispersion, conversion and deposition of ammonia and ammonium. Recently, several instruments were developed suitable for measuring concentrations in ambient conditions even at very low levels, such as ammonia sensors suitable for monitoring and research, deposition measuring systems and aerosol samplers for on-line measurement of aerosol composition. These instruments have been tested and applied in a number of "eld studies. These studies include dry deposition measurements, ammonium nitrate studies in relation to the (in)direct aerosol e!ect, emission studies and policy evaluation with concentration and deposition monitoring data. The policy evaluation study showed that the measures to reduce ammonia emissions were not as successful as projected beforehand by statistical studies.

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Nature of the problem Reactive nitrogen (N • r) is of fundamental importance in biological and chemical processes in the atmosphere-biosphere system, altering the Earth's climate balance in many ways. Th ese include the direct and indirect emissions of nitrous oxide (N 2 O), atmospheric N r deposition and tropospheric ozone formation (O 3), both of which alter the biospheric CO 2 sink, N r supply eff ects on CH 4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NO x) and ammonia (NH 3). Human production and release of N • r into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an eff ect European N r emissions are having on net warming or cooling. Approaches Th is chapter summarizes current knowledge of the role of N • r for global warming. Particular attention is given to the consequences of atmospheric N r emissions. Th e chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric N r emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe. Th e use of N • r fertilizers has major additional eff ects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect eff ects are not assessed here. Key fi ndings/state of knowledge Due to its multiple, complex eff ects on biospheric and atmospheric processes, the importance of N • r for the European greenhouse gas balance has so far received insuffi cient attention. Th e main warming eff ects of European anthropogenic N • r emissions are estimated to be from N 2 O (17 (15-19) mW/m 2) and from the reduction in the biospheric CO 2 sink by tropospheric O 3 (4.4 (2.3-6.6) mW/m 2). Th e main cooling eff ects are estimated to be from increasing the biospheric CO 2 sink by atmospheric N r deposition at −19 (−30 to −8) mW/m 2 and by light scattering eff ects of N r containing aerosol (−16.5 (−27.5 to −5.5) mW/m 2), in both cases resulting from emissions of NO x and NH 3. Th e production of O • 3 from European emissions of NO x is estimated to have a modest warming eff ect (2.9 (0.3-5.5) mW/m 2), which is largely off set by the cooling eff ect of O 3 in reducing the atmospheric lifetime of CH 4 (−4.6 (−6.7 to −2.4) mW/m 2), giving an uncertain net warming of +1.7 (−6.4 to +3.1) mW/m 2). Overall, including all of these terms, European N • r emissions are estimated to have a net cooling eff ect, with the uncertainty bounds ranging from a substantial cooling eff ect to a small warming eff ect (−15.7 (−46.7 to +15.4) mW/m 2). Major uncertainties/challenges Th e largest uncertainties concern the aerosol and N • r fertilization eff ects, and the estimation of the European contributions within the global context. Published estimates suggest that the default N • 2 O emission factor of 1% used by IPCC for indirect emissions from soi...