All forms of reactive nitrogen deposition to Natura 2000 sites should not be treated equally: effects of wet versus dry and reduced versus oxidised nitrogen deposition

• Atmospheric nitrogen deposition occurs in several different forms, including wet deposition of ammonium and nitrate, and dry deposition of ammonia. Each of these inputs occurs intermittently, according to patterns of precipitation, long range pollutant transport and local ammonia dispersion from point sources. • Evidence is presented from a nitrogen manipulation study, undertaken using ‘real world’ treatment scenarios, on an ombrotrophic bog where the effects of gaseous ammonia are compared with wet deposited nitrogen, as ammonium or nitrate. • Per unit N deposited, ammonia is found to be much more damaging to nitrogen sensitive plant species than wet deposited ammonium, which, in turn, is found to be more damaging than wet deposited nitrate. • Damage is related to the likelihood of nitrogen accumulation in the plant tissue, which is greater with ammonia > ammonium > nitrate. • Ammonia effects on lower plants are thought to be related to physiological damage associated with the intermittent high ammonia concentrations. • Thresholds for damage effects from ammonia reduce logarithmically with the logarithm of time, indicating a memory effect. • Ammonia damage to Calluna vulgaris appeared to be mediated indirectly through interaction with stress, winter desiccation, pests and pathogens. • Wet ammonium deposition at N doses > 24 kg N ha-1y-1 significantly increases N accumulation in lower plants leading to reduced growth in the pleurocarpous mosses Hypnum jutlandicum and Pleurozium schreberi. • By comparison no significant effects of nitrate have been detected except in Sphagnum capillifolium, which is sensitive to N dose. • The results clearly demonstrate that the form of nitrogen deposition affects nitrogen impacts on an acid peatland ecosystem, with adverse effects per unit N input in the order: dry ammonia > wet ammonium > wet nitrate. These differences need to be recognized in the development of air pollution policies, although the order may vary with ecosystem type, particularly with respect to nitrate versus ammonium.