Summary The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi‐natural and natural ecosystems are reviewed. The severity of these impacts depends on abiotic conditions (e.g. buffering capacity, soil nutrient status and soil factors that influence the nitrification potential and nitrogen immobilization rate) in the particular system. The sensitivity of fresh water ecosystems, wetlands and bogs, species‐rich grasslands, heathlands and field layer of forests, all of which have conservational value, are discussed in detail. The most important effects of nitrogen deposition are: (i) accumulation of nitrogenous compounds resulting in enhanced availability of nitrate or ammonium; (ii) soil‐mediated effects of acidification; and (iii) increased susceptibility to secondary stress factors. Long‐term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclusion of characteristic species by more nitrophilic plants, especially under oligo‐ to mesotrophic soil conditions. Soil acidification (with losses of buffering capacity and increased concentrations of toxic metals) is especially important after nitrification of ammonium in weakly buffered environments: acid‐resistant plant species then become dominant at the expense of the often rare plants typical of intermediate pH. The related change in the balance between ammonium and nitrate may also affect the performance of several species. The susceptibility of plant species to secondary stress factors (pathogens; frost and drought) may be affected by air‐borne nitrogen but data are only available for a few communities (e.g. dry heathlands). Most global biodiversity is contained within natural and semi‐natural vegetation. It is thus crucial to control emissions of nitrogenous compounds to the atmosphere, in order to reduce or prevent effects on diversity in these systems. Most research has focused on acidification in forestry stands and lakes and on the effects on trees. We highlight serious gaps in knowledge of other ecosystems.

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