THE EFFECTS OF AIR-BORNE NITROGEN POLLUTANTS ON SPECIES-DIVERSITY IN NATURAL AND SEMINATURAL EUROPEAN VEGETATION

1 The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi-natural and natural ecosystems a re reviewed. The severity of these impacts depends on abiotic conditio ns (e.g. buffering capacity, soil nutrient status and soil factors tha t influence the nitrification potential and nitrogen immobilization ra te) in the particular system. The sensitivity of fresh water ecosystem s, wetlands and bogs, species-rich grasslands, heathlands and field la yer of forests, all of which have conservational value, are discussed in detail. 2 The most important effects of nitrogen deposition are: (i ) accumulation of nitrogenous compounds resulting in enhanced availabi lity of nitrate or ammonium; (ii) soil-mediated effects of acidificati on; and (iii) increased susceptibility to secondary stress factors. 3 Long-term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclu sion of characteristic species by more nitrophilic plants, especially under oligo- to mesotrophic soil conditions. 4 Soil acidification (wit h losses of buffering capacity and increased concentrations of toxic m etals) is especially important after nitrification of ammonium in weak ly buffered environments: acid-resistant plant species then become dom inant 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. 5 The susceptibility of plant species to secondary stress factors (pathogens; frost and dro ught) may be affected by air-borne nitrogen but data are only availabl e for a few communities (e.g. dry heathlands). 6 Most global biodivers ity is contained within natural and semi-natural vegetation. It is thu s crucial to control emissions of nitrogenous compounds to the atmosph ere, in order to reduce or prevent effects on diversity in these syste ms. Most research has focused on acidification in forestry stands and lakes and on the effects on trees. We highlight serious gaps in knowle dge of other ecosystems.