Impact Type: Direct exposure to pollutant
Bryophytes are highly sensitive to atmospheric nitrogen deposition though some species are more tolerant than others, and are similarly sensitive to exposure to high concentrations of NH3 (UKCLAG 1996). While part of the effect of NH3 may occur as a consequence of the increased availability of nitrogen, there is also some evidence of direct toxic effects of NH3. For example, van der Eerden et al. (1991) and Greven (1992) reported, respectively, chlorosis or tissue death in Racomitrium lanuginosum following exposure to 25-30 µg m-3 NH3 for around 20-30 days. Other species affected were Dicranum polysetum, Hypnum cupressiforme and Campylopus flexuosus, while Pleurozium schreberi was less sensitive (Greven 1992).
Although critical levels for NH3 exposure have been set, Burkhardt et al. (1998) have shown that the contribution of NH3 to nitrogen deposition means that the critical load will almost always be exceeded before the critical level. Hence habitat specific critical loads for nitrogen are the mo re sensitive threshold.
Pitcairn et al. (1998) showed substantial changes in bryophytes in the vicinity of intensive farms subject to high NH3 concentrations. However, they related the impacts observed to elevated nitrogen deposition rather than exposure to toxic levels of NH3. Continuous exposure of Polytrichum commune at 9 µgm-3 NH3 caused visible damage (Leith pers comm.) It may be difficult to establish a critical level for NH3 as actual concentrations measured may be modified up or down by associated flora.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Indication of exceedance||Reference|
3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)
Direct visible injury; species composition changes. Ecosystems where sensitive lichens and bryophytes are an important part of the ecosystem integrity, the critical level is set at 1 µg NH3 m-3.