NH3 :: Mosses and liverworts (general)

Latin name: 
Bryophytes (general)

Impact Type: Direct exposure to pollutant

Key Concerns:

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).

Additional Comments:

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. 

Critical Load/level: 
Habitat/ Ecosystem Type Critical Load/ Level Status Indication of exceedance Reference
Higher plants

3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)

UNECE, 2007

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.

Burkhardt, J.; Sutton, M.A.; Milford, C.; Storeton-West, R.L.; Fowler, D. 1998 Ammonia concentrations at a site in southern Scotland from 2 yr of continuous measurements Atmospheric Environment 32 325-331
Greven, H.C. 1992 Changes in the Duch bryophyte flora and air pollution Dissertationes Botanicae Band 194
Pitcairn, C.E.R.; Leith, I.D.; Sheppard, L.J.; Sutton, M.A.; Fowler, D.; Munro, R.C.; Tang, S.; Wilson, D. 1998 The relationship between nitrogen deposition, species composition and foliar nitrogen concentrations in woodland flora in the vicinity of livestock farms. Environmental Pollution 102 41-48
Van der Eerden, L.J.M.; Dueck, T.A.; Berdowski, J.J.M.; Greven, H.; Dobben, H.F. 1991 Influence of HN3 and (HN4)2 SO4 on heathland vegetation Atic Bot Neerl 40 281-296

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