NH3 :: Mosses and liverworts (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.