The effect of atmospheric ammonia on ecosystems has been the subject of ongoing research. Its adverse effects as an air pollutant are well characterised, and may be even more widespread than previously thought (see Aber et al. 2003; Erisman et al. 2003; Krupa 2003; Purvis et al. 2003). The most important sources of NH3 in Europe are agricultural activities, mainly crop fertilization and cattle management (Galloway et al. 2003; EPER 2004). Livestock housing facilities are recognised to be large point sources of NH3 emissions. Close to such facilities, atmospheric NH3 concentrations are very high, decreasing rapidly with distance over a few hundreds of meters to a few kilometres (Sutton et al. 1998). Measurement of atmospheric NH3 in the vicinity of livestock housing include those by Pitcairn et al. (1998, 2003), with reported values in Scotland of 24 –59 μg m−3 close to source, which declined to background values of 1.6 –5 μg m−3 at 1 km. In order to assess the range of effects of NH3 in natural ecosystems, that can be used for effective NH3 mitigation policies (Dragosits et al. 2006), one can rely on two distinct approaches: (i) direct measurements of atmospheric NH3 concentrations, which provide an estimate of dry NH3-N deposition, but require intensive and costly operations; (ii) monitoring of effects on the biotic component. The latter approach should be carried out using groups of biota that are more sensitive to the pollutant of interest. Lichens have been reported as the most sensitive group to NH3 emissions (e.g. Wolseley et al. 2006a; van Herk 1999). Lichens are symbiotic organisms widely used as biomonitors of environmental changes (e.g. Nimis et al. 1991; Vokou et al. 1999; Geebelen and Hoffmann 2001; Giordani et al. 2002; Pirintsos and Loppi 2003; Geiser and Neitlich 2007). \ud Monitoring atmospheric pollutants using lichens may be undertaken in three ways: (1) measuring variations in lichens diversity and/or abundance, (2) using variations in physiological parameters, and/or using lichens as accumulators of pollutants (Branquinho 2001), and (3) considering functional groups related to nutrients tolerance, such as the division between nitrophytic/oligotrophic (or nitrophytic/ acidophitic) groups (see van Dobben and ter Braak 1999; Ruisi et al. 2005; Wolseley et al. 2006b)
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