One of the vital roles played by fungi in ecosystems lies in their being prime agents in the decomposition of plant material, thereby recycling nutrients (Swift, Heal & Anderson, 1979). It has been suggested that one insidious but important effect of pollution may be a reduction in the activity of decomposer communities (Ineson & Gray, 1980; Wookey, Ineson & Mansfield, 1991). Historically, the most significant atmospheric pollutant has been SO2, whose adverse effects on fungal decomposers are well documented (Magan, 1993, also Magan, Chapter 7). In the UK the trend is for SO2 to decline in importance as a pollutant (but see Boddy et al., Chapter 6) while tropospheric ozone and nitrogenous pollutants are increasing (UK Terrestrial Effects Review Group, 1988; UK Review Group on the Impact of Atmospheric Nitrogen, 1994). The effects of these latter two classes of pollutants on fungi are still largely unresearched.

There have been several reports of decomposer activity in litter being adversely affected by treatment with simulated acid precipitation in which pH was controlled by sulphuric acid (Brown, 1985; Skiba & Cresser, 1986). By contrast, ambient rainwater has a significant component of nitric acid, amounting to approximately 30% of total acidity (Warren Spring Laboratory, 1990). Nitrogen has long been known as a promoter of the decay of plant material by fungi (Garrett, 1963), and this additional pollutant load might be expected partially to offset deleterious effects due to sulphurous pollutants.