EFFECTS OF ATMOSPHERIC NITROGEN DEPOSITION ON ERICOID MYCORRHIZAL INFECTION OF CALLUNA-VULGARIS GROWING IN PEAT

In a greenhouse experiment, Calluna moorland peat microcosms from five sites along a pollution gradient (2-10 kg N ha(-1) year(-1) in wet de position) have been subjected over 18 months to uniform amounts of sim ulated polluted precipitation with compositions appropriate to each mi crocosm sampling site, thus eliminating temperature and precipitation amount as variables. Maximum mycorrhizal infection (80.5% +/- 3.11) oc curred at the second most polluted site (2.36 kg N ha(-1) year(-1)), a nd thereafter there was a significant decrease in mycorrhizal infectio n rate along the pollution gradient, down to 14.5% +/- 2.64 at the mos t polluted site. This is strong circumstantial evidence for a depressi ve effect of acid and/or ammonium deposition upon the extent of ericoi d mycorrhizal infection of Calluna roots. In a second greenhouse exper iment, Calluna moorland peat microcosms from a single site (thus elimi nating residual soil effects as a variable) were subjected over 12 mon ths to NH4+ (as ammonium sulphate) and NO3- (as nitric acid) concentra tions which were one, two and six times the ambient values of 12 kg N ha(-1) year(-1). The high ammonium sulphate treatment significantly re duced the extent of mycorrhizal infection of both new Calluna roots an d of the Callina total root system. The nitric acid treatment signific antly reduced the infection on new roots.