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 deposition) have been subjected over 18 months to uniform amounts of simulated polluted precipitation with compositions appropriate to each microcosm sampling site, thus eliminating temperature and precipitation amount as variables. Maximum mycorrhizal infection (80.5% +/- 3.11) occurred at the second most polluted site (2.36 kg N ha(-1) year(-1)), and thereafter there was a significant decrease in mycorrhizal infection rate along the pollution gradient, down to 14.5% +/- 2.64 at the most polluted site. This is strong circumstantial evidence for a depressive effect of acid and/or ammonium deposition upon the extent of ericoid mycorrhizal infection of Calluna roots. In a second greenhouse experiment, Calluna moorland peat microcosms from a single site (thus eliminating residual soil effects as a variable) were subjected over 12 months to NH4+ (as ammonium sulphate) and NO3- (as nitric acid) concentrations which were one, two and six times the ambient values of 12 kg N ha(-1) year(-1). The high ammonium sulphate treatment significantly reduced the extent of mycorrhizal infection of both new Calluna roots and of the Callina total root system. The nitric acid treatment significantly reduced the infection on new roots.