Long-term effects of ammonium sulphate on Calluna vulgaris.
The results of the first 4 years of an experiment which has involved frequent applications of ammonium sulfate within a 1-km2 tract of nitrogen-poor dry heathland in southern England are reported. Experimental additions of ammonium sulfate at deposition rates of 7.7 and 15.4 kg ha-1 year-1 of N plus background deposition give total nitrogen deposition rates similar to the critical load suggested for the conversion of dry heathland to grassland. By investigating long-term biological and chemical changes in the experimental system it was hoped to determine whether the proposed critical loads are soundly based. Despite an increase in shoot nitrogen content of Calluna vulgaris following only 1 year's addition of ammonium sulfate at a rate of 7.7 kg ha-1 of N, no statistically significant effects were found in the subsequent 3 years. However, upward trends in shoot nitrogen from the control to the high-nitrogen treatments were apparent in the third and fourth years of the experiment. Application of ammonium sulfate at rates of 7.7 and 15.4 kg ha-1 year-1 of N resulted in significant stimulations in shoot growth, flowering and litter production from the second year of the experiment onwards. However, given the low nitrogen status of the experimental site, it is suggested that the positive effects demonstrated in this study may not be wholly typical of responses which may be expected in heathlands of higher nitrogen status.