Long-term effects of ammonium sulphate on Calluna vulgaris

1. This paper documents the results of the first 4 years of an experiment which has involved frequent applications of ammonium sulphate within a 1-km(2) tract of nitrogen-poor dry heathland in southern England. Experimental additions of ammonium sulphate 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. 2. Despite an increase in shoot nitrogen content of Calluna vulgaris following only 1 year's addition of ammonium sulphate 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. 3. Application of ammonium sulphate 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, the positive effects demonstrated in this study may not be wholly typical of responses which may be expected in heathlands of higher nitrogen status. 4. Continuation of the experiment is necessary to cover a greater part of the life cycle of Calluna in order to investigate how continued nitrogen inputs modify responses to natural stresses, and ultimately to determine the validity of the critical load suggested for dry heathlands.