LONG-TERM EFFECTS OF AMMONIUM-SULFATE ON CALLUNA-VULGARIS

1. This paper documents the results of the first 4 years of an experim ent which has involved frequent applications of ammonium sulphate with in 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 tot al nitrogen deposition rates similar to the critical load suggested fo r 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 ba sed. 2. Despite an increase in shoot nitrogen content of Calluna vulga ris 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 i n the subsequent 3 years. However, upward trends in shoot nitrogen fro m the control to the high-nitrogen treatments were apparent in the thi rd and fourth years of the experiment. 3. Application of ammonium sulp hate at rates of 7.7 and 15.4 kg ha(-1) year(-1) of N resulted in sign ificant 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 demons trated in this study may not be wholly typical of responses which may be expected in heathlands of higher nitrogen status. 4. Continuation o f the experiment is necessary to cover a greater part of the life cycl e of Calluna in order to investigate how continued nitrogen inputs mod ify responses to natural stresses, and ultimately to determine the val idity of the critical load suggested for dry heathlands.