AN EXPERIMENTAL INVESTIGATION INTO THE EFFECTS OF ATMOSPHERIC NITROGEN DEPOSITION ON 2 SEMINATURAL GRASSLANDS

1 A series of ammonium nitrate additions have been made to calcareous and acidic grasslands in the Peak District of Great Britain to simulate increasing loads of nitrogen deposition. Supplementary plots investigated (i) the effects of glucose addition, (ii) the role of phosphorus limitation, and (iii) compared the impact of ammonium nitrate with that of ammonium sulphate. 2 Ammonium sulphate caused a decline in the moss Rhytidiadelphus squarrosus on the acid site, but no other change in vegetation composition was detected after 3 years of any treatment. 3 Plant nitrogen concentration and nitrate reductase activity increased with increasing nitrogen supply in most of the vascular plants, and there was no loss of the capacity for induction in nitrate reductase activity. In contrast, nitrate reductase activity declined in the moss R. squarrosus and there was a loss of induceability. Leaf and shoot growth did not change with nitrogen additions. 4 Plant growth appeared to be colimited by phosphorus and nitrogen supply on the calcareous site. Any changes in vegetation composition would result from subtle changes in chemical composition and physiology, rather than differences in growth response of different species, and would be expected to take many years. 5 Data gathered at the site suggest that atmospheric deposition is a larger source of plant nitrogen supply than mineralization in the soil. Soil nitrogen mineralization rate was also stimulated by additional nitrogen input.