ARE CALCAREOUS GRASSLANDS IN THE UK UNDER THREAT FROM NITROGEN DEPOSITION - AN EXPERIMENTAL-DETERMINATION OF A CRITICAL LOAD

1 Nitrogen was applied to artificial chalk grasslands as either ammoni um-N or nitrate-N to simulate recent increases in atmospheric N deposi tion. 2 Inputs of nitrogen above ambient stimulated the growth of the sward, but there was no further increase in growth when N inputs were increased from 20 kg N ha(-1) year(-1) to over 80 kg N ha(-1) year(-1) . The increase in dry weight of the sward was a result of a greater ab ove-ground yield of certain forbs, particularly Filipendula, Scabiosa and Thymus. N treatment did not significantly increase the growth of o ther forbs, but there were no adverse effects even at N inputs as high as 80 kg N ha(-1) year(-1). 3 The forbs which responded positively to N addition had low Ellenberg Indicator Values. This index is unlikely to be an accurate predictor of how a species will respond to increase s in N deposition. 4 The growth of the grass Brachypodium pinnatum did not increase significantly as a result of nitrogen addition. There wa s no evidence that increases in atmospheric nitrogen deposition will r esult in grass dominance and a loss of species diversity. 5 Growth of both grasses and forbs was probably limited by the availability of pho sphorus. In these circumstances, high levels of N deposition may prove a greater threat to ground water quality than to species composition. 6 The growth of Brachypodium was adversely affected by a simulated gr azing treatment. The response of forbs was variable; grazing reduced t he growth of some species but had beneficial effects on others. Manage ment practices such as grazing are likely to be a key factor in mainta ining species diversity in chalk grasslands. 7 The critical load for a grazed, P-limited chalk grassland is calculated to be between 42 and 55 kg N ha(-1) year(-1). Neither Brachypodium nor any of the forbs sho wed different growth responses according to whether N was applied as a mmonium or nitrate. A critical load for total N deposition should be c alculated for chalk grasslands, and modified for local characteristics such as past and present management practises, and factors which may limit N assimilation (e.g. P status).