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 ammonium-N or nitrate-N to simulate recent increases in atmospheric N deposition. 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 above-ground yield of certain forbs, particularly Filipendula, Scabiosa and Thymus. N treatment did not significantly increase the growth of other fortes, but there were no adverse effects even at N inputs as high as 80 kg N ha-1 year-1. 3 The fortes 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 increases in N deposition. 4 The growth of the grass Brachypodium pinnatum did not increase significantly as a result of nitrogen addition. There was no evidence that increases in atmospheric nitrogen deposition will result in grass dominance and a loss of species diversity. 5 Growth of both grasses and forbs was probably limited by the availability of phosphorus. 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 grazing treatment. The response of fortes was variable; grazing reduced the growth of some species but had beneficial effects on others. Management practices such as grazing are likely to be a key factor in maintaining 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 showed different growth responses according to whether N was applied as ammonium or nitrate. A critical load for total N deposition should be calculated for chalk grasslands, and modified for local characteristics such as past and present management practices, and factors which may limit N assimilation (e.g. P status).