The relative sensitivity of ecosystems in Europe to the indirect effects of acidic depositions.

The need for sulfur and nitrogen emission abatement in Europe has been established, and several strategies have been used or conceived in order to accomplish this. Strategies have been based on flat rate reductions (“30 per cent club”), varying percentage reductions for different countries or by the implementation of best available technology. Much effort has recently concentrated on the derivation of ecologically based target deposition levels worked out from critical load estimates for ecosystems. These targets are goals and may be used in cost-effective strategies which abate emissions with reference to the sensitivity of the receptor regions. The relative sensitivity of ecosystems to the acidic deposition has been worked out by dividing a limited number of variables controlling site sensitivity (bedrock lithology, soil type, land use, rainfall) into a small number of categories and combining them using weights. The resulting distribution of five relative sensitivity classes in Europe is shown. With reference to the critical load estimates for specific sites, targets have been applied to the relative sensitivity classes. 20 keq H+ km−2yr−1 is the target applied to the most sensitive regions. Due to complex nitrogen cycling within the ecosystem, modifications need to be carried out in order to incorporate the acidifying effect of nitrogen deposition. It is necessary to work out the degree to which deposited nitrate leaches from sites and a method is suggested using estimates of plant uptake and site factors affecting drainage, mineralization, nitrification and denitrification.