Critical loads for nitrogen deposition and their exceedance at European scale

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In Europe serious problems associated with emissions of nitrogen oxides and ammonia are related to eutrophication and acidification. Impacts of eutrophication in terrestrial ecosystems are associated with changes in floristic composition and in ecosystem function and stability. Various activities have been initiated to assess the risk posed by nitrogen deposition for negotiations concerning the UN/ECE NO x Protocol. As there are difficulties in mapping critical loads for nitrogen, two different methods are presented here at European scale. One method uses a sensitivity approach linked to data for empirically derived critical loads; the other method uses a nitrogen saturation approach which sums quantitative values for acceptable long-term removal rates of nitrogen from the ecosystem and accumulation in soil organic matter. Resulting critical load maps show significant areas across Europe that are potentially sensitive to nitrogen deposition. Generally, the spatial distribution of sensitivity shows similarities using both approaches. However, the range of empirical critical loads in the literature determined for certain vegetation types is greater than those calculated using the saturation approach. A comparison with 1990 total nitrogen deposition does reveal a similar distribution of exceedance, but only when the lower end of the empirical critical load range is used. There are uncertainties associated with both methods nevertheless they do illustrate that substantial areas across Europe may be at risk from deleterious impacts caused by nitrogen deposition.