Critical loads for nitrogen deposition and their exceedance at European scale

In Europe serious problems associated with emissions of nitrogen oxides and ammonia are related to eutrophication and acidification. Impacts of eutrop hication in terrestrial ecosystems are associated with changes in floristic composition and in ecosystem function and stability. Various activities ha ve been initiated to assess the risk posed by nitrogen deposition for negot iations concerning the UN/ECE NOx Protocol. As there are difficulties in ma pping critical loads for nitrogen, two different methods are presented here at European scale. One method uses a sensitivity approach linked to data f or empirically derived critical loads; the other method uses a nitrogen sat uration approach which sums quantitative values for acceptable long-term re moval 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 spati al distribution of sensitivity shows similarities using both approaches. Ho wever, the range of empirical critical loads in the literature determined f or certain vegetation types is greater than those calculated using the satu ration approach. A comparison with 1990 total nitrogen deposition does reve al 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 ac ross Europe may be at risk from deleterious impacts caused by nitrogen depo sition.