Deposition and critical loads of nitrogen in Switzerland

Many ecosystems in Switzerland suffer from eutrophication due to increased atmospheric nitrogen (N) input. In order to get an overview of the problem, critical loads for nutrient N were mapped with a resolution of 1x1 km applying two methods recommended by the UN/ECE: the steady state mass balance method for productive forests, and the empirical method for seminatural vegetation, such as natural forests, (sub-)alpine or species-rich grassland and raised bogs. The national forest inventory and a detailed atlas of vegetation types were used to identify the areas sensitive to N input. The total N input was calculated as the sum of NO3-, NH4+, NH3, NO2 and HNO3 wet and dry deposition. Wet deposition was determined on the basis of a precipitation map and concentration measurements. Dry deposition was calculated with inferential methods including land-use specific deposition velocities. The concentration fields for NH3 and NO2 were obtained from emission inventories combined with dispersion models. Reduced N compounds account for 63% of total deposition in Switzerland. As indicated by exceeded critical loads, the highest risk for harmful effects of N deposition (decrease of ecosystem stability, species shift and losses) is expected on forests and raised bogs in the lowlands, where local emissions are intense. At high altitudes and in dry inner-alpine valleys, deposition rates are significantly lower.