Critical loads of nitrogen and sulphur to avert acidification and eutrophication in Europe and China

Forests and other (semi-)natural ecosystems provide a range of ecosystem services, such as purifying water, stabilizing soils and nutrient cycles, and providing habitats for plants and wildlife. Critical loads are a well-established effects-based approach that has been used for assessing the environmental consequences of air pollution on large regional or national scales. Typically critical loads of sulphur (S) and nitrogen (N) have been derived separately for characterizing the vulnerability of ecosystems to acidification (by S and N) and eutrophication (by N). In this paper we combine the two approaches and use multiple criteria, such as critical pH and N concentrations in soil solution, to define a single critical load function of N and S. The methodology is used to compute and map critical loads of N and S in two regions of comparable size, Europe and China. We also assess the exceedance of those critical loads under globally modelled present and selected future N and S depositions. We also present an analysis, in which the sensitivity of the critical loads and their exceedances to the choice of the chemical criteria is investigated. As pH and N concentration in soil solution are abiotic variables also linked to plant species occurrence, this approach has the potential for deriving critical loads for plant species diversity.