Soil response to acid deposition at different regional scales : field and laboratory data, critical loads and model predictions

Enhanced soil, ground water and surface water acidification by elevated deposition of S and N compounds is one of the most important large-scale environmental problems today. This thesis deals with the quantification of:(i) natural and man-induced sources of acidification in agricultural soils and forest soils in the Netherlands;(ii) present impacts of atmospheric S and N deposition on the solution chemistry of acid sandy forest soils in the Netherlands;(iii) various buffermechanisms (i.e. mineral weathering, cation exchange and AI dissolution) in acid sandy soils in the Netherlands;(iv) average critical deposition levels (loads) for N and acidity (N and S) for forests, heathlands, ground water and surface water in the Netherlands;(v) spatial variability in critical loads for N, S and acidity and the degree by which these loads are exceeded at present on forests in the Netherlands and in Europe;(vi) long-term impacts of acidic deposition on representative non-agricultural soils;(vii) spatial variability in long-term impacts of acidic deposition on forest soils in the Netherlands and in Europe. Quantification was performed on the basis of interpretation of literature information, combined with field research (i and ii), laboratory research (iii), and model research, (iv, v, vi and vii). In order to derive critical loads, steady-state soil models were developed, i.e. a one-layer model (START) for application on a European scale and a multi-layer model (MACAL) for application on a national scale. Similarly, two dynamic soil models were developed to assess the long-term soil response to acidic deposition, i.e. a onelayer model (SMART) for application in Europe and a multi-layer model (RESAM) for application in the Netherlands.Results showed that:(i) the contribution of acid deposition to soil acidificatiom in the Netherlands is dominant in non- calcareous forest soils (≥80%), intermediate in non-calcareous agricultural soils (≤50%) and minor in calcareous soils (≤20%);(ii) S0 ₄ behaves conservative in Dutch forest soils, whereas N is largely retained. Despite the high N deposition, actual soil acidification, which is mainly manifested by leaching of AI associated with SO ₄ and NO ₃ leaching, is dominantly caused by S deposition;(iii) dissolution of AI from secondary inorganic AI compounds is the dominant buffermechanism in acid sandy (forest) soils. The dissolution rate of AI can be described well as a function of the secondary AI pool and the degree of undersaturation with respect to gibbsite;(iv) average critical N loads for forests, heathlands, ground waters and surface waters in the Netherlands generally vary between 500 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>('sensitive' heathlands and surface waters) and 3600 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>('insensitive' ground waters). Average critical acid loads are generally lower and range between 400 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>('sensitive' ground- and surface waters) to 1700 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>('insensitive' forests and ground waters);(v) critical loads are largely exceeded in Central and Western Europe both in N (up to 3500 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>) and S (up to 12000 mol _c ha <sup>-1</SUP>yr <sup>-1</SUP>). In the Netherlands largest exceedances occur in areas with intensive animal husbandry. There emission reductions of more than 80% are needed to meet the critical loads;(vi) long-term continuation of present atmospheric deposition causes a depletion of the pool of secondary AI compounds, both in forest soils and dune soils of the Netherlands, leading to extremely low pH values. However, reduction of atmospheric deposition levels leads to a fast improvement of the soil solution quality (decreased concentrations in SO ₄ , NO ₃ and AI and increased pH);(vii) deposition scenarios including current reduction plans with respect to S and N emission lead to an improvement in the soil solution quality below forests in the Netherlands, but not in Europe.