Soil acidification in southern Switzerland between 1987 and 1997: A case study based on the critical load concept

Southern Switzerland with its mainly acidic bedrock is potentially very sen sitive to the effects of atmospheric proton input. In addition, this region suffers from relatively high acid deposition originating from the greater Milano area. The soil solution composition at different depths of a crypto- podzolic soil was monitored over a 10 year period with samples collected fo rtnightly. On the basis of the concept of critical loads of acidity, the mo lar ratio of base nutrient cations (BC) to aluminum was used as the paramet er to assess acidification. Despite characteristic seasonal shortterm varia tions of BC and Al, which could be! attributed qualitatively to specific so il chemical processes, statistically significant trends over the entire obs ervation period were observed. In all horizons a depletion of BC concentrat ions indicated an impoverishment of the nutrient status of the vegetation. Aluminum decreased in the upper soil, whereas a significant increase was ob served at the lowest soil depth. The BC/Al ratio decreased significantly in all mineral soil horizons, indicating rapid soil acidification. Considerin g the constant decrease of acid deposition at the site during the observati on period with values significantly below calculated critical loads, the ob served acidification is attributed to a memory effect of the high acid load s between 1965 and 1985.