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 sensitive 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 cryptopodzolic soil was monitored over a 10 year period with samples collected fortnightly. On the basis of the concept of critical loads of acidity, the molar ratio of base nutrient cations (BC) to aluminum was used as the parameter to assess acidification. Despite characteristic seasonal short-term variations of BC and Al, which could be attributed qualitatively to specific soil chemical processes, statistically significant trends over the entire observation period were observed. In all horizons a depletion of BC concentrations indicated an impoverishment of the nutrient status of the vegetation. Aluminum decreased in the upper soil, whereas a significant increase was observed at the lowest soil depth. The BC/Al ratio decreased significantly in all mineral soil horizons, indicating rapid soil acidification. Considering the constant decrease of acid deposition at the site during the observation period with values significantly below calculated critical loads, the observed acidification is attributed to a memory effect of the high acid loads between 1965 and 1985.