Acidification of the sandy percolation zones under heathland and oak wood in Denmark

Heathland has existed for more than 3000 years in one field area, and the oak wood has probably not been deforested since the last glaciation about 14000 years ago in the other field area. This vegetation history provides a unique opportunity for considering the effect of vegetation-type on the weathering processes of the percolation zones of fluvio-glacial sediments. The role of atmospheric deposition and organic internally produced acidity in controlling the weathering processes of the percolation zones is examined. Soil texture, carbonate content, CEC, and the soil water chemistry of the percolation zones under heathland and oak wood were studied from 6 and 7 m deep wells in which suction cells were installed at different depths. The carbonate dissolution front of the sandy Weichselian (14000 BC) sediments under the oak wood was found at a depth of 3.5 m below ground level, whereas under heathland the percolation zone down to 6.3 m depth was decalcified. An important finding from this investigation is that alkalinity matches the sum of Ca⁺²+Mg⁺² in equivalents in the percolation zone of the oak wood. This means that carbonic acid and/or other organic acids, and not acid deposition causes the dissolution of carbonate in the percolation zone under the oak. The oak wood ecosystem is able to neutralize the percolating soil solution better than the heather vegetation at the expense of a high degree of acidification of the solid phase of the percolation zone. Simulation with the hydro-geochemical model PHREEQM showed that vegetation-type, via the chemical composition of infiltration water, controlled the dissolution of carbonates as are present. Additionally, preferential flow in the percolation zone also seems to influence the dissolution process.