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Agricultural University, Wageningen, The Netherlands
Proton budgets of four soil-vegetation systems in a 3 ha woodland with acidic and calcareous soils were derived from three years of hydrochemical monitoring of rain- and canopy throughfall water, soil solutions at various depths and measurements on litterfall and tree growth. Soil water fluxes were obtained from dynamic simulation of water transport, calibrated with measured water potentials and water contents in the soil. Atmospheric input is predominantly ammonium sulphate. Only 5–20% of the annual uptake of nutrient elements from the soil is stored in wood, bark and roots of trees. Free H+ inputs from the atmosphere are almost completely neutralized in the tree canopy by exchange with cations from the vegetation. The forest floor and the 0– 10 cm mineral surface layer at the acidic soil sites, however, produce free H+, mainly by nitrification, partly by organic acid dissociation. Most of this H+ is neutralized at greater depth by nitrate uptake or denitrification, and by dissolution of cations from soil minerals, mainly aluminium in the acid soils. The dominant role of N transformations in the H+budget of these soil vegetation systems is clearly due the high anthropogenic inputs of ammonium sulphate. As a result, the rate of weathering of acidic soils has probably increased severalfold.
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