A comparison of measured and calculated effects of artificial acid rain on cations in soil solution and exchange sites

In this paper, we report the results of a comparison of soil solution and exchangeable cation data from a laboratory experiment with those predicted by MIDAS, a simple soil acidification model based on Gaines‐Thomas cation exchange. In the laboratory experiment three different acid treatments (pH 3.3, 3.8 and 4.6) were used. The treatment solution was first allowed to equilibrate with the humus layer sample. After this an aliquot of the leachate was taken for chemical analyses and the rest of the leachate was used as a treatment solution for eluvial layer soil samples. The procedure was continued for ¡lluvial layer samples in the similar way. The soil samples used in the experiment were collected from Scots pine and Norway spruce stands.

The laboratory experiment described was found to be useful for testing the dynamic behaviour of soil cation exchange and acidification models, and for narrowing the range of exchange parameter values used by such models. The MIDAS model adequately explained the changes in H⁺, Al³⁺, Ca²⁺ and Mg²⁺ concentrations in the equilibrium solution of humus layers induced by acid treatments in the laboratory experiment. The model somewhat over estimated the leaching of exchangeable Ca²⁺ and Mg²⁺ from the humus and failed to give comparable results for the amounts of exchangeable H⁺ in the two most acidic treatments. For the mineral soil horizons studied more realistic model descriptions of the dynamics of Al‐compounds other than simple cation exchange, e.g. organic complexes, are needed for long‐term predictions of changes in the chemical characteristics of the mineral soil and drainage waters in response to acid deposition.