A long-term soil leaching column experiment investigating the effect of variable sulphate loads on soil solution and soil drainage chemistry

A series of leaching column experiments were set-up to investigate the effects of increasing and decreasing the sulphate load on the uppermost mineral horizon of an acidified podzolic soil from NE Scotland. The soils showed signs of recovery when the sulphate load was reduced and acidified further when the sulphate load was increased. For the soils in which simulated precipitation inputs were less acidic than the soil pH, cation adsorption was still occurring at the end of the experiment. Adsorption was probably limited by the rate of diffusion of ions from the bulk soil water to the surfaces of soil particles. The leachate and soil solution from the experiment in which the least acidic input was applied were more acidic than the equivalent solutions from the experiment with the intermediate pH input. This was due to the acidifying effect of the desorption of protons from soil particles. The leachate and soil solution chemistry and the weathering rate of the soil subject to the most acidic simulated precipitation were modelled using the PROFILE model. The predicted bulk weathering rate was over estimated by a factor of 1.2. The predicted weathering rates of individual ions varied from the measured values by factors of between 0.06 and 10. Both the measured and PROFILE predicted weathering rates fall in the critical load of acidity class suggested for the soil on the basis of soil parent material by the Skokloster classification. PROFILE failed to predict accurately the measured soil solution and leachate chemistry. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.