Influence of the soil solution composition on retention and release sulfate in acid forest soils

The potential for sulfate retention is an important soil feature for buffering of atmospheric acid deposition. We studied the effects of increasing additions of different neutral salts and acids on mobilization and retention of SO42- in acid forest soils. Soils containing up to 11 mmol SO42- kg(-1) were equilibrated with H2O, NaCl, MgCl2, and HCl. Release of SO42- was highest with H2O and NaCl additions and lowest when HCl was used. Increasing the ionic strength of the added solutions caused decreasing SO42- concentrations in equilibrium solution. Decreasing pH in equilibrium solution was found to be the reason for the decrease in release. Even when the pH was < 4, the SO42- release decreased. We assume that this finding resulted from the fact that in the;soils studied the SO42- sorption was controlled by the high contents of Fe oxides/hydroxides. Experiments with Na2SO4, MgSO4, and H2SO4 demonstrated that the B horizons already containing high amounts of SO42- were still able to retain SO42-. Sulfate retention increased in the order Na2SO4 < MgSO4 < H2SO4, which corresponds to increasing H+ availability The higher SO42- retention along with MgSO4 compared to Na2SO4 may be caused by higher potential of Mg to mobilize soil acidity compared to Na.