Aluminium mobility at an acid sensitive site with high S-deposition

Soil water at an acid-sensitive forested catchment in southwestern Poland h as been studied for four years. Median base saturation (BS) is only 5% in t he podzol B-horizons. Very low pH values in the soil water from the O-horiz ons (10- and 90 percentiles pH 3.5 and 4.3) increased to a typical median p H in the B-horizons of 4.4, mainly by release of inorganic labile aluminium (Ali). Median concentrations in the B horizons were 3.4mg Al-i L-1. Al-soi l/soilwater interactions were studied over a large span of sulphate concent rations resulting from both a generally decreasing S-deposition during the last decades and an increase in precipitation during the study period. Thes e changes led sulphate to leach from the mineral soil. Aluminium mobilisation is better described by jurbanite- than by gibbsite s olubility. For the soils with aluminium saturation (AlS) >90%, there is a t endency that the concentration of Al3+ decreases less than divalent base ca tions with a decrease in SO42- concentration. This causes the critical load molar ratio (R-CL={Al3+}/{Ca2++Mg2+}) to increase with a decrease in the s ulphate concentration in soil water, which is not in agreement with a simpl e cation-exchange model.