Solution chemistry in acid forest soils: Are the BC : Al ratios as critical as expected in Switzerland?

Abstract

AbstractThe molar ratio of base nutrient cations to total dissolved aluminum (BC : Altot) in the soil solution was measured at six forest sites in Switzerland in acid mineral soils to determine whether the ratio measured in the field was lower than the critical value of 1, as predicted by the mapping of exceedances of critical loads of acidity. The soil chemistry was then related to the soil solution composition to characterize the typical effective base saturation (BS) and BC : Al ratio in soil leading to critical BC : Altot in the soil solution. The median BC : Altot ratio in the soil solution never reached the critical value in the root zone at any sites for the whole observation period (1999–2002), suggesting that the BC : Altot ratios measured in the field might be higher than those modeled for the determination of critical loads of acidity. The gibbsite model usually applied for the calculation of critical loads was a poor predictor of the Al3+ activity at the study sites. A curvilinear pH‐pAl3+ relationship was found over the whole range of pH (3.8–6.5). Above a pH of 5.5, the slope of the pH‐pAl3+ relation was close to 3, suggesting equilibrium with Al(OH)3. It decreased to values smaller than 1.3 below a pH of 5.5, indicating complexation reactions with soil organic matter. The BS and the BC : Al ratios in the soils were significantly correlated to the BC : Altot ratios in the soil solution. The soil solutions with the lowest BC : Altot ratios (≤ 2) were typically found in mineral soils with a BS below 10 % and a BC : Al ratio in the soil lower than 0.2. In acid pseudogleyed horizons overlying a calcareous substrate, the soil solution chemistry was strongly influenced by the composition of the underlying soil layers. The soil solutions at 80 cm had pH values and BC : Altot ratios much higher than expected. This situation should be taken into account for the calculations of critical loads of acidity.