Jürgen Prenzel

Acid deposition in the German solling area: Effects on soil solution chemistry and Al mobilization

The Al chemistry of soil solutions was evaluated in two forest ecosystems in the North-German Solling area which is heavily impacted by acidic deposition. The principal H+ buffering process in these soils is the release of Al ions. Within the stand of Norway spruce, Al concentrations increase with soil depth up to 370 umol/L. Ca/Al ratios of the soil solution decrease with depth and suggest high risk of Al toxicity to tree roots and potential antagonistic effects for ion uptake. The Al concentrations of the soil solution in the upper horizons do not appear to be in equilibrium with mineral phases of Gibbsite, Alunite and Jurbanite as suggested by the depth gradients and temporal patterns in ion activity products. Depletion of extractable soil Al in the upper horizons is occuring. The release of Al to the soil solution under these conditions seems to be restricted by kinetic constraints.

Describing soil SO42− dynamics in the solling roof project with two different modelling approaches

The release of previously stored soil SO42− is tightly connected with the reversibility of soil and water acidification. Thus soil SO42− dynamics have to be included when predicting the reversibility of acidification. Our aim was to compare two modelling approaches: The model MAGIC (Cosby et al., 1985) describes SO42− dynamics with the Langmuir sorption isotherme. In the SO-MODEL (Prenzel, 1991) a precipitation/ dissolution of jurbanite is defined.Even though it was possible to calibrate both models to lysimeter data of the Solling D1 site in 1 m depth, the prognosis for SO42− concentrations in the soil solution differed significantly. While MAGIC predicted the observed gradual decrease of SO42− concentration with decreasing deposition, the SO-MODEL calculated stable concentrations up to the year 2026 followed by a sudden drop. Because the prognosis established with the SO-MODEL is incompatible with observed field data, we concluded that the predicted SO42− dynamic of the SO-MODEL was unrealistic.

Reconstruction of water acidification in a forested catchment, western Harz Mountains, Germany

Abstract A dynamic model based on the precipitation of aluminium hydroxosulf ate minerals was used to reconstruct a hypothetical history of catchment acidification. The effects of 140 a of increased atmospheric S deposition into the Lange Bramke valley, western Harz Mountains, was simulated. The model includes the following processes: complexation of several hydroxo- and sulfato-compounds ; ion exchange based on selectivity constants for all uncomplexed cations; carbonic acid equilibria reactions system assuming a given partial pressure of CO2; and the precipitation/dissolution equilibria for gibbsite (Al(OH)3), alunite (KAl3(OH)6(SO4)2) and jurbanite (AlOHSO4). The adequacy of the model was demonstrated by applying it to 4 a of deposition, soil water and streamwater chemistry data from the Lange Bramke area. The model indicated that the measured S retention in the catchment can be explained by precipitation of alunite and jurbanite.