Measured and modelled retention of inorganic sulfur in soils and subsoils (Harz Mountains, Germany)

Atmospheric deposition has resulted in an accumulation of inorganic sulfur (S) in many forest soils. At Sosemulde (Hart Mountains) samples from 5-240 cm depth were analysed. Most sulfate (SO4) is accumulated at about 30-60 cm depth: 8.5-9.5 mmol(c) kg(-1). Large amounts can also be retained in < 100 cm. To assess changes in SO4 dynamics in time,adsorption isotherms have been included in several process-oriented models, e.g., in MAGIC. The Lange Bramke (LB) Model is the first model used on the catchment scale containing solubility products for the hydroxosulfate minerals jurbanite and alunite. By reconstructing the long-term acidification history (140 years) both models were successfully calibrated to a 14-year deposition, soil and streamwater data set at Lange Bramke catchment (Harz Mountains). According to MAGIC the present accumulation of SO4 in 0 - 80 cm is 8.7 mmol(c) kg(-1), while according to the LB-Model 10.2 mmol(c) kg(-1) are stored as jurbanite. Both models predicted 4.5 mmol(c) kg(c) SO4 in the subsoil layer, retained as alunite in the LB Model. These values correspond to the amounts measured in soil and subsoil samples at Sosemulde, respectively. However, for future scenarios with decreasing S inputs the models show different developments in SO4 concentrations. Changes in MAGIC are gradual whereas the LB model predicts stepwise decreasing SO4 values as soon as previously stored hydroxosulfates are fully dissolved. Such concentration "jumps" have not been observed.