S-retention and S-release play an important role in controlling the chemistry of soils affected by atmospheric deposition of sulfuric acid. Therefore knowledge of the dynamics of sulfur in soils is critical in understanding long-term developments in acidified terrestrial ecosystems. Until now, however, it is not clear as to whether also defined Al-Hydroxo-sulfates are part of the inorganic sulfur-reservoir in soils. Additionally, formation and dissolution kinetics as well as in-situ stability constants are unknown.
Several authors proposed the existence of defined Al-hydroxo-sulfates based on calculations of ion activity products (Prenzel, 1982; Nordstrom, 1982; Courchesne and Hendershot, 1990). However, ion activity products may indicate but cannot prove the existance of a solid phase. Therefore we intend to investigate solid phases of sulfur under environmental conditions (pH, etc.) by means of spectroscopic methods. Some of these methods have been applied in the investigation of soil- and rock-forming minerals and weathering kinetics (Berner and Holdren, 1979; Eggleston et al., 1989; Goosens et al., 1989) which represent single solid-state systems. In preliminary experiments we successfully tested Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), and Infrared Spectroscopy (IR).
Additional methods will be applied in the future: Secondary Ion Mass Spectrometry (SIMS), Differential Thermal Analysis (DTA), X-ray Fluorescence Analysis (XFA), Atomic Force Microscopy (AFM) and Laser Microprobe Mass Analysis (LAMMA).
Column experiments have been set up where two rock-forming minerals (gibbsite and albite) are reacted with sulfate and Al3+ under various pH-conditions typical for the chemistry of acidified soil solutions. The experiments are run at constant temperature (10°C). Both the solid phase and the solution are analyzed once a month with respect to the weathering products formed.