Clay mineral transformations in soils affected by fluorine and depletion of organic matter within a time span of 24 years

Data on soil chemistry and especially on clay mineral changes are presented, which were obtained from a soil monitoring site in a forested ecosystem near Möhlin (north-western part of Switzerland) in a time span of 24 years (1969–1993). The development in soil chemistry and clay mineralogy with respect to time was compared in two soil horizons at depths of 0–5 cm and 30–35 cm. A striking feature of the chemical changes observed in the soil was the distinct decrease of soil organic matter in the subsoil during the investigation period. These changes are most presumably due to past pollution of the area with F caused by a nearby aluminium industry. The observed decomposition of organic matter strongly influenced the behaviour of major and minor chemical constituents of the soil. In the subsoil, mica was intensively weathered and contributed, therefore, to the formation of smectites. The significant decrease in chlorites also traces the formation of smectite back to the weathering of chlorite. Al removal from the interlayers seemed to be enhanced by dissolved F and especially by organic complexing agents, which must have been present in the soil solution during this period of organic matter decomposition in the soil. Vermiculitic compounds had substantial changes towards lower content only in the topsoil, while minerals, such as interstratified chlorite/HIS, remained more or less unaffected. We conclude that the decomposition of organic matter and to a certain extent also changes in the acidity within the time span 1969–1993 have led to, or initiated, a kind of weathering, which has similarities to podzolisation. Due to the decrease in exchangeable F, the process of desalumination has additionally been enhanced by fluorine. Usually, mineralogical changes—under oxic environmental surrounding conditions—are believed to be a long-term process. Our investigation, however, clearly shows that significant clay mineral alterations may occur within relatively short periods of time.