Some effects of plant ash and heating of soil minerals on soils affected by bushfires

[Truncated abstract] Bushfires are very common in Australia and severely modify ecosystems including soils. The effects of heating on soil chemical, mineralogical and morphological properties including the growth of plants have been investigated using laboratory, glasshouse and field observations. This thesis focuses on three soil minerals that are affected by heating: kaolinite, gibbsite and goethite. The thesis also considers the nature of ash created by bushfires and its reaction with soil. A laboratory study investigated dehydroxylation of pure kaolinite, gibbsite and goethite and their rehydroxylation under wet conditions. Rehydroxylation of heated gibbsite was extensive at 95oC with bayerite and gibbsite forming during 14 days, the process was much slower at 55oC. Metakaolinite formed from kaolinite and hematite formed from goethite by heating did not recrystallise but did aquire structural water during incubation in water. The specific surface area of all three dehydroxylated minerals was increased substantially by wet incubation. Dehydroxylated minerals and probably their partly rehydroxylated forms may exist in soils heated by fire and thus affect the chemical behaviour of these soils. Soil heated a day earlier in a bushfire at Wundowie in the Darling Range, Western Australia was taken from under and adjacent to burnt Eucalyptus and grass tree (Xanthorrhoea pressii) logs. Conventional and synchrotron XRD patterns of heated and unheated soil show that the main crystalline compounds of unheated soil are quartz, kaolinite, gibbsite and goethite. In heated soil, kaolinite had dehydroxylated to form metakaolinite, gibbsite altered into an amorphous phase, while goethite transformed into hematite (hydrohematite). The bushfire added calcite in plant ash to the soil which considerably increased the pH.