Changes in the mineralogy and chemistry of a lateritic soil due to a bushfire at Wundowie, Darling Range, Western Australia

Bushfires are very common in Australia as seasonally arid conditions prevail and many Australian native plants are highly combustible. Soil is heated in bushfires but the effects of heating are poorly understood. Soil heated in an intense bushfire was collected from Wundowie in the Darling Range, Western Australia. The soil samples were from under and adjacent to burnt eucalyptus (Eucalyptus marginata) and grass tree (Xanthorrhoea pressii) logs. Conventional and synchrotron XRD patterns of heated and unheated soil show the effect of fire on soil minerals. The main crystalline compounds of unheated soil are quartz, kaolinite, gibbsite and goethite. In heated soil, kaolinite had dehydroxylated to form metakaolinite, gibbsite had altered into an amorphous phase, while goethite had transformed into hematite (hydrohematite). The bushfire added calcite and other salts in plant ash to the soil, which considerably increased the pH and EC of the soil. Heated soil had increased amounts of oxalate extractable Al, Fe and Si due to crystalline hydroxylated minerals becoming amorphous and soluble in oxalate as they were dehydroxylated by heating. Clearly dehydroxylated minerals and possibly their naturally rehydroxylated forms are likely to be present in soils heated by bushfires and these minerals may have significant effects on the chemical behavior of the soil.