The Ash-Bed Effect in Eucalyptus regnans Forest: Chemical, Physical and Microbiological Changes in Soil After Heating or Partial Sterilisation

The ash-bed effect (the enhanced growth of plants on soil which has been heated) following fire in Eucalyptus regnans forest is dramatic. The results are presented of studies of the effects of a range of heating and partial sterilization treatments on chemical, microbiological and physical properties in soil from a 250-year-old E. regnans forest in Victoria. Soil treatments not involving heat (chemical sterilization, γ -irradiation and air-drying) and the lower temperature heat treatments (100 and 200°C) had no marked effects on physical characteristics. All treatments produced more or less similar effects on microbial populations. On the other hand, heating the soil to 400-600°C produced large, significant and sustained increases in the availability of nitrogen and phosphorus and these increases were enhanced by a decrease in clay colloid. The results support the hypothesis that the ash-bed effect following fire in E. regnans forest is due to an increase in the availability of nutrients, and in the availability of nitrogen and phosphorus in particular. A transitory increase in the concentration of manganese caused by heating the soil may account for initial toxicity in plants grown in soils which have been heated. Since species within the subgenus Monocalyptus are characterized by lower tissue concentrations of manganese than those within Symphyomyrtus, it could be hypothesized that the potential for toxicity following bushfire varies between the two subgenera. The literature on the effects of soil-sterilising treatments is highly variable; the causes of variability include soil type and moisture content, treatment (sterilizing by steam, chemicals or heat) and the method of treatment (time, how the soil was contained, and how the treatment was applied).