Soil moisture thresholds for combustion of organic soils in western Tasmania

The present study aimed to determine moisture thresholds for combustion of organic soils sampled from various vegetation types at 63 locations in Tasmania, Australia. To observe whether the soil sample sustained smouldering combustion, moisture content was experimentally manipulated and heat was applied. Combustion was primarily determined by moisture content, but was also influenced by soil bulk density and organic content: the gravimetric moisture content corresponding to a 50% probability of burning ranged from 25 to 94% as organic content varied from 34 to 96%. There was no evidence of differences among vegetation types in the relationship between soil combustibility and organic content. Combustion in Tasmanian organic soils occurred with moisture levels similar to those reported elsewhere, despite differences in vegetation and environment. It was also found that a hand-held meter that measured volumetric moisture content using time domain reflectometry could be used to satisfactorily predict organic soil combustion. Finally, combining the data with estimates of volumetric soil moisture based on high-resolution gridded weather data (Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia, or BARRA), it was demonstrated that most Tasmanian organic soils are likely to be combustible at some time almost every summer (December to February).