As well as the obvious destruction of vegetation, human infrastructure, domestic animals and native fauna and their habitat, bushfires also have severe and long-lasting effects on the soil which supports them all.
Soils in burned areas are roasted by the high temperatures, incinerating plant roots and soil organic matter and killing seeds, microfauna and microflora, eggs and spores (yet stimulating the seeds of some species to germinate when it next rains). Plant nutrients in the soil are lost and some soil minerals may even be altered, adversely affecting soil structure. Surface soil may become more alkaline (raised pH) because of ash on the surface, thereby changing the availability to plants of the remaining nutrients.
All these factors greatly increase the vulnerability of soils to water erosion, which is probably the worst consequence of bushfires for soils, due to raindrop impact and runoff from bare soil surfaces once it rains. Protective groundcover has been lost in the fire and some soils form a surface seal, which reduces or prevents the infiltration of rainwater.
Measures to control bushfires often involve the urgent grading or bulldozing of firebreaks and access tracks. These, along with the windrows created, can also lead to the concentration or diversion of runoff water and increased soil erosion.
As well, storms occurring after intense bushfires can wash volumes of ash, charcoal and topsoil into waterways and dams, causing pollution and reduced storage capacity.
Wind erosion is generally a lesser problem, except on exposed sandy soils.
A major way of reducing post-fire erosion is by changing the fire regime, specifically the intensity of fire. More intense fires create a greater hazard than low intensity fires. Lower intensity fires cause less damage to vegetation, the soil surface and soil organic matter, allowing some measure of protection. Geographic analysis can identify priority areas where
fire regimes most need to be altered.
There are several post-fire measures that can be taken to reduce the potential for soil loss and water pollution, applicable in different landscapes and climatic regions. It is important to take these measures as soon as possible after fires have been extinguished, though the window of opportunity can be brief and there are many other urgent demands on the time and efforts of land managers at this time.
Soil Knowledge Network Position
The Soil Knowledge Network recognises the extreme effects of bushfires and subsequent rainfall events on;
- soil chemical and physical properties, organic matter, plants and their propagules, and microfauna and microflora;
- vulnerability of soil to erosion, and;
- pollution of waterways.
Geographic analysis should be used to identify priority areas where fire management regimes may be altered to less intense burns, to reduce infrastructure and vegetation damage, soil erosion and sedimentation.
Post-fire measures need to be taken as soon as possible after fires have been extinguished, and include:
- rebuild erosion control structures disturbed by heavy machinery whilst fire fighting;
- respread windrowed material and mulch or reseed temporary tracks and firebreaks and any topsoil disturbed by heavy machinery during fire fighting;
- leave timber debris undisturbed or, if feasible, arrange it into windrows along contours;
- in open country, and subject to suitable soil conditions, construct shallow single furrows along the contour to trap or slow runoff water and catch ash and soil;
- construct contour banks to control runoff where feasible;
- construct temporary short banks to divert the first runoff water and ash from entering farm dams;
- delay restocking until pasture regenerates or is re-sown;
- sow cover crops, with fertiliser, where feasible, after rain to provide ground cover and stock feed;
- seek local technical advice.