- Title:
-
Effects of Fire Retardant on Soils of Heathland in Victoria
- Date:
- November 2003
- Organisations
- BCRC
- Authors:
- Peter Hopmans and Ross Bickford
- Location:
- Australia, VIC, Australia
Overview
Fire retardants have been used operationally for the control of wildfires in Victoria’s parks and forests since 1967. One of the most effective retardants used is Phos-Chek D75-R (Phos-Chek), which contains ammonium sulphates, ammonium phosphates, guar gum, iron oxide and performance additives (CSIRO 2000). While the use of this product in natural environments has been approved by the United States Department of Agriculture, a recent review by CSIRO (2000) identified that no experiments have been conducted to examine the effects of this retardant specifically on native flora and fauna in Australia.A study was therefore initiated in 2000 by Fire Management of the then Department of Natural Resources and Environment to assess the effects of fire retardant on vegetation, soil chemistry and surface-active invertebrates in fire-prone heathland communities near the coast at Marlo and in the Victoria Valley of the Grampians. The impact of retardant on soil chemistry is reported here, the effects on vegetation (Bell 2003) and invertebrates (Collett & Schoenborn 2003) are reported separately.
The application of retardant at rates required for effective control of fire (1.5 L/m2 of a solution of 14% Phos-Chek) equate to broadcast applications of elemental nitrogen, phosphorus and sulphur of approximately 340, 90 and 330 kg/ha respectively. Monitoring of properties of surface soils to a depth of 20 cm was conducted at 2, 6 and 12 months after treatment and provided an insight into the immediate and short-term effects on properties of Podosols at these sites. The impact of Phos-Chek on surface soils at Marlo and the Grampians can be summarised as:
- Retardant decreased soil acidity (pH) by up to 0.5 unitsto pH 4.8 at Marlo and by 0.3 units to pH 4.9 at the Grampians site. The effect on soil pH was still evident after 12 months and further soil monitoring is required to determine the long-term impact in the soil profile.
- Soil salinity increased immediately at both sites, this was followed by a rapid decline to pre-treatment values within 12 months. Changes in salinity were related mainly to increases in soluble sulphate from the retardant.
- The impact of retardant on total carbon and nitrogen in the soil was relatively minor and within the range of natural variation of these elements in the surface soil at both sites.
- Levels of readily available or labile forms of nitrogen increased about three-fold at Marlo and nearly ten-fold at the Grampians. This substantial increase in labile nitrogen was of short duration and levels declined rapidly to background values after 12 months.
- A significant five-fold increase in labile phosphorus was found in the surface soil after 12 months. At Marlo, the application of retardant progressively increased extractable phosphorus in the surface soil from 2 to 9 mg/kg over 12 months. In contrast, a rapid increase in extractable phosphorus from 2 to 36 mg/kg was observed after 2 months at the Grampians site followed by a decline to 10 mg/kg after 12 months. These results indicate that a large proportion of the phosphate applied in retardant has been leached into the subsoil.
- Retardant applied at the highest
rate caused a three-fold increase in labile sulphate after 2 months at
Marlo followed by a rapid decline to background levels. In contrast,
labile sulphate increased nearly twenty-fold after 2 months at the
Grampians site followed by a rapid decline to background levels.
Results indicate that sulphate applied has leached rapidly into the
subsoil increasing the level of soluble salts stored in the soil
profile.
It is therefore important to determine the accumulation of phosphate in the soil profile at least to the maximum depth of the root zone and the changes this may have induced in the vegetation. It is recommended that these experimental sites be maintained to allow further monitoring of soil profiles and vegetation in 2005, four years after application of the retardant. This should provide a better insight into the longer-term impact of retardant on soil chemistry of these Podosols and the availability of nutrients to plants on these heathlands.
