Effect of fire intensity on solution chemistry of surface soil under a Eucalyptus pauciflora forest

Abstract

The chemical composition of soil solutions (field percolates collected in situ and laboratory saturation extracts) was measured at three sites subjected to widely varying fire intensity in subalpine Eucalyptus paucfiora forest near Canberra. The sites were unburnt forest, areas prescribed burnt resulting in almost complete canopy scorch, and ashbeds (intensely heated soil). Saturation extracts were obtained 1, 58, 375, 745 and 1095 days after the fire, and soil percolates were collected on 17 occasions during the initial year after burning. Large quantities of cations (Ca2+, Mg2+, K+ , NH+4) and anions (Cl-, SO24-) and soluble silica were mobilized by burning, especially under ashbeds. Mobilization resulted from deposition of water-soluble elements in ash, immediate effects of soil heating, and enhanced rates of mineralisation of soil organic matter indicated by high concentrations of NH+4 which persisted for more than one year in surface soils under the ashbeds. After burning Ca2+ became the dominant cation in saturation extracts of surface (0-5 cm) soils for the entire 3-year study period. In the 5-15 cm soil layer, firstly NH+4 and later K+ replaced some of the Na+ in the solution phase. Most of the Cl- deposited in ash was leached below 15 cm depth within one year and was probably accompanied by transport of K+, Mg2+, Na+ and NH+4, but very little transfer of Ca2+ occurred. Concentrations of NO-3 and phosphate were always low in saturation extracts and soil percolates, and levels were unaffected by burning, despite the presence of large amounts of exchangeable NH+4 in the soil and the deposition of significant amounts of phosphate in ash. Burning increased the concentrations of soluble silica and SO24- in saturation extracts for at least 3 years after the fire. Most of the changes in soil solution chemistry measured would increase nutrient availability to the vegetation during the initial year after burning, but these changes must be balanced against losses of organic matter and nutrients during and after fires.