EFFECTS OF SLASH BURNING ON SOIL-PHOSPHORUS FRACTIONS AND SORPTION AND DESORPTION OF PHOSPHORUS

Soil P fractions and P sorption-desorption characteristics were studie d 7 months after clearfelling and slash burning at a mixed Eucalyptus forest site in eastern Australia. Depending on the fire intensity, thr ee different microsites were generated: unburnt, burnt and intensely b urnt (ashbed). Phosphorus fractions were extracted from the soil with NH4F (0.03 N) + HCI (0.025 N) (Bray 1), NaHCO3 (0.5 N), NaOH (0.5 N), and H2SO4 (1 N). Adsorption isotherms were obtained by equilibrating s oils with solutions having different concentrations of P, and desorpti on of the adsorbed P was studied by extracting the soils with Bray I e xtract. The effects of fire on soil P were greatest in the surface soi l horizons and depended upon fire intensity. Ashbed soils differed fro m unburnt soils for P fractions and P sorption and desorption characte ristics. Labile inorganic P (Bray I) increased from less than 1 mg kg- 1 in the unburnt soil to 5-13 mg kg-1 in the ashbed. Inorganic P (NaOH and H2SO4 extractions) increased markedly after fire, especially in t he surface layers. The increase in labile organic P (NaHCO3-extractabl e) contrasted with a decrease in total organic P (H2SO4) and less labi le organic P (NaOH) in ashbed soils, suggesting marked transformation of organic P pools after intense fire. After incubation for 2 months, labile organic P in the unburnt soil increased, whereas large decrease s were observed in ashbeds and surface (0-5 cm) burnt soils. The ashbe d soil showed an increase in sorption capacity in the 0-5 cm soil laye r, but the sorbed P was generally less tightly bound to the solid phas e. Seedling growth and foliage P concentrations were greatest in ashbe d soils. Harvesting and burning increased the spatial distribution of soil P in the field. The ashbed and burnt microsites represented 19% a nd 18% respectively, of the surface area of the slash burnt coupe, and about 8 kg P ha-1 was deposited in ash.