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

Soil P fractions and P sorption-desorption characteristics were studied 7 months after clearfelling and slash burning at a mixed Eucalyptus forest site in eastern Australia. Depending on the fire intensity, three different microsites were generated: unburnt, burnt and intensely burnt (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 soils with solutions having different concentrations of P, and desorption of the adsorbed P was studied by extracting the soils with Bray I extract. The effects of fire on soil P were greatest in the surface soil horizons and depended upon fire intensity. Ashbed soils differed from unburnt soils for P fractions and P sorption and desorption characteristics. 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 the surface layers. The increase in labile organic P (NaHCO3-extractable) contrasted with a decrease in total organic P (H2SO4) and less labile 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 decreases were observed in ashbeds and surface (0-5 cm) burnt soils. The ashbed soil showed an increase in sorption capacity in the 0-5 cm soil layer, but the sorbed P was generally less tightly bound to the solid phase. Seedling growth and foliage P concentrations were greatest in ashbed soils. Harvesting and burning increased the spatial distribution of soil P in the field. The ashbed and burnt microsites represented 19% and 18% respectively, of the surface area of the slash burnt coupe, and about 8 kg P ha-1 was deposited in ash.