PHOSPHATASE-ACTIVITY AND PHOSPHORUS FRACTIONS IN KARRI (EUCALYPTUS-DIVERSICOLOR F MUELL) FOREST SOILS

Karri forest soils contain negligible concentrations of labile-P, low concentrations of total P and more P in organic forms than inorganic. The ratio of organic P to inorganic P was lowest (1:2) in recently burnt surface soils and greatest (7:1) at depth in soil that had been undisturbed for long periods of time. Phosphomonoesterase and phosphodiesterase activities (to 10 cm depth, phosphomonoesterase 700-1300; phosphodiesterase 2000-2400 mug nitrophenol released h-1 g-1 fresh weight) were comparable to those in other, organically rich forest soils. The optimum pH for phosphatase activities were within 1-2 units of soil pH (approximately 6) and little reduction in activity was observed over the pH range 4-8. Phosphatase activity was reduced by air-drying (up to 20-fold reduction) and was almost entirely absent in soils that were heat-affected as a result of logging/burning operations. Neither phosphomonoesterase nor phosphodiesterase were directly related to soil P fractions or total P. A reduction in P demand is postulated as the cause of reduced phosphatase activity and the increased concentration of organic P with increasing soil depth.