EFFECTS OF POSTHARVEST RESIDUE MANAGEMENT ON LABILE SOIL-PHOSPHORUS IN A PINUS-RADIATA PLANTATION

The effects of clearfelling followed by four logging-residue and litter-management treatments (retention or removal of slash and litter; retention of litter only, with or without ploughing) on NaHCO3-extractable inorganic and organic soil P were measured over a 14-month period after clearfelling a 37-year-old stand of Pinus radiata D. Don growing on a podzolized sand in South Australia. Inorganic P in soils increased by an average of 7.5 kg P ha-1 from the levels in uncut forest in all four management treatments. Removal of slash and litter resulted in slightly lower levels of P in available forms than the other treatments. Where slash and litter, or litter only, were left undisturbed, the accumulation of inorganic P was initially slow but increased by the 14th month after clearfelling. Ploughing more than doubled the net accumulation of inorganic P in the first 8 months, but subsequent accumulation was slower. Greater P availability in the litter-ploughed treatment was related to increased concentrations of P in decomposing needle litter in litter bags buried in the soil, and in the foliage of young trees. Phosphorus availability in all clearfelled treatments was adequate to sustain high early growth rates of young trees. Results are discussed in relation to the management of slash, litter, weeds and P fertilizer.