UTILIZATION OF LABELED MINERAL AND ORGANIC PHOSPHORUS SOURCES BY GRASSES COMMON TO SEMIARID MULGA SHRUBLANDS

We examined whether the differing growth responses to varying concentrations of soil P by grasses common to the mulga shrublands of south-west Queensland were due to differences in their ability to utilize sparingly soluble sources of P in two glasshouse trials. The uptake of three sparingly soluble sources of mineral P (amorphous FePO4, crystalline FePO4 (strengite), and amorphous AlPO4) Was compared with that of a soluble P source, KH2PO4, over 34 days. All P sources were uniformly labelled with P-32. For the four grasses studied (Cenchrus ciliaris, Aristida armata, Digitaria ammophila and Thyridolepis mitchelliana), the addition of strengite produced the lowest yields and plant P content, followed by FePO4, and then AlPO4 and KH2PO4. Recovery of the labelled P by the grasses was in the order: strengite < FePO4 < AlPO4 < KH2PO4. C. ciliaris recovered more of the added strengite, but less FePO4 and AlPO4, than the other grasses. There were no differences in specific activity between the grasses for KH2PO4. Where soil treatments were the same, there was no evidence that these species differed in their ability to utilize sparingly soluble sources of mineral P. The reutilization of P from P-32 labelled plant residues by two grasses was examined in a second experiment. The experiment consisted of a factorial combination of two species (D. ammophila and A. armata ) and a control (non-planted pots), two phosphorus systems (low and high P), and four harvests (15, 23, 31, and 39 days). The net release of P from the residues was significantly increased in the presence of plants but was not affected by the species present. D. ammophila and A. armata obtained similar proportions of their P content from the added residues (6.6%).