CHANGES IN SOIL PHOSPHATE FRACTIONS IN THE RHIZOSPHERE OF SEMIARID PASTURE GRASSES

The comparative ability of several species of grass common to the semi-arid mulga shrublands of south-western Queensland, namely Cenchrus ciliaris cv. U.S.A., Aristida armata, Digitaria ammophilla and Thyridolepis mitchelliana, to deplete various fractions of soil inorganic/organic phosphorus was assessed using a rhizosphere thin section methodology. All species significantly depleted inorganic phosphorUS extractable with 0.5 M NaHCO3, 0.1 M NaOH, and 1.0 M HCl. The grasses were able to deplete 39, 32, and 29%, respectively, of the inorganic component of each of these extracts. In contrast, the organic phosphorus fraction of all three extracts either remained unchanged or increased in the presence of the grasses compared with unplanted control sections. Depletion of sparingly soluble soil phosphorus coincided with rises in rhizosphere pH. This result contrasts with previous studies that attributed plant depletions of phosphorus pools that are sparingly soluble to an acid hydrolysis process mediated by the root. The liming of the soil had only a minor effect on the distribution of added phosphate between the various soil inorganic phosphorus fractions but resulted in a depletion in the organic fraction of the 0.5 M NaHCO3 and 1.0 M HCl extracts. There was no difference between the grasses in ability to extract the various soil phosphorus pools. These results confirm earlier studies using radioactively labelled compounds of mineral phosphate that these grasses have the ability to utilize sparingly soluble pools of soil phosphorus that are not traditionally considered to be 'plant available'.