Phosphate solubilizing microorganisms in the rhizosphere of native plants from tropical savannas: An adaptive strategy to acid soils?

Abstract

Savannas are natural ecosystems that predominate in the tropics. These systems usually have acid soils with low fertility in which nutrients, specially phosphorus, are scarce. Phosphorus is generally fixed in insoluble forms that cannot be rapidly incorporated by plants. In acid soils phosphorus is fixed as aluminum and iron phosphates; in calcareous soils phosphorus is fixed as calcium phosphate. In both cases these phosphorus forms need to be solubilized in order to make phosphate ions available to the plant in soil solution. Besides the natural soil acidity, organic acids produced through microbial mechanisms or plant roots have been proved to solubilize these phosphates. I investigated if native plant rhizospheres of acid or calcareous soils are enriched with phosphate solubilizing microorganisms, in order to find mechanisms to improve plant nutrition and agrosystem sustainability. The rhizosphere of a typical and dominant grass from savannas, Trachypogon plumosus Ness, was studied in order to corroborate the former hypothesis. Furthermore different phosphorus forms in rhizospheric soil were determined applying Hedley et al., (1982) P-fractionation method. T. plumosus growing in acid and neutral-calcareous soil rhizospheres were compared in terms of microbial populations and phosphate fractions. My results show that in many of the rhizospheres considered P-Al and P-Fe solubilizing organisms predominate when P-Al and P-Fe are important P fractions present in soil. This was not the case in calcareous soils where P-Ca solubilizing organisms P-Ca fractions predominate in soil. Through this approach I elucidate mechanisms operating in plant rhizospheres to make hardly soluble phosphates available to plants. The implications of such mechanisms on biotechnological and agricultural approaches are discussed.