Speciation and Fractionation of Phosphorus Affected by Enterobacter cloacae in the Rhizosphere of Sugarcane ( Saccharum officinarum L.)

The purpose of this study is to improve relative phosphorus (P) distribution in various proportions by greater solubility in the rhizosphere of plants influenced by phosphate solubilizing bacteria (PSB), P availability, and fertilizer management. For this purpose, a pot experiment was conducted with three phosphorus concentrations including 0 (P0), 20.5 (P50), and 41 (P100) mg P kg−1 and two PSB strains as well as Enterobacter cloacae R13 (R13) and E. cloacae R33 (R33) as factors which were evaluated at three harvesting times: 60, 95, and 140 days after planting (DAP) sowing. In the non-fertilized condition, treatments inoculated by R13 and R33 had higher P uptake in comparison with the non-inoculated treatment. Compared with the bulk soil, the chemical composition of the soil solution sample was affected by the dynamic properties of rhizosphere. In the inoculation-treated rhizosphere, the soluble P concentration was higher than that in the non-inoculated treatment. However, P uptake was greater in the inoculated treatments. Also, higher P free ions (H2PO4− and HPO42−) activity and lower calcium (Ca2+) and magnesium (Mg2+) activity were observed in soil solution inoculated rhizosphere as compared with the non-inoculated treatment. At all harvesting times, octacalcium and β-tricalcium phosphate minerals controlled P concentration in the rhizosphere. The pedogenic Ca-P or low soluble P (Ca8-P) was higher by up to 72.97% compared with other fractions, which can be ascribed to octacalcium and β-tricalcium phosphate minerals. In the current study, we conclude that the reduction of pH is not a major mechanism in dissolving insoluble P by E. cloacae although phosphate solubility may be attributed to the chelation process. In comparison, rhizosphere characteristics were more effective than PSB in converting the type of P-containing minerals. The results suggest that Ca8-P fraction is equivalent to octacalcium phosphate and β-tricalcium phosphate minerals in the rhizosphere of sugarcane.