Phosphorus (P) is an essential nutrient in sustainable production of sugarcane. Due to low labile P in soil under sugarcane cultivation, evaluation of the efficiency of P uptake and the application of phosphate solubilising bacteria (PSB) play important roles in management of P fertiliser. To investigate the effect of using PSB on P uptake in sugarcane (variety CP57–614), a pot experiment was conducted with three replications in greenhouse conditions. The treatments were a combination of three P rates (0 (P0), 50 and 100% (~40 mg kg−1) as triple superphosphate, and two PSB strains (Enterobacter cloacae R13 (R13) and R33 (R33)) which were applied independently and simultaneously. Morphological characteristics of sugarcane and some biochemical parameters were evaluated in the rhizosphere at three harvesting times: 60, 95 and 140 days after planting (DAP). Whereas in low available P (P0), bacterial strain R33 improved P uptake along with sugarcane ageing, P uptake was diminished in non-inoculated treatment over time. Activity of PSBs in the rhizosphere (especially strain R33) prevented the sharp fall of P influx after 95 DAP in low available P condition. Indeed, activity of R33 in the rhizosphere decreased the dependence of P uptake on root development via improving P uptake. Therefore, influx was the main mechanism of P uptake in sugarcane. Sugarcane inoculated by PSBs acquired 76 and 81% of total P uptake from non-Olsen-P fraction in P0R13 and P0R33 respectively at 95 DAP. However, this amount was lower (70.4%) in P0R0. Furthermore, strain R33 improved P uptake efficiency in sugarcane by changing root morphology (e.g. specific root length and root length) and reducing soil limitations (e.g. enhancement of P compound solubility and P influx).

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 (P 0), 20.5 (P 50), and 41 (P 100) mg P kg −1 and two PSB strains as well as Enterobacter cloacae R13 (R 13) and E. cloacae R33 (R 33) 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 R 13 and R 33 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 (H 2 PO 4 − and HPO 4 2−) activity and lower calcium (Ca 2+) and magnesium (Mg 2+) 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 CaP or low soluble P (Ca 8-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 Ca 8-P fraction is equivalent to octacalcium phosphate and β-tricalcium phosphate minerals in the rhizosphere of sugarcane.

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