Exploring the Phosphate Solubilization Potential of Rhizobia Isolated from Sesbania grandiflora

Abstract

Rhizobia, are known to exhibit the ability to solubilize phosphate (P) in the soil, apart from their primary function of nitrogen fixation. Hence this study was conducted to evaluate and characterize rhizobia isolates obtained from root nodules of Sesbania grandiflora for their potential in P solubilization. Two experiments were conducted under laboratory conditions arranged in a Complete Randomized Design (CRD) with three replicates. All data were analyzed using analysis of variance (ANOVA) and means were separated using the LSD test. Four bacterial strains (a,b,c,d) were isolated from root nodules using the trap plant method. They were initially identified as Rhizobia based on morphology and authenticated through gram staining, acid-alkaline tests, Congo Red Yeast Mannitol Agar (CRYMA), and Bromothymol blue Yeast Mannitol Agar (BRYMA) tests. The most effective rhizobia for P solubilization was identified based on the Phosphorus Solubilization Index (PSI) and solubilized P in Pikovaskya (PVK) solid and liquid media. Subsequently, optimization of PVK liquid media was conducted for effective P solubilizer, varying with carbon (C), nitrogen (N), and P sources. All isolates were gram-negative and exhibited acid production, authenticated as rhizobia. The significantly (p<0.05) highest PSI was recorded with isolate “c” and it also exhibited the highest (p<0.05) solubilized P. The isolate “c” showed potassium dihydrogen phosphate (35.1±0.46 ppm), glucose (25.47±0.49 ppm), and ammonium sulfate (3.20±0.17 ppm) as the optimum sources of P, C, and N, respectively, for achieving significantly higher phosphorus solubilization in PVK medium by thriving rhizobia “c” as an effective phosphorus solubilizer. However, further field studies are required to assess performance of rhizobia “c” before introducing it as a P solubilizing inoculum.