Studies on Microbial Phosphate Solubilization and Development of Inoculum Formulations

Abstract

Efficient phosphate solubilizing bacteria and fungi were isolated from rhizosphere of Stevia rebaudiana grown in organic farming. Selected bacterial and fungal isolates were significantly solubilizing insoluble form of tri-calcium phosphate and rock phosphate into soluble P with reduction in pH of the growth medium. All the bacterial and fungal isolates were produced organic acid, acid phosphatase, alkaline phosphatase and phytase enzyme in growth medium. On the basis of 16S rRNA gene sequencing two most efficient bacterial isolates were identified as Pantoea cypripedii (PSB-3), Pseudomonas plecoglossicida (PSB-5). And on the basis of ITS sequence analysis fungal isolates were identified as Aspergillus tubingensis (PSF-4) and Aspergillus niger (PSF-5, PSF-6 and PSF-7). Maximum P solubilization in bacterial and isolates was observed with glucose as a carbon source and ammonium sulphate as a nitrogen source. The mechanism of P solubilization might be due to pH reduction, organic acid production, acid phosphatase, alkaline phosphatase and by phytase enzyme production in both bacterial and fungal isolates. All the selected bacterial and fungal isolates were positive for indole acetic acid production and siderophore production. Two year field study of maize and wheat crop was conducted and results showed that bio-inoculation treatments of bacterial (P. cypripedii and P.plecoglossicida) and fungal isolates (A. tubingensis and A. niger) along with RP fertilization have more significant effects for the improvement of crop yield and soil fertility compared to DAP treatments, bio-inoculation alone and RP fertilization alone in organic farming. It can be concluded from the present study that Pantoea cypripedii, Pseudomonas lecoglossicida, Aspergillus tubingensis and Aspergillus niger can be used as bio-inocuant along with RP fertilization in organic farming and they showed their consistent performance at multilocational sites. Results showed that RP sustain the viability of both fungal and bacterial isolates at different temperature (4 ºC and 37 ºC) up to 270 days of storage compared to other carrier materials tested such as fly ash, charcoal and vermiculite. Chemical fertilizers might be substituted by cheaply available RP and P-solubilizing microorganisms. These results ensure the long term fertility of the soil in relation to P and would provide a cost effective, sustainable and environmental friendly production system for crops in organic farming.