Efficacy of organic acid secreting bacteria in solubilization of rock phosphate in acidic alfisols

Abstract

Many phosphate solubilizing microorganisms (PSMs) solubilize calcium phosphate (CaP) complexes by the secretion of organic acids. Alfisols, however, are rich in ferric phosphate (FeP) and aluminium phosphate (AlP) complexes. The efficacy of organic acid secreting PSMs at releasing P in alfisols is not well understood. The potential of naturally existing PSMs at solubilizing mineral phosphates of alfisols was investigated. Native soil bacteria grew well on alfisol and alfisol amended with rock phosphate (RP) when supplemented with 100 mM glucose and 10 mM KNO 3: but did not release P in soil solution. Two CaP solubilizing bacteria, Enterobacter asburiae PSI3 and Bacillus coagulans, were tested for their ability to grow on minimal media containing AlP or FeP as sole P sources. E. asburiae PSI3 could grow on and acidify the medium with either of the P sources, whereas B. coagulans was unable to grow. E. asburiae PSI3: B. coagulans and another RP solubilizing bacterium, Citrobacter koseri, failed to release P from alfisol as well as alfisol amended with RP. Addition of citric, oxalic, tartaric, gluconic, succinic, lactic and acetic acids at concentrations sufficient to decrease the pH to less than 4.0 failed to release P from alfisol. However, addition of organic acids to alfisol supplemented with RP brought about release of P to different extents, oxalic acid being the most effective. After the addition of 2 mM KH 2 PO 4 to alfisol, a rapid decrease in soluble P to about 60 lM was observed within 30 min indicating very high P fixing ability. In order to extrapolate the difference of P solubilising ability of E. asburiae PSI3 in alfisol and vertisol, mung bean plants were seed inoculated with E. asburiae PSI3. Significant decrease in the root length was found in vertisols but not in alfisol.