Phosphate solubilising fungi (PSF) play an important role in increasing phosphorus (P) bioavailability and the fertility of soil. However, the mechanism by which PSF solubilise insoluble P using carboxylic acid… Click to show full abstract
Phosphate solubilising fungi (PSF) play an important role in increasing phosphorus (P) bioavailability and the fertility of soil. However, the mechanism by which PSF solubilise insoluble P using carboxylic acid is not fully understood. In this study, Penicillium oxalicum PSF-4 was isolated and shown to have satisfactory solubilisation performance towards tricalcium phosphate (TP) and iron phosphate (IP). In treatments with TP or IP, the soluble P (Psol) concentrations were positively correlated with the carboxyl groups in organic acids but negatively correlated with the pKsp of insoluble P sources and pH. Additionally, the compositions of the organic acids (considering the lowest pKa value for acids with several pKas) were remarkably different between the treatments: oxalic (pKa = 1.27), malic (pKa = 3.46), and formic (pKa = 3.75) acids in TP treatments (maximum 381 mg/L); and lactic (pKa = 3.85), gluconic (pKa = 3.86), and citric (pKa = 3.12) acids in IP treatments (maximum 1634 mg/L). The addition of H+ without P. oxalicum PSF-4 inoculation markedly improved Psol concentrations. The above results offer new insights that the pKsp of P sources, compositions of carboxylic acids, and pH regulation are the key factors influencing P solubilisation of different insoluble P sources.
               
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