New Insight into Carboxylic Acid Metabolisms and pH Regulations During Insoluble Phosphate Solubilisation Process by Penicillium oxalicum PSF-4.

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.