Hyphosphere interactions between an arbuscular mycorrhizal fungus and a phosphate solubilizing bacterium promote phytate mineralization in soil

Both arbuscular mycorrhizal (AM) fungi and phosphate solubilizing bacteria (PSB) are involved in phosphorus (P) mobilization and turnover but the influence of their interaction on organic P mineralization in the root free soil (hyphosphere) have been little studied. We investigated the interactive effects of an AM fungus (Rhizophagus irregularis, RI) and/or PSB (Pseudomonas alcaligenes, PA) on phytate mineralization and subsequent transfer to the host plant (Medicago sativa) using a two-compartment microcosm with a central 30 μm nylon mesh barrier. The root growth compartment containing 5 mg inorganic P (Pi, KH2PO4) kg−1 soil was inoculated with RI or uninoculated and the AM fungal hyphal soil containing 75 mg organic P (Po, Na-phytate) plus 0 or 5 mg Pi kg−1 soil was inoculated with PA or uninoculated. Sole inoculation with RI increased shoot P content compared with the uninoculated treatment and dual inoculation with both RI and PA did not increase shoot P compared with sole RI inoculation. Sole PA inoculation significantly increased microbial biomass P (MBP). Compared with sole PA inoculation soil MBP increased under zero-Pi addition but decreased under 5 mg Pi kg−1 soil addition in the dual inoculation RI/PA treatment. The uninoculated microcosms had the lowest acid phosphatase activity and the highest phytate-P remaining in the soil. Inoculation with PA led to higher acid phosphatase activity and lower phytate-P than did RI. Dual RI/PA inoculation had the highest acid phosphatase activity and the lowest phytate-P remaining in the soil. Addition of 5 mg Pi kg−1 soil to the hyphal compartment decreased phytate-P remaining in the RI and/or PA treatments. The phytate-P remaining in the soil was negatively correlated with soil acid phosphatase activity or MBP in the presence of RI but there was no correlation between shoot P and soil phytate-P. In conclusion, our results indicate that the mineralization of soil phytate was promoted by the interaction between the AM fungus and its hyphosphere PSB.