Optimization for rock phosphate solubilization by phosphate-solubilizing fungi isolated from phosphate mines

The optimization for rock phosphate (RP) solubilization by phosphate-solubilizing fungi ( Candida krissii , Penicillium expansum and Mucor ramosissimus ) isolated from phosphate mines (Hubei, PR China) was investigated. The content of soluble phosphorus (P) released by these isolates was tested on the National Botanical Research Institute's phosphate growth medium (NBRIP) containing RP as sole P source. Results showed that the optimum conditions were at temperature, 32 °C; shaking speed, 160 rpm; RP concentration, 2.5 g l −1 ; EDTA concentration, 0.5 mg ml −1 . The content of soluble P gradually decreased with a larger particle size of RP with all the isolates. The content of soluble P was the highest when the initial pH for RP solubilization was 5.5 in the medium inoculated with C. krissii , which was different from that of 7.0 in the medium inoculated with P. expansum and 7.5 in the medium inoculated with M. ramosissimus . All the isolates showed lower tolerance with increasing concentration of silver ion (Ag + ). Moreover, the highest content of soluble P was achieved when released by C. krissii at 20 of C:N ratio, P. expansum at 40 of C:N ratio and M. ramosissimus at 30 of C:N ratio.