Phosphate solubilization activity of rhizobia native to Iranian soils

Agricultural soils in Iran are predominantly calcareous with very low plant available phosphorus ( P) content. In addition to their beneficial N-2-fixing activity with legumes, rhizobia can improve plant P nutrition by mobilizing inorganic and organic P. Isolates from different cross- inoculation groups of rhizobia, obtained from Iranian soils were tested for their ability to dissolve inorganic and organic phosphate. From a total of 446 rhizobial isolates tested for P solubilization by the formation of visible dissolution halos on agar plates, 198 (44%) and 341(76%) of the isolates, solubilized Ca-3( PO4) 2 ( TCP) and inositol hexaphosphate ( IHP), respectively. In the liquid Sperber TCP medium, phosphate- solubilizing bacteria ( Bacillus sp. and Pseudomonas. fluorescens) used as positive controls released an average of 268.6 mg L-1 of P after 360 h incubation. This amount was signi. cantly ( P< 0.05) higher than those observed with all rhizobia tested. The group of Rhizobium leguminosarum bv. viciae mobilized in liquid TCP Sperber medium signi. cantly ( P < 0.05) more P ( 197.1 mg L-1 in 360 h) than other rhizobia tested,. This group also showed the highest dissolution halo on the TCP solid Sperber medium. The release of soluble P was signi. cantly correlated with a drop in the pH of the culture. ltrates indicating the importance of acid production in the mobilization process. None of the 70 bradyrhizobial isolates tested was able to solubilize TCP. These results indicate that many rhizobia isolated from soils in Iran are able to mobilize P from organic and inorganic sources and this bene. cial e. ect should be tested with crops grown in Iran.