Investigation and determination of some phosphate solubilizing bacteria characteristics from agricultural soil of wheat field located in Yazd province

The widespread use of chemical fertilizers became common by coming the Haber-Bosch reaction product to the market in the second decade of the twentieth century, and thereafter the agricultural industry flourished. Nowadays, increasing usage of chemicals has caused adverse and harmful consequences against the environment and also increased costs of production of crop plants. The use of biofertilizers, which potentially solubilizes phosphorus compounds in the agricultural soil, is considered as an environmental friendly option and a superior alternative to further usage of chemical fertilizers. New collections of phosphate-solubilizing bacteria with a vast biodiversity are required to be used for greater varieties of crop plants in more different climates. The aim of this study was verification and characterization of some active phosphate-solubilizing bacteria isolated from several areas of wheat arable soils located in Yazd province.

In this study, soil microorganisms were screened and all the isolates were examined in PVK agar culture medium for halo formation and then phosphate solubilization index and efficiency were calculated. Phosphate release rate in liquid culture medium was measured in µg/ml. Nitrogen fixation test using two carbon sources (glucose and sucrose) was evaluated comparatively. All the isolates were examined for tolerance to various environmental conditions such as temperature, salinity and pH. Finally, the production of several types of organic acids by the isolates was examined using thin layer chromatography.

Among the bacteria, 10 isolates created a halo zone in culture medium containing calcium phosphate and showed growth and discoloration in nitrogen-free medium with bromothymol blue reagent. Five isolates out of 10 showed the highest efficiency in phosphate solubilization (equal to or more than 200% Z/C) and 5 isolates showed the highest phosphate solubility index (between 2- 4). The highest index belonged to YC isolate (4.1), while statistical analysis showed no significant difference between the means of solubility efficiency of different isolates. The quantitative study showed that the amount of phosphate released for 4 superior isolates was 150.25-203.91 µg/ml. Statistical analysis shows that among the 10 strains tested, YS and YG showed the highest efficiency with the release of 203.9 and 194.8 μg/l phosphate, respectively. These two strains were not significantly different from YK and YC isolates. Isolates YX, YA, YP, YH, YJ and YI also showed the least ability at the same statistical level. All the isolates were able to fix nitrogen in the presence of glucose and 4 isolates were able to fix nitrogen in the presence of sucrose as two different carbon sources. Experiments carried out to show the resistance of the isolates to environmental factors and indicated that almost all isolates can grow adequately in the range of acidic to alkaline pH (5-9) and temperatures of 25-37 ºC. The isolates were also able to tolerate salinity up to 10% (w/v). The use of thin layer chromatography confirmed the excretion of organic acids such as citric acid and oxalic acid by the examined bacteria.

The present study indicates that the isolated microorganisms with high phosphate solubilizing ability show significant potential as a suitable substitute to chemical fertilizers with harmful side effects.