Effect of Phosphate-solubilizing bacteria (PSB) on distribution of phosphorus forms in a calcareous soil

Recognition of the distribution of various phosphorus forms influenced the activity of microorganisms in soil is essential for better understanding of their capacity to maintain and provide phosphorus for plants.  In this study to investigate the role of phosphate solubilizing bacteria (PSB) in the distribution of different forms of soil phosphorus, a surface soil sample (0-30 cm) were selected in West Azarbaijan with a loamy texture, Belongs to the Typic clacixerpts subgroup. An experiment was conducted as a factorial in a completely randomized design with two factors: Microbial treatments (control (no inoculation) and bacterial strains (Pseudomonas fluorescens and Bacillus megaterium) and sampling time (0, 7, 21, 45 days) in six replications. Soil samples were incubated at 28 ° C for a period of 45 days. Then, different forms of inorganic phosphorus, organic phosphorus and Olsen phosphorus were determined by sequential extraction method. Soil pH value 7.7, Clay soil tested 17% and activated calcium carbonate in soil 3.5%. Ca10-P formed was 45% of total phosphorus in the soil and indicates its importance in the calcareous soil. The results showed that the bacterial treatment caused significant changes in the amount of phosphorus, organic phosphorus and mineral phosphorus, the amount of available phosphorus, organic phosphorus and mineral form (di-calcium phosphate (Ca2-P)) in bacterial inoculation compared to control treatment increased 66, 15 and 92%, respectively.  However, the octa-calcium phosphate (Ca8-P) and apatite (Ca10-P) forms in the treatment of bacteria were reduced 13 and 5%, respectively. In addition, the Al -P form was increased 10% compared to control in bacteria treatment. In the treatment of bacterial significant correlation (r=0.84**) between P (Olsen-P) and (Ca2-P, O-P, and Al-P) was present in the soil. The amount of phosphorus in different forms in the treatment of bacteria was as O-P> Ca10-P> Ca8-P> Olsen-P> Ca2-P> Al-P at the end of incubation. The results showed that inoculation of bacterial, increase the solubility of the available phosphorus forms in the soil.