Quantitative Evaluation of Dissolved and Microbial Biomass Phosphorus Released from Insoluble Phosphates by some Strains in order to Select Efficient Bacteria

The efficiency of phosphorus (P) chemical fertilizers in agriculture is relatively low (5-25%). This has led to more P fertilizers’ application which not only increases production costs but also threatens the environment. Therefore, it was necessary to introduce suitable alternatives to replace part of these fertilizers consumption. Phosphate solubilizing bacteria have been identified as a promising option to reduce P chemical fertilizers’ application due to their ability to solve insoluble P in soil environment. These bacteria can assimilate part of the solubilized P, known as the microbial biomass P. This mechanism allows natural ecosystems to be self-sustainable, without the application of phosphate fertilizers. Given the role of potent bacterial strains in dissolving insoluble phosphates and the importance of microbial biomass P in providing part of the plant needed P in the long term, this study was aimed to select efficient strains by quantitative measurement of dissolved P and microbial biomass P after inoculation of bacteria in sperber medium with different phosphate sources.

This study was conducted as factorial experiment in a completely randomized design format using two factors in three replications. The first factor was bacteria inoculation including strains: Curtobacterium flaccumfaciens Tkd/4, Pantoea agglomerans Ggd/4 and Sphingobium yanoikuyae Rpd/4, all isolated from Giroud Shemshak Phosphate mine, Bacillus pumilus RPY isolated from Yazd Asfordi Phosphate mine, Pseudomonas putida Ps/14 prepared from Soil and Water Research Institute (isolated from maize farm soil) and blank (contains phosphate sources without bacteria inoculation) and the second factor was three phosphate sources: tricalcium phosphate (TCP), calcium phytate (CPhy) and phosphate main soil (PMS). The measured traits were included pH, EC, dissolved P and microbial biomass P. Total solubilized P from the phosphate source (sum of dissolved P and microbial biomass P), the ratio of dissolved P to microbial biomass P and biosorption percentage of P were calculated as well.

The effect of bacteria strain, posphate source and their interactions was significant on all measured traits (P ≤0.01). The ability of Ps/14 strain to dissolve phosphate was higher than other strains in all three phosphate sources. All strains absorbed more phosphorus from TCP than the other two phosphate sources. Microbial biomass P was higher in phosphate soil treatments inoculated with all strains except Tkd/4 strain than in CPhy treatments inoculated with those strains. The lowest pH (3) and the highest EC (1.53 dS m-1) were observed in the PMS and TCP treaments inoculated with Ps/14 strain, respectively.

The studied strains had different effects on both dissolved P and microbial biomass P properties from different P sources. The ability of Ps/14 strain to dissolve P from all three phosphate sources was higher than the other strains. The results revealed that the strain that has more potential in dissolving P from one source does not necessarily have the same potential in assimilating P from the same source. This can be attributed to the solubility nature of material and the solubilizing capability of the microorganisms. The results of this study also showed that the total solubilized P could be more strongly attributed to dissolved P rather than microbial biomass P. Overall, the results showed that the potent of some of the studied bacteria in both dissolved P and microbial biomass P properties can be considered as a promising option in reducing the application of P fertilizers and increasing their effectiveness.