Effect of Thiobacillus, Sulfur and Phosphorus on Population of Phosphate Solubilizing Bacteria in Soil

According to WHO and FAO studies, the diseases caused by contaminated foods are of the most widespread threats to human health in developing and developed countries. Therefore, in recent years, researchers have been trying to use soil microorganisms to solve this problem and maintain the health of plants and the environment. Phosphorus after nitrogen, is a major macronutrient in plants which controls the growth, seeding and fruit production and involves in basic biological functions such as cell division, nucleic acids synthesis, photosynthesis and respiration and energy transfer. However, high amount of soluble inorganic phosphate is annually applied to the soil as chemical fertilizer but a large portion of it is immobilized rapidly after application due to phosphate fixation by aluminum, calcium, iron, magnesium and soil colloids and becomes unavailable to plants. The use of biological agents especially phosphate solubilizing microorganisms, can play an important role in supplying plant nutrients and improves crop health and productivity without causing any harm in agricultural and natural ecosystems. Bacteria and fungi are the two important groups of phosphate solubilizing microorganisms. Phosphate solubilizing bacteria in soil include Rhizobium, Bacillus, Pseudomonas, Agrobacterium, Achromobacter, Enterobacter and Burkholderia, and the most important ones i.e., Bacillus sp. and Pseudomonas flourescens.

In order to evaluate the effect of Thiobacillus, sulfur and phosphorus applicationon population of phosphate solubilizing bacteria in soil, a field experiment was conducted at Zarghan Agricultural and Natural Resources Research Center of Iran in a factorial, based on complete randomized block design with 3 replications. Treatments consisted of three levels of sulfur fertilizer with biofertilizer containing Thiobacillus bacteria (without sulfur and biofertilizer containing Thiobacillus (S0), application of 500 kg S + 10 kg biofertilizer containing Thiobacillus (S1), 1000 kg S + 20 kg biofertilizer containing Thiobacillus (S2) and 2000 kg S + 40 kg biofertilizer containing Thiobacillus (S3) per hectare), three levels of triple super phosphate (without phosphorus (P0), 100% (P1) and 65% (P2) percent phosphorus recommended based on the soil test) in two corn planted and not planted states. After harvesting, 72 soil samples were collected from each plot and transferred to the biology laboratory of soil and water research institute of Karaj. Soil samples were stored in sterile conditions at 4◦C. In order to isolate phosphate solubilizing bacteria, 10 gram of soil from each sample was suspended in 90 ml of sterilized water to make 1:10 dilution. Then, series of dilution were made (101 – 107) and 0.1 ml of suspensions of the diluted soil sample were transferred to petri dishes containing pikovskaya medium and incubated at 28- 30˚C. To identify PSP from halos surrounding characterized colonies was used and counting was performed 1-14 days after cultivation. The colonies were isolated on the basis morphological characteristics such as shape, color and size and then purified by linear culture. Finally, 60 strains were purified that were used to compare phosphate solubilizing capability.

The results of this study showed that the main and interaction effects of sulfur fertilizer and biofertilizer treatments of Thiobacillus, phosphorus and plants on the population of phosphate-solubilizing bacteria (cells per gram of dry soil) in Pikovskaya medium were significant at 0.01 level probability. The highest population of bacteria was obtained at the lowest level (S1).  Increasing the level of sulfur fertilizer and Thiobacillus biofertilizer decreased the population of phosphate-solubilizing bacteria and the highest level of sulfur and Thiobacillus biofertilizer led to the lowest bacterial population. Also, the study of phosphorus application on the bacterial population showed that phosphorus fertilizer at both levels significantly increased the bacterial population compared to the control (no application phosphorus). The best fertilizer treatment for phosphorus application was P1 which had the greatest effect on bacterial population compared to P2 treatment in Pikovskaya environment. In this experiment, the population of bacteria in corn planted conditions was higher than in non-planted conditions and this population increase was observed in almost all different levels of sulfur and phosphorus fertilizers. The highest bacterial population was observed in combined treatment of S1P2 under corn planted conditions. The results of microscopic, physiological and biochemical tests of the strains showed that all 60 bacterial strains were capable to form clear zone in Pikovskaya medium. Among them, 15 strains (7, 3, 2 and 3 strains belonging to Bacillus megaterium, Bacillus subtilis, Bacillus cereus and Pseudomonas fluorescent, respectively) had higher phosphate solubility than the others.