Characterization of plant growth promoting rhizobacteria isolated from organic soybean farmland

Biofertilizer is a type of fertilizer that uses beneficial microorganisms to enhance plant growth and increase production. In this study, plant growth promoting bacteria and nitrogen fixation bacteria were isolated from organic soybean soil. Among them, the plant growth promoting bacteria with highest activities for 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, indole-3-acetic acid (IAA) production, and insoluble phosphate solubilization were selected. N6 (Bacillus cereus) and N48 (Neobacillus mesonae) have the highest production abilities of ACC deaminase and IAA production, respectively. N23 (Bacillus altitudinis) has the highest ability to solubilize insoluble phosphate. N57 (Enterobacter asburiae) isolated from soybean root nodules showed the highest nitrogen fixation ability. The maximum growth values of microorganisms were compared under various temperature (23–35°C) and pH (3–9) conditions. As a result, N6 and N23 showed the highest maximum growth value at 25°C, while N48 and N57 showed the highest maximum growth value at 35°C. Additionally, pH 7 was the optimal growth condition for four microorganisms. Under optimal growth conditions (35°C and pH 7), the ACC deaminase production of N6, IAA production of N23 and nitrogen fixation ability of N57 were approximately 1.2, 4 and 1.6 times higher, respectively, compared to the previous conditions (30°C and pH 7). Furthermore, the mixed strains showed enhanced plant growth promoting activity and nitrogen fixation ability more effectively than the individual strains. Therefore, these results suggest the possibility that biofertilizers using mixed strains may increase plant growth efficiency more than individual strains.