Effects of Co-Inoculation of Rhizobium and Plant Growth-Promoting Rhizobacteria on Common Bean (Phaseolus vulgaris) Yield, Nodulation, Nutrient Uptake, and Microbial Activity under Field Conditions

Rhizobacteria are promising as biotechnological methods to improve soil fertility and legume production under field conditions. A field experiment, organized in a randomized complete block design with three replications, was conducted to evaluate the effects of inoculation with plant growth-promoting rhizobacteria (PGPR) in combination with Rhizobium on nodulation, growth, yield components, nutrient and protein content of common bean (Phaseolus vulgaris cv. Nebraska) as a model legume plant. The effect of co-inoculation of Rhizobium and PGPR on soil microbial activity and availability of soil N, P, and Fe was also investigated. Three PGPR strains (Micrococcus agilis KMe7, Serratia marcescens BM1 and Pseudomonas fluorescens TW2) were selected based on their ability to produce indole acetic acid and siderophores as well solubilization of inorganic phosphate. Generally, the results obtained from this study indicated that the co-inoculation of Rhizobium with each of the three tested PGPR significantly increased (P <0.05) the nodulation (nodule number and nodule dry weight), root and shoot dry weight as compared to Rhizobium-inoculated alone and uninoculated control plants at 70 days after sowing. The uptake of N, P, K, and Fe in shoot tissues increased significantly in the co-inoculation treatment and plant growth was enhanced as compared with the single inoculation treatment and uninoculated control. Co-inoculation with Rhizobium and PGPR significantly increased soil dehydrogenase activity compared with single inoculation. Additionally, co-inoculation of Rhizobium and PGPR also had a positive effect on the composition of the rhizosphere microbial populations and significantly increased populations of bacteria, phosphate solubilizing bacteria and siderophores producing bacteria as compared with individual inoculation. The results show that the dual inoculation of PGPR and Rhizobium significantly increased common bean yield and yield components (pods per plant, seeds per pod, and 100-seed weight as well as straw, seed, and biological yields) compared with single inoculation and uninoculated control. Results showed that the best co-inoculation treatment was Rhizobium + Pseudomonas fluorescens with relative increase of seed, straw, and biological yield reaching 19.6, 35.4 and 22.2%, respectively, over the Rhizobium alone treatment. Finally, co-inoculation significantly increased the percentage of protein in common bean seeds as compared with single inoculation or uninoculated control. These results indicate that co-inoculation with PGPR and Rhizobium had a positive effect on common bean nodulation, growth, soil nutrients status, nutrients uptake, and yield. The present study concluded that PGPR can be used to promote legume plants in Egypt and to increase soil fertility when it is inoculated with symbiotic N₂-fixing bacteria