Efficiency of different formulations of Bradyrhizobium japonicum and effect of co-inoculation of Bacillus subtilis with two different strains of Bradyrhizobium japonicum

A key constraint in successfully obtaining an effective inoculant is overcoming difficulties in formulating a viable and user-friendly final product and maintaining the microbial cells in a competent state. Co-cultures of rhizobia and PGPR (Plant Growth Promoting Rhizobacteria) are a logical next subject for formulation researchers as they can influence the efficacy of rhizobia. A greenhouse experiment was set to assess the formulation effect of one strain i.e. Bradyrhizobium japonicum, 532c (granules, liquid and broth) and also to determine the efficiency of co-inoculation of Bacillus with two commercial strains of B. japonicum (532c and RCR 3407) on 2 soybean (Glycine max L.) varieties. PCR–RFLP analysis was used to determine the nodule occupancy in each treatment. Most of the inoculants showed increased nodulation and biomass yields (by approximately 2–5 and 4–10 g plant−1 respectively) as compared to the uninoculated controls. TGx1740-2F showed no significant differences in nodule fresh weights for the formulation effect while the co-inoculants increased the nodule fresh weights by up to 4 g plant−1. The liquid and granule-based inoculants induced higher biomass yields (4–8 g plant−1) suggesting a possible impact of formulation on the effectiveness of the inoculants. The co-inoculants also gave higher yields but showing no significant differences to the rhizobial inoculants. Nodule occupancy was 100 % for the rhizobial inoculants as well as the co-inoculants emphasizing the infectivity and high competitiveness of 532c and RCR 3407 strains despite the high population of indigenous rhizobia.