Effects of inoculation with plant growth promoting rhizobacteria (PGPRs) and Sinorhizobium fredii on biological nitrogen fixation, nodulation and growth of Glycine max cv. Osumi

We investigated the effects of three plant growth promoting rhizobacteria (PGPR), on Biological Nitrogen Fixation (BNF), nodulation and growth promotion by soybean (Glycine max) var. Osumi plants. The strains. Aur 6. Aur 9 and Cell 4, belong to Psedomonas fluorescens, Chryseobacterium balustinum and Serratia fonticola respectively, Inoculation modes for the PGPRs and sinorhizobium fredii (carried out through irrigation), were examined. In the first mode, PGPRs and S. fredii were co-inoculated. In the second mode. we first inoculated S. fredii and after the PGPRs, which were added 5 or 10 days later (each inoculation being an independent treatment). In the third mode, the PGPRs were inoculated first, and the S. fredii was inoculated 5 days later. We also included treatments inoculated with only the PGPRs (one PGPR per treatment) and only with S. fredii. Plants were maintained in a greenhouse under controlled environmental conditions. and were sampled 3 months after sowing. The results obtained showed the effects of the inoculation sequence. The most significant effects on growth parameters (stem plus leaf weight and fresh root weight) were found when inoculations with PGPR and S. fredii were at different times or when we inoculated only with PGPR and the plants were watered with nitrogen. Co-inoculation had no positive effects on any parameter. probably due to competition between the PGPR and S. fredii. Our results indicate that the inoculation modes with PGPR and rhizobia play a very important role in the effects produced. Thus, although plant growth promoting rhizobacteria may interact synergistically with root-nodulating rhizobia plant growth promoting rhizobacteria selected for one crop should be assessed for potentially hazardous effects on other crops before being used as inoculants.