Phytobeneficial bacterial inoculants for common bean growth and productivity in nitrogen and phosphorus deficient soils

Most of the soils are N and P-deficient and require high applications of chemical fertilizers to ensure optimal crop production. However, this practice poses in most cases serious environmental issues. In recent years, inoculation with beneficial bacteria has emerged as a safe and sustainable alternative to chemical fertilizers. In this context, we investigated the effect of two symbiotic Agrobacterium radiobacter strains (LMR670 and LMR676) and two plant growth-promoting rhizobacteria Bacillus sp. (M131) and Enterobacter sp. (P1S6), as single or combined inoculants, on common bean growth and yield under N and P-deficient conditions. In a first trial, Agrobacterium strains’ symbiotic efficiency with common bean was evaluated in a low phosphorus and nitrogen soil under greenhouse conditions. Strain LMR670 recorded the highest nodules number (53 nodules per plant) and shoot dry weight (0.553 g plant-1). This strain was then used in combination with the PGP rhizobacteria in a common bean co-inoculation assay under sufficient and deficient P levels (80 kg ha-1of P and No P added). Single inoculation with LMR670 recorded the highest shoot dry weight (82% increase compared to non-fertilized control) compared to combined inoculants (46-47% increase). To corroborate the obtained results, a field experiment was conducted using the same treatments. LMR670 as a single inoculant or mixed with M131 was consistently effective leading to common bean yields comparable to N and P fertilized plants (13.07 t ha-1 and 12.35 t ha-1 respectively). In addition, single inoculation with the multi-PGP strain M131 showed positive effects on all common bean growth parameters and yield value (14 t ha-1); exceeding even N and P fertilized control plants. These results suggest that the strains LMR670 and M131 can be used, in a single or combined inoculation, as effective biological fertilizers for common bean cultivation to replace phosphorus and nitrogen fertilizers. Globally our results highlight the potential of native phytobeneficial strains for successful nodulation, growth and yield of common bean under N and P-deficient conditions.