Co-inoculation of soybeans and common beans with rhizobia and azospirilla: strategies to improve sustainability

Plant-microorganism associations have long been studied, but their exploitation in agriculture partially or fully replacing chemical fertilizers is still modest. In this study, we evaluated the combined action of rhizobial and plant growth-promoting rhizobacteria inoculants on the yields of soybean and common bean. Seed inoculation with rhizobia (1.2 x 10(6) cells seed(-1)) was compared to co-inoculation with Azospirillum brasilense in-furrow (different doses) or on seeds (1.2 x 10(5) cells seed(-1)) in nine field experiments. The best in-furrow inoculant dose was 2.5 x 10(5) cells of A. brasilense seed(-1) for both crops. Inoculation with Bradyrhizobium japonicum increased soybean yield by an average 222 kg ha(-1) (8.4 %), and co-inoculation with A. brasilense in-furrow by an average 427 kg ha(-1) (16.1 %); inoculation always improved nodulation. Seed co-inoculation with both microorganisms resulted in a mean yield increase of 420 kg ha(-1) (14.1 %) in soybean relative to the non-inoculated control. For common bean, seed inoculation with Rhizobium tropici increased yield by 98 kg ha(-1) (8.3 %), while co-inoculation with A. brasilense in-furrow resulted in the impressive increase of 285 kg ha(-1) (19.6 %). The cheaper, more sustainable inoculated treatment produced yields equivalent to the more expensive non-inoculated + N-fertilizer treatment. The results confirm the feasibility of using rhizobia and azospirilla as inoculants in a broad range of agricultural systems, replacing expensive and environmentally unfriendly N-fertilizers.