Benefits of soybean co-inoculation with Bradyrhizobium spp. and Azospirillum brasilense: Large-scale validation with farmers in Brazil

The global contribution of biological nitrogen fixation (BNF) to soybean production ranks Brazil as the leading country. In 2013/2014, a new technology based on soybean co-inoculation with Bradyrhizobium spp. and Azospirillum brasilense strains Ab-V5 and Ab-V6 was released, and here we report a five-year set of extension activities in Paran ' a state, southern Brazil, to demonstrate the benefits of co-inoculation. In total, 273 technical reference units (TRUs) were installed in two main soybean macro-regions, all showing naturalized populations of soybean bradyrhizobia. In each TRU, side-by-side plots consisting of soybean seeds non-inoculated or coinoculated with Bradyrhizobium spp. and A. brasilense were installed, and 260 TRUs were evaluated for nodulation and 242 for grain yield. Consistent nodulation increases, of up to 350% were observed in every cropping season, with an average significant (p < 0.01) increase of 35% in the five-year period. Similarly, grain yield increased up to 80% due to co-inoculation, with an average significant (p < 0.01) increase of 8%, corresponding to 273 kg ha (-1). The increases were confirmed in the two macro-regions and different levels of grain yield, from low (<3000 kg ha (-1)) to high (>4000 kg ha (-1)). In the five-year period, 3299 small farmers were reached (farms of similar to 50 ha) and the average profit due to co-inoculation was estimated at US$ 111.5 ha (-1) per cropping season. In addition, we discuss environmental benefits associated with the mitigation of greenhouse gases emissions, estimated at 350 kg ha (-1) of CO2-e, in addition to soil N enrichment.