Secondary Metabolites of Rhizobium tropici CIAT 899 Added to Bradyrhizobium spp. Inoculant Promote Soybean Growth and Increase Yield

Products based on beneficial microorganisms, such as rhizobia, are used in agriculture to reduce or even completely replace the use of mineral fertilizers. In Brazil, rhizobia inoculants have been widely used in agriculture in the cultivation of legumes such as soybeans. Currently, the development of new formulations based on microorganisms has been made to improve the effectiveness of these products. Among these new technologies, the use of secondary bacterial metabolites has been studied to compose new formulations of inoculants. Inoculants enriched with metabolic extract from rhizobia have been showing promising results in plant growth promotion, but little is known about the isolated effects of the molecules contained in this extract, such as the lipochitoligosaccharides (LCOs), chitoligosaccharides (COs), and exopolysaccharides (EPS), when they are purified. In this study, one greenhouse and two field experiments were carried out to evaluate the effects of the addition of secondary metabolites from Rhizobium tropici CIAT 899 to the standard inoculation (SI) with Bradyrhizobium spp. on soybean nodulation, growth, and yield. Total metabolic extract containing LCOs (ME-LCO) and other secondary metabolism molecules, such as purified LCOs, COs, and EPS, were tested. In the greenhouse, it was observed that the addition of ME-LCO increased nodule number and shoot, root, and nodules dry weight in comparison to the SI. The SI + ME-LCO treatment significantly increased soybean grain yield by 7.6% compared to SI in the 2018/2019 cropping season. In 2019/2020, an increase of 3.1% was observed, but it was not statistically significant. The use of ME-LCO of R. tropici CIAT 899 in supplementation of Bradyrhizobium spp. soybean inoculants is very promising for the formulation of new generation inoculants.