Co-Inoculation with Bradyrhizobium and Humic Substances Combined with Herbaspirillum seropedicae Promotes Soybean Vegetative Growth and Nodulation

The effect of humic substances (HSs) in combination with or without plant-growth-promoting bacteria on soybean vegetative growth and root nodulation was examined in this study. Seeds were inoculated with Bradyrhizobium japonicum SEMIA 5079 in the presence of HSs from leonardite and Herbaspirillum seropedicae HRC54. Additional HSs and H. seropedicae application at the substrate surface was conducted at the V3 stage. The experiment was carried out in a greenhouse using pots filled with a top layer of an Oxisol soil, and plants were harvested at the R1 stage. The HS and H. seropedicae treatments significantly promoted plant shoot and root growth. The number and weight of soybean nodules were higher in the treated plants when compared to a control. The plant nodulation process was affected by the treatments that included activities of malate dehydrogenase (MDH), nitrate reductase (NR) and plasma membrane H+-ATPase (MHA). At low concentrations, the HSs and H. seropedicae increased the nodule quantity, size and weight, favoring plant growth. Combining humic substances and plant-growth-promoting bacteria (PGPB) could be a promising approach to promoting soybean nodulation and increasing crop production.