A combination of humic substances and Herbaspirillum seropedicae inoculation enhances the growth of maize (Zea mays L.)

Endophytic diazotrophic bacteria colonize several non-leguminous plants and promote plant growth. Different mechanisms are involved in bacteria-induced plant growth promotion, including biological nitrogen fixation (BNF), mineral solubilization, production of phytohormones, and pathogen biocontrol. Herbaspirillum seropedicae is a broad-host-range endophyte that colonizes sugarcane, rice, wheat, sorghum, and maize, and has been used as a biofertilizer. Contrasting results between greenhouse and field experiments have prompted efforts to improve the consistency of the plant response to microbial stimulation. The aim of this study was to evaluate the effect of the presence of humic substances on inoculation of maize (Zea mays L.) with H. seropedicae. Two experiments were conducted: one in the greenhouse using sand and nutrient solution and the other a field trial in soil with low natural fertility and to which was applied N in the form of urea (50 kg ha(-1)). In the greenhouse, pre-emerging seeds were inoculated with a solution of H. seropedicae (10(9) cells mL(-1)) in the presence of humic substances isolated from vermicompost (10, 20, or 30 mg C L-1); in the field trial, bacteria combined with humate were added as a foliar spray (450 L ha(-1)). At early stages (7 and 45 days old) in the greenhouse, the treatment activated plant metabolism including enhancement of plasma membrane H+-ATPase activity, alteration of sugar and N metabolism, and greater net photosynthesis. The number of viable bacterial cells was higher in root tissues when inoculation was in the presence of soluble humic substances. Foliar application of endophytic diazotrophic bacteria and humic substances increased maize grain production 65 % under field conditions. These results show a promising use of humic substances to improve the benefit of endophytic diazotrophic inoculation.