Plant growth-promoting rhizobacteria inoculation and nitrogen fertilization increase maize (Zea mays L.) grain yield and modified rhizosphere microbial communities

Plant growth-promoting rhizobacteria (PGPR) were used as inoculants of cereal crops to improve their growth and grain yield. The crops responses to inoculation are complex because are defined by plant-microorganisms interactions, many of them still unknown. Thus, it is necessary to improve the knowledge about the microbial ecology of the rhizosphere of crops under different agricultural practices. The aim of this study was to evaluate the effects of certain PGPR inoculants and nitrogen fertilization on maize (Zea mays L.) production and some associated microbial communities under field conditions in order to increase the knowledge about microbial ecology to improve crop response to PGPR inoculation. A field experiment of maize was performed to evaluate five PGPR inoculation treatments -including commercial and experimental inoculants of Azospirillum brasilense or Pseudomonas fluorescens- and three levels of nitrogen fertilization. Particular microbial groups belonging to the carbon and nitrogen soil cycles were analyzed. Nitrogen fertilization and PGPR inoculation increased maize grain yield. Inoculation only modified the number of microaerophilic nitrogen fixing (MNF) microorganisms at the reproductive stage of the crop, while fertilization modified the amount of cellulolytic, nitrifying and MNF microorganisms, only in the vegetative stage of maize. In addition, it was observed that both inoculation and fertilization modified the physiology of the rhizosphere microbial communities in the reproductive stage. Physiological changes observed in different ontogenetic stages of the crop had higher impact than both agricultural practices. All the results demonstrate that changes in the relationships between plant and microorganisms are due to different management decisions. This work gives a better understanding of maize-rhizosphere microbial ecology which can be used to improve PGPR inoculation response in order to obtain a sustainable agricultural production.