Microorganisms reveal what plants do not: wheat growth and rhizosphere microbial communities after Azospirillum brasilense inoculation and nitrogen fertilization under field conditions

Azospirillum brasilense is one of plant growth promoting bacteria used to improve plant growth and grain yield of cereal crops. The level of inoculation response is defined by complex plant-microorganism interactions, many of them still unknown. Thus, we evaluated both agronomic response and microbial ecology of wheat crop under A. brasilense inoculation and nitrogen fertilization at field conditions in order to improve inoculation efficiency. Treatments were: control, nitrogen fertilization and inoculation with 40M and 42M strains. Functional and structural diversity of rhizosphere bacterial communities were evaluated by community-level physiological and terminal restriction fragment length polymorphism profiles. Besides, aerial biomass, grain yield and counts of microaerophilic diazotrophic rhizobacteria were determined. Plant ontogeny modified the number of culturable microaerophilic diazotrophic rhizobacteria. Although agronomic response did not show differences, plant ontogeny and the agricultural practices modified both physiology and genetic structure of rhizosphere microbial communities. Interestingly, these differences due to the treatments were observed at jointing stage but not at grain-filling stage of wheat. Our results demonstrate how different management decisions can change plant- microorganism relationships. While wheat could not show differences between some agricultural treatments, under the soil surface microbial communities could show them.