Effect of inoculation with nitrogen-fixing bacterium Pseudomonas stutzeri A1501 on maize plant growth and the microbiome indigenous to the rhizosphere

•Inoculation with diazotrophic P. stutzeri A1501 substantially improved maize growth.•The A1501-inoculated plants received 31.6% (at best) of their nitrogen from BNF.•Inoculation altered composition of the diazotrophic community in the rhizosphere.•Inoculant stimulated the population and activity of N2-fixer and ammonia oxidizers. Plant growth promoting diazotrophs with the ability to associate with plant roots are in common use as inoculants to benefit crop yield and to mitigate chemical nitrogen fertilization. However, limited information is available in understanding to what extent the plant growth-promoting effect of the inoculum has on the plant’s nitrogen acquisition as well as on the impact of inoculation on the indigenous rhizosphere microbial population. Here we reported on experiments that assessed how endophytic Pseudomonas stutzeri A1501 inoculated on maize improved plant growth and plant nitrogen content using a 15N dilution technique under two water regime conditions. The effects of inoculation and different water regimes were also assessed for the maize rhizospheric and surface soil communities by MiSeq community sequencing combined with qPCR of functional genes and transcripts (nifH and amoA) related to nitrogen cycling. Results support maize inoculated with P. stutzeri A1501 grew better and accumulated more nitrogen with a lower δ15N signature after 60 days than did plants inoculated with nifH-mutant and sterilized A1501 cells (non N2-fixing controls). Inoculant contribution to the plant was estimated to range from 0.30 to 0.82g N/plant, depending on water conditions. Inoculation with P. stutzeri A1501 significantly altered the composition of the diazotrophic community that P. stutzeri became dominant in the rhizosphere, and also increased the population of indigenous diazotrophs and ammonia oxidizers and functional genes transcripts. Redundancy analysis revealed that soil compartment and A1501 inoculation treatments were the main factors affecting the distribution of the diazotrophic community.