Improving maize sustainability with partial replacement of N fertilizers by inoculation with Azospirillum brasilense

Maize (Zea mays L.) is a major cereal crop worldwide and demands N fertilizers, which can be replaced partially by plant growth-promoting bacteria. In Brazil, inoculants carrying Azospirillum brasilense strains Ab-V5 and Ab-V6 have exponentially grown, but a quantitative study on the replacement of N fertilizer by inoculants is still missing. Under greenhouse conditions, we investigated the effects of A. brasilense cells and metabolites applied via seeds or leaf-sprayed at the V3 growth stage on roots and confirmed strong benefits in traits that improve the uptake of water and nutrients. In the field, 30 trials were performed at 13 sites in Brazil over 10 yr, comparing noninoculated and inoculated maize receiving 0, 50, 75, and 100% (90 kg ha(-1) of N) of N fertilizer (urea) applied as side-dress 35 d after emergence. Overall, inoculation significantly increased grain yield by 3.1%. Increases were confirmed in lower and higher yield levels, subtropical and tropical conditions, and clayey and sandy soils, with low and higher soil organic carbon levels. Yield increases due to inoculation were statistically significant at all levels of N fertilizer, except for at 50%. Inoculation with the supply of 75% of N increased yield by 4.6% compared with the noninoculated counterpart and, noteworthy, did not differ statistically from the non-inoculated control receiving 100% of N. These results confirmed the feasibility of replacing 25% of N-fertilizer applied as side-dress by seed inoculation with A. brasilense, besides the possibility of avoiding the emission of 4.95 million Mg of CO2-e per year.