Evidence of the Role of Plant Growth-Promoting Bacteria in Mitigating N2O Emissions from Maize Cultivation

Nitrous oxide is a potent greenhouse gas, with N fertilizers being one of its major sources. Plant growth-promoting bacteria have been used to mitigate N2O emissions by improving N use efficiency in plants. In addition, some of these microorganisms are capable of reducing N2O to N2, a process that could be further explored as a complementary mitigation strategy. This study aimed to test whether Azospirillum brasilense strains Ab-V5 and Ab-V6, and strain Wa3, as well as Nitrospirillum viridazoti strain BR 11145, already used in commercial inoculants for N-fertilized crops and known to carry the nosZ gene encoding nitrous oxide reductase, could act as biological sinks for this gas. In the pot experiment, soils fertilized with N and inoculated with N. viridazoti exhibited consistently lower N2O emissions, but not when A. brasilense strains were used. The mitigation effect was observed both in bare soil (72% emission reduction) and in the presence of millet plants (60% emission reduction), confirming the ability of N. viridazoti strain BR 11145 to consume N2O. In a field experiment conducted with maize, inoculation with N. viridazoti again reduced N2O fluxes during the first two weeks after fertilization compared with the urea-only treatment. However, no significant differences were detected comparing emission factors, whose calculation requires consideration of the entire monitoring period, thereby adding more variability. While N2O mitigation was observed, no significant effects of inoculation or N fertilization were found on maize growth or yield. Nonetheless, the consistent reduction in N2O emissions achieved with N. viridazoti strain BR 11145 suggests that inoculants with this bacterium represent a promising biological sink for N2O, offering a novel nature-based solution to enhance the sustainability of tropical crop management.