Evaluation of the Agronomic Efficiency of Azospirillum brasilense Strains Ab-V5 and Ab-V6 in Flood-Irrigated Rice

The rational use of nitrogen fertilization is fundamental, not only to increase recovery efficiency, but also to increase crop productivity and reduce the production costs and risks of environmental impacts. In the State of Rio Grande do Sul in Brazil, irrigated rice productivity can surpass 8 tonne·ha−1 as a result of the technification of the crop and favorable environmental conditions, yet there is great variability in the agronomic efficiency of chemical nitrogen fertilizers, which rarely exceed 50% of the applied dose. Biological nitrogen fixation is one of the technological alternatives for reducing the use of nitrogen in this crop. In this study, the agronomic efficiency of Azospirillum brasilense strains Ab-V5 and Ab-V6 in terms of biological nitrogen fixation in flood-irrigated rice cultivars in a lowland agroecosystem was evaluated through five field experiments. A. brasilense combined with reduced nitrogen fertilization (reduction of 30 kg N·ha−1) increased the dry mass of the aerial part of rice plants by 3.2%, and promoted an increase in N concentration in stems and leaves and in the N content exported by grains by 43% and 27.5%, respectively, in relation to the absence of N and inoculant, and promoted an average increase of 30% in rice production.