A technique that is beginning to be explored in soybean [Glycine max (L.) Merrill] is the inoculation of bacteria of genus Azospirillum with the Bradyrhizobium genus, which is made in order to achieve multiple benefits, higher than those achieved only by seed inoculation with Bradyrhizobium. The benefits would be the root growth of culture, and/or the stimulus of nodulation and biological nitrogen fixation of Bradyrhizobium. The aim of this work is to improve the understanding of the action of co-inoculation on biological nitrogen fixation, root growth and absorption of this nutrient from the soil. The experiment will be conducted in a greenhouse in Piracicaba - SP. In this study soybean plants are sown in pots with 25 dm3 filled with soil. The experimental design is a randomized block design with five replications. The treatments are: T1: without inoculation (NI); T2: inoculation with Bradyrhizobium (100 ml 50 kg-1 of seed); T3: inoculation with Bradyrhizobium + Azospirillum 2.5 x 105 seed-1 cells; T4: inoculation with Bradyrhizobium + Azospirillum 5.0 x 105 seed-1 cells; T5: Azospirillum Inoculation with only 2.5 x 105 seed-1 cells; T6: Azospirillum Inoculation with only 5.0 x 105 seed-1 cells. In order to simulate the mineral soil nitrogen, will be added in all vessels a little amount of nitrogen (10 kg ha-1). We will evaluate the parameters of root growth and volume of soil explored with the help of WinRhizo software; absorption of mineral N in the soil, with the help of isotope technique of 15N marker; biological fixation of nitrogen of Bradyrhizobium by the method of ureide and concentration and extraction of N in the roots and shoots. This research will provide information about the benefits of co-inoculation in respect of root development, soil exploration and improvement of biological nitrogen fixation; and enable the understanding of how these two processes are affected by the presence of Azospirillum.
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