Co-inoculation of Azospirillum brasilense and late nitrogen fertilization for no-till soybean production

Soybean [Glycine max (L.) Merr.] is highly efficient in the biological N₂ fixation (BNF) process through the association of bacteria of the genus Bradyrhizobium in the root nodules of the plants. However, there are still doubts about the need to complement soybean N demand through N fertilization in high-yield environments. In addition, the real impact of co-inoculation of soybean with Azospirillum brasilense and Bradyrhizobium spp. is not yet clear in such environments. A field experiment was conducted from 2012 to 2021 with six soybean cropping seasons in a crop rotation scheme with black oat (Avena strigosa Schreb), maize (Zea mays L.), and wheat (Triticum aestivum L.) under no-till (NT) in Southern Brazil. Soybean seeds were co-inoculated with A. brasilense (strains Ab-V5 and Ab-V6) shortly after inoculation with Bradyrhizobium japonicum, and different levels of N fertilization were used in top dressing at the start of pod formation (R₃). Soybean nutritional status and grain yield were not benefited by co-inoculation with A. brasilense. Since the increased inoculum rate of A. brasilense co-inoculated with rhizobia in soybean compromised both N nutrition and grain yield, this practice should not be encouraged. There was no need to complement soybean N demand through N fertilization during the reproductive stage. Soybean achieved grain yields of 5.0–5.7 Mg ha⁻¹ and, even so, there was no need to complement N demand through N fertilization. The results suggest that soybean N demand in a high-yielding environment under NT could be satisfied exclusively through the optimization of BNF.