Sulfur fertilization and inoculation of soybean with Azospirillum brasilense and Bradyrhizobium spp. can improve grain yield and quality

The success of biological nitrogen fixation (BNF) in soybean depends on two crucial factors, viz., seed inoculation and soil nutritional balance. The macronutrient sulfur (S) is vital to the formation of ferredoxin, a common source of electrons that controls the proper functioning of the subunits of the enzyme nitrogenase, responsible for the conversion of atmospheric nitrogen (N₂) to ammonia (NH₃⁺). However, as the S dynamics is a complex process in soil, it may cause to the plants to be sulfur limited. This study aims at assessing the relationship between S fertilization through the use of elemental-S, and bacterial inoculation (Bradyrhizobium spp.) and co-inoculation (Bradyrhizobium spp. and Azospirillum brasilense) on nodulation, production and quality of soybean. The study was performed on the 2017/2018 and 2018/2019 crop seasons, involving four experiments where two were carried out in Santa Maria and two in Augusto Pestana, Rio Grande do Sul, Brazil. Adopting the randomized experimental block design, the treatments included a 3 × 4 factorial design, with three inoculations (Non-inoculated control, Inoculation and Co-inoculation) together with four doses of S (0, 20, 40 and 60 kg ha^− 1). Evaluations were done of the plant nodulation, accumulation of shoot dry matter, yield constituents, and quality of the soybean grain. S fertilization and co-inoculation promote an increase in plant nodulation, proving to be an important strategy to support nitrogen supply to soybean crops. The application of elemental sulfur in doses between 20 and 40 kg ha^− 1 promotes nodulation, the accumulation of dry mass of plants, the productivity and quality of soybeans, in addition to benefiting nodulation when combined with co-inoculation of Bradyrhizobium ssp. and Azospirillum brasilense.