Soybean Response to N Fertilization Compared with Co-Inoculation of Bradyrhizobium japonicum and Azospirillum brasilense

The soybean [Glycine max (L.) Merrill] relationship with the bacteria Bradyrhizobium japon-icum is responsible for providing around 60% of the nitrogen (N) required for the crop and the remaining N comes from the soil or supplemental fertilization. To investigate if higher yields are possible, supplemental N studies and co-inoculation of Rhizobium with Azospirillum are necessary. This N rate (0, 30, 56,112, 336 kg N ha(-1)) and inoculation study was conducted across eight envi-ronments in eastern North Dakota, USA, in 2021 and 2022. Also, the effect of supplemental N and co-inoculation on nodulation was evaluated. When N was applied at 112 kg N ha(-1), nodulation was significantly inhibited. Co-inoculation increased the number of large nodules and the volume of nodules; however, the yield was not different from inoculation with B. japonicum. Nitrogen at 112 and 336 kg ha(-1) increased grain yield, protein yield, and seed weight; however, the higher N rate decreased plant population. There were significant positive relationships between yield and protein content and seed weight, and negative relationships between oil and protein content, and yield and oil content. Based on a polynomial relationship, the highest yield (3711 kg ha(-1)) would be achieved at 273 kg N ha(-1). The application of N resulted in a yield increase but using current prices may not be an economical choice. Additional research is necessary to verify if co-inoculation with efficient strains can improve biological N fixation.