Synergistic Effect of Microbial Biostimulants Arthrobacter pascens and Bradyrhizobium japonicum, as the Primary Driver of Climate-Resilient Soybean Productivity at 55° N Latitude in Europe

Climate change and increasing demand for local protein resources offer a significant reason to introduce soybean (Glycine max (L.)) in Lithuania, which is beyond soybean’s typical distribution area in Europe. This study was carried out to examine the effect of microbial biostimulant inoculation in combination with chemical micronutrients on soybean under the edaphic condition of Lithuania. A three-year field trial (2023–2025) tested five treatments, including uninoculated control, Arthrobacter pascens (AP), Bradyrhizobium japonicum (BJ), BJ + AP, and BJ + AP combined with micronutrients (BJ + AP + MN) on soybean biomass, nodulation, growth, quality, yield, and yield components. BJ alone promoted most of the crop traits because the northern soil lacks biological nitrogen fixation rhizobia. It established nodulation, increased SPAD value, shoot biomass (81%), plant height (19.16%), root biomass (40.1%), protein (6.9%), pods per plant, and grain yield (33.2%) over the uninoculated control. Moreover, AP also elevated biomass and yield components significantly without nodule formation. The effects of combined biostimulant treatments in many cases were not significantly different from the single treatment of BJ. All the treatments exhibited stable performance in growing seasons, although with year-to-year climatic variation. The year 2025 gave the highest biomass and yield because of the good precipitation and temperature over that year. The most positive response of soybean grain yield was observed with inoculation of BJ + AP, which increased grain yield by 35.65% over the untreated control. This study highlights the potential of the novel epiphyte Arthrobacter pascens 13LEP5 as a highly efficient, nodule-independent biofertilizer that can match or exceed the performance of traditional inoculants in soybean production. This study suggests identifying rhizobial inoculation as a strong climate-smart strategy for the launch of a productive, low-input, organic soybean system as an expansion poleward cultivation under the edaphic condition of Lithuania.