Objective This study aimed to isolate and systematically evaluate a multifunctional free-living nitrogen-fixing bacterium from farmland black soil in Northeast China, capable of both nitrogen fixation and plant growth promotion, and to investigate its roles in soil nitrogen transformation and plant growth stimulation.

Method A functional strain was isolated from farmland soils using Ashby nitrogen-free medium combined with dilution plating. Candidate strains were screened based on nitrogenase activity and indole-3-acetic acid (IAA) production. The optimal growth conditions of the selected strain were analyzed across gradients of temperature, pH, salinity, and nutrient sources. Functional verification was performed through soil microcosm incubations and soybean pot experiments, assessing soil nutrient dynamics, plant biomass, microbial communities, and greenhouse gas emissions.

Result A novel strain, designated Arthrobacter sp. 0A17, was identified, exhibiting nitrogenase activities of 198.85 ± 4.65 U L⁻¹ and IAA production of 180.86 ± 11.49 mg L⁻¹. The strain grew optimally at 25°C, tolerated pH 5 ~ 9 and salinity up to 50 g L⁻¹ NaCl. Following soil inoculation, 0A17 effectively significantly increased total nitrogen (TN, + 15.30%) and ammonium-N (NH₄ ⁺ -N, + 7.99%) content. It promoted soybean root growth, enhancing root length, volume, and surface area, and significantly increased soybean plant biomass within 10 and 20 days. Microbial community analysis revealed significant shifts following inoculation, including increased abundance of Actinobacteriota and reduced proportions of Proteobacteria, indicating modulation of rhizosphere microbiota. Early-stage inoculation also reduced N₂O emissions by approximately 29%, although CO₂ emissions were moderately elevated, reflecting increased microbial respiration.

Conclusion Arthrobacter sp. 0A17 exhibits robust ecological adaptability and multifunctional potential, positioning it as a promising candidate for enhancing total nitrogen and ammonium nitrogen (NH₄ ⁺ -N) content in black soil regions while promoting soybean growth. Its capacity to improve nitrogen utilization efficiency and soil health underscores its practical value as a region-specific microbial inoculant, highlighting significant potential for large-scale implementation to advance sustainable agriculture in these ecosystems.