Cadmium-immobilizing bacteria utilize octanoic acid and two synthetic compounds to enhance nitrogen fixation in soybeans under cadmium stress

Rhizobia-soybean symbiosis is essential for agriculture and ecosystems, yet it is vulnerable to disruption by cadmium (Cd) pollution. Here, we investigated the protective role of non-nitrogen-fixing bacteria on soybean (Glycine max) under Cd-induced stress. Three Cd-immobilizing bacteria were identified: Arthrobacter sp. CC3, Pseudarthrobacter sp. CC12, and Mesorhizobium sp. CC13. These bacteria reduced the bioavailable Cd content in the soil, decreased Cd accumulation in soybeans, and increased nodule nitrogenase activity. However, no nitrogenase genes were identified in the genomes of the three strains. Soil metabolomics showed increased levels of octanoic acid, propafenone, and levonorgestrel after introducing bacteria. Subsequent soybean pot experiments demonstrated the ability of these strains and compounds to enhance nodule nitrogenase activity and reduce Cd content in soybeans. The analysis of bacterial abundance in harvested soybean nodules revealed a significant decline in the Bradyrhizobium population, accompanied by a notable increase in Xanthobacteraceae abundance. Co-inoculation with Ancylobacter sp. QY-1, a bacterium belonging to the Xanthobacteraceae family, and Bradyrhizobium sp. USDA110 resulted in enhanced nitrogenase activity in soybean root nodules. Our findings reveal a cooperative mechanism wherein both non-nitrogen-fixing bacteria and specific compounds support the nitrogen-fixation function of soybean under Cd stress by regulating bioavailable Cd and rhizobial abundance.