Co-inoculation of Bradyrhizobium and Azospirillum mitigates the deleterious effects of waterlogging in soybean plants in a scenario of enhanced atmospheric CO2

Rising atmospheric CO₂ concentrations directly influence photosynthesis and productivity in C₃ plants, contributing to global warming and altering hydrological cycles, which in turn increase extreme rainfall events. Soybean, a waterlogging-sensitive crop, exhibits marked yield reductions under such conditions. Legumes establish symbioses with diazotrophic bacteria and are increasingly co-inoculated with plant growth-promoting bacteria (PGPB) to enhance stress resilience. While elevated CO₂ (e[CO₂]) and PGPB generally stimulate photosynthesis and growth, waterlogging often counteracts these benefits by intensifying photorespiratory activity. This study investigated the effects of Bradyrhizobium inoculation (IB) and co-inoculation with Azospirillum brasilense (CA) on soybean carbon and nitrogen metabolism under waterlogging and e[CO₂] (750 μmol mol⁻¹ vs. ambient 420 μmol mol⁻¹). At the V4 stage, plants were subjected to seven days of waterlogging followed by four days of drainage. Measurements included gas exchange, glycolate oxidase (GO), glutamine synthetase (GS), glutamate synthase (GOGAT), and biometric traits. e[CO₂] significantly enhanced gas exchange, an effect further amplified by CA. The synergistic interaction between e[CO₂] and CA improved photosynthetic performance during both stress and recovery. GO activity was reduced under CA and e[CO₂], though it increased transiently under waterlogging. Waterlogging upregulated GS-GOGAT activity, which returned to baseline after the post-drainage. Under e[CO₂], GS-GOGAT activity declined, but CA maintained higher activity than IB. Co-inoculated plants improved the growth metrics under all conditions, with e[CO₂] further enhancing performance. Overall, e[CO₂] improved photosynthesis and suppressed photorespiration, while CA mitigated waterlogging-induced photorespiratory stress and preserved nitrogen metabolism. These results demonstrate that co-inoculation with Azospirillum and e[CO₂] synergistically enhances soybean resilience to waterlogging, offering a sustainable strategy for climate-smart agriculture.