Elevated CO2 Drives Cadmium Phytostabilization in the Robinia pseudoacacia–Rhizobia Symbiosis by Altering Cadmium Bioavailability, Nutrient Uptake and Antioxidant Systems

Elevated atmospheric carbon dioxide (ECO2) is a key climatic factor influencing the resilience of plant–microbial symbiotic systems against heavy metal contamination. Robinia pseudoacacia–rhizobia symbiosis shows great potential for cadmium (Cd) remediation. However, the mechanism by which ECO2 regulates Cd phytostabilization in symbiosis remains unclear. This study conducted a 90-day experiment in growth chambers to investigate the effects of ECO2 on the growth, Cd accumulation and chemical forms, as well as nutrient uptake and antioxidant system in Robinia pseudoacacia–rhizobia symbiosis. Results indicated that ECO2 significantly increased plant biomass and photosynthetic efficiency while significantly raising Cd content in roots (34.5%, p < 0.001) and decreasing it in shoots (31.4%, p < 0.001). This resulted in a significant reduction in Cd translocation factor (TF). Meanwhile, ECO2 markedly increased Cd accumulation in roots (81.2%, p < 0.001) and reduced the bioavailability of Cd in the symbiosis. Moreover, ECO2 promoted the content of nutrients and stimulated the antioxidant system. The random forest model indicated that root weight, Cd and Mn contents are the core factors for ECO2-driven Cd phytostabilization. This study demonstrates that ECO2 enhanced Cd phytostabilization by optimizing the resistance of symbiosis to Cd, offering a novel perspective for predicting plant–microbe joint restoration of heavy metal pollution under global climate change scenarios.