Concentration-dependent alleviation of lead toxicity in rapeseed by elevated CO₂ is associated with rhizosphere...

Interactive effects of elevated CO₂ (eCO₂) and heavy metal pollution on plant-microbe systems are critical for predicting ecosystem responses under future climate scenarios. We investigated how eCO₂ modulates lead (Pb) toxicity in rapeseed (Brassica napus) and its rhizosphere microbiome using a two-factor experiment with three CO₂ concentrations (400, 550, and 700 ppm) and three Pb levels (0, 70, and 400 mg·kg^-1). The alleviating effect of eCO₂ on Pb toxicity was strongly concentration-dependent. Moderate eCO₂ (550 ppm) significantly increased biomass, photosynthetic rate, nutrient uptake, and non-enzymatic antioxidant capacity (glutathione and ascorbate), thereby mitigating Pb-induced oxidative damage. Conversely, 700 ppm eCO₂ induced photosynthetic acclimation and provided limited protection. Concurrently, eCO₂ reshaped rhizosphere bacterial community in a concentration-dependent manner, enriching stress-tolerant taxa including Proteobacteria and Paenibacillus, and enhancing predicted functional pathways related to carbon and energy metabolism. Mantel analysis revealed strong positive correlations between plant nutrient status and microbial functional potential. We propose a "plant-microbe interaction framework" in which moderate eCO₂ increases photosynthetic carbon inputs, fostering beneficial microbial communities that in turn support plant tolerance to Pb stress. These findings highlight the non-linear nature of CO₂-heavy metal interactions and provide new insights for microbe-assisted phytoremediation strategies under future climate conditions.