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

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.