Effects of elevated CO₂ on rhizosphere characteristics of Cd/Zn hyperaccumulator Sedum alfredii.

The effects of elevated CO2 on the metal bioavailability and the rhizosphere characteristics of hyperaccumulator are not well understood. In this study, soil pot experiment was carried out to contrast the effects of elevated CO2 on rhizosphere characteristics between a hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of Sedum alfredii grown under ambient (350 μL L(-1)) or elevated (800 μL L(-1)) CO2. Elevated CO2 facilitated the growth of both ecotypes of S. alfredii, but the promotion in the HE was much greater than in the NHE. No significant (P<0.05) changes in soil pH, dissolved organic matter (DOM) and microbial biomass (Cmic) were observed in the rhizosphere of NHE under both CO2 level. For HE, however, elevated CO2 reduced soil pH by 0.3 units, increased DOM (especially for hydrophilic acid (HiA) fractions) by 19.2% and Cmic by 19%, as compared to ambient CO2. Mobile Cd and Zn (extractable with 1M NH4NO3) in the rhizosphere of HE decreased considerably, but the decreases were greater under ambient CO2 than under elevated CO2. Phytoextraction efficiency of Cd and Zn by HE was increased significantly by elevated CO2 (P<0.05). The results suggest that elevated CO2 can change soil microenvironment, increase bioavailability of Cd and Zn and thus facilitate metal uptake by the HE. This work highlights that elevated CO2 may be a useful way to improve phytoremediation efficiency of Cd/Zn-contaminated soil by hyperaccumulating ecotype S. alfredii.