Elevated CO2 concentration increase the mobility of Cd and Zn in the rhizosphere of hyperaccumulator Sedum alfredii

The effects of elevated CO₂on metal species and mobility in the rhizosphere of hyperaccumulator are not well understood. We report an experiment designed to compare the effects of elevated CO₂on Cd/Zn speciation and mobility in the rhizosphere of 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) CO₂conditions. No difference in solution pH of NHE was observed between ambient and elevated CO₂treatments. For HE, however, elevated CO₂reduced soil solution pH by 0.22 unit, as compared to ambient CO₂conditions. Elevated CO₂increased dissolved organic carbon (DOC) and organic acid levels in soil solution of both ecotypes, but the increase in HE solution was much greater than in NHE solution. After the growth of HE, the concentrations of Cd and Zn in soil solution decreased significantly regardless of CO₂level. The visual MINTEQ speciation model predicted that Cd/Zn–DOM complexes were the dominant species in soil solutions, followed by free Cd²⁺and Zn²⁺species for both ecotypes. However, Cd/Zn–DOM complexes fraction in soil solution of HE was increased by the elevated CO₂treatment (by 8.01 % for Cd and 8.47 % for Zn, respectively). Resin equilibration experiment results indicated that DOM derived from the rhizosphere of HE under elevated CO₂(HE-DOM-E) (90 % for Cd and 73 % for Zn, respectively) showed greater ability to form complexes with Cd and Zn than those under ambient CO₂(HE-DOM-A) (82 % for Cd and 61 % for Zn, respectively) in the undiluted sample. HE-DOM-E showed greater ability to extract Cd and Zn from soil than HE-DOM-A. It was concluded that elevated CO₂could increase the mobility of Cd and Zn due to the enhanced formation of DOM–metal complexes in the rhizosphere of HE S. alfredii.