Effects of elevated CO₂ and Pb on phytoextraction and enzyme activity

The wide-ranging impacts of globally increasing carbon dioxide (CO ₂) concentration and rising metal-contaminated soils are serious problems in terrestrial ecosystems. In this study, we investigated the effects of elevated CO₂ on the lead (Pb) uptake of pine seedlings and the microbial activity in Pb-contaminated soil. Three-year-old pine seedlings were exposed to ambient, as well as elevated levels of CO₂ (380 and 760 ppmv, respectively) in 500 mg/kg Pb-contaminated soil. Growth rates, C/N ratios and Pb uptake of the pine seedlings were determined. Dissolved organic carbon (DOC) content and microbial activity were also measured in the rhizosphere soil. Elevated CO₂ significantly increased the total biomass and accumulation of Pb in roots and shoots. In addition, the accumulation of Pb in the roots under elevated CO₂ concentration was four times higher than those in the roots under ambient CO₂ concentration. Elevated CO ₂ levels also affected C/N ratios in the pine seedlings and soil enzyme activities. Decline in the overall nitrogen content and increases in the C/N ratios of pine needles were observed. Soil enzyme activity increased in the rhizosphere soils, including those of β-glucosidases, N- acetylglucosaminidases, and phosphatases. Quality of the DOC was affected by elevated CO₂, while the quantity of DOC was affected by Pb additions under elevated CO₂ conditions. Two major conclusions can be drawn from this study: (1) elevated CO₂ significantly increased biomass and metal uptake of pine seedlings and (2) chemical metabolism on pine tissue and processes of organic decomposition were more affected by elevated CO₂ levels than by Pb contamination.