Fate and Behavior of Lead in Soils Planted with Metal‐Resistant Species (River Birch and Smallwing Sedge)

Phytoremediation of metal‐contaminated soils requires an understanding of the interactions between metal‐tolerant plant species and soil chemical properties controlling the bioavailability of metals. We conducted controlled laboratory studies to investigate the effects that river birch (Betula occidentalis Hook.) and smallwing sedge (Carex microptera Mack.) had on the fate and behavior of Pb in a contaminated soil (3000 mg Pb/kg) and tailings (13 000 mg Pb/kg) collected from an abandoned mining site in Utah. Significant Pb accumulation in aboveground tissue was observed in smallwing sedge (≥1000 mg/kg dry wt.) in both the soil and tailings, but Pb was primarily excluded by birch (≤300 mg/kg dry wt.). Lead exclusion in birch resulted in elevated concentrations of Pb in the rooting zone in both the soil and tailings. In the soil, the exchangeable Pb concentration of the unplanted control was not significantly different than the birch rhizosphere but was higher than the birch bulk (nonrhizosphere) soil fraction. This suggested that plants using exclusionary mechanisms of metal resistance may promote soil Pb stabilization by sequestering normally mobile fractions of Pb in the rhizosphere. However, both birch and smailwing sedge increased the leachate Pb concentration by 2 mg/L and decreased the pH by one unit in the tailings compared with unplanted controls. Leachate Pb concentrations and pH were not significantly affected by plants in the soil. This indicated that the ability of metal‐resistant plants to promote soil Pb stabilization is soil specific and depends on the level of Pb contamination and soil characteristics controlling the solubility and mobility of Pb.