Cadmium availability to three plant species varying in cadmium accumulation pattern

Plant response to soil Cd is highly variable. Plants varying in Cd accumulation pattern were evaluated to determine if Cd was taken up from similar or different pools in soils. Three metal-contaminated soils (pH 8) and a noncontaminated soil were labeled with Cd-109 using a carrier-free (CdCl2)-Cd-109 solution Rye grass (Lolium perenne L.), lettuce (Lactuca sativa L.), and the Zn- and Cd-hyperaccumulator Thlaspi caerulescens J. Presl & C. Presl were grown on these soils and harvested. The isotopic composition of Cd in plant shoots (IC,) was measured and the isotopic composition in the soil solution (ICs) was calculated using the isotopic exchange kinetics (IEK) method. The Cd concentrations in shoots ranged from 0.1 to 2.3 mg kg(-1) in rye grass, 0.4 to 8.3 mg kg(-1) in lettuce, and 8.7 to 647 mg kg(-1) in T. caerulescens. The ICp values were similar among species, suggesting that the same pool of soil isotopically exchangeable Cd was accessed despite variations in Cd uptake pattern. The ICP values were similar to the ICs, suggesting that the IEK method assesses the pool that is phytoavailable. Despite the high soil pH, more than 50% of the total soil Cd was isotopically exchangeable within 90 d. Lettuce and rye grass took up less than 1% of the exchangeable Cd while T. caerulescens extracted up to 22% of this pool. In conclusion, plants representing a wide array of behavior toward Cd take up the metal from a similar pool of soil Cd, which is assessed by the IEK method.