Cadmium Isotope Fractionation in Soil–Wheat Systems

Analyses of stable metal isotope ratios constitute a novel tool in order to improve our understanding of biogeochemical processes in soil–plant systems. In this study, we used such measurements to assess Cd uptake and transport in wheat grown on three agricultural soils under controlled conditions. Isotope ratios of Cd were determined in the bulk C and A horizons, in the Ca­(NO<sub>3</sub>)<sub>2</sub>-extractable Cd soil pool, and in roots, straw, and grains. The Ca­(NO<sub>3</sub>)<sub>2</sub>-extractable Cd was isotopically heavier than the Cd in the bulk A horizon (Δ<sup>114/110</sup>Cd<sub>extract–Ahorizon</sub> = 0.16 to 0.45‰). The wheat plants were slightly enriched in light isotopes relative to the Ca­(NO<sub>3</sub>)<sub>2</sub>-extractable Cd or showed no significant difference (Δ<sup>114/110</sup>Cd<sub>wheat–extract</sub> = −0.21 to 0.03‰). Among the plant parts, Cd isotopes were markedly fractionated: straw was isotopically heavier than roots (Δ<sup>114/110</sup>Cd<sub>straw–root</sub> = 0.21 to 0.41‰), and grains were heavier than straw (Δ<sup>114/110</sup>Cd<sub>grain–straw</sub> = 0.10 to 0.51‰). We suggest that the enrichment of heavy isotopes in the wheat grains was caused by mechanisms avoiding the accumulation of Cd in grains, such as the chelation of light Cd isotopes by thiol-containing peptides in roots and straw. These results demonstrate that Cd isotopes are significantly and systematically fractionated in soil–wheat systems, and the fractionation patterns provide information on the biogeochemical processes in these systems.