Towards an understanding of the Cd isotope fractionation during transfer from the soil to the cereal grain

Cd in soils might be taken up by plants, enter the food chain and endanger human health. This study investigates the isotopic fractionation of major processes during the Cd transfer from soils to cereal grains. Thereto, soil, soil solution, wheat and barley plants (roots, straw and grains) were sampled in the field at three study sites during two vegetation periods. Cd concentrations and d114/110Cd values were determined in all samples. The composition of the soil solution was analyzed and the speciation of the dissolved Cd was modelled. Isotopic fractionation between soils and soil solutions (D114/110Cd20-50cm-soil solution ¼ 0.61 to 0.68‰) was nearly constant among the three soils. Cd isotope compositions in plants were heavier than in soils (D114/110Cd0-20cm-plants ¼ 0.55 to 0.31‰) but lighter than in soil solutions (D114/110Cdsoil solution-plants ¼ 0.06e0.36‰) and these differences correlated with Cd plant-uptake rates. In a conceptual model, desorption from soil, soil solution peciation, adsorption on root surfaces, diffusion, and plant uptake were identified as the responsible processes for the Cd isotope fractionation between soil, soil solution and plants whereas the first two processes dominated over the last three processes. Within plants, compartments with lower Cd concentrations were enriched in light isotopes which might be a consequence of Cd retention mechanisms, following a Rayleigh fractionation, in which barley cultivars were more efficient than wheat cultivars.