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

Abstract 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 δ 114/110 Cd values were determined in all samples, soil solution compositions measured and dissolved Cd speciation modelled. Isotopic fractionation between soils and soil solutions (Δ 114/110 Cd 20-50cm-soil solution  = −0.61 to −0.68‰) was nearly constant among the three soils. Cd isotope compositions in plants were heavier than in soils (Δ 114/110 Cd 0-20cm-plants  = −0.55 to −0.31‰) but lighter than in soil solutions (Δ 114/110 Cd soil solution-plants  = 0.06–0.36‰) and these differences correlated with Cd plant-uptake rates. In a preliminary model, desorption from soil, soil solution speciation, 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 whereat the first two processes dominated over the last three processes. Within plants, compartments with lower Cd concentrations were enriched in light isotopes what might be a consequence of Cd retention mechanisms, following a Rayleigh fractionation, in which barley cultivars were more efficient than wheat cultivars.