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 δ114/110Cd 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 (Δ114/110Cd20-50cm-soil solution = −0.61 to −0.68‰) was nearly constant among the three soils. Cd isotope compositions in plants were heavier than in soils (Δ114/110Cd0-20cm-plants = −0.55 to −0.31‰) but lighter than in soil solutions (Δ114/110Cdsoil solution-plants = 0.06–0.36‰) and these differences correlated with Cd plant-uptake rates. In a conceptual 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 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. [Display omitted] •The chemical characteristics of soil and solution does not affect isotope fractionation.•Desorption and solution speciation control Cd isotopic composition of the plants.•Cd retention controls the plant internal Cd isotope distribution.•Barley plants are more effective in preventing Cd to be transferred to the grain. The isotopic fractionation between soil and soil solution is mainly driven by shorter bond length of aqueous than sorped Cd, while the fractionation between soil and plant depend on pool size effects and the plant internal fractionation is controlled by Cd retention mechanisms which are more efficient in barley than wheat plants.