18Analyses of stable metal isotope ratios constitute a novel tool to improve understanding of 19 biogeochemical processes in soil-plant systems. In this study, we used such measurements 20 to assess Cd uptake and transport in wheat grown on three agricultural soils under 21 controlled conditions. Isotope ratios of Cd were determined in the bulk C and A horizons, in 22 the Ca(NO 3 ) 2 extractable Cd soil pool and in roots, straw and grains. The Ca(NO 3 ) 2 23 extractable Cd was isotopically heavier than the Cd in the bulk A horizon (Δ

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