Cadmium transfer in contaminated soil-rice systems: Insights from solid-state speciation analysis and stable isotope fractionation

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (delta Cd-114/110 = -0.37 to -0.39%) compared to the plants that grew on non-flooded soils (delta Cd-114/110 = -0.45 to -0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl2 root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Delta Cd-114/110(shoot-root) 0.16-0.19 parts per thousand). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Delta Cd-114/110(shoot-root) -0.02 to 0.08 parts per thousand). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd-S complexes in the vacuole. Cd-S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems. (C) 2020 The Author(s). Published by Elsevier Ltd.

Automated summary

The study found that soil flooding promotes the binding of Cd to S and results in the enrichment of heavy Cd isotopes in plants in soil-rice systems rich in Cd and S. Excluder rice roots strongly retain light Cd isotopes, while the difference in isotope composition between roots and shoots is smaller in non-excluder rice plants.