Staple crops such as rice are our major sources of cadmium (Cd). Therefore, a fundamental understanding of processes that control Cd accumulation in crops is crucial. Currently, we are investigating Cd behaviour in rice by combining synchrotron solid state speciation analysis with Cd isotope process tracing in contaminated soil-rice systems. The main principle of the combined approach is that (i) synchrotron solid state speciation techniques (XANES) identify snapshots of Cd speciation in soil and rice compartments at the time of sampling (ii) while mass balances and isotope ratios provide information on how Cd speciation control the mobility of Cd during plant growth. With this combined approach, we aim to provide a holistic view on major processes that determine the Cd distribution in distinct soil-rice systems.
To this end, we grew rice (Oryza sativa L.) in pots that contained wet and flooded soils that were spiked with 15 mg kg-1 Cd. We grew two rice accessions: Taichung and TCM213, which respectively have functional and non-functional OsHMA3, a tonoplast-localized Cd transporter. At flowering stage, we collected bulk soil, roots, and shoots. During plant growth, we frequently collected soil solution and analyzed pH, Eh, TOC, major cations and anions, Fe, Zn, and Cd concentrations as well Cd isotopes to model Cd speciation in soil solution using PHREEQC. Prior isotope analysis using MC-ICPMS, soil and plant samples were purified using a one stage anion exchange chromatography. For bulk XANES analysis, samples were frozen with liquid nitrogen immediately after their harvest.
At the conference, we will show Cd mass balances, Cd isotope, and Cd speciation data of flooded and non flooded soil-rice systems. Overall, we expect that S ligands will strongly determine the mobility of Cd in the entire soil-rice system. First soil solution data showed that the different soil redox states caused contrasting soil properties that control the solubility of Cd in the soil such as pH, Fe2+ concentrations, and total dissolved organic carbon. We expect that dissolution of Fe-oxides and sulfide formation in the flooded soils will alter Cd binding sites in the soil and thus also the Cd isotope ratios in soil solution and plants. Within the rice, we expect that the retention of Cd in the roots is not only determined by its functional HMA3 transporter but also by a Cd immobilization through enhanced binding to S ligands in vacuoles. Consequently, a larger fraction of Cd bound to S should be accompanied by a strong enrichment of light isotopes in roots in wild type compared to mutant rice. The outcomes of this pot trial will be integrated into current conceptual models of Cd isotope fractionation and Cd transport in soil-plant systems.