Tracing atmospherically deposited cadmium accumulation in rice using stable isotopes

Understanding the plant uptake of atmospherically deposited cadmium (Cd) is limited. Factorial soil and atmospheric exposure experiments conducted in both field and greenhouse settings, in conjunction with stable Cd isotopes and synchrotron microscopic X-ray fluorescence (SR-μXRF), were performed to differentiate the impact of newly deposited Cd from legacy Cd in the soil. Root uptake of newly deposited Cd in soils contributed 43-48 %, 6.3-30 %, and 6.3-30 % to the leaves, stems, and grains, respectively. In contrast, foliar uptake contributed the most to rice leaves, which were subsequently limited in their translocation to stems and grains, accounting for 18-31 %, 5.7-12 %, and 5.7-11 % of Cd in leaves, stems, and grains, respectively. Stem nodes appear to restrict Cd translocation from leaves to grains while simultaneously facilitating both bi-directional xylem and phloem transport. Geochemical analyses and diffusive gradients in thin films extractions indicated that newly deposited Cd constituted the major bioavailable Cd fractions in soil solutions. In the rice growing season, the atmospherically deposited Cd only accounted 0.8-5.5 % of soil pools, but they substantially contributed 27-45 % to grains, raising important questions about how atmospheric Cd deposition may impact food security. The accumulation of Cd in rice grains from atmospheric deposition primarily occurs during the filling stage, making the management of emissions during this period crucial compared to the remediation of affected soils.