Calcium–magnesium synergy in reducing cadmium bioavailability and uptake in rice plants

The synergistic application of calcium (Ca) and magnesium (Mg) was investigated to mitigate cadmium (Cd) uptake and translocation in rice grown in Cd-contaminated soil. A pot experiment was conducted using different Ca:Mg molar ratios (Ca1:Mg2, Ca2:Mg1, and Ca1:Mg1) to evaluate their effect on Cd uptake. The results showed that the Ca1:Mg1 treatment achieved the highest reduction in grain Cd content (54.7%, <i>p</i> &lt; 0.05), followed by Ca2:Mg1 (47.6%), and Ca1:Mg2 (40.7%), all below China’s National Food Safety Standard (0.2 mg kg⁻¹). Significant reductions were also observed in roots, stems, and leaves (<i>p</i> &lt; 0.05). Ca1:Mg1 minimized Cd translocation by decreasing stem-to-grain transfer by 61.0% and xylem sap Cd by 50.1% (<i>p</i> &lt; 0.05). It also reduced mobile Cd fractions in roots (F_E from 25% to 18%, F_Di from 44% to 37%) and increased DCB-extractable Fe (DCB-Fe) on roots, enhancing Cd immobilization. Ca:Mg treatments raised soil pH by 23.6–25.7% (<i>p</i> &lt; 0.05), shifting Cd from bioavailable forms (F_EX reduced by 9.3%, F_CB by 17.8%) to more stable forms (F_Fe/Mn increased by 15.5%, F_OM by 1.9%). Strong negative correlations (<i>p</i> &lt; 0.05, 0.01) between soil pH, DCB-Fe, Ca, Mg_TF, F_Fe/Mn, and grain Cd indicating their effect in reducing Cd uptake. To the best of our knowledge, this is the first study to evaluate the synergistic effects of different calcium:magnesium (Ca:Mg) ratios on cadmium (Cd) accumulation in rice. Our findings showed that optimal Ca:Mg ratios (1:1, 2:1, and 1:2) reduced Cd accumulation below safety thresholds through mechanisms such as increased soil pH, altered soil chemistry, enhanced iron plaque formation, and reduced Cd translocation factors. This study highlights the importance of balanced Ca:Mg ratios for effective Cd mitigation and offers a cost-effective and eco-friendly solution.