<p>Heavy metal contamination in agricultural soils poses severe risks to ecosystems and public health. This study evaluated rice residue biochar as a soil amendment to immobilize cadmium (Cd) and lead (Pb) in contaminated soils. A pot experiment with Indian mustard (<i>Brassica juncea</i>) assessed biochar application rates (0.5%, 1%, 2%, and 4%) under nine Cd–Pb spiking combinations. Biochar at 2% achieved the highest immobilization efficiencies, reducing diethylene triamine pentaacetic acid (DTPA)-extractable Cd and Pb by 32.3% and 28.4%, respectively. Dry matter yield (DMY) decreased under high Cd–Pb spiking (25;50, 5.06&#xa0;g pot⁻¹) but increased by 18% with 1% biochar. However, 2% biochar reduced DMY due to potential nutrient imbalances. Nutrient uptake was maximized at 1% biochar, with increases of 45.3% for phosphorus, 42.9% for potassium, and 87% for calcium compared to the control. Shoot Cd and Pb levels were reduced by 31% and 32%, respectively, under 2% biochar. Post-harvest soil analysis confirmed effective immobilization, showing reductions of 59.7% in Cd and 51.9% in Pb. Principal component analysis highlighted strong positive correlations between nutrient uptake and DMY, while heavy metals negatively impacted biomass. Although biochar reduced hazard quotient (HQ) values, Cd and Pb levels in Indian mustard exceeded dietary safety thresholds. These findings demonstrate that rice residue biochar is a promising, cost effective amendment for mitigating heavy metal risks and enhancing nutrient use efficiency in contaminated soils. The study emphasizes the importance of optimizing biochar application rates to balance crop productivity and food safety under combined Cd–Pb stress. However, the residual bioavailability of metals highlights the need for integrated, long-term remediation strategies to ensure sustainable agricultural production.</p> Graphical Abstract <p></p>