Synergistic effects of modified biochar and selenium on reducing heavy metal uptake and improving pakchoi growth in Cd, Pb, Cu, and Zn–contaminated soil

Heavy metal stress poses a significant threat to both crop production and human health. The application of biochar and selenium (Se) to remediate soil Cd pollution and alleviate plant Cd toxicity has been extensively investigated. However, under Cd, Pb, Cu, and Zn stress, the synergistic mechanisms of key components in lignin–based modified biochar (BC) and foliar selenium fertilizer promoting plant growth and development still need further investigation. The results indicated that, compared to the control, the simultaneous application of BC and foliar selenium fertilizer significantly increased the soil dissolved organic carbon, available silicon, and exchangeable magnesium, while reducing the bioavailability of Cd, Pb, Cu, and Zn by 27.35%, 42.50%, 14.50%, and 11.79%, respectively. The synergistic interaction between silicon silicate minerals in BC and selenium plays a crucial role in enhancing chlorophyll content, improving photosynthesis, and boosting both enzymatic antioxidant systems (CAT and SOD) and non–enzymatic antioxidant systems (GSH). Interestingly, the combined application of BC and Se shows a more pronounced effect, reducing the absorption of Cd, Zn, Pb, and Cu by pakchoi by 56.04%, 53.21%, 32.54%, and 18.22%, respectively. This study provides an effective and practical approach to alleviate the composite toxicity of plants to multiple metals such as Cd, Pb, Cu, and Zn. The synergistic effects of BC and Se in reducing heavy metal uptake by pakchoi have significant implications for sustainable agriculture and environmental remediation.