Tailored biochar for effective cadmium immobilization in soil and reduced accumulation in plants: a meta-analysis and experimental validation study

Soil available cadmium (Cd) governs its phytoavailability, thereby threatening food and medicinal plant safety. While biochar offers a promising remediation approach due to its high adsorption capacity and cost-effectiveness, its efficacy for Cd passivation requires a customized design. Here, a meta-analysis was conducted to examine how biochar preparation factors influence soil available Cd. The results indicate that biochar derived from herbaceous single Chinese medicine residues (SCMR), prepared at moderate-to-low pyrolysis temperatures (< 500 °C) with extended duration (> 3 h), achieved the highest Cd passivation efficiency. Adsorption kinetics experiments confirmed that biochar prepared from the decoction residue of Houttuynia cordata (i.e., a typical herbaceous SCMR) at 450 °C and 3.5 h exhibited optimal Cd adsorption capacity, verifying the meta-analysis. Pot experiments with lettuce ( Lactuca sativa L.) further validated the findings of the meta-analysis: the 2% H. cordata biochar application led to the greatest reductions in soil available Cd and plant Cd accumulation by 17.6% and 52.3%, respectively. In heavy metal-accumulating medicinal plants ( Taraxacum officinale F.H.Wigg. and Lilium davidii var. willmottiae ), this biochar decreased soil available Cd by 18.0%–55.5% and plant Cd accumulation by 42.9%–68.6%. These results indicate the promising cross‑species potential of the tailored biochar in mitigating Cd uptake in plants. In summary, this study establishes a meta‑analysis‑guided framework for tailoring biochar production, offers an evidence‑based strategy for soil Cd remediation, and proposes a practical pathway for the valorization of Chinese medicine residues.