Abstract Soil contamination by Cd and Pb poses significant risks to ecological security and human health. Herein, a series of modified biochar (AMBC) were synthesized from Astragalus membranaceus residues, and subsequently employed as carriers of Atlantibacter hermannii XJ08 to construct biochar-microbe composites for soil remediation. Among the tested materials, brown sugar-modified AMBC (BS-AMBC) showed the highest microbial loading capacity (9.2 × 107 CFU·g–1) and superior Cd and Pb adsorption performance. The BS-AMBC@A. hermannii XJ08 composite effectively reduced DTPA-extractable Cd and Pb concentrations to 0.26 mg·kg–1 and 68 mg·kg–1, respectively. Notably, > 56% of labile fractions (water-soluble/acid-soluble) were transformed into stable oxidizable and residual forms, indicating persistent metal immobilization. In addition, the composite significantly improved soil quality, increasing soil organic matter, cation exchange capacity, and available nitrogen/phosphorus/potassium to 22.3 g·kg–1, 30.4 cmol·kg–1, and 122/44.0/235 mg·kg–1, respectively. Urease, phosphatase, and dehydrogenase activities were markedly elevated by 243%, 527%, and 685%, respectively. Microbial community analysis further revealed the enrichment of functional taxa (e.g., Cytobacillus) associated with heavy metal immobilization, alongside the regulation of key metabolic pathways involved in biogeochemical cycling. This study demonstrates a sustainable approach for valorizing medicinal plant residues into high-performance biochar-microbe composites, offering an effective strategy for the ecological restoration of multimetal contaminated soils.
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