The native soil microbiome mediates the synergistic effects of arbuscular mycorrhizal fungi and biochar on cadmium phytoremediation by willow

While the potential of arbuscular mycorrhizal fungi (AMF) and biochar to enhance phytoremediation of cadmium (Cd)-contaminated soils is well established, the mechanisms by which the native soil microbiome mediates their synergy remain unclear. To elucidate these mechanisms, a full-factorial pot experiment was conducted using willow ( Salix spp.) to investigate the interactive effects of soil microbial status (non-sterilized, NS, vs. sterilized, S), AMF inoculation, and biochar amendment. The combined AMF and biochar treatment (BA) significantly reduced leaf Cd accumulation (by 16.42%), the translocation factor (by 14.39%), and increased soil nutrients in NS soil relative to CK. Conversely, in S soil, the same treatment increased leaf Cd accumulation by 22.79%, indicating the critical role of the native microbiome in mediating synergistic remediation. Transcriptomic analysis revealed that AMF inoculation activated defense pathways (e.g., plant-pathogen interaction) in S soil, whereas the native microbiome suppressed stress responses and promoted metabolic homeostasis in NS soil. The BA treatment restructured the soil microbial community, enriching beneficial taxa (e.g., Myxococcota, Basidiomycota) and enhancing enzyme activities (e.g., dehydrogenase), thereby increasing soil organic matter and nutrient availability. The native soil microbiome facilitates the AMF-biochar synergy by enhancing microbial functions and regulating plant adaptations, consequently improving phytoremediation efficiency. Therefore, microbiome management should be an integral part of field-scale phytoremediation.