Multi-omics analysis reveals the mechanisms on biochar-mediated cadmium transport in Salix : insight into rhizosphere phosphorus-iron coupling and transporter expression

Biochar addition promoted cadmium (Cd) phytoremediation of woody plants, especially phosphors (P)-modified biochar. However, the underlying mechanism on the uptake and transport of Cd mediated by biochar remains unclear. Here, we integrated the physiological, metagenomics, transcriptomics, and in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) imaging analysis to investigate how bamboo biochar (BBC) and phytic acid modified biochar (PABC) impact Cd accumulation and transport in Salix J1010 through root-soil interface. Our results showed that PABC significantly increased Cd translocation from roots to aboveground by 77.9% and total Cd accumulation in plan by 203%, respectively. Iron plaque emerged as a key factor, with PABC-mediated inhibition of iron plaque (-44.6%) accelerating Cd uptake. This iron plaque decrease is closely accompanied by the decreased soil redox potential (Eh), enriched resin-P and inorganic P fractions, and potential coupling of P mineralization and Fe(III)-reducing processes in the rhizosphere soil. Transcriptomics analysis further revealed that PABC influenced root metal transporter expression, downregulating vacuolar sequestration-related ABC, CAX, MTP gene families, while upregulating most ZIP, HMA, and YSL genes families involved in xylem loading. LA-ICP-MS imaging corroborated the enhanced Cd transport in xylem tissue. PABC enhanced leaf cell-wall Cd binding and antioxidant defenses, thereby promoting Cd detoxification and accumulation. Collectively, the enhanced phytoremediation capacity of willow was driven by coordinating trade-offs across multiple levels, including the rhizosphere, subcellular scales, and whole-plant. The results provide a mechanistic basis for biochar-assisted phytoremediation strategies in Cd-contaminated soils.