Biochar-loaded AM fungi coupled with Arundo donax enable targeted red mud remediation via valency—specific metal detoxification and soil function recovery

Abstract Red mud, a saline-alkaline and metal-contaminated byproduct, poses severe ecological risks. This study elucidates the synergistic remediation mechanisms of biochar (BC) combined with arbuscular mycorrhizal fungi (AMF) in an Arundo donax- soil system. We specifically investigated biochar loaded with Funneliformis mosseae (BC–FM) and that loaded with Rhizophagus intraradices (BC–RI). The BC–FM treatment significantly enhanced the plant antioxidant system and photosynthetic capacity while reducing the content of exchangeable arsenic (As) and soil pH, thereby inducing a “photosynthesis enhancement-As immobilization” synergy. In contrast, the BC–RI treatment markedly increased plant biomass and soil microbial α–diversity, while simultaneously reducing the contents of soil lead (Pb) and sodium ions (Na⁺) and enhancing alkaline phosphatase activity—thus demonstrating a “Pb fixation–microbial diversity–soil phosphorus (P) activation” cascade. Rhizosphere network analysis identified key bacterial genera, with Longimicrobiaceae driving soil organic carbon accumulation in the BC–FM treatment and Lechevalieria enhancing alkaline phosphatase (ALP) activity in the BC–RI treatment. These findings support a novel “fungal species–heavy metal valency matching” principle, where RI preferentially targets cationic Pb, while FM targets anionic As. This principle establishes a three-dimensional synergistic model: “Heavy metal transformation–concurrent salinity-alkalinity mitigation–microbial function activation”. The results provide a foundational strategy for zonal red mud remediation: applying BC–RI in Pb–dominated areas and BC–FM in As–contaminated areas. Graphical Abstract Highlights Proposed “Fungal species−metal valency matching” enables zonal RM remediation: BC−RI for Pb, BC−FM for As. The BC–FM synergy boosts photosynthesis and immobilizes As (V), thereby alleviating saline-alkali stress. BC−RI enhances biomass, immobilizes Pb, diversifies the microbiota, and activates P. Rhizosphere microbial networks reveal key genera (Longimicrobiaceae, Lechevalieria) driving C accumulation and ALP enhancement.