Shared and Distinct Mechanisms of Two Bioorganic Fertilizers Combined with Arbuscular Mycorrhizal Fungi in Improving the Phytostabilization of Heavy-Metal-Contaminated Saline Soils

Heavy-metal-contaminated saline soil (HCSS) threatens ecosystem health. This study explores the synergistic mechanisms of arbuscular mycorrhizal fungi (AMF) combined with two bioorganic fertilizers [BOFs: manure bioorganic fertilizer (MBOF) and lignite bioorganic fertilizer (LBOF)] in enhancing the phytostabilization efficiency of Stipa grandis in HCSS. The coapplications of AMF and BOFs increased plant biomass (86.87–123.58%), root accumulation of Na and heavy metals (103.62–335.53%), nutrient uptake (79.43–1368.42%), soil organic carbon (SOC; 33.24–50.13%), available nutrients (8.87–361.30%), and macroaggregates, while reducing translocation factors (5.26–60.00%). The coapplication reshaped the rhizosphere microbiota, enhanced the interaction of microbial networks, and altered keystones. Although both AMF+MBOF and AMF+LBOF improved the phytostabilization efficiency by modulating soil properties and rhizosphere microbial communities, structural equation modeling revealed a divergence in their primary mode of action: AMF+MBOF primarily by modulating microbial communities (β = 0.535), while AMF+LBOF predominantly by altering soil properties (β = 0.503). These findings provide a foundation for customized HCSS remediation strategies.