Sustained hydrogen delivery enhances microbiome-associated cadmium and copper immobilization in co-contaminated soils

Molecular hydrogen (H₂) is an important energy source for soil microbes and has been implicated in biogeochemical regulation; however, whether-and how-exogenous H₂ inputs promote Cd and Cu immobilization in co-contaminated soils remains unclear. We engineered MgH₂-biochar granules capable of sustained H₂ release for up to 448 h. The granules exhibited high adsorption capacities in aqueous Cd-Cu systems (Cu: 59.76 mg g⁻¹; Cd: 35.76 mg g⁻¹). In soil, MgH₂-biochar treatment was associated with enhanced microbial hydrogenase activity, reduced oxidative stress, and enrichment of functional taxa, together with significant reductions in available Cd (10.9-22.5%) and Cu (14.2-30.1%) (P < 0.05). Notably, Mg-associated alkalization elevated soil pH and contributed to physicochemical immobilization. Microbial community analyses further revealed H₂-associated shifts in community structure and significant associations between H₂ metabolism and metal resistance (Cd: P < 0.001; Cu: P < 0.03). Genome-resolved metagenomics identified 10 metagenome-assembled genomes (MAGs) that co-harbour [NiFe]-hydrogenases (Groups 1 and 4) and Cd/Cu resistance genes (e.g., actS, czcA, and copC). Exploratory pathway modeling further indicated associations of Cd/Cu immobilization with H₂-associated microbial processes and Mg-associated physicochemical effects. This work provides integrated material, geochemical, and microbial evidence supporting an H₂-associated contribution to heavy-metal immobilization in soils.