Beyond Metal(loid) Immobilization: Redox-Stratified Biocrusts Shield Humid Mining Regions.

Biological soil crusts (biocrusts) develop vertical redox-microbial-nutrient stratification that regulates hydrological and elemental cycles and contributes to ecological restoration in extreme environments, including mining regions. However, the roles of this heterogeneity in metal(loid) immobilization remain unclear, particularly in humid regions, where pronounced redox and microbial stratification may foster unrecognized stabilization mechanisms. We integrated physicochemical characterization with bioinformatic analysis to reveal stratified microbial communities and metabolic potentials in humid tailings biocrusts. Biocrusts exhibited stratified functionality through the upper photoautotrophic layer (PL) and the lower heterotrophic layer (HL). In the PL, Cyanobacteria and SWB02 formed a self-reinforcing oxygen barrier through clay-silt enrichment (2.8-fold higher than bare tailings sand) and extracellular polysaccharide accumulation (18-fold), which swelled upon hydration to physically hinder oxygen infiltration, confining Gammaproteobacteria-associated iron-manganese oxide immobilization to this layer. Beneath this barrier, the HL harbored sulfidogenic potential through microbes enriched in hydB (17.4-fold) and phsC (3.4-fold) genes, including Bacteroidota and Desulfobacterota, supporting a potential mechanism for metal(loid) sequestration via sulfide formation in underlying tailings, where sulfur occurred exclusively as sulfides at 5 cm depth. This barrier-mediated effect may outweigh metal(loid) immobilization within biocrusts. Our findings elucidate biocrust-mediated protection against metal(loid)s and provide theoretical support for remediation in humid mining regions.