Hidden acidification beneath soil-covered mine waste dumps: links to oxygen-limited sulfur oxidation

Soil covering is widely applied in mine-site restoration to suppress sulfide oxidation by limiting oxygen diffusion and promoting the enrichment of sulfate-reducing bacteria. However, increasing evidence indicates that acidification can recur beneath soil covers, raising concerns about the long-term stability of restored mine sites. In this study, we combined geochemical profiling with microbial community and ecological assembly analyses to investigate the processes associated with subsurface acidification in soil-covered waste dumps. Our results show that, despite initial lime neutralization, strong acidity re-emerged in amended layers within two years (pH < 4), accompanied by elevated metal (loid) bioavailability and upward acid migration. These geochemical changes were associated with a restructuring of the microbial community toward acidophilic taxa, including Sulfobacillus and Acidithiobacillus, and an increased representation of sulfur-related metabolic potentials inferred from functional prediction. Null model and network analyses further revealed a shift toward more deterministic community assembly and tightly connected microbial networks under low-pH conditions. Together, these findings suggest that subsurface acidification beneath soil covers is sustained by coupled geochemical processes and a specialized acidophilic microbiome. This study provides insight into the mechanistic underlying soil-cover restoration failure and highlights the need for restoration strategies that integrate long-term geochemical control with microbial process management.