Vertical stratification of mine tailings microbial communities along geochemical gradients one decade after organic cover application

Organic amendments that consume oxygen can function as reactive barriers, limiting sulfide mineral oxidation in acid-generating tailings; however, they may also promote metal mobilization in surface-oxidized horizons. One decade after applying a 1-m thick organic cover, we observed notable yet stratified biogeochemical and microbial transformations down the reclaimed Anthroposol profile. The surface organic cover layer showed increased nutrient availability alongside declines in organic matter and C:N ratio, reflecting ongoing mineralization and decomposition, and became slightly acidic (pH decreasing from 7.1 to 6.7). Beneath this layer, the interface zone exhibited microbial and geochemical convergence toward more soil-like conditions, where bioavailable P, Zn, Fe, and Cu increased by up to two orders of magnitude. Deeper tailings were slightly acidified, enriched in sulfur- and iron-oxidizing microbial taxa, potentially indicating continued sulfide weathering. Despite the improvements in the interface layer, switchgrass roots largely remained confined to the organic cover, hinting at possible chemical or physical barriers limiting deeper rooting. These findings highlight the short-to-medium-term success of organic covers in enhancing nutrient cycling and fostering microbial succession near the surface. However, persistent weathering underscores the need for complementary strategies, such as deeper amendments or reactive barriers. Given projected climate warming could accelerate organic matter decomposition and sulfide oxidation, increasing the risk of acid generation over time, these findings underscore the importance of sustained organic inputs and adaptive management to ensure long-term reclamation success. • Long-term study traced tailings biogeochemical shifts under 1 m cover. • Organic cover boosted nutrient status but showed ongoing organic matter decomposition. • Interface zone showed soil-like microbial and geochemical traits over time. • Deeper tailings had sulfur- and iron-oxidizing microbes, showing ongoing weathering. • Metal mobilization in deep tailings show cover's surface effect.