Decade-Long Soil Cover Promotes Mineral Weathering and Organic Matter Accrual in Fe Ore Tailings But Deteriorates Topsoil

Soil cover is widely adopted to initiate ecological rehabilitation on tailing deposits, yet its influence on geochemical processes and organic matter (OM) dynamics in underlying tailings remains poorly understood. This study investigated the physicochemical and mineralogical evolution of iron (Fe) ore tailings in Hebei Province, China, following a decade of soil cover and revegetation. Soil cover and revegetation generally neutralized alkalinity, fostered microaggregate formation, and improved OM stabilization in the underlying tailings. Synchrotron-based Fe K-edge X-ray absorption spectroscopy (XAS) identified a transformation of primary biotite-like minerals into Fe(III) oxyhydroxides. Nanoscale secondary ion mass spectrometry further delineated the submicrometer association of these secondary Fe and aluminum (Al)-bearing minerals with OM, which underpins the mechanisms of OM stabilization. Notably, soil cover with grass rehabilitation proved more effective than that with cropland in driving the above-mentioned processes. Conversely, the underlying tailings exerted a deleterious influence on the soil cover, manifested by increased alkalinity and metal(loid) concentrations, alongside a reduction in total organic carbon, nitrogen, and aggregate stability relative to those of natural soils. These findings demonstrate that while soil cover and revegetation partially ameliorate unfavorable conditions in Fe ore tailings, they degrade soil cover quality, implying the requirement of an integrated strategy to preserve the functional stability of the entire profile.