Microbial Processing and Mineral Stabilization of Organic Matter in Iron Ore Tailings: A Four-Year Microcosm Study via Isotopic Labeling and Submicron-Scale Analysis

Microbial utilization and formation of organic matter (OM) are key driving processes in the eco-engineered pedogenesis in iron (Fe) ore tailings, underpinning sustainable ecological rehabilitation. This four-year microcosm study aimed to unravel the mechanisms of relatively long-term OM transformation and stabilization subject to microbial processing in the OM amended Fe-ore tailings. The method of isotopic tracing (¹³C-glucose and ¹³C/¹⁵N-labeled spring wheat biomass) and a high-resolution Orbitrap mass spectrometry and nanoscale secondary ion mass spectrometry (NanoSIMS) were employed to characterize molecular composition of OM and organo-microbial-mineral interactions in the resultant tailings technosol at the submicron scale. It was revealed that both soluble (i.e., glucose) and solid (plant biomass) OM used to amend tailings for soil formation generated a diverse range of molecules, including protein-, lipid-, and lignin-like compounds. These organics were predominantly stabilized by Fe- and Al-rich minerals heterogeneously. Meanwhile, microbial ¹⁴N₂ fixation was observed in tailings primed with ¹³C-glucose, resulting in microbial OM enriched with ¹³C and ¹⁴N. A dynamic OM turnover in the tailings amended with ¹³C/¹⁵N-labeled spring wheat biomass was observed, which was characterized by the decrease of exogenous ¹³C/¹⁵N and the emergence of organic compounds containing atmospheric sources of ¹²C/¹⁴N. These microbial and mineral-mediated OM formation and stabilization processes indicate the emergence of developing soil ecological resilience, indicated by in situ microorganism-driven C/N biogeochemistry in the initially OM-amended tailings.