How non-indigenous material input governs the accumulation of organic carbon and its fractions in reclaimed soils

Non-indigenous materials have been introduced into soil systems during land reclamation in mining areas, yet the impact of the introduced materials on soil organic carbon (SOC) accumulation remains unknown. Through a 360-day mesocosm experiment comparing coal gangue (CG), lake mud (LM), and Yellow-river sand (YS), we examined their impacts on SOC and its fractions-passive (POC), intermediate (IOC), and active organic carbon (AOC). Our results showed that the CG treatment demonstrated better SOC recovery, increasing from 8.2 to 14.9 g kg^-1 during the incubation period, significantly surpassing LM and YS treatments. POC constituted the dominant fraction, with CG maintaining consistently higher POC concentrations (7.0-10.3 g kg^-1) than the LM and YS treatments. Structural equation modeling revealed that in the CG treatment, higher total nitrogen concentration and C/N ratio promoted the accumulation of SOC and POC (GFI = 0.985, P = 0.255), while its greater proportion of large macro-aggregates also enhanced AOC formation (GFI = 0.988, P = 0.630), thereby establishing an effective balance between SOC stabilization and mineralization. Meanwhile, rising temperature reduced SOC stability by increasing AOC and IOC concentrations while decreasing the proportion of POC (GFI = 0.987, P = 0.426). These findings provide a valuable foundation for understanding carbon sequestration dynamics in reclaimed soils and support the prioritization of extraneous materials for soil reclamation in mining areas.