Primary succession of microbial community and natural pedogenic process in two abandoned red mud depositories

The foundation for ecological restoration in red mud (bauxite residue) depositories lies in pedogenic process, where microorganisms assume a crucial role. In this study, we examined two abandoned red mud depositories (Bayer and sintering process) undergoing natural pedogenesis over 25 years. Despite the varying physicochemical property changes and microbial community succession between the two types of red mud, our results showed that as stacking age increased, the physical, chemical, nutrient, and biochemical properties of both red mud approached the typical ranges observed in natural saline-alkali soils, which was accompanied by a transformation of the saline-alkaline oligotrophic microbiota present in the red mud into a copiotrophic, soil-like community. The key bacteria signifying pedogenic early- and later-stages were Leptolyngbya and Nocardioidaceae, respectively. The critical thresholds of pH, EC, and ESP restricting microbial community development were approximately 11.0, 6.0 mS/cm, and 30 %, respectively. In conclusion, nutrient level augmentation, stable aggregate formation, and salinity and alkalinity reduction constitute the major microbial driving forces to regulate the natural pedogenesis of red mud.