Stable Aggregate Formation and Microbial Diversity Resilience in Soil Formation of Bauxite Residue: Roles of Extracellular Polymeric Substances Secreted by Penicillium oxalicum

Microbially driven aggregate formation plays a sustainable role during the soil formation process in bauxite residue, which may realize the large-scale treatment of the typical alkaline solid waste. However, the potential mechanisms of microbial metabolites on the formation of organic–alkaline mineral complexes and stable aggregates are usually neglected. Here, we developed a novel microbially driven strategy to improve aggregate formation and enhance ecological service resilience by the inoculation of Penicillium oxalicum (P. oxalicum, an alkali-tolerant and acid-producing fungi). The intensive application of P. oxalicum reduced the contents of electrical conductivity (EC) and exchangeable sodium percentage (ESP) while increasing the proportions of macroaggregates and microaggregates in bauxite residue compared to the application of chemical amendments. Furthermore, even inoculated with the same dose of P. oxalicum, multiple inoculations achieved higher microbial extracellular metabolic activity by increasing the contents of metabolites than single inoculation. Simultaneously, P. oxalicum guided oligotrophic-dominated assemblages (including Actinobacteria, Proteobacteria, and Ascomycota members) and increased microbial network complexity. The spectroscopic analysis demonstrated that extracellular polymeric substances (EPSs) promoted the formation of organic–alkaline mineral complexes due to the high adsorptive functional groups (e.g., −OH, −COOH, N–H). The existence of hydrogen bonds led to high viscosity exhibition and stable organic–alkaline mineral complexes through the binding effects of polysaccharides to alkaline minerals (e.g., sodalite, cancrinite). The findings enhanced the understanding of the binding role of EPSs on aggregate stabilization and demonstrated the application potential of microbially driven aggregate formation during the soil formation process and ecological rehabilitation in bauxite residue.