Pre-culturing soil microbial inoculum in plant residues enhanced the resilience of tolerant bacteria and bioneutralization efficacy in alkaline bauxite residues

Bioneutralization of alkaline bauxite residues (BR) may be achieved through in situ organic acids produced from fermentative decomposition of carbohydrates-rich organic matters (e.g., plant residues), which are driven by organophilic and heterotrophic prokaryotes tolerant of extremely saline and alkaline conditions. The present study investigated if the resilience of tolerant prokaryotes in soil microbial inoculums could be improved by pre-culturing them in carbohydrate-rich plant residues, leading to enhanced bioneutralization efficacy in strongly alkaline BR. In a 2-week microcosm experiment with BR (pH 10.5), it was found that the resilience of prokaryotic communities and their functional modules and bioneutralization efficacy were significantly boosted in BR admixed with plant residues (i.e., SM: sugarcane mulch, LH: Lucerne hay) pre-cultured with soil microbial inoculum. The results showed that 10-20% of the initially inoculated soil prokaryotic features were recovered in treatments with pre-cultured plant residues. Besides, the enriched diverse prokaryotes formed highly clustered networks in the amended BR. These modules actively drove C and N mineralization and sustained 0.8-2.0 units of pH reduction, despite the buffering effects of alkaline minerals in BR solid phase. In contrast, soil microbial inoculation cultured in the growth medium lost >99% of the original prokaryotic features in soil inoculums, resulting in merely 0.2-0.7 unit pH reduction in the treated BR. Therefore, pre-culturing soil inoculum in plant residues would be preferred as an integral system to treat BR for effective bioneutralization.