Coupling microbial and abiotic amendments accelerates in situ remediation of bauxite residue at field scale.

Bauxite residue is a highly saline-sodic tailings material formed as a by-product of the Bayer process for alumina production. In situ remediation of bauxite residue has the potential to provide an effective means for accelerated rehabilitation of residue storage areas. However, previous work has predominantly only used chemical and physical amendments to date, limiting rates of pH neutralisation and extent of remediation. Combining these abiotic amendments with recently developed microbial biotechnology for pH neutralisation may transform bauxite residue into a productive soil material in a shorter timeframe. Here we investigated the effects of microbial and abiotic amendments (compost plus tillage), both in isolation and combined, on remediation of key bauxite residue properties in field scale trials (10 × 15 m × 2 m deep field plots). Triplicate residue samples were collected to 30 cm depth from each plot in quarterly field sampling campaigns. Changes in chemical and physical properties were monitored to assess remediation performance under different amendments. After one year, field plots amended with a microbial treatment had significantly (p < 0.05) lower average pH (8.99-9.46) in the upper 20 cm than the control (10.3). The combined microbial-abiotic treatment also had improved physical structure, higher organic C and lower electrical conductivity than the microbial treatment alone. The strong performance of the microbial-abiotic treatment is attributed to the combined benefits of bioneutralisation from microbial fermentation products, enhanced leaching of alkaline pore water and salts due to tillage and compost, and addition of highly stable C and N in compost. Combining novel microbial biotechnology with common abiotic amendments is therefore suggested for accelerating in situ remediation progress towards a material amenable for plant growth.