USE OF MICROBIAL INDICATORS IN THE ASSESSMENT OF BAUXITE RESIDUE REHABILITATION

One of the best strategies for long-term management and potential in situ remediation of bauxite residue held in storage facilities globally is through vegetation establishment (often termed phytostabilisation). Sustainable vegetation growth on bauxite processing residues involves the reconstruction of an entire ecosystem and is dependent on the development of functional communities both above- and below-ground. The reconstruction of ecosystems directly on residues can be considered analogous to a primary successional ecosystem created following a major disturbance. However, there are many constraints to plant growth on bauxite residues that must be overcome; including poor physical structure, high alkalinity, sodicity and salinity coupled with a lack of organic matter, mineral nutrients and microorganisms. In Western Australia, which supplies almost one-third of the world’s bauxite, Alcoa of Australia has successfully established growth of local coastal dune plant species in the residue sand fraction. The rehabilitation strategies used have alleviated, to some extent, the physical and chemical limitations to plant growth. However, there is a significant knowledge gap on the development of the below-ground microbial community, which is necessary to drive microbially-mediated organic matter turnover and nutrient cycling for the long-term provision of plant nutrients. We analysed bauxite residue sand under early rehabilitation for the presence of two functionally-distinct microbial populations that are ubiquitous in soil: i) arbuscular mycorrrhizal fungi and ii) prokaryotic nitrifiers. These populations are involved in plant root symbioses to aid nutrient acquisition and soil nitrogen cycling, respectively. Species belonging to each of these functional groups were found in bauxite residue sand in 1-2 year-old rehabilitation. Both populations were found to have lower diversity compared to a reference coastal sand ecosystem. However, comparison of different rehabilitation ages, up to 6 years old, provided evidence of successional development in the microbial populations which were related to changing chemical characteristics of the bauxite residue.