Nitrogen and Phosphorus Dynamics in Highly Alkaline Bauxite-processing Residue Sand

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The demand for lightweight metal such as aluminium (Al) has substantially increased worldwide. Alumina is extracted from bauxite ore through the Bayer process, which uses hot caustic soda, resulting in the production of highly alkaline, saline and sodic bauxite residue. Global bauxite residue production is estimated at 120 million tons per annum with the establishment of sustainable vegetation cover systems (e.g. dust emission control crops) in residue storage areas (RSAs) as the key objective in rehabilitation activities. In Western Australia, Alcoa of Australia employs the dry-stacking method for managing bauxite processing residue at RSAs. In this method, residue is separated into mud ( 150 μm) fractions with the latter being used for constructing the RSAs’ perimeter embankments. The sand fraction, referred to in this study as bauxite-processing residue sand (BRS), exhibits a low nutritional status [i.e. nitrogen (N) and phosphorus (P) < 1 mg kg-1] and poor water holding capacity, which can severely restrict the availability of key plant nutrients. The key objectives of this study were to a) assess analytic and diagnostic methods for determining N and P availability indices suitable for use in highly alkaline BRS for an improved and efficient fertilization strategies in the RSAs; b) determine the most suitable organic or inorganic amendment materials that would enhance the growth environment by improving N and P availability, plant N and P uptake by plants, and vegetation growth in BRS; c) assess the fate (including chemical and biological transformation) of applied 15N fertilizer, plant growth response and the role of the microbial community in the dynamics of N in BRS with and without greenwaste compost; and, d) determine which type of fertilizer is most suitable to improve the nutrient status and plant growth in BRS.