Micronutrient fractionation and plant availability in bauxite-processing residue sand

Bauxite-processing residue must be disposed of in specifically designed facilities for long-term management. Consideration of alkalinity, salinity, sodium content, and poor nutritional status is essential for successful rehabilitation of residue disposal areas (RDA). The aim of this study was to examine the availability and distribution of the micronutrients, B, Cu, Fe, Mn, and Zn, in (i) fresh bauxite-processing residue sand (particle size > 150 mu m) with and without gypsum amendment, and (ii) aged residue sand from a 4-year-old rehabilitated RDA that had received past gypsum and fertiliser addition. Samples of fresh residue sand from India and Australia exhibited high alkalinity, high salinity, and sodicity. Gypsum addition significantly lowered pH, soluble Na, and alkalinity. Aged residue sand had low levels of all micronutrients, with low extractability for Zn and Mn followed by B, Cu, and Fe. Fractionation showed that 30-78% of Zn and Mn and 40-60% of B existed in non-available (residual) forms. The next most dominant fractions were the Fe and Mn oxide-bound and carbonate-bound fractions. Plant-available fractions (i.e. exchangeable and organically bound) contributed < 1% of the total concentration. Total concentration was found to be a reliable indicator for Zn, Cu, and B extractability but not for DTPA-extractable forms of Fe and Mn. Leaf analysis of vegetation grown on aged residue sand indicated deficiencies of Mn and B. Results demonstrated that bauxite-processing residue sand contained very low levels of B, Mn, and Zn and these concentrations may be limiting to plant growth. Distribution of micronutrients among chemical pools was significantly influenced by pH, organic carbon, exchangeable Na, and alkalinity of residue. Nutrient management strategies that account for the characteristics of residue sand need to be developed for residue rehabilitation. Importantly, strategies to limit the conversion of nutrients to non-available forms are required to minimise micronutrient disorders.