Soil quality and vegetation performance indicators for sustainable rehabilitation of bauxite residue disposal areas: a review

The generation of bauxite residue, the by-product of alumina manufacture from bauxite ore, has increased to a global stockpile of some 3 billion tonnes. In the absence of significant reuse options, the bulk of this residue is contained within bauxite residue disposal areas (BRDAs), which can occupy a significant footprint and pose potential environmental risk. Rehabilitation (amendment and vegetation establishment) is viewed as a significant strategy for eventual closure of the BRDAs. Major limitations to plant growth in residue include high pH, salinity, and sodicity, as well as deficiencies of macro- and micronutrients and potentially elevated levels of trace elements. The physical properties are also problematic as residue mud consolidates to form a solid mass that waterlogs easily or dries to form a massive structure, whereas sand has a very low water- and nutrient-holding capacity. A variety of techniques have been trialled at the pot level and at the field scale to bring about reductions in residue alkalinity and sodicity to promote plant establishment, with gypsum amendment viewed as the most promising. Other amendment strategies include use of organic additions or fertiliser applications, and a combined approach can lead to improved residue properties and successful plant establishment. Few reports have focused on longer term plant growth, self-propagation, and residue interactions under field conditions. There is some evidence that rehabilitated residue can support vegetation growth and soil development in the short to medium term (similar to 15 years), but key issues such as nutrient availability and plant uptake require further study. Although rehabilitated residue can support diverse microbial communities and demonstrate trajectory analogous to soil, the ability of rehabilitated residue to support soil biota and key ecosystem processes warrants further study. The bioavailability of trace elements within rehabilitated sites and potential food chain transfer are relatively unexplored. These areas need careful study before definitive statements can be made regarding the sustainability of residue rehabilitation strategies.