Amendment of bauxite residue sand can alleviate constraints to plant establishment and nutrient cycling capacity in a water-limited environment

Abstract

Improving below-ground physical, chemical and biological properties is crucial to developing sustainable vegetation cover on mine tailings or mineral processing residues. This study evaluated amendments with the potential to alleviate constraints to plant growth in residue generated from the refining of bauxite to produce alumina. The effects of textural (carbonated residue mud or soil-derived clay), organic (raw-state or mature compost) or combined textural–organic amendment of the bauxite residue sand fraction on key physical, chemical and microbial properties and growth of annual ryegrass were studied in a glasshouse trial. Two watering treatments were utilised to assess the efficacy of amendments under predominantly nutrient-limited or water-limited growth environments. Amendment of residue sand with carbonated residue mud, although able to improve water retention, was detrimental to plant growth as a result of added alkalinity, salinity and sodicity. Amendment with a kaolinitic clay did not improve water retention and was also detrimental to plant growth, most likely due to increased micro-nutrient cation sorption, which was not overcome by combining with organic amendment. Organic amendment significantly increased the organic C and N content, inorganic nutrient availability (P and K) in residue sand with up to 20-fold increases in the size of the microbial biomass (depending on compost type and amendment rate). When organic amendment was used in addition to an initial inorganic nutrient application, plant growth increased. The optimal organic amendment rate (field relevant rates of 2 and 5% by weight were tested) for rapid plant establishment was dependant on whether nutrient-limitation or water-limitation was the dominant growth constraint. Under a regular rainfall scenario, root and shoot growth was maximised with the lower organic amendment rate. A negative correlation between plant growth and microbial biomass or activity suggested that plant–microbial nutrient competition slowed plant growth at the higher organic amendment rate. Conversely under a drought stress scenario, the higher organic amendment rate maximised plant growth, the result of significantly improved water retention capacity. In conclusion, organic amendment was able to alleviate nutrient and water availability constraints to plant establishment that could not be achieved through fine-fraction textural amendment.