Aluminate toxicity as a factor controlling plant growth in bauxite residue

Abstract

AbstractMillions of metric tons of alkaline bauxite residue (red mud), produced annually in aluminum refineries at numerous locations around the world, are stored in large, diked impoundments. Bauxite residue is characterized by high pH (9 to 12), high concentrations of soluble sodium, carbonate and aluminate and low concentrations of the major plant nutrients. Establishment of a functioning ecosystem on these derelict lands requires an understanding of the ecological factors limiting plant growth. A series of organic amendments to bauxite residue was tested in order to increase the growth of Distichlis spicata var. stricta (desert saltgrass), an alkaline‐tolerant rhizomatous grass. Sewage sludge‐amended residue increased shoot growth more than 10 times than did unamended or nutrient‐amended residue. Yield was negatively correlated with shoot Al and Fe concentrations' and positively correlated with shoot N, K, P, Ca and Mg content. A hydroponic culture experiment was used to test the effects of high pH (9.5 and 10.5) and aluminate addition (1.0 mM) on the growth of desert saltgrass. Although elevated pH levels reduced Mn uptake, there was very little reduction in growth. The addition of aluminate, however, reduced shoot weight by 67 and 77% at pH 9.5 and 10.5, respectively. Growth reductions were associated with significant declines in shoot P, N, Mg, Ca and K concentrations. At pH 9.5, reductions in shoot P concentrations were associated with large increases in root Al and P levels. At pH 10.5, however, precipitation of an Al‐Mg hydroxycarbonate mineral was associated with a decline in shoot Mg levels. Aluminum toxicity appears to be a major factor controlling plant growth in bauxite residue. Possible Fe toxicity needs to be investigated further. Sewage sludge may reduce the toxicity of bauxite residue by lowering pH, precipitating Al and increasing cation availability via complexation.