Plant-available manganese in bauxite residue sand amended with compost and residue mud

Abstract

Manganese (Mn) deficiency has been a constraint for revegetation on bauxite residue sand and there is still no effective strategy to remedy this problem. The effect of addition of organic amendments (piggery waste, biosolids, and commercial compost) and mineral amendments (unamended, seawater-neutralised residue mud, and carbonated bauxite residue mud) on Mn forms and availability in residue sand was studied. Incubation of residue sand with organic amendments (applied at rates of 0, 10, and 50 t/ha) over a 30-day period found little change in DTPA-extractable Mn concentrations, which remained below the critical level of 1 mg/kg. The DTPA-extractable Mn concentrations were comparable to those in the exchangeable fraction (DTPA-Mn = 0.931 × Exch-Mn + 0.358, r2 = 0.84) and, therefore, may provide an estimation of plant-available Mn. The highest Mn concentrations were consistently associated with the carbonate fraction, suggesting that Mn was either retained by surface adsorption reactions and/or co-precipitated with calcium carbonate. The addition of residue mud amendments generally reduced DTPA-extractable Mn, probably through adsorption by hydrous Fe and Al oxides. Leaching did not cause significant (P > 0.05) movement of Mn in residue sand columns, possibly due to the alkaline pH and specific adsorption reactions. Given the difficulty of increasing plant-available Mn by organic amendments, residue mud additions, leaching, and/or fertilisers, overcoming Mn deficiency in vegetation on bauxite residue sand may depend on using Mn-efficient species that are able to efficiently extract Mn associated with carbonate and Fe/Al oxyhydroxide fractions.