Role of soil covers in establishment of vegetation on gold oxide refining residues

Abstract

Revegetation of ore refining residue may be limited by the physical and chemical constraints of the material, necessitating the use of soil covers and ameliorants to improve conditions for plant growth. The aim of this study was to establish the depth of soil cover needed to establish vegetation on saline, sodic, alkaline residue in a mediterranean climate, Western Australia. The effects of three soil covers (10cm topsoil plus nil, 15 or 30cm of gravel-rich subsoil (referred to as gravel), on the residue) compared to no cover, of residue ameliorants (broadcast gypsum or compost), and the influence of shrinkage-cracks on plant density, species richness, leaf stress symptoms and root abundance were examined in a 2-year field experiment. Seedling density on topsoil covers, recruited from the topsoil seedbank and broadcast seeding, was 10-fold higher than that on bare residue. Live canopy cover with topsoil-only over residue was consistently higher than with topsoil over gravel. Plant roots preferentially grew along the faces of the vertical shrinkage-cracks, as well as fine horizontal cracks and through coarser textured residue strata and had greatest abundance and depth in residue covered by topsoil-only. However, seedling density and species richness declined with topsoil-only cover compared to the thicker cover treatments in the second year. Leaf symptoms from salt damage developed in plants established with topsoil-only on residue. Increasing gypsum broadcast on residue from 30 to 60tha−1 increased root abundance and penetration, but had no effect on seedling density. We conclude that the 10cm cover of topsoil-only, while potentially cheaper to establish, and providing greater initial vigour and vegetation cover, had diminished species diversity after 2 years relative to thicker covers. By contrast the thicker cover (10cm topsoil plus 15–30cm gravel) slowed initial growth, but combined with 60tgypsumha−1 broadcast on the residue, resulted in the most diverse plant community and most vigorous root growth on saline gold oxide processing residue.