Soil properties and earthworm populations associated with bauxite residue rehabilitation strategies

Abstract

It is recognised that the establishment and function of soil biota is critical for successful mine residue rehabilitation. Bauxite residues are alkaline, saline and sodic and, whilst methods for establishing vegetation are well studied, little is known about key soil fauna such as earthworms. At a bauxite residue disposal area in Ireland, a 12-year-old rehabilitated residue was examined for evidence of earthworm populations. Five species of earthworm, dominated by Allolobophora chlorotica, were recorded in the rehabilitated residue representing the endogeic, epigeic and epi-anecic ecological groups. To further understand the potential for rehabilitated residues to support earthworm communities, a series of exposure tests was conducted. Whilst unamended residues (pH 10.2, EC 0.629 mS cm−1, ESP 54) was hostile to A. chlorotica survival, 100% survival was observed after 90 days for gypsum and organic-amended residue at salinity of up to 2.9 mS cm−1, possibly due to calcium becoming the dominant cation. Survival of earthworms at salinities higher than anticipated tolerance levels suggests that specific ion dominance plays a role in earthworm survival in saline soils. Percent mass change was negatively correlated with pH, EC and sodium content of the residues. Residue from the 12-year-old site also supported the anecic species Aporrectodea longa over 100 days. Percent mass change in residue samples retrieved from the 12-year-old site was significantly greater (p < 0.05) to that observed for a control soil. Capability of the rehabilitated residue to support earthworm populations indicates the development of a functioning soil system in rehabilitated residues.