Impact of earthworms on trace element solubility in contaminated mine soils amended with green waste compost

Abstract

The common practice of remediating metal contaminated mine soils with compost can reduce metal mobility and promote revegetation, but the effect of introduced or colonising earthworms on metal solubility is largely unknown. We amended soils from an As/Cu (1150 mgAs kg⁻¹ and 362 mgCu kg⁻¹) and Pb/Zn mine (4550 mgPb kg⁻¹ and 908 mgZn kg⁻¹) with 0, 5, 10, 15 and 20% compost and then introduced Lumbricus terrestris. Porewater was sampled and soil extracted with water to determine trace element solubility, pH and soluble organic carbon. Compost reduced Cu, Pb and Zn, but increased As solubility. Earthworms decreased water soluble Cu and As but increased Pb and Zn in porewater. The effect of the earthworms decreased with increasing compost amendment. The impact of the compost and the earthworms on metal solubility is explained by their effect on pH and soluble organic carbon and the environmental chemistry of each element.

Introduction

The combination of large areas of metal or metalloid contaminated soils associated with former mining and smelting activities and the generation of green waste from domestic, agricultural and silvicultural management of vegetation has resulted in the practice of remediating mine contaminated soils with green waste composts and other organic wastes (van Herwijnen et al., 2007b, Pichtel and Bradway, 2008, Farrell et al., 2010). Composts generally increase plant growth which can prevent wind and water erosion of contaminated soils (Tordoff et al., 2000). In addition, cationic metals bind to exchange sites on the surface of organic matter which reduces metal leaching from soils (Soler-Rovira et al., 2010).

Earthworms represent a significant proportion of the soil fauna and are considered ecosystem engineers owing to the role that they play in organic matter degradation, nutrient cycling and hydrology (Jones et al., 1994). For these reasons they have been the subject of innoculation programes during the reclamation of degraded soils (Butt, 1999). Earthworm inoculation therefore has the potential to become a commonly used practice during remediation and revegetation of metal contaminated mine soils. Earthworms are also able to colonise contaminated land if climatic and material (organic matter, texture, pH, contaminant) conditions are suitable (Eijsackers, 2010) and so, when organic amendments are incorporated into contaminated soils, it is likely that earthworms will colonise leading to changes in the chemical, biological and physical properties of the soil.

We reviewed the impact of earthworms on the mobility and availability of metals and found that in the majority of studies earthworms increase the mobility of metals (Sizmur and Hodson, 2009). Recent experiments have identified that this may be due to the impact of earthworms on the degradation of organic matter and subsequent release of organically bound elements and dissolved organic acids that lower the soil pH and lead to further mobilisation of potentially toxic elements (Gomez-Eyles et al., 2011, Sizmur et al., 2011b). In contrast, Beesley and Dickinson (2011) showed in an experiment with an urban soil contaminated with As, Cd, Cu, Pb and Zn, that Lumbricus terrestris reduced dissolved organic carbon in porewater and thereby reduced the solubility of As, Cu and Pb in a compost amended soil.

Different trace elements bind with organic compounds to varying degrees and behave differently to changes in soil pH. Therefore, the impact of compost or earthworm additions on the solubility of trace elements depends not only on the changes in soluble organic carbon and pH, but also on the chemistry of the element in question. Copper and Pb both bind strongly with organic carbon and therefore their solubility is much affected by changes in soluble organic compounds (McBride et al., 1997). Zinc, however, does not bind so strongly with organic carbon and so its solubility is relatively more affected by changes in pH (McBride, 1994). The solublity of Cu, Pb and Zn is increased with decreasing pH because these elements are cationic (McBride et al., 1997), but As solubility is decreased with decreasing pH because As forms an oxy-anion in solution and binds to positively charged soil surfaces such as iron oxyhydroxides (Masscheleyn et al., 1991).

Lukkari et al. (2006) gave evidence that earthworms increase the extractability of Cu and Zn in their faeces, but decrease the overall extractability of metals in the bulk earthworm inhabited soil. This indicates that there are probably at least two separate conflicting mechanisms by which earthworms impact metal mobility. Earthworms burrow and create casts that have elevated concentrations of extractable trace elements (Sizmur et al., 2011a). In addition, they also release mucus into the soil solution which may decrease the solubility of metals (Sizmur et al., 2010). Mucus is produced in greater quantities during copulation (Edwards and Bohlen, 1996) and so this effect would be observed to a greater extent in experiments where two or more earthworms are incubated in each test vessel.

In the current study we used anecic L. terrestris to determine the impact of earthworms on the remediation of contaminated soils with green waste compost. Treatments of either one or two earthworms, with a constant earthworm:soil mass ratio, were applied to determine the impact of earthworm interactions on the solubility, extractability and speciation of As, Cu, Pb and Zn in two highly contaminated mine soils.