Effects of a native earthworm species (Amynthas morrisi) and Eisenia fetida on metal fractions in a multi-metal polluted soil from South China

Abstract

The effects of the native species Amynthas morrisi and the composting earthworm Eisenia fetida were evaluated on the forms of Zn, Cd, Pb and Cu in a long-term contaminated Chinese soil, with and without added organic matter (OM), in a 60-day laboratory experiment (25 °C field capacity). Four metal fractions were extracted using a sequential procedure: water-soluble exchangeable and carbonate-bound fraction (WAEX), Fe and Mn oxides fraction (FMOX), organic compounds (ORGA), and residual non-extractable fraction (RESI). The sum of available fractions (AVA = WAEX + FMOX + ORGA) and the total contents of the surface casts and non-ingested soil were calculated. In all treatments, all casts of the two earthworm species had higher pH and DOC (Dissolved organic carbon) contents and lower Eh values than those of their respective controls. Amynthas morrisi casts contained higher concentrations of available forms (AVA) of Cd (+84.1%) and Zn (+39.9%) and lower concentrations of available Cu (−10.1%) than those in the control soil (P < 0.05). Organic matter addition limited the variations of pH, Eh and DOC and had the opposite effect to earthworms on metal speciation. However, in the soils of all experimental units and compared with control soil, earthworms increased organically bound Pb (+16.0%) and Cu (+46.1%, A. morrisi; 64.0%, E. fetida), whereas other extractable forms of elements decreased, depending on the metals and their chemical forms (P < 0.05). Amynthas morrisi had similar effects on metal dynamics to the exotic E. fetida and reproduced when organic matter was added; both are therefore good candidates for soil phytoremediation technologies based on increased earthworm activities.