Heavy metal accumulation by two earthworm species and its relationship to total and DTPA-extractable metals in soils

Abstract

The concentrations of Zn, Cd, Pb and Cu in earthworm tissues were compared with the total and DTPA-extractable contents of these heavy metals in contaminated soils. Samples were taken from a pasture polluted by waste from a metallurgic industry over 70 y ago. Three individuals of Aporrectodea caliginosa and Lumbricus rubellus and soil samples were collected at six points along a gradient of increasing pollution. Total metal contents of earthworms, soil, and metals extracted by DTPA from the soil were measured. Total heavy metal contents of the soils ranged from 165.7 to 1231.7 mg Zn kg −1, 2.7 to 5.2 mg Cd kg −1, 45.8 to 465.5 mg Pb kg −1 and 30.0 to 107.5 mg Cu kg −1. Their correlations with metals extracted by DTPA were highly significant. Contents of the metals in earthworm tissues were higher in A. caliginosa than in L. rubellus, with values ranging from 556 to 3381 mg Zn kg −1, 11.6 to 102.9 mg Cd kg −1, 1.9 to 182.8 mg Pb kg −1 and 17.9 to 35.9 mg Cu kg −1 in A. caliginosa, and from 667.9 to 2645 mg Zn kg −1, 7.7 to 26.3 mg Cd kg −1, 0.5 to 37.9 mg Pb kg −1 and 16.0 to 37.6 mg Cu kg −1 in L. rubellus, respectively. Correlations between body loads in earthworms with either total or DTPA-extractable contents of soil metals were significant, except for Cd in L. rubellus and Cu in A. caliginosa. Considering its simple analytical procedure, DTPA-extractable fraction may be preferable to total metal content as a predictor of bio-concentrations of heavy metals in earthworms. Biota-to-Soil Accumulation Factor (BSAF) of these four metals are Cd>Zn>Cu>Pb, with range of mean values between: Cd (6.18–17.02), Zn (1.95–7.91), Cu (0.27–0.89) and Pb (0.08–0.38) in A. caliginosa, and Cd (3.64–6.34), Zn (1.5–6.35), Cu (0.29–0.87) and Pb (0.04–0.13) in L. rubellus. The BSAF of Ca, Fe and Mn are Ca>Mn>Fe, with mean values of: Ca (0.46–1.31), Mn (0.041–0.111), Fe (0.017–0.07) in A. caliginosa and Ca (0.98–2.13), Mn (0.14–0.23), Fe (0.019–0.048) in L. rubellus, respectively. Results of principal component analysis showed that the two earthworm species differ in the pattern of metal bioaccumulation which is related to their ecological roles in contaminated soils.