Etude des relations Lumbricidae – éléments traces métalliques en conditions naturelles et contrôlées.

Abstract

The bank soils of Rhumel wadi subject to anthropogenic pressures from human activities. These activities are a source of soil contamination by trace metals (TM). As soil chemical measures are insufficient, the determination of biological parameters can tell us about the state of soils contamination by these elements. In this case, earthworms are the most representative population in the soil. In this context, relations between earthworms and traces were studied under natural and controlled conditions, with the main objective to highlight the bioindication importance of earthworm taxa Aporrectodea trapezoides (Dugès, 1828) in the biomonitoring of metallic contamination of Rhumel wadi bank soils. Initially, study focused on soils characterization and evaluation of the degree of contamination by TM. The results indicate that calcareous fraction is the most important factor contributing to increase pH, stabilization of organic matter and slowing humification. For TM, total contents were 1.14, 62.7, 19.8, 26.4, 31.0 and 97.7 μg.g-1 for Cd, Cr, Cu, Ni, Pb and Zn, respectively. The majority (> 80%) of these elements is of natural origin, while the rest (about 20%) is mobilized fraction. These levels varied depending on the soil parameters, especially the calcareous fraction. Overall, Constantine city would be the main source of metal enrichment of studied soils. Among the six analyzed elements, only Cd and Pb contents exceeded contamination limits. This enrichment is likely due to use phosphate fertilizer and/or urban areas. Secondly, it is proposed to study the chosen biological compartment to explain the different relationships with physicochemical parameters. The results show that earthworm population is represented by two families and 10 taxa, however A. trapezoides represents most abundant species, most dominant, better distributed, and therefore the more representative. It seems to be adapted to the particular conditions of studied soils. Earthworm metal concentrations were 4.7, 4.2, 10.16, 3.15, 34.85 and 210.31 μg.g-1 for Cd, Cr, Cu, Ni, Pb and Zn, respectively. According to the bioaccumulation factor, A. trapezoides accumulate Cd, Pb and Zn, and do not accumulate Cr, Cu and Ni. Zn is the most concentrated because it’s essential role. On the other hand, Cd and Pb accumulation suggest these to be potentially exposed to risks associated with their levels increase in soils. Moreover, earthworms are not content to the mobilizable fraction but accumulate these metals from the total fraction. The last part of this work was devoted to study the impact of the two TM that contaminate studied soils (Cd and Pb) on chosen species, through an ecotoxicological approach where earthworms were exposed to different TM concentrations. The results confirmed the strength of these worms at high metal concentrations and also revealed their avoidance behaviour to the presence of these metals in soils. In conclusion, this study showed that A. trapezoides is a powerful tool for biomonitoring of contaminated soils by TM due to its performance in number and biomass, and its individuals accumulate TM in their body, which supports the idea of a model for this type of study.