Résumé:
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.
