[Influence of the earthworm Lumbricus terrestris on soil solution complexation capacity]

Four soil samples highly contaminated with metals of urban and mine origin (SE1, SE2, SM1, SM2) and having different physico-chemical proprieties were selected to study copper complexation capacity (LT) of soil solution. The effect of Lumbricus terrestris on copper complexation capacity of soil solution was investigated on SE1 and SE2. The complexation capacity was estimated by amperometric titration of soil solution by copper. Free hydrated cation and labile complexes of copper were determined by DPASV. The results show that the copper complexation capacity variation depends on the physico-chemical characteristics of soils, particularly pH. Thus, the values of copper complexation capacity are 0; 0.6 x 10(-7); 1.8 x 10(-7) and 5.5 x 10(-7) mol l-1 respectively for SM2; SM1; SE1 and SE2 which are pH 5; 5.4; 6.5 and 7.4. Based on these results, the bioavailability levels of heavy metals show the following pool ranking: SM2 > SM1 > SE1 > SE2. The copper complexation capacity of soil solution increases with the soil disturbance by Lumbricus terrestris. This is more obvious when the time of disturbance by lumbrics is longer. Indeed, average values determined for 1 month and 3 months are 3.8 x 10(-7) and 7.8 x 10(-7) mol l-1 for SE1; 7.7 x 10(-7) and 15.2 x 10(-7) mol l-1 for SE2 respectively. It seems that the action of earthworm on soil can contribute to the decrease of bioavailability of heavy metals, particularly copper.