Effects of historic metal(loid) pollution on earthworm communities.

The effects of metal(loid)s (Pb, Cd, Cu, Zn, As and Sb) from atmospheric fallout on earthworm communities were investigated in a fallow meadow located close to a 60-year-old lead recycling factory. We examined abundance and species diversity as well as the ratio of adult-to-juvenile earthworms, along five 140 m parallel transects. The influence of soil pollution on the earthworm community at the plot scale was put in context by measuring some physico-chemical soil characteristics (OM content, N content, pH), as well as total and bioavailable metal(loid) concentrations. Earthworms were absent in the highly polluted area (concentration from 30,000 to 5000 mg Pb·kg(-1) of dried soil), just near the factory (0-30 m area). A clear and almost linear relationship was observed between the proportion of juvenile versus mature earthworms and the pollution gradient, with a greater proportion of adults in the most polluted zones (only adult earthworms were observed from 30 to 50 m). Apporectodea longa was the main species present just near the smelter (80% of the earthworms were A. longa from 30 to 50 m). The earthworm density was found to increase progressively from five individuals·m(-2) at 30 m to 135 individuals·m(-2) at 140 m from the factory. On average, metal(loid) accumulation in earthworm tissues decreased linearly with distance from the factory. The concentration of exchangeable metal(loid)s in earthworm surface casts was higher than that of the overall soil. Finally, our field study clearly demonstrated that metal(loid) pollution has a direct impact on earthworm communities (abundance, diversity and proportion of juveniles) especially when Pb concentrations in soil were higher than 2050 mg·kg(-1).