Diagnostic de contamination des agrosystèmes périurbains de Dakar par les éléments traces métalliques

Abstract

Description of the subject. Urban and periurban market gardening is widespread in the Dakar area of Senegal. Repeated long-term application of organic matter results in high inputs of trace elements (TE). Objectives. The aim is to assess TE contamination in suburban agrosystems. Method. At four sites, we compared the Cu, Zn, Pb and Cd content in the surface layers and sublayers of plots cultivated over a long period with the content found in adjacent uncultivated plots. Results. The two prevailing soil types (Arenosol and Fluvisol) were characterized by significantly different background soil geochemical TE content. In the 0-20 and 20-40 cm soil layers at the four studied sites, TE content was significantly higher (p < 0.05) in the cultivated plots than in the non-cultivated plots. Arenosols showed a significantly higher (p < 0.05) TE content than Fluvisols. An exception to these results was found in cultivated Arenosols, where Cd, Cu, Zn and Pb contamination was observed. This could be explained by the frequent input into these soils of sewage sludge, poultry droppings, and irrigation with only partially treated wastewater. Cadmium contamination of cultivated Fluvisols was the result of regular natural phosphate inputs, as observed throughout the soil profile. Conclusions. Analysis of soils and organic matters would enable adjustment of the quantities of organic matter applied and could lead to a reduction in TE contamination within these agroecosystems.