Fate and behaviour of Cu and Zn from pig slurry spreading in a tropical water-soil-plant system

Pig slurry is commonly spread on crop fields as a means of managing this agricultural waste product. However, this practice has an impact on the environment, e.g. increasing soil copper (Cu) and zinc (Zn) concentrations. Many studies have assessed the fate of these elements, but some questions remain, especially with respect to tropical agrosystems which have yet to be studied in depth. The aim of this study was to determine the fate of Cu and Zn from pig slurry spreading while also focusing on describing the dynamics of these elements in a tropical system and accounting for the three compartments of the water-soil-plant system. We observed that all of the Zn accumulated within the 20-60 cm soil layer. Although the uncertainty calculated for these results was high, these findings were confirmed by the absence of uptake by the plant cover and of leaching via water flows. This pattern for Zn in a tropical setting differed from findings generally reported in temperate areas. The Zn accumulation mechanism in tropical soil seems to be a reversible sorption phenomenon, suggesting the possibility of long-term Zn leaching. The Cu mass derived from pig slurry spreading was stored in the 0-20 cm soil layer. This result obtained in a tropical environment was similar to that noted in temperate areas. This could be explained by Cu speciation in the pig slurry (insoluble copper sulfide), and was therefore relatively independent of the soil-climate system. (Résumé d'auteur