Mobility of Cu and Zn in Soil Amended with Composts at Different Degrees of Maturity

Purpose This work tackles the mobility of heavy metals in soil after compost addition, with particular attention to the effects of compost feedstock and compost maturity. Methods Three types of organic wastes: MSW (municipal solid waste), MSW + GW (municipal solid waste + green waste) and SS + GW (sewage sludge + green waste), were composted and sampled at three different composting times (0, 6 and 12 weeks). These samples were mixed with a cultivated horizon from a silt loam soil in order to pursue column leaching experiments. Columns were built up in three replicates and submitted to leaching with dilute CaCl2 solution, during 1 h, nearly each week during 2 months. Leachates were analysed for organic and inorganic C, pH, Zn and Cu. Results The leaching of Cu and dissolved organic C were closely related, while Zn showed a different behaviour. Compost feedstock was the factor with the highest influence in element leaching, which decreased in the sequence MSW > MSW + GW > SS + GW. Within composts obtained from the same feedstock, maturity also had an influence on element leaching, which was expressed at two levels: (1) during the first days after compost addition to soil, leaching decreases at increasing degrees of compost maturity; (2) cumulated amounts of Cu and organic C leached during the 2-month experiments increased with compost maturity. The limited variations of pH during leaching experiments showed no clear relationship neither with the maturation of compost, nor with the mobility of the studied heavy metals.