The organic carbon derived from sewage sludge as a key parameter determining the fate of trace metals

In a sandy agricultural soil of south-west of France, continuously cultivated with maize and amended with sewage-sludge over 20 years, the behavior of three trace metals (Cu, Pb, and Zn) was studied during the sludge applications (1974-1993) and after its cessation (1993-1998). Using the delta13C analysis, the dynamics of different sources of organic matter were followed in order to elucidate the influence of the sludge-derived organic matter on the fate of trace metals in the soil and its particle size fractions. This study revealed that sludge-derived organic matter contributed to the formation of macroaggregates through the binding of preexisting microaggregates. These macroagreggates were thus responsible for the accumulation of trace metals in the coarsest fraction as well as for the protection of maize-derived organic matter against biodegradation. After sludge application ceased, the disaggregation of macroaggregates occurred simultaneously with high losses in Cu and Pb. On the contrary, Zn appeared less affected by the cessation of sludge application, with only a location change from coarse to fine fractions.