Evolution of heavy metals in marsh areas under rice farming

Total and EDTA-extractable concentrations of Cd, Co, Cu, Ni, Pb, and Zn in rice-farming soils and their distribution in the profile depth were evaluat ed. The temporal variability of metal concentrations in the superficial hor izon was studied during 1991-1995. Results show that the highest metal conc entrations correspond to Zn, Pb, and Cu, for total and EDTA-extractable fra ction. In general, heavy metals showed an accumulation in soil surface, exc ept for Co and Ni that have their highest values in the deeper horizons. pH and organic matter were the key parameters that regulate the distribution of metals in these soils. All the studied metals were significantly correla ted with available P, N, and sand textural fraction. The study of the tempo ral evolution of these metals shows that the most significant variations co rrespond to Cd, Cu, Ni, and Zn. Although Cd shows an increase, Cu and Zn te nd to decrease in these soils. (C) 1999 Elsevier Science Ltd. All rights re served.