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 evaluated. The temporal variability of metal concentrations in the superficial horizon was studied during 1991–1995. Results show that the highest metal concentrations correspond to Zn, Pb, and Cu, for total and EDTA-extractable fraction. In general, heavy metals showed an accumulation in soil surface, except 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 correlated with available P, N, and sand textural fraction. The study of the temporal evolution of these metals shows that the most significant variations correspond to Cd, Cu, Ni, and Zn. Although Cd shows an increase, Cu and Zn tend to decrease in these soils.