Temporal Trends of Metal Extractability in Calcareous soils Affected by Soil Constituents and Metal Contamination Levels

The role played by soil constituents in governing temporal trends of metal mobility in calcareous Mediterranean soils deserves special attention due to the particular soil and climate characteristics. Reactions occurring during the aging modify the metal mobility over time, and the rate of metal application and type of soil can be decisive in the outcome of aging reactions in soils. The aim of the present work was to investigate the role that both metal dose and soil constituents play in temporal trends of (potential) metal mobility in soils offering a natural gradient of carbonate and whose remaining soil constituents differed. Soil samples were spiked with a mixture of Cd, Cu, Pb and Zn at two levels and then left aging for 12 months incubation. Metals were extracted at different time intervals (1 day, 1, 3, 6 and 12 months) with one-step extraction methods to estimate immediate metal mobility (NaNO3) and potential metal mobility (diethylene triamine pentaacetic acid –DTPA–). At both levels of contamination, NaNO3-extractable Cd, Cu and Zn concentration values reached equilibrium within the period of incubation. Temporal trend of immediate metal mobility was governed by carbonate fraction for Cd and Cu and by the finest carbonate fraction for Zn. In the case of potential metal mobility, DPTA-extractable metal concentrations did not reach equilibrium within the incubation time. In this case, the combined action of carbonate, organic, Fe-oxide and clay fractions were decisive to define the different temporal trends observed for each metal and level.