INFLUENCE OF CATTLE-MANURE SLURRY APPLICATION ON THE SOLUBILITY OF CADMIUM, COPPER, AND ZINC IN A MANURED ACIDIC, LOAMY-SAND SOIL

The concentrations of Cd, Cu, and Zn were evaluated bimonthly in the soil solution of five acidic loamy-sand soil plots during an 18-mo period, before and after application of 25 Mg ha-1 cattle-manure slurry on top of the plots. pH, Cd, Zn, Cu, and Fe concentrations, electrical conductivity (EC), and dissolved organic carbon (DOC) were monitored in the soil-solution samples extracted from soil down to 80-cm depth. Dissolved metal concentrations ranged from 9 x 10(-7) to 9 x 10(-5) mmol m-3 for Cd, from 0.14 to 33 mmol m-3 for Zn, from 0.005 to 5 mmol m-3 for Cu, and from 0.54 to 4.6 x 10(3) mmol m-3 for Fe. Maximum soil solution concentrations of Cd (9 x 10(-5) mmol m-3) and Zn (33 mmol m-3) were found between 0- and 40-cm depth 6 wk after cattle-manure slurry application; Cu and Fe concentrations were not elevated. Except for the peak concentrations of Cd and Zn following the cattle-manure slurry application, no statistically significant temporal trends in heavy metal concentrations were observed. Multiple linear regression analysis showed a positive correlation between the metal concentrations with DOC and a negative correlation with pH. High EC levels in topsoil layers, resulting from manuring were also found to increase metal concentrations. In the topsoil the Cd and Zn concentrations were affected by soil solution pH, and in the subsoil by DOC variations. The Cu concentration was affected by DOC variations in all soil layers. Supercentrifugation of selected subsoil samples from one sampling date removed 80% of the Cd, Zn, and Cu from the solution along with DOC and Fe. Cation-exchange column experiments as well as anodic stripping voltammetry measurements (speciation) of selected soil solution samples, showed that 30 to 70% of the dissolved Cu and all Cd was bound in relatively fast dissociating metal complexes. The observed variations in soil solution heavy metal concentration and speciation show that a distribution coefficient that describes the partition of metals over soil solution and solid phase is not a constant but depends on the physicochemical situation.