Trace metal solubility and speciation in a calcareous soil 18 years after no-till sludge application

To understand the long-term fate of heavy metals applied to agricultural so ils via sewage sludges, it is necessary to measure metal speciation and sol ubility in the soil for many years after application, With this as our obje ctive, we measured total and dissolved trace element concentrations includi ng potentially toxic heavy metals, in the contaminated 0 to 5-cm surface so il layer of a long-term, no-till continuous bromegrass experiment about 18 years after the last application of three chemically different sewage sludg es. For each particular sludge, long-term heavy metal solubility was genera lly linearly correlated to the remaining heavy metal concentration in the s oil, with the nature of the sludges applied affecting the partition coeffic ient (K-D) for some elements and heavy metal. Most of the dissolved Cu and Pb (generally >75%) was not labile by anodic stripping voltammetry (ASV), i ndicating a high degree of complexation of these two metals by soluble orga nic matter. A smaller degree of complexation (<50%) of dissolved Cd and Zn in nonlabile organic complexes was measured. Some reduction of both organic matter content and heavy metal concentrations in the 0 to 5-cm layer was m easured in the time interval between 1979 and 1997. The K-D values for Cu, Zn and Cd were all close to 10(4) for the sludge-treated soils with the hig hest organic matter content, indicating a strong metal retention that may b e attributable to binding to organic matter, but free calcium carbonate in the soil was probably important in limiting metal lability and solubility. A strong linear correlation between total Cd and sulfur in these soils sugg ests that Cd may be associated with organosulfur ligands. Heavy metal ion a ctivities in the high-organic matter surface layer at this site were low co mpared with activities measured at other sites with comparable total metal loadings, suggesting the importance of the role of continuous grass and no- till management in maintaining high organic matter and consequent low metal activity in high-lime soils.