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 soils via sewage sludges, it is necessary to measure metal speciation and solubility in the soil for many years after application. With this as our objective, we measured total and dissolved trace element concentrations, including potentially toxic heavy metals, in the contaminated 0 to 5-cm surface soil layer of a long-term, no-till continuous bromegrass experiment about 18 years after the last application of three chemically different sewage sludges. For each particular sludge, long-term heavy metal solubility was generally linearly correlated to the remaining heavy metal concentration in the soil, with the nature of the sludges applied affecting the partition coefficient (KD) for some elements and heavy metals. Most of the dissolved Cu and Pb (generally >75%) was not labile by anodic stripping voltammetry (ASV), indicating a high degree of complexation of these two metals by soluble organic 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 measured in the time interval between 1979 and 1997. The KD values for Cu, Zn and Cd were all close to 10 for the sludge-treated soils with the highest organic matter content, indicating a strong metal retention that may be 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 suggests that Cd may be associated with organosulfur ligands. Heavy metal ion activities in the high-organic matter surface layer at this site were low compared with activities measured at other sites with comparable total metal loadings, suggesting the importance of the role of continuous grass and notill management in maintaining high organic matter and consequent low metal activity in high-lime soils.