Behavior of Heavy Metals in Soil: Effect of Dissolved Organic Matter

Heavy metal contamination of soils has received much attention with regard to plant uptake, deterioration of soil microbial ecology, and contamination of groundwater or surface waters (Cunningham et al., 1975; Riekerk and Zasoski, 1979). The increased application of pesticides, urban wastes such as municipal refuse and sewage sludge, and animal wastes on farmland or orchards led to heavy metal accumulation in soils. Cu concentrations of as high as 1000 mg/kg in poultry litter and pig manure were not uncommon due to the supplement of Cu in animal feed as a common practice for many years (Van der Watt et al., 1994; Giusquiani et al., 1998). In many orchard soils, especially for the well-aged orchard, the Cu level has exceeded more than 300 mg/kg due to the application of Bordeaux mixture as pesticide for decades (Aoyama, 1998). The application of organic wastes such as animal manure, crop residues, green manure, and forest residues is very common practice to provide nutrients and to improve soil physical properties in many countries. In China, the practice of land application of farmyard manure can be traced back 2000 years, which effectively maintains high soil fertility and productivity. It is generally considered that these materials can immobilize metals by sorption of metal in particulate organic matter, which reduces the metal bioavailability in the contaminated soil. However, the effectiveness of in situ immobilization of metals by organic wastes depends on the origins and properties of the waste types used.