Differences in the Influence of Cadmium on the Decomposition of Various Types of Organic Materials in Soil

The influence of Cd on the decomposition of various types of organic materials in soil was studied. CdCl₂ or CaCl₂ (control) was added to a Gley soil at a level of 10 mmol kg^-1 soil. Three days later, organic materials including glutamic acid, glucose, casein, starch, cellulose, lignin, rice straw, rice straw compost, or 3 kinds of sludges were mixed with the soil in a proportion of 1%, respectively. During an 8-week period of incubation at 28°C, CO₂ evolution was measured periodically. At the end of the incubation period, the form of Cd in the soil was analyzed by successive extractions with water, CaCl₂, CH₃COOH, Na₄P₂O₇, and with hot HCl after HNO₃-HClO₄ digestion.

The decomposition of all the organic materials was inhibited by the addition of Cd, but the degree of inhibition varied considerably among the types of organic materials. The decomposition of rice straw, rice straw compost, and sludges was markedly inhibited by Cd. The amount of water-soluble Cd was less in the soils treated with rice straw, rice straw compost, and sludges than in the soils treated with other types of organic materials, while the amounts of CaCI₂-extractable Cd were much larger in the latter soils. In the case of rice straw, rice straw compost, and sludges Cd was easily adsorbed from the CdCl₂ solution.

These results suggest that the inhibition of organic matter decomposition by Cd is caused by the adsorption of Cd onto organic matter.