INFLUENCE OF HEAVY-METALS ON SOIL MICROBIAL ACTIVITIES

Studies were carried out to evaluate the influence of Cd, Cr, Cu, Ni, Pb, or Zn on the decomposition of organic matter in soils and on their microflora. The heavy metals were added to samples of Gley soil and Light-colored Andosol at the level of 10-mu-mol/g soil, respectively. Three days after the addition, sewage sludge was mixed with the samples at a 2% rate. During the 4 week period of incubation at 28-degrees-C, CO2 evolution was measured periodically. At the end of the incubation period, the soil samples were analyzed to determine the number of bacteria and fungi, and the amount of heavy metals extracted successively with water and other reagents. The toxicity of heavy metals to soil microorganisms was also analyzed. The dilute suspensions of Gley soil and Light-colored Andosol were inoculated into albumin medium and rose bengal medium containing heavy metals of various concentrations, and the influence of the metals on colony formation by the bacteria and fungi was investigated. All the metals inhibited the CO2 evolution from both soils. However, the effect varied considerably among the metals and between the soils. CO2 evolution from the Gley soil was significantly lower than that from the Light-colored Andosol. Among the heavy metals, the effect exerted by Cd, Cu, and Ni was highly significant while that of Pb was the least significant. The toxicity of heavy metals to colony formation by soil bacteria was high for Cd and Cu, which inhibited CO2 evolution substantially. The amount of water-soluble heavy metals in the Gley soil was higher than that in the Light-colored Andosol, and the decrease in the CO2 evolution was associated with the increase in the amount of water-soluble heavy metals divided by the ED90 values of the heavy metals in relation to colony formation by the bacteria. These results indicate that the decrease of the inhibition of CO2 evolution by the heavy metals depended mainly on the degree of toxicity of the metals to soil bacterial growth and the amount of water-soluble heavy metals in soil.