| Summary: | 碩士 === 國立交通大學 === 環境工程所 === 88 === Microorganisms are vital for soil fertility and for the degradation of organic matter and pollutants in soils. Due to their function microorganisms can acts a very suit index of pollution in soil environment. In this study, the toxic effect of Hg, Cd and Cu were evaluated follow the respiration, nitrification activity, dehydrogenase activity, ATP content and biomass C of soil microflora. Although the EC50s of 14-day exposure experiment was roughly lower than that of 3-day exposure, there was no significant different between the two experiment stage while the 95% confidence interval of EC50s from each stage overlap obviously. 3-day experiment seems to be enough to provide the toxic effect of heavy metal in soil. Five parameters were compared on the basis of EC50s, it was clearly nitrification and dehydrogenase activity are more sensitive ones to heavy metal than biomass C. However, the low toxic effect, say EC10, shows different result from EC50. Microbial biomass C with smooth concentration-response curve which is probably due to the contribution of diverse microorganisms can be more sensitive than nitrification ( or dehydrogenase ) activity on the basis of EC10. Impressionable species included in these diverse microorganisms is the cause of the lower EC10 of microbial biomass C. Also, The same eventuality comes on basal respiration.
Extrapolative DC5 ( Dangerous Concentration 5% ) to heavy metal can be caculated based on EC10s of five soil functions. The concentrations 0.033, 3.125 and 3.132 mg/kg soil of Hg, Cd and Cu respectively are considered hazardous for 5 % of soil functions. In the other words, above these concentrations, called DC5s, 95 % of soil functions is relatively safe while 5 % of soil functions may not work properly or even worse. As comparing our results with the fourth level concentrations of EPA standard, the standard seems incomplete to protect normal functions of soil practicability, especially for Hg and Cu. More functional concepts to be incorporated into the standard of soil quality is indispensable in future works.
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