重金屬對土壤中微生物之危害性研究

Abstract

微生物在土壤中扮演分解者角色，反應出有機物或污染物質的降解作用，且其活性可為土壤肥沃性的參考，因此這些處於生態系基層的微生物可作為良好的污染指標。本研究以土壤中微生物的呼吸作用、硝化活性、脫氫□活性、ATP含量與生質碳量為觀測參數，探討重金屬Hg、Cd、Cu存在時對土壤微生物之危害；實驗分別進行三天與十四天，雖然經十四天暴露後部份實驗之EC50有降低，但幅度不大，且兩階段實驗中，各個EC50的95%信賴區間皆有相當大的重疊，在統計上並非有明顯的差異，因此三天的實驗已能描繪出微生物受重金屬毒物影響之輪廓，暴露時間的增長對敏感性似乎幫助不大；在五種參數的EC50比較下，硝化活性與脫氫□活性對各重金屬毒物顯得較為敏感，而生質碳量受影響程度較小，然而在微量毒性效應EC10下之敏感性卻並非全如前所述，與其他參數相較之下，生質碳量之濃度-反應曲線之斜率低，呈現平緩趨勢，顯示對此參數貢獻之微生物較具有生物多樣性，其中包含相當敏感的物種，在非常低的濃度下即造成傷害，使得EC10有時反而較其他參數來的低，相同的情況在斜率較低的呼吸作用也可發現。 將五種微生物參數視為土壤的五種功能，以此為基礎，模擬重金屬Hg、Cd、Cu對所有土壤功能造成5%危害的濃度DC5 (Dangerous Concentration 5%)分別為0.033、3.125與3.132 mg/kg soil，換句話說，在此濃度下可保證至少有95%的土壤功能受到保護，因此亦可視為一安全值，與民國八十九年所公佈之土壤含量等級區分規範比較，本研究土樣對Hg與Cu之DC5皆遠小於第四級觀察值，顯示觀察值所規範之濃度尚無法對土壤功能的完整性提供良好的保護，因此建議法規標準之建立，應可考慮將土壤中微生物受影響的程度納入參考，以期能達到最全面性的保障。

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.