Effects Of Cadmium, Lead And Mercury Pollution On Soil Microorganisms And Their Activities

This thesis investigates the effects of cadmium, lead- and mercury contamination on soil microorganisms and their activities. A greenhouse pot experiment was carried out to evaluate the changes of soil microbial biomass and community structure during Chinese cabbage (Brassica chinensis) growth in two soils contaminated by cadmium and lead. A laboratory incubation experiment was conducted to elucidate the effects of cadmium and mercury applied singly and in combination of two on the soil microorganisms and their activities. The main objective of this study was to provide some useful information for bioremediation, environmental quality evaluation and especially for soils contaminated by compound heavy metals. The main results of this study were presented as follows:1. A greenhouse pot experiment was conducted to evaluate the impact of different concentrations of cadmium (Cd) and lead (Pb) on soil microbial activities and community, structure during growth of Chinese cabbage (Brassica chinensis) in two different soils, namely, the marine sediment silty loam soil (S1) and the yellowish red soil (S2). The field soils were used for a small pot, short-term 60-day growth chamber study. The soils were amended with different Cd and Pb concentrations, ranging from 0 to 30 mg Cd kg^-1 soil and 0 to 900 mg Pb kg^-1 soil, respectively. The results indicated that the application of Cd and Pb at lower concentrations (<15 mg Cd kg^-1 soil and <500 mg Pb kg^-1 soil, respectively) appeared to result in a slight increase in soil microbial activities, whereas Cd concentrations > 15 mg kg^-1 and Pb concentrations > 500 mg kg^-1 simultaneously caused an immediate gradual significant decline in soil microbial biomass carbon and the C_mic/C_org ratio and increase in the metabolic quotient (qCO₂), indicating that soil microorganisms were affected by heavy metal stress. Moreover, soil microbial biomass carbon and the C_mic/C_org ratio were significantly negatively correlated with the available Cd and Pb contents, while the microbial metabolic quotient (qCO₂) was positively correlated with the available Cd and Pb contents. Therefore, soil microbial biomass the C_mic/C_org ratio and the metabolic quotient (qCO₂) were sensitivebiological indicators to reflect heavy metal pollution. The results also showed that the degree of impact on soil microbial activities by Cd and Pb was related to management (plant vegetation) or the contents of clay and organic matter in soils. Fewer shifts of microbial biomass and two ecophysiological parameters affected by Cd and Pb in planted S2 soil was found. In this study, it was speculated that, with the growth of Chinese cabbage, the critical levels of cadmium and lead in agricultural soils were 15 mg.kg"'and 500 mg.kg'1 respectively.Meanwhile, soil microbial population and functional diversities of microbial community have changed to some extent under the stress of cadmium and lead pollution. The results of the functional diversity of soil microbial community indicated by BIOLOG ECO plantes showed that soil microbial metabolism quotient (AWCD), microbial community richness, Shannon index and envenness all decreased significantly in two soils polluted by Cd and Pb, compared with non-polluted soils. The Cd and Pb pollution caused the decreases in the functional diversity of microbial community and reduced the microbial numbers utilizating different carbon resources, thereby leading to the changes of the soil microbial community structure. The profiles of 19 phospholipid fatty acids (PLFAs) were also used to assess whether observed changes in functional microbial parameters were accompanied by changes in the composition of the microbial communities after Cd and Pb application. The results of principal component analyses (PCA) indicated that the composition of the microbial community was also altered by Cd and Pb levels, with increases in PLFA biomarkers for fungi and actinomycetes, and increases in the ratio of Gram-positive to Gram-negative bacteria. In a word, soil microbial biomass and community structure, therefore, may be sensitive indicators reflecting environmental stress in soil-Cd/Pb-plant system. However, further studies will be needed to better understand how these changes in microbial community structure might actually impact soil microbial community function.2. A laboratory incubation experiment was carried out to elucidate the effects of cadmium (Cd) and mercury (Hg) applied singly and in combination of two on the soil microorganisms and their activities. The activities of urease and acid phosphatase and basal respiration were determined at 0, 14, 28, 42, 56 days after heavy metal application (DAA). The results showed the application of Cd at lower concentration (1 mg Cd kg'1 soil) appeared to enhance soil microbial activities, whereas, higher concentrations of Cd and Hg applied singly and in combination of two had remarkable inhibitory effects on soil microbial activities from the start to 28 DAA. From 28 to 56 DAA, the activities of urease and acid phosphatase were declined gradually and the basal respiration was enhanced with increasing concentrations of heavy metals,accompanying with an order of Hg + Cd > Hg > Gd. Under the same condition, Hg was more toxic to soil microorganisms than Cd. In the case of Hg + Cd, the inhibitory effect was synergism of the two elements. Hg and Cd showed weak synergism in multiple treatments. At the same time, the incubation time could enhance heavy metals' inhibitory effects on microbial activities.On 42 DAA, substrate utilization pattern of soil microbial community was determined by inoculating BIOLOG ECO plantes. The results indicated that Cd and Hg applied singly and in the combination of two inhibited the functional activity of soil microbial communities as indicated by the intensity of average well color development (AWCD) during 168 h of incubation and microbial community richness, Shannon index and envenness, compared with non-polluted soils. Principle component analysis of sole carbon source utilization pattern demonstrated that higher levels of Cd and Hg applied singly and in the combination of two had significantly affected soil microbial community structure. The results of phospholipid fatty acids (PLFAs) analysis also observed changes in the composition of the microbial communities after different treatments of Cd and Hg applied singly and in the combination of two. The results of principal component analyses (PCA) indicated that the structure of the microbial community was also significantly altered by Cd and Hg levels, with increases in PLFA biomarkers for fungi and actinomycetes, and increases in the ratio of Gram-positive to Gram-negative bacteria.