Cd^2+和Pb^2+对崇明岛东滩湿地土壤脲酶的毒物兴奋效应研究

Hormetic Effects of Cd^(2+)and Pb^(2+)on the Activity of Urease in Soils of Dongtan Wetlands in Chongming Island

以崇明岛东滩湿地土壤为研究对象,分别在土壤中添加Cd^(2+)质量比为0.0~2.0 mg/kg的CdCl_2溶液和Pb^(2+)质量比为0.0~1 000.0 mg/kg的Pb(NO_3)_2溶液,观测土壤脲酶活性随着培养时间(0~120 h)的变化。结果表明,培养48 h时,在Cd^(2+)质量比为0.001~0.01 mg/kg处理下,0~30 cm深度土壤脲酶活性比对照处理高19.9%~55.7%,当Cd^(2+)的质量比大于1.0 mg/kg时,土壤脲酶活性受到抑制;培养6 h时,在Pb^(2+)质量比为0.5~5 mg/kg处理下,土壤脲酶活性比对照处理高19.5%~56.8%,当Pb^(2+)的质量比大于50.0 mg/kg时,0~30 cm深度土壤脲酶活性明显降低。Cd^(2+)和Pb^(2+)含量与土壤脲酶之间存在显著的毒物兴奋效应关系,该关系与培养时间有关;与对照处理相比,具有毒物兴奋效应的处理的酶促反应动力学参数Km(米氏常数)和Vmax(最大反应速率)同时降低,土壤脲酶纯酶活性几乎不受培养时间影响。因此,Cd^(2+)和Pb^(2+)含量与土壤脲酶活性的毒物兴奋效应可能是土壤微生物群体过度补偿效应的表现。

So far little has been reported on low-dose hormetic effects of heavy metals on soil enzymes. Soil samples were taken from Dongtan wetlands in Chongming Island, treated with different doses of Cd^2＋and Pb^2＋, making the samples 0-2 mg/kg in Cd^2＋concentration and 0-1 000 mg/kg in Pb^2＋concentration, separately,and then incubated for 0-120 h. The results indicated that urease activity was 19.9%-55.7% higher than that of the control with the addition of 0.001-0.01 mg/kg Cd^2＋, but decreased when Cd^2＋concentration was 1.0 mg/kg above. In contrast, a significant increase by about 19.5%-56.8% of urease activity was observed at the doses of 0.5-5.0 mg/kg Pb^2＋added in soils. When Pb^2＋exceeded to 50.0 mg/kg, the activity of urease was substantially inhibited. It showed that a hormetic phenomenon exists between heavy metals and soil enzyme, however,the hormetic effect was closely related to the incubation time between soils and the agents. The presence of Cd^2＋and Pb^2＋on the enzymatic kinetic parameters of urease was also tested. Compared with control samples,both Vmax（maximum reaction velocity） and Km（Michaelis constant） fall simultaneously with the treatment of a low-dose hormetic effect. Moreover, the activity of purified urease was not almost affected by the incubation time（6-120 h）. The overall results seem to suggest that the hormetic effects of heavy metals on soil urease was possibly resulted from the integrated overcompensation of microorganisms in soils.