Effects Of Cadmium Mercury On Soil Enzyme Activities In Soil And Soil-Greengrocery Systems

Based on the Laboratory incubation and greenhouse pot experiments,this research investigated the effects of cadmium and mercury on soil urease,acid phosphatase and dehydrogenase activities in soil and soil-greengrocery systems in two soil,which were colletcted from the Changjiang Delta-area,namely marine sediment silty loam and yellowish red soil.The main objective was to provide some useful information for the environmental risk evaluation and the foundation for warning indictors of heavy metal contamination.The main results were presented as follows:1.Under the laboratory incubation experiment condition,this study investigated the single and combined impacts of Cd and Hg on soil urease,acid phosphatase and dehydrogenase activities, which was continued from 0 day to 56d.The results showed:1)There was a significant inhibitory influence of single and combined contamination of Hg,Cd on soil urease,acid phosphatase and dehydrogenase activities in both marine sediment silty loam and yellowish red soil.And the order is Hg+Cd＞Hg＞Cd at the same content of heavy metal.2)There was a slight synergism effect of Cd and Hg in two soils(namely,the net changes of soil urease,acid phosphatase and dehydrogenase activities were more than 0)during the whole incubation time,and the net changes of soil acid phosphatase and dehydrogenase activities were over urease.Based on these results,it indicated that the combined of Cd,Hg could increase the inhibition effect,and soil acid phosphatase and dehydrogenase activities were the more sensitive indicators reflecting environmental stress of heavy metal.3)The activities of the three enzymes were almost increasing at low concentration of Cd during the whole incubation time,which shows that low lever of Cd had a stimulating effect on the three soil enzyme.4)Compared to the marine sediment silty loam which contains lower organic matter and clay particle,yellowish red soil showed higher enzyme activities in all treatments during the whole incubation time,Which demonstrated that higher content of organic matter and clay particle could mitigate the inhibition effects of heavy metal on urease and acid phosphatase.2.A greenhouse pot experiment was conducted to investigate the single and combined impacts of Cd and Hg on soil urease,acid phosphatase in soil-greengrocery system when 0d,15d and 45d of Brassica Chinensis growth,and also researched the heavy metal chemical forms at 0d and 45d.The results showed: 1)Consistent with that of in laboratory incubation experiment,the single and combined contamination of Hg,Cd had an significant inhibitory influence on soil urease,acid phosphatase activities in both marine sediment silty loam and yellowish red soil in soil-greengrocery system. Under the same concentration,the inhibition effect on soil enzyme activities had an order:Hg+Cd＞Hg＞Cd.There was a slight synergism effect of Cd and Hg in two soils(namely,the net changes of soil urease,acid phosphatase activities were more than 0)during the whole growth time of greengrocery,which indicated that the combined of Cd,Hg could increase the inhibition effect on soil enzyme.2)During the whole growth time,enzyme activities in yellowish red soil were higher than that of in marine sediment silty loam in all treatments of Cd,Hg,which showed that higher content of organic matter and clay particle could mitigate the inhibition effects of heavy metal on urease and acid phosphatase.According to the vary relationship between enzyme activity and incubation time,it presented increasing trends in soil urease and phosphatase activities with longer growth time,which was contrary to the results of laboratory incubation.3)It was proved that the validity of Cd and Hg in soil was lower because of low proportion of extractable shape which were mainly presented in residual form in all treatments of two soils.There was approximately similar tendency in both marine sediment silty loam and yellowish red soil under the same Cd treatment from the 0 d to 45d,that is exchangealbe and carbonate forms decreased all the time,organic form was decreased with growth time,Fe-Mn oxide and residual forms were increased;under the same Hg treatment,exchangealbe and carbonate forms decreased a litte with growth time,organic form almost kept invariant,whereas,Fe-Mn oxide and residual forms were increased a bit.4)Under the same concentration,content of exchangeable form in combined contamination was higher than that in single contamination when Brassica Chinensis grower od in the two soils,which further proved that composite pollution exhibits a stronger inhibition effect than single pollution because of its higher content of exchangeable form.There was significant differences of proportion between Fe-Mn oxide form and organic form Cd and Hg,content of which in Yellowish red soil were higher than that in marine sediment silty loam.5)The firing relationship between different forms of Hg and soil urease,phosphatase activities in the two soils took on non-significant difference,but there were amostly significant difference between different forms of Cd and soil urease,phosphatase activities,especially,soil urease, phosphatase activities bulk had a negative correlation with the contents of exchangealbe Cd,and a positive correlation with the contents of residual Cd,which further proved that soil enzyme activities decreased with the increasing of the content of exchangeable heavy metal.