Effects Of Uranium And Associated Heavy Metals On Soil Enzyme Activities

This study was supported by the national key governance project of nuclear decommissioning facilities and radioactive waste(14ZG6101). The relationship between soil enzyme activities and uranium and associated heavy metals was investigated in the view of ecological and biological effects of uranium and associated heavy metal pollution. The research objects included the typical tailings uranium soil and potting soil which was added exogenous heavy metal. This paper is aimed at providing a scientific basis for soil environmental quality monitoring, assessing and early warning of similar contamination. The main conclusions can be drawn as follows:(1) The environment in the uranium tailings was polluted seriously by uranium and associated heavy metal and the pollution index PN is far greater than 3. Among this, the pollution of 226 Ra, U, Cd and Pb is serious, while Th, Mn and Cu pollution is relatively light. The assessment results of multifactorial potential ecological risk index(RI) revealed that uranium and associated heavy metals could cause serious ecological hazard, and the order of pollution degree is 226Ra> Cd> U>Pb> Cu, Th, Mn. U-Th. Besides, the interaction of U-Th, Mn-Cd and Mn-Cu is significant and the correlation coefficient is over 0.5. The pollution had a significant influence on the fertility and the physical and chemical properties of the soil. Among them, the total nitrogen, cation exchange capacity decreased rapidly, and its largest decline was 47%, 80.8% and 87.2%, respectively.(2) The amount of bacteria, fungi and actinomycetes in uranium tailings environmental soil significantly reduced. The largest decline amount of bacteria, fungi and actinomycetes reached 85.1%, 90.9% and 83.8%, respectively, and the total number of microorganisms decreased from 43.8% to 83.5%. The order of the total number of these microorganisms is bacteria microbes> fungi> actinomycetes. The number of microorganisms correlated negatively with U and 226 Ra, while positively correlated with Th. Which can be used as the biological indicators of uranium tailings pollution.The activity of urease, acid phosphatase and invertase in the soil significantly reduced, the largest decline amountis 95.9%, 81.8% and 79.4%, respectively. While the activity of peroxidase increased sharply, with the largest increase rate at 55.1% in uranium tailings environment. U and 226 Ra have similar effect on four kinds of soil enzymes while Th plays an opposite role. Urease is most sensitive to uranium contamination, followed by phosphatase and peroxidase, and the three soil enzymes activity could be used as the bio-indicators of uranium tailings environmental pollution.In addition to the peroxidase, the number of soil microorganisms and activity of soil enzyme in summer season is higher than that in winter and spring and the number of soil microorganisms and activity of soil enzyme decreased as the soil depth increased. The number of microorganisms is the largest when the p H value is from 4 to 5. The activity of urease and peroxidase is generally enhanced with the increase of p H, while the activity of phosphatase and sucrose is reduced. Organic matter can mitigate the effects of uranium and associated heavy metals on soil microbial quantity and enzyme activity.(3)The single pollution of uranium and associated heavy metal has different effect on soil enzyme activities. The urease, phosphatase, dehydrogenase and peroxidase are sensitive to uranium and associated heavy metals, and the activities of urease, phosphatase, dehydrogenase are mostly inhibited by these metals, while the activities of peroxidase was promoted by them. In the combined pullution of uranium and associated heavy metal, Mn, Pb and U + Cd interaction significantly inhibited urease activity, U and associated heavy metals have significant inhibitory effect on the phosphatase activity, U impact invertase activity significantly, U, Mn and interaction significantly promoted peroxidase activity and Mn and Pb + Cd interaction significantl y inhibited dehydrogenase activity. Phosphatase, peroxidase and dehydrogenase are sensitive to the compound pollution of U, Cd, Mn, Pb, while the urease is less sensitive and the invertase is not sensitive to it at all.(4) Urease, phosphatase, peroxidase and dehydrogenase are the sensitive enzymes to the contamination of uranium and associated heavy metal. The results of main component analysis showed that the total distribution of first and second main components in the sensitive soil enzymes(urease, phosphatase and peroxidase) index system of soil polluted by uranium tailings reaches 94.53%. The results of using sensitive soil enzymes index system as basic to classify various samples coincide basically with that based on co mprehensive pollution index and potential ecological risk index. Thus, it is feasible to use sensitive soil enzymes index system constructed with sensitive enzymatic activity to indicate the pollution situation by uranium and associated heavy metal combine d pollution in the soil of uranium tailings environment. The Uranium and associated heavy metals pollution early warning system based on sensitive soil enzymes activity index can be used to monitoring the uranium tailings environmental pollution roughly.