The Research On Soil Biochemical Activity Affected By Different Valence Of Chromium

With the development of social economy and the growth of world population, the soil, as the natural resource on which human beings live, was more and more unable to withstand the load. Nearly 1/5 of total existing arable land about 1×108hm2 in our country was polluted with different degree. The heavy metal was a major kind of pollutants. So it was one of the most important environmental researches to monitor and remediate soil polluted by heavy metal. Chromium as one of the four major of soil pollution,the biological toxicity of Cr6+ is 100～1000 times larger than Cr3+.but their ecotoxicity are different from that.Soil enzyme was one of the principal components in soil, which took part in all of the biochemical reactions in soil and played an impotant part in nutrient circulation, energy metabolism, pollutant monitoring and remediation in nature. Heavy metal entered the soil by various ways, and then reacted with the soil enzyme.The study of ecotoxicological of heavy metals, slecting soil enzyme as the research target, could reveal the mechanism of soil enzymes and heavy metal in theory and provide the theoretical basis for removal, treatment and monitoring of heavy metal in practice. So it was of great theoretical and practical significance to work on this area. However, in the past decade most researches only focused on the relationship between some certain heavy metal and the soil enzyme activity, Rare study on different valence of heavy metals, and the conclusions were not the same, it was seldom to find report on the kinetics and mechanism of soil enzyme.By simulative method, we systematically studied not only the relationships between urease, alkaline phosphatase and dehydrogenase and the heavy metals[Cr3+,Cr6+,Combined chromium]in several major soil types of Northern China, but also the reaction mechanism between enzymes and the metal by means of enzymatic kinetics. The results were as follows:1. Chromium decreased soil pH remarkably. With the increase of Cr3+ concentration, the pH value of soil solution decreased continuously, the biggest drop was 3.03 units. While, when adding different concentration of Cr6+, the pH value decreased gradually, but the sample No.5, the pH value showed alkaline or alkalescent.2. Cr3+could be adsorbed heavily by soil, and there was less than 1%remaining in the soil solution. A small number of Cr6+was adsorbed when adding Cr6+ to soil, apart from part of Cr6+ which were reduced to Cr3+and adsorbed quickly by soil, most of Cr6+ existed mainly in the form of water soluble hexavalent chromium. When adding combined chromium to the soil samples, Cr3+was quickly adsorbed by soil, Part of the Cr6+were reduced to Cr3+and quickly adsorbed by soil, the remaining Cr6+ existed in the form of water soluble. It was hard to detect water soluble Cr3+in the soil solution.3. Chromium inhibited the soil alkaline phosphatase activity, The order of inhibition was that:Cr3++Cr6+> Cr3+> Cr6+. The model U=A/(1+B×C) (in which A, B were composite parameters) could describ the relationship better between soil enzyme activity and chromium concentration. The mechanism of different valence state of chromium on the soil alkaline phosphatase was completely inhibition. The ecological dose ED10 of alkaline phosphatase in the same type of soil decreased continuously with the decline of soil organic matter content. There was an interaction between trivalent chromium and hexavalent chromium on the soil alkaline phosphatase.4. Chromium inhibited the activities of Jackbean urease, The order of inhibition was that: Cr3+> Cr3++Cr6+> Cr6+. The critical concentrations (ED10) of Cr3++Cr6+,Cr3+and Cr6+in Jackbean urease were 0.1,416.1,295.0 mg/kg respectively. With the increasing of chromium concentration, the kinetic parameter Km of Jackbean urease were kept in the same level, which revealed that the mechanism of chromium on Jackbean urease was noncompetitive inhibition.5. Different valence state of chromium had the distinct influence on soil urease activity. Cr3+ inhibited the activities of soil urease, while Cr6+ activated the activities of soil urease. The order of inhibition is Cr3+> Cr6+> Cr3++Cr6*. There were an interaction between Cr3+ and Cr6+ on the soil urease activity, and it showed synergism. within the range of the test chromium concentration, the kinetic parameters Km of soil urease were keep in the same level. The Vmax value falled with the increase of Cr3+concentration, and rised with the increase of Cr6+concentration, the kinetic parameters of lou soil were different from Cinnamon soil and Sandy soil. The reason will be further studyed.. It revealed that the mechanism of Cr3+ on soil urease was noncompetitive inhibition.6. chromium inhibited the dehydrogenase activity of soil, The order of inhibition was Crb +> Cr3++Cr6+> Cr3+, Which maybe cause that dehydrogenase activity is the overall performance of microbial activities. The model U=β0-β1×C could describ the relationship between soil dehydrogenase activity and chromium concentration. It revealed that Dehydrogenase can be characterized as an index of chromium contamination in certain extent. There were interaction between trivalent chromium and hexavalent chromium on the soil dehydrogenase activity.7. According to the principle of that the smaller ecological dose, the more sensitiv, the critical concentrations (ED10) of Cr3+,Cr6+and Cr3++Cr6+in soils were 37.90.10.19 and 33.7 mg/kg respectively. The critical concentrations (EC10) of water soluble Cr3+,Cr6+and Cr3++ Cr6+ in soils were 0.08,795.88,191.63 mg·l-1 respectively.