Influential Factors For Soil Cadmium Ecological Toxicity As By Soil Biochemical Active

Cadmium pollution has become one of the most widespread and harmful heavy metalpollutions. It put a potential risk to human health, as well as to the soil environmental quality.Soil enzyme and microorganism are preconditions of organic carbon and nutrientdecomposition, regulating the energy supply to maintain soil nutrient cycle in a steady state.Therefore they are considered to be comprehensive indices to reflecting soil health andtoxicity of pollutants. Previous researches revealed that ecological doses calculated fromactivities of soil enzyme and microorganism under Cd pollution had significant difference,and the toxicity of Cd estimated by lab experiments was far higher than the field experiment.But no explicit conclusions had drewed on how the soil properties affect Cd toxicity toenzymes and microorganisms. Therefore, researching on the relationship between Cd and soilbiological activity, the function mechanism and soil properties effects on Cd toxicity to soilbiological activity, had important theoretical and practical significance for revealing the riskof Cd pollution, monitoring pollution level and remediation. In our studies,18different soils,respresentative of soils in China, calf intestinal alkaline phosphatase, clay minerals, andlong-term cadmium (Cd) contaminated soil we chose as the materials.The relationshipbetween soil properties and toxicity of Cd, mechanism of effect of Cd on soil enzyme, andcharacteristics of soil enzyme activity and soil microbial diversity in Cd contaminated soilwere investigated by experiments on measuring the toxicity of added Cd to soil enzymesactivities and potential nitrification rate, batch adsorption experiments, and the methods ofkinetics and thermodynamics. The major conclusions are as follows:1. We studied systematically and quantified the relationship between ecological dosagevalue (ED25and ED50) of Cd toxicity and the soil properties. Research shows that the soilorganic carbon (TOC) is the main factor that control the inhibition of water soluble Cd toalkaline phosphatase activity, while for the total Cd, the main factor is cation exchangecapacity (CEC), but the fitting precision of both two regression equations were not high. Inorder to improve the accuracy of fitting equation, soils were classified as two parts accordingto pH. In the acid soil (pH <6.27), the ED25Tand ED50T(based on total Cd) were increasedwith the increment of pH, which explained up to85%of the variation for regression model. Inthe orther soils (pH>6.27), however, the value of EDW(based on water soluble Cd) and EDT were decreased with the increment of pH, which explained up to46%and82%of thevariation for regression models respectively.The main factor affecting the toxicity of the water soluble Cd to dehydrogenase activity(TTC) was pH, and TOC was the main factor affecting the toxicity of total Cd, but two factorscan control the variation of regression models were less than50%. The most important factoraffecting the toxicity of water soluble Cd to dehydrogenase activity (INT) was pH, ED50wandED25wwere decreased while the soil pH increased, which explained up to57%and42%ofthe variation for regression models respectively. As for total Cd, TOC was the main controlfactor, next to pH and CEC, three factores explained up to93%of regression model.Soil nitrification rate is not sensitive to Cd pollution, the nitrification rate can besignificantly reduced when soil Cd content reached28mg kg-1. The developed regressionmodel based on the main soil properties, such as pH, organic matter concentration and CEC,can accurately predict the Cd toxicity in the soils as determined by nitrification rate.2. We illuminated the inhibitory mechanisms of Cd to alkaline phosphatase by kineticsand thermodynamics methods. The results shows that Cd had no effect on Michaelis constant(Km) of alkaline phosphatase, but reduced the maximum reaction rate (Vmax), which belong tothe noncompetitive inhibition. Under different temperature, the values of inhibition constant(Ki) ranged from1.80to16.01,1.11to13.37,0.80to5.28and5.28to19.89mmol L-1respectively. The Kivaried soil types, Kiin weak acid and neutral soils were higher thanalkaline soils. As temperatures rised, Kishowed a trend of decline. In alkaline soil the mainfactor affected Kiwas CEC content, pH may play a major factor in acidic and neutral soil.Cd could increased the response of soil alkaline phosphatase to temperature in low doses,and high doses reduced enzyme sensitivity to temperature. Cd had a strong effect on thepre-exponential factor A, when the content of Cd increased to25mg kg-1, the pre-exponentialfactor A declined to at least60%of the control in most soil, as the concentration of Cdincreased the pre-exponential factor (A) declined to only5%of the control. Activation energy(Ea) decreased with the increased of Cd concent. In conclusion, Cd reduced the Ea of reaction,but the pre-exponential factor is more sensitive to Cd, therefore the pre-exponential factor Acan control the reaction rate constant of alkaline phosphatase, so the apparent activities ofalkaline phosphatase decreased with the increased of Cd. Due to the specific contact of thepre-exponential factor and activation entropy ΔS*, showed the effects of Cd on soil alkalinephosphatase activity belongs to entropy control process.3. The inhibition mechanism of Cd to dehydrogenase was studied by dynamics. Theinhibition types of Cd to soil dehydrogenase varied with soil types, for red soil in Yunnanprovince and brown soil in Liaoning province were noncompetitive inhibition, however, in other soils Cd inhibition dehydrogenase was linear mixed inhibition. The inhibition constant(Ki) of red soil dehydrogenase in Yunnan and brown soil in Liaoning were12and4.7mmolL-1respectively. The Kiof Cd to other soil dehydrogenase are between0.67to0.67mmol L-1,the affinity of Cd and dehydrogenase was significantly higher than the affinity of the enzymeand substrate (INT).4. Revealing the inhibition mechanism of Cd on the activity of free and adsorbedalkaline phosphatase. In the concentration of0～2.5mg mL-1, the effect of Cd on alkalinephosphatase is reversible; Once the concentration was greater than25mg mL-1, the inhibitionof Cd on alkaline phosphatase was irreversible. The toxicity of Cd on free enzyme andadsorbed alkaline phosphatase was affected by time, the enzyme activity strongly depressedas exposure time extends. Clay minerals can weaken the Cd toxicity on alkaline phosphatase.Cd had little impact on the Kmof alkaline phosphatase and obvious inhibition of the Vmax,which was the characteristic of the noncompetitive inhibition. The Kiof pure enzyme,montmorillonite and goethite adsorbed alkaline phosphatase had been calculated, which was2.3,2.3,2.6mmol L-1（17℃）separately, it was decreasing while temperature going up.Regarding to the free enzyme and absorbed states of alkaline phosphatase, activation energy(Ea) decreased as the concentration of Cd increased. Comparing to the free enzyme andgoethite absorbed alkaline phosphatase, the Ea of montmorillonite adsorbed alkalinephosphatase was more susceptible to Cd, the maximum inhibition rate of the three forms ofenzyme was-24%,-71%,-23%separately. Cd had a strong effect on the pre-exponentialfactor. At the concentration of0.25mg kg-1, the pre-exponential factor of the free enzymedeclines55%and for two kinds of adsorbed enzyme, it came to a higher value; At theconcentration of0.5mg kg-1, however, the factor of montmorillonite and goethite adsorbedenzyme decreased92%and84%respectively.The activation free energy, activation enthalpyand activation entropy of those three forms of enzyme had the same variation trend.Activation free energy remains unchanged as the Cd concentration increased.Within a range of17℃and47℃, the average inhibition rates of Cd on ΔH*and ΔS*offree enzyme were1.1%～-18%and0.56%～-12%; For montmorillonite adsorbed enzyme,rates were24%～-67%and1.7%～-9%; For goethite adsorbed state enzyme, rates rangedform10%to-19%、1.4%to-9%.5. Revealing the enzyme activity and microbial diversity characteristics on long-term Cdcontaminated soil. In the long-term heavy metal pollution, soil microbial community andenzyme activity are affected by both of heavy metal and soil properties (pH and organiccarbon). After getting rid of the effect of organic carbon on the soil enzyme, the total Cd, Znin soil had significant negative correlation with special enzyme activity which expressed enzyme activity by TOC. The soil enzyme diversity index (TEI) established by using thespecial enzyme activity can reflect the degree of soil heavy metal pollution. The analysis fromBiolog indicates that in the long-time Cd and Zn pollution soil, the style of carbon utilizationpattern in the microorganism changed significantly and mainly on the utilization of aminoacids carbon source.The following conclusions are summarized based on above-mentioned experimentalresults:(1) Soil TOC, CEC and pH are the main factors which affect Cd on the toxicity of soilbiochemical activity. The ecological dose value of Cd on soil biochemical activity can besuccessfully predicted by using these soil properties;(2) The inhibition mechanism of Cd onalkaline phosphatase and dehydrogenase is mixed by non-competitive inhibition and linearinhibition. The effect of Cd on soil alkaline phosphatase depends on entropy control;(3) Clayminerals can abate the toxicity of Cd on alkaline phosphatase;(4) The degree of long-term Cdpollution in soil can be appropriately reflected by utilizing soil enzyme diversity index (TEI)constructed from relative enzyme activity.