INTRODUCTION

Genetically modified(GM)crops were first approved for commercialization in 1994 and for large scale commercialization in USA in 1996(Hung et al.,2016).The plantation area of GM crops has reached 179.7 million hectares(ISAAA,2016).There is considerable public concern about the environmental and health impacts of their use.Approximately 25%of GM crops contain the insecticidal trait,derived from the spore-forming bacterium Bacillus thuringiensis(Bt)(Helassa et al.,2011).There is a wide range of issues being discussed in the context of risk assessment of these Bt plants.Transgenic plants encoding Bt toxins release these proteins into soil through root exudates during the vegetative period and through the incorporation of root and aerial plant biomass into soil after harvest(Stotzky,2005),and some input comes from pollen(Kim et al.,2008).The toxin accumulates in soil when its concentration exceeds the consumption by insect larvae and the degradation by soil microbes(Stotzky,2005).In a large-scale agricultural field,the Cry1Ab protein expressed by Bt maize was transported to waterways adjacent to agricultural fields by runoffand the Cry1Ab protein persisted for up to 2 months(Strain and Lydy,2015).The persistence of the Bt toxin in soil is further enhanced through association with soil particles.Previous studies showed that the toxin could rapidly bind to clay minerals,humic acids,and organo-mineral complexes(Stotzky,2000;Zhou et al.,2007,2011;Fu et al.,2012),and the content of clay minerals in soil had a significant in fluence on the adsorption and desorption of Bt protein;the vertical movement of Bt protein in the soil generally decreased as the clay content increased(Saxena et al.,2002).The toxins bound to soil components had decreased bioa-vailability and were less susceptible to degradation.Therefore,the accumulated toxins were exposed to non-target organisms in the soil environment(Muchaonyerwa et al.,2006).

据了解，农业农村部将在这次颁证的基础上，向全国的农村集体经济组织颁发登记证书。农村集体经济组织可以凭登记证到相关部门办理公章刻制和银行开户等相关手续，以便发挥好管理集体资产、开发集体资源等作用。

Soil organic matter(SOM)shows high sorption characteristics because of its high specific surface area(SSA)and cation exchange capacity(CEC).Therefore,SOM could play a major role in the interactions of Bt toxins with soil particles(Crecchio and Stotzky,1998).Because soil adsorbing surfaces are highly heterogeneous,the in fluence of soil components on the adsorption of toxins can be examined in two different ways.One involves studying adsorption in simple semisynthetic systems associated with various soil colloid materials such as clays(Tapp et al.,1994),organic matter(Crecchio and Stotzky,1998),oxides(Sander et al.,2010;Madliger et al.,2011),and organomineral associations(Crecchio and Stotzky,2001).The other involves removing a specific soil component in order to establish its effect on adsorption(Pagel-Wieder et al.,2007;P´erez-Novo et al.,2008;Fu et al.,2012).Soil structure can be defined as an arrangement of mineral soil particles and organic compounds that form aggregates of different sizes and stability.Pagel-Wieder et al.(2007)reported that destruction of organic matter with hydrogen peroxide(H2O2)was related to an increase in the adsorption of Cry1Ab protein.However,Muchaonyerwa et al.(2006)observed that the clay fraction of H2O2-treated Vertisol adsorbed less toxin than the natural clay fraction.The varying results from these two studies indicate that more detailed studies are needed to con firm the in fluence of organic matter on toxin adsorption,as well as on the physicochemical properties of soils.

In this study,we examined the adsorption and insecticidal activity of Bt toxin in the natural and H2O2-treated soils.In order to compare different soil types,variable-(Ultisol)and constant-charge(Al fisol)soils were chosen.The objective of this study was to identify changes in the adsorption patterns and insecticidal activity of Bt toxin due to a reduction in the organic matter content of soils.

解决孩子的“问题”，并不等于消除现象（情绪/行为/想法），并且需要从理解孩子成长的需求出发，养育的终极目标是孩子的成长。

MATERIALS AND METHODS

Bt toxin and soil samples

The transgenic insecticidal toxin(Cry1Ab)was provided by the Institute of Plant Protection,Chinese Academy of Agricultural Sciences.The Cry1Ab protein has a molecular weight of 66 kDa and consists of 618 amino acid residues.

Heliothis armigera( first instar)with a length of 2 to 3 mm was used to determine the insecticidal activity of Bt toxin.Free and adsorbed toxins were diluted to 200,100,50,25,and 12.5 mg L−1.Each concentration of toxin needed one incubation plate with 24 wells(1.7 cm high,1.6 cm in diameter);therefore, five concentrations of each sample needed five incubation plates(120 wells in total).The in flection feed was prepared as follows∶agar powder(4.5 g)was dispersed in 220 mL sterilized water,and maintained at boiling temperature for 10 min;yeast powder(12 g),soybean powder(24 g),sodium benzoate(0.42 g),and vitamin C(1.5 g)were mixed and soaked with 80 mL sterilized water,and then transmitted into the agar solution;during stirring,the temperature of the suspension solution dropped to 65°C,and 3.9 mL acetic acid(36%,weight∶weight)was added;the in flection feed was obtained by mixing 2 mL Bt toxin samples with 18 mL suspension feed.Twenty milliliters of in flection feed were dispersed into the 24 wells of incubation plate.After air-drying for 12 h,water on the walls of the wells disappeared(feed was still wet),and one larva was placed into each well and incubated at 30± 2°C for 72 h.The incubation plate was covered tightly with a lid.

For SOM removal,20 g soil was transferred into a beaker(500 mL),and mixed with 200 mL deionized water.Five milliliters of H2O2solution(30%,weight∶weight)were added.The suspension was stirred periodically at room temperature until bubbling ceased,and then 5 mL H2O2solution(30%,weight∶weight)was added again.This procedure was repeated until no visible frothing was observed after H2O2 addition,indicating a completed reaction(Mikutta et al.,2005).The soil slurry was heated to 70–90 °C to remove excess H2O2.The samples were washed out with deionized water and centrifuged at 5000×g for 10 min.This washing procedure was repeated 5 times.Finally,the samples were air-dried and stored at room temperature for further use.Soil organic carbon was analyzed by using the K2Cr2O7digestion method.The surface morphology of soils before and after H2O2 treatment was examined in a Hitachi S-4800 scanning electron microscope(Hitachi,Japan)equipped with an X-ray dispersive analyzer.The basic characteristics of the natural and H2O2-treated soils are summarized in Table I.

Adsorption kinetics and isotherm experiments

Two isotherm equations were tested in the present study.The Langmuir model assumes that adsorption occurs at specific homogeneous sites on the adsorbent surface and has been successfully applied to many monolayer adsorption processes.The Freundlich model is an empirical relationship describing the adsorption of solutes from a liquid to a solid surface,and assumes that different sites with various adsorption energies are involved.The equations for the two models are given below∶

Total carbon of the natural brown soil was much higher than that of the natural red soil(Table II).After treatment with H2O2,the amount of carbon in the red and brown soils decreased by 71.26%and 82.82%,respectively.With respect to soil texture,the sand content(including coarse and fine sand)slightly decreased,while the clay and silt contents slightly increased.There was no significant difference in the mineral composition of the natural soils and H2O2-treated soils according to the XRD identification(Fig.1).After removing SOM,the CEC and pH decreased,and the SSA,PZC,and zeta potential increased(Table II).

浙江专员办党组认为，从本质上说全面实施预算绩效管理是政府治理方式的深刻变革。预算是政府活动和宏观政策的集中反映，也是规范政府行为的有效手段。预算绩效是衡量政府绩效的主要指标之一，反映的是各级政府、各部门的工作绩效。全面实施预算绩效管理，着重解决财政资源配置和使用中的低效无效问题，有利于夯实各地区、各部门、各单位绩效主体责任，推动政府效能提升。加强对部门和单位预算绩效管理的监督是财政监督队伍义不容辞的责任，必须切实履行。

TABLE I Basic characteristics of the natural and H2O2-treated red and brown soils

Soil Clay Silt Sand pH TotalCoarse Fine%Natural red soil 54.2 29.4 16.4 3.9 12.5 4.61 Treated red soil 54.7 33.2 12.1 1.6 10.5 4.05 Natural brown soil 17.7 36.7 45.6 10.5 35.1 5.99 Treated brown soil 20.4 37.6 42.0 7.1 34.9 4.87

Determination of insecticidal activities

Red soil(Ultisol)was collected from the 0–40 cm layer of cultivated land in Xianning,Hubei Province,China.Brown soil(Al fisol)was sampled from the 0–40 cm layer of forest land in Tianwai Village,Taishan,Shandong Province,China.Neither soil was planted with Bt crops.After air-drying,the soils were ground to pass a 0.2-mm mesh.Soil pH was measured in a soil∶water ratio of 1∶2.5(weight∶volume).The point of zero charge(PZC)was determined using the salt titration method(Gillman and Uehara,1980).Mineralogy of the soils was determined by X-ray diffraction(XRD)using a diffractometer with Cu Kα radiation(Rigaku D/max-2500,Japan).The apparent zeta potential of the soil particles was estimated from electrophoretic mobility measurements with a Zetasizer Nano-ZS90(Malvern Instruments,UK).The CEC was determined by extraction with NH4C2H3O2.The SSA was measured using the N2adsorption method.The texture of soil samples was analyzed with a wet sieving and sedimentation technique.

The adsorption isotherm is the equilibrium relationship between the concentrations in the fluid phase and in the adsorbent surface at a given temperature.Within the range of Bt toxin concentrations studied(0–1.0 g L−1),the adsorption amount increased rapidly at the initial stage and then gradually reached equilibrium(Fig.3).

where MRis the rectification mortality of larvae in the testing sample,and MDis the direct mortality of larvae in the testing sample,MCis the mortality of larvae in the control sample.The soils were used as the control,and the mortality in it was almost zero.

To calculate the lethal concentration required to kill 50%of the larvae(LC50),the mortality values were converted into probability values.A plot of probability values versus the logarithmic concentrations of Bt toxin yielded a straight line.The LC50values were obtained using the least square method.The bioassay was repeated three times;each soil sample required 15 incubation plates and 270 mL liquid feed in total.

Data analysis

The Bt toxin was dissolved in Tris-HCl buffer(pH 7.0).The adsorption kinetics was determined at a toxin concentration of 0.2 g L−1and a soil concentration of 0.5 g L−1.The mixtures of soil and Bt toxin were shaken at 300 r min−1and 25± 1°C,pipelined out at designated time intervals(from 0.5 to 180 min),and then centrifuged at 20000×g for 20 min;the absorbance of supernatants was measured at 280 nm(Zhou et al.,2007).The amount of Bt toxin adsorbed was calcula-ted from the concentration difference.

where qe,Land qe,Fare the equilibrium adsorption capacity of the adsorbent in the Langmuir and Freundlich models,respectively(g g−1),qmaxis the maximum adsorption capacity of the adsorbent(g g−1),KLis the Langmuir adsorption constant related to free energy of adsorption(L g−1),Ceqis the equilibrium concentration of Bt toxin in solution(g L−1),and KFand n are the Freundlich constants.The experimental data for Bt toxin adsorption were analyzed using the Langmuir and Freundlich models with SigmaPlot software(Systat Software Inc.,USA).The parameters of the Langmuir and Freundlich models can be easily obtained by choosing the modified hyperbola I and power equations,respectively.

All the experiments were conducted in three replications.Data are expressed as means±standard errors.

RESULTS AND DISCUSSION

Selected properties of natural and H2O2-treated soils

Adsorption isotherm experiment was carried out with an initial concentration of Bt toxin from 0 to 1.0 g L−1(pH 8.0)and of soil at 1.0 g L−1.The mixtures of soil and Bt toxin were shaken at 200 r min−1and 25±1°C,and a 3-h adsorption period was sufficient for equilibrium conditions to be reached.The mixtures were centrifuged at 20000×g for 20 min and the amount of Bt toxin adsorbed was calculated according to the adsorption kinetics experiment.

然而，现有的各类方法大都基于白天的视频检测，对于夜间视频检测的适用性并不高。对于粒子滤波算法而言，该算法对目标的识别依赖于视频图像中的色彩直方图信息，当视频中光线不足、被识别目标和背景区分度不高时，粒子滤波方法的可靠性并不高。另外，人类视觉系统使用的HSI 色彩空间与视频色彩直方图所使用的RGB 空间不同，需要进行有效地统一。

Adsorption kinetics

It was found that the adsorption rate of Bt toxin on the H2O2-treated soils was higher than that on the natural soils(Fig.2).Furthermore,the equilibrium timefor adsorption of Bt toxin became shorter.Adsorption of Bt toxin on the H2O2-treated soils was rapid up to 15 min and then reached equilibrium.For the natural soils,the adsorption of Bt toxin reached equilibrium at around 30 min.

TABLE II Selected propertiesa)of the natural and H2O2-treated red and brown soils

a)SSA=specific surface area;CEC=cation exchange capacity;PZC=point of zero charge;TOC=total organic carbon.

Soil SSA CEC PZC Zeta TOC Clay mineral composition potential m2g−1 cmol kg−1 mV g kg−1 Natural red soil 48.5 21.2 3.3 −18.4 14.73 Hydromica(20%),1.4-nm mineral(35%),kaolinite(45%)Treated red soil 52.7 17.5 3.6 −16.3 4.23 Hydromica(20%),1.4-nm mineral(35%),kaolinite(45%)Natural brown soil 31.7 46.2 4.1 −17.6 26.33 Hydromica(45%),vermiculite(35%),kaolinite(20%)Treated brown soil 37.5 41.2 4.3 −15.8 4.52 Hydromica(45%),vermiculite(35%),kaolinite(20%)

Fig.1 X-ray diffraction analysis of the natural and H2O2-treated red and brown soils.

Fig.2 Adsorption kinectics of toxin from Bacillus thuringiensis on the natural and H2O2-treated red and brown soils.Vertical bars indicate standard errors of the means(n=4).

Adsorption isotherms

The mortality was then determined using a sterilized toothpick.If the insect body became black or could not move,it was con firmed as dead.The mortality of the testing sample needed rectification using the following equation∶

当采用集中外售的方式时，由于需要用车辆进行运输，增加了运输成本，且外售需委托有资质的固废处置公司进行处理，需交付一定处置费用，这无疑给公司带来了一定的处置成本；当采用回炉处理时，由于砷滤饼含硫较高，会造成放热，影响炉温、渣温以及烟气温度，而且会使闪速炉的冰铜品位有小范围的波动，与此同时，回炉处理时，造成砷的循环，致使废酸中砷含量增长较快。

The Freundlich constants KFand n indicate the adsorption capacity and adsorption intensity,respectively(¨Ozcan et al.,2005;Bulut and Tez,2007).The KFand 1/n values of the H2O2-treated soils were higher than those of the natural soils,which is consistent with the variation in qmax(Table III).The KL value from the Langmuir model is also used for expressing the adsorption affinity(Cai et al.,2006).However,in the present study,the KLvalue for the H2O2-treated brown soil was lower than that for the natural brown soil,which is in contrast to the variation in qmax.Therefore,for adsorption affinity,the Freundlich model had a better fit than the Langmuir model.It is also reported that the coefficient of determination(R2)of adsorption data can be used to determine which model has a better fit(Chen et al.,2007).As shown in Table III,the R2values of the Langmuir model were higher than those of the Freundlich model.According to our experimental results,both models had different advantages.As the adsorbent had a limited adsorption capacity,the amount of maximum adsorption could be better described by the Langmuir model than by the Freundlich model.Nevertheless,the Freundlich model was better for the expression of adsorption affinity.

因为酒店管理这一专业有着很强的实用性以及实践性，因此学生在进行这门专业的课程学习过程中，最主要的目的就是在未来的工作过程中可以对酒店管理工作做得更好。但是因为当今我国很多院校的酒店管理专业在教学的过程中都有着很多的问题存在，不仅有着十分复杂的教学内容，而且随着经济的不断发展，以及社会对人才的要求不断更新，使得这一专业的教师在进行实践教学的过程中也要面临越来越巨大的挑战。

Fig.3 Adsorption isotherms of toxin from Bacillus thuringiensis on the natural and H2O2-treated red and brown soils.Vertical bars indicate standard errors of the means(n=4).

TABLE III Parametersa)of the Langmuir and Freundlich models used for the natural and H2O2-treated red and brown soils

a)qmaxis the maximum adsorption capacity of the adsorbent;KLis the Langmuir adsorption constant related to free energy of adsorption;R2is the coefficient of determination;KFand n are the Freundlich constants.

Soil Langmuir model Freundlich model qmax KL R2 KF 1/n R2 g g−1 L g−1 Natural red soil 0.4267 4.290 0.9973 0.3550 0.3288 0.9917 Treated red soil 0.4987 4.331 0.9875 0.4185 0.3385 0.9841 Natural brown soil 0.5353 2.424 0.9975 0.3904 0.4560 0.9908 Treated brown soil 0.5994 2.364 0.9983 0.4352 0.4662 0.9929

Insecticidal activity of adsorbed Bt toxin

Both free and adsorbed toxins were toxic to the larvae of H.armigera(Table IV).The LC50values for the adsorbed Bt toxin were lower than that of free Bt toxin;i.e.,the insecticidal activity of adsorbed Bt toxin increased,which is in agreement with a previous study(Crecchio and Stotzky,2001).The LC50values for Bt toxin in the H2O2-treated soils were slightly lower than those in the natural soils,suggesting that the environmental risk from toxins could increase if the organic matter of soils decreased.The measurement of insecticidal activity using insects used in this study is expensive and time consuming.A rapid and convenient in vitro method of enzyme-linked immunosorbent assays(ELISA)(Mueting et al.,2014)has currently been used for evaluating toxin degradation in soils.

TABLE IV Insecticidal activity of toxin from Bacillus thuringiensis before and after adsorption by natural and H2O2-treated red and brown soils

a)Lethal concentration required to kill 50%of the larvae of Heliothis armigera.

Toxin LC50a)µg mL−1 Before adsorption 9.12±1.39 After adsorption Natural red soil 3.08±1.13 Treated red soil 2.76±0.94 Natural brown soil 4.16±1.50 Treated brown soil 3.24±1.22

Effects of SOM removal on mineral structure

There are three reagents used for SOM removal∶H2O2,sodium hypochloride(NaOCl),and disodium peroxodisulfate(Na2S2O8)(Mikutta et al.,2005).Hydrogen peroxide was introduced by Robinson(1922)for soil texture analysis and has become the most widely used chemical reagent for organic matter destruction.Generally,NaOCl and Na2S2O8are more effective in organic carbon removal than H2O2.The NaOCl procedure is carried out at pH 9.5,with three consecutive cycles including boiling each for 15 min(Anderson,1963).Despite the higher electronic potential of NaOCl with decreasing pH,carbon removal efficiency is maximal at pH 9.5.However,the NaOCl procedure(pH 9.5)may dissolve Al-hydroxides,although alkaline conditions favor the precipitation of metals released upon destruction of organic matter(Mikutta et al.,2005).Furthermore,at higher temperatures,the NaOCl treatment may induce changes in extractable pedogenic Al and Fe due to transformation into more crystalline forms.The efficiency of Na2S2O8in removing organic matter has been proposed as superior to H2O2 and NaOCl(Meier and Menegatti,1997).With the measurements using XRD,infrared spectroscopy,and N2adsorption,no structural alteration of Na2S2O8-treated illite,kaolinite,and montmorillonite reference minerals was observed(Menegatti et al.,1999).The red soil used in the present study contains iron-aluminum oxides;therefore,the Na2S2O8procedure was not suitable for our experiment.

The results in Table III indicate that the adsorption isotherm can be described by both the Langmuir(R2≥0.9875)and Freundlich models(R2≥0.9841).The maximum adsorption amount of Bt toxin on the H2O2-treated soil was higher than that on the natural soils.For example,the maximum adsorptions of Bt toxin on the natural and treated red soils were 0.4267 and 0.4987 g g−1,respectively,and on the natural and treated brown soils were 0.5353 and 0.5994 g g−1,respectively.The results of the present study are in contrast to the study of Muchaonyerwa et al.(2006),who reported that the adsorption of toxin from Btt(Bacillus thurigiensis subsp.tenebrionis)on the carbon fraction was much higher than that on clay minerals.In view of the contrasting results,we speculate that the effects of SOM on adsorption of Bt toxin may not be limited to the carbon content,and are also attributed to soil texture and other factors,such as the SSA,CEC,PZC,and zeta potential.

Both red and brown soils are composed of nonuniform particles.After treatment with H2O2,the size of soil particles decreased(Fig.4),supporting the results that the SSA increased after SOM removal(Table II),but the basal spacing of soil minerals was not observed(Fig.1).

Fig.4 Scanning electron microscope images of the H2O2-treated red soil(a),natural red soil(b),H2O2-treated brown soil(c),and natural brown soil(d).

Effects of SOM removal on Bt toxin adsorption

There are several potential mechanisms for interactions between Bt proteins and the surfaces of soil particles.It can be assumed that electrostatic forces(Sander et al.,2010),ligand exchange(Fu et al.,2007),hydrogen bridges(Crecchio and Stotzky,2001),ionic bonds and complex formation(Tapp et al.,1994),hydrophobic interactions(Helassa et al.,2011),and van der Waals forces are responsible for the binding of Bt toxin.The adsorptive forces operating between the bound toxin and the clay-sized fractions are not clearly understood.Sander et al.(2010)reported that electrostatic interactions governed the adsorption of monomeric Cry1Ab toxin to charged polar surfaces,whereas Helassa et al.(2011)reported that hydrophobic interactions may play an important role in Cry1Ab toxin adsorption.

池水深度保持在50～80cm，施肥的次数和数量以适当的水色和透明度为准，保持水色呈豆绿色或茶褐色、透明度在30～40cm。

The isoelectric point(IEP)of Cry1Ab toxin was between 6.0 and 6.4(Sander et al.,2010),and the PZC of the studied soils ranged from 3.3 to 4.3(Table II).At the experimental pH of 7.0,the soils and Cry1Ab toxin are negatively charged,and there are repulsive electrostatic interactions between the negatively charged surfaces of soil particles and the anionic functional groups of the toxin.When the negative charges on the surface of soil particles decrease,the electrostatic repulsion between toxin and soil particles weakens,resulting in an increasing affinity of toxin for soil particles.The zeta potential of the H2O2-treated soils was higher than that of the natural soils(Table II),which is consistent with the results of toxin adsorption(Table III).On the other hand,when the pH value is above the IEP of soils,the lower the PZC values are,the higher the net negative charge on their surface is(McLaren et al.,1958).We found a high correlation between the qmax and PZC values(R2=0.9357);therefore,our results support the electrostatic driving mechanism for toxin adsorption.

Cry1Ab toxin has a higher affinity for hydrophobized surfaces compared to hydrophibic ones(Janot et al.,2010).Soil organic matter may considerably enhance hydrophobicity,leading to macroscopic changes in soil properties(Doerr et al.,2007).However,the adsorption capacity of Bt toxin in the H2O2-treated soils(lower hydrophobility)was higher than that in the natural soils(higher hydrophobility)(Table III),which does not appear to support the hydrophobic interaction mechanism for toxin adsorption(Helassa et al.,2011).There are two possible reasons for the contradictory results.First,the SSA of the H2O2-treated soils was higher than that of the natural soils(Table II).The adsorption amount for a biomacromolecule depends on its size in the medium and the SSA of the adsorbent.The relationship between the adsorption amount and the SSA can be described as in Eq.4 when the section area of the adsorbate is constant(Physical Chemistry Teaching and Research Section of Tianjing University,1983).

where Γ∞ is the saturation adsorption amount(mol kg−1),Awis the specific surface area of the adsorbent(m2kg−1),L is the Avogadro’s number,and Amis the section area of the adsorbate molecule(m2).It can be seen that the greater the SSA is,the higher the adsorption amount is.Although the hydrophobicity of the H2O2-treated soils may decrease,the SSA increases due to the changes in soil texture.If a decrease in the hydrophobicity and an increase in the SSA work against each other,the adsorption may show an increase or a decrease,depending on which factor is predominant.Second,the PZC and zeta potential of the soils increased after SOM removal,resulting in a decrease of electrostatic repulsion between toxin and soil particles.

Effects of SOM removal on insecticidal activity

Previous studies showed that the insecticidal activity of bound toxin was higher than that of free toxin(Tapp and Stotzky,1995;Crecchio and Stotzky,2001).The increase in apparent toxicity may have been a result of the toxin being localized by binding on the complexes;i.e.,more toxin was ingested by larvae as clay-toxin complexes than the free toxin uniformly spread over the surface of food medium(Crecchio and Stotzky,1998,2001).Zhou et al.(2005)assumed that the adsorbed toxin showed higher insecticidal activity due to its higher stability.However,Hung et al.(2016)suggested that toxicity may not have been enhanced by adsorption,but the larvae consumed less feed when it contained soluble Cry than when Cry was adsorbed on soils.Their results con firmed that the consumption amount of feed containing adsorbed toxin was 7%higher than that of feed containing soluble toxin.Another reason for the enhanced toxicity of adsorbed toxin was due to the less marked mandible paralysis of larvae and hence less cessation of feeding for adsorbed Cry with respect to soluble Cry in feed(Hung et al.,2016).The adsorbed Cry would not be solubilized in the insect mouth,but could be desorbed in the alkaline solution of the midgut.In the present study,the toxin adsorbed on the H2O2-treated soils had a higher toxicity than that on the natural soils.The possible reason is that the H2O2-treated soils had a higher adsorption capacity of toxin,leading to a lower desorption of toxin in feed,and thus larvae consumed more feed,resulting in higher mortality.

CONCLUSIONS

After the removal of SOM by H2O2treatment,the mineral composition of the soils showed no significant changes,but there was a slight change in soil texture;the CEC and pH decreased,while the SSA,PZC,and zeta potential increased.The adsorption isotherm experiment showed that the toxin adsorption on the natural and H2O2-treated soils fitted both the Langmuir model(R2≥0.9857)and the Freundlich model(R2≥0.9841).The adsorption amount of toxin on the H2O2-treated soils was higher than that on the natural soils,and there was a high correlation between the maximum adsorption of toxin and the PZC of soils(R2=0.9357).Therefore,the adsorption of Bt toxin was not only in fluenced by the SOM content,but also by soil texture as well as the SSA,CEC,PZC,and zeta potential.The current study supports the electrostatic driving and cation exchange mechanisms for toxin adsorption.The LC50values for the toxin in the H2O2-treated soils were slightly lower than those in the natural soils,suggesting that the environmental risk from the toxin could increase if the organic matter of soils decreased.Because the measurement of insecticidal activity using insects is expensive and time consuming,a rapid and convenient in vitro method of ELISA is recommended for evaluating the toxin degradation in soils in future studies.

ACKNOWLEDGEMENT

This research was supported by the National Natural Science Foundation of China(Nos.31470573 and 31070478).

领舞者持钓鱼鞭进入场中央，鞭打四方，四名斗虎英雄分别与四虎缠斗，四名斗虎英雄踏鼓点一起按逆时针做“白虎洗脸”，原地分别向虎脸踢出四脚转一周，四虎也分别以斗虎英雄为轴按逆时针转一周（头里尾外），扑向斗虎英雄，斗虎英雄来不及躲闪，就地十八滚从老虎腹下窜出，引领老虎，向前跃出。

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