| With the development of modern industrial technology,the types and applications of plastics are more and more extensive.Generally,plastic waste will not disappear naturally when it enters the environment,but will be decomposed into microplastics with a particle size of less than 5mm after being subjected to ultraviolet radiation and biodegradation.Plastic film is widely used in agricultural production.The residues of agricultural plastic film remain in the soil after weathering and degradation for many years,leading to a significant increase of polyethylene microplastics（PE）residues in the soil,which has seriously polluted the soil environment.In addition,as a broad-spectrum fungicide,carbendazim is widely used in the prevention and control of fungal diseases in the agricultural production process,especially in the growth process of cotton.Due to the multiple and high incidence of diseases,carbendazim is used in large quantities and has high application frequency,and its half-life in soil is long.With the extension of its service life,carbendazim residue accumulates seriously in farmland soil.Pollution of soil ecological environment.Studies have shown that microplastics can act as carriers of toxic chemicals,increasing their environmental risks,but the adsorption effects of microplastics with pesticides and their combined toxicology on plants and soil are rarely reported.Therefore,PE and carbendazim were selected as the research objects in this experiment.The adsorption behavior of PE to carbendazim was studied through adsorption test,and the adsorption mechanism was analyzed by adsorption model,and the influence of PE on the half-life of carbendazim in soil and the control of cotton fusarium wilt was studied.On this basis,pot experiment was designed to systematically explore the single and compound pollution effects of polyethylene microplastics and carbendazim(2 mg·kg-1,5 mg·kg-1)with different particle sizes（100 mesh,500 mesh）and different concentrations（1%,5%m/m）on cotton and soil.The main results are as follows:1.The adsorption behavior of PE to carbendazim in soil system was simulated.The results showed that the adsorption behavior of PE to carbendazim was consistent with second-order kinetics and Freundlich isothermal adsorption model.In the environmental impact test,the adsorption effect of carbendazim was enhanced under acidic conditions.When pH increased from 2.0 to 7.0,the adsorption capacity of carbendazim in different treatment groups decreased by 55.7%～80.9%.As the concentration of sodium chloride increased from 0 to 1%,the adsorption capacity of PE on carbendazim increased by 7.2%～13.5%,but with the increase of carbendazim concentration（1%～3.5%）,the adsorption capacity on PE did not change significantly.However,fulvic acid inhibits the adsorption to some extent.When the concentration of fulvic acid(1～50 mg L-1)increases,the adsorption capacity decreases by25.2%～38.3%.In addition,the effect of carbendazim on cotton fusarium wilt was weakened due to the adsorption of PE,and the half-life of carbendazim in soil was prolonged.The half-life of carbendazim(5 mg·kg-1)in soil treated with 5%PE（500mesh）was prolonged the longest,from 6.31d to 14.20d.2.The effect of PE on cotton and soil:With the increase of PE concentration,the biomass and chlorophyll content of cotton plant decreased gradually.The effect of high concentration of PE（500 mesh）was the most significant.The plant height,root length,fresh weight and dry weight of cotton in 5%PE group decreased by 4.8%,3.5%,17.7%and 12.7%,respectively,compared with 1%PE group.The chlorophyll content of cotton in high concentration 5%PE treatment group decreased by 3.8%compared with low concentration.Single PE treatment significantly inhibited the root activity of cotton（P<0.05）,and increased the antioxidant oxidase activity and malondialdehyde content in cotton plants.The lowest root activity of cotton treated with 5%PE was 160.52μg TTF/g FW/h;The activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）in cotton were significantly increased,which were 10.41,2.71 and 2.80 times higher than those in CK group,respectively.The highest malondialdehyde content was found in the 5%PE（100 mesh）treatment group 29.04μM?g-1FW.PE treatment can reduce the pH value of soil,improve the activity of phosphatase in soil,and reduce the activities of cellulase,urease,sucrose,catalase,dehydrogenase and protease.The inhibition effect of small particle size PE at 5%concentration reached a significant level（P<0.05）,and the inhibition effect was stronger than that of large particle size PE treatment.3.Effects of carbendazim on cotton and soil:With the increase of carbendazim concentration,cotton plant biomass and chlorophyll content decreased significantly.Compared with the control group,the plant height,root length,dry weight and fresh weight of cotton in the high concentration carbendazim treatment group(5 mg·kg-1)decreased by 15.1%,26.7%,31.9%and 32.7%,respectively,and the total chlorophyll content of leaves decreased by 39.6%.At the same time,high concentration carbendazim treatment significantly increased the activity of SOD,POD and CAT in cotton,which reached 76.9%,15.3%and 69.3%,respectively,and the content of malondialdehyde was 1.22 times that of the control cotton.The minimum root activity was 204.08μg TTF/g FW/h under high concentration carbendazim treatment.Carbendazim treatment decreased soil pH and increased soil cellulase activity,and the enzyme activity increased by 4.2%under high concentration.Protease showed a trend of promotion at low concentration and inhibition at high concentration.The activity of dehydrogenase did not change significantly,while the activities of urease,phosphatase,sucrase and catalase decreased significantly under high concentration of carbendazim（P<0.05）,and the inhibitory effect on sucrase was the most significant,with the inhibitory rate reaching 47.8%.4.Compound stress effect of PE and carbendazim on cotton and soil:The toxic effects of carbendazim and PE treatment on cotton plant health and soil physical and chemical properties were intensified in the combined pollution group.The cotton biomass in the combined treatment group with high concentration carbendazim(5mg·kg-1)and small particle size PE（500 mesh 5%）was the lowest.The inhibitory effects on plant height,root length,fresh weight and dry weight were 1.57,1.76,2.12and 2.22 times higher than those in carbidazm(5 mg·kg-1)treatment group,and 2.42,2.74,2.08 and 2.42 times higher than those in PE（500 mesh 5%）treatment group.The chlorophyll content of cotton in the compound treatment group was the lowest.Compared with CK group,the reduction of 45.2%was higher than that of PE（500mesh 5%）group（0.1%）,carbendazim(5 mg·kg-1)group（39.6%）,carbendazim(2mg·kg-1)combined with PE（500 mesh 5%）group（22.0%）,carbendazim(5 mg·kg-1)and PE（100 mesh 5%）combined treatment group（6.2%）.In the 100 mesh PE combined pollution group,the effect of carbendazim on cotton root activity was reduced by 47.3%to a certain extent,while in the 500 mesh PE combined pollution group,the effect of carbendazim on cotton root activity was increased by 27.3%to a certain extent.Combined stress generally increased the effect of single pollutant addition on the activities of soil urease,catalase,dehydrogenase and protease.The inhibition degree of above enzyme activity in carbendazim(5 mg·kg-1)and PE（500 mesh 5%）combined treatment group decreased by 60.0%,38.1%,46.8%and 13.2%compared with carbendazim(5 mg·kg-1)alone group,respectively.They were 61.9%,60.0%,14.8%and 12.7%lower than those treated with PE alone（500 mesh 5%）.The combined treatment group with high concentration of carbendazim(5 mg·kg-1)and PE（500 meses 5%）enhanced the activation of cotton sucrase and cellulase by 12.6%and 9.7%,respectively,while the phosphatase activity of the combined pollution group had no significant inhibitory effect compared with the single carbendazim pollution group.5.From the analysis of the previous two chapters,it was known that the high-concentration and small-particle pollution group had the most significant toxic effect on the physicochemical properties of cotton plants and soil.Therefore,the bacterial community structure of cotton rhizosphere in soil treated with carbendazim(5mg·kg-1)and PE（500 mesh,5%）was analyzed.Illumina Mi Seq3000 sequencing analysis showed that the number of bacteria in the single carbendazm,PE and composite pollution groups was less than that in the CK group at the phylum and genus level,and the number of bacteria in the composite pollution group at the genus level was the least,which was 36 less than that in the CK group.α,βdiversity and ANOSIM/Adonis analysis showed that the combined pollution group significantly decreased the bacterial community richness（P<0.05）and changed the community structure.The number of unique OTUs in CK group was 768,which was 197,186 and 295 more than that in carbendazim,PE and compound treatment groups,respectively.In the analysis of species difference,the relative abundance of beneficial bacteria in the soil of CK group,such as Bacillus,RB41,Vicinamibacteraccae,etc.,which had antibacterial function and maintained the stability of bacterial community,was higher.The relative abundance of degrading actinomycetes such as Nocardia was increased in the pollutant treatment group.Changes in bacterial community will inevitably lead to changes in soil bacterial function.Further analysis of FAPROTAX function showed that pollutant treatment group weakened some biochemical functions of bacteria in cotton rhizosphere soil,such as urea decomposition,fermentation and nitrate reduction.Genus level species correlation network analysis showed that the pollutant treatment group weakened the rhizosphere bacterial interaction,reducing the number of positive correlation connections,increasing the number of negative correlation connections,simplifying the network structure,and changing the key flora.The results showed that PE adsorption of carbendazim was a multilayer adsorption on non-uniform surface affected by chemical and physical adsorption processes.PE adsorption of carbendazim formed complex stress,changes the physical and chemical properties of soil and carbendazim bioavailability,affects plant growth parameters and soil enzyme activity,and changes the structure and function of bacterial community.The combined stress effect varied with PE particle size and carbendazim concentration.This study can provide important data for the systematic evaluation of the adsorption mechanism of pesticides on microplastics and the impact on the ecological security of soil environment,and can also provide ideas for the study of the combined pollution of other microplastics and hydrophobic pollutants. |
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