Effects Of Polyethylene Microplastics On Soil Aggregate Properties And Microbial Diversity

Generally,plastic particles with particle size less than 5 mm are defined as microplastics.As a new pollutant,microplastics can exist in the soil environment for a long time and affect the soil ecosystem.Soil aggregates,as the basic unit of soil,have significant heterogeneity and affect the distribution and function of microorganisms in soil environment.The presence of microplastics in soil environment will affect the physical and chemical properties of soil aggregates,change the microbial community structure in soil aggregates,and then affect the ecosystem function of soil.Therefore,it is of great significance to study the effects of microplastics on soil aggregate properties and microbial diversity.In this study,soil was divided into soil aggregates with three particle sizes of > 250 ?m,53~250 ?m and <53 ?m by dry sieve method.Polyethylene microplastics(PE-MPs)less than 100 ?m were selected for soil culture experiment.The soil physical and chemical indexes such as the soil pH,cation exchange capacity(CEC),total organic carbon(TOC),total nitrogen(TN),total phosphorus(TP),total potassium(K),available phosphorus,dissolved organic carbon(DOC),and dissolved organic nitrogen(DON)were determined.The soil enzyme activities such as catalase,polyphenol oxidase,urease,manganese peroxidase,laccase and ?-glycosidase were also examined.These determination of soil physical and chemical indexes and enzyme activities were used to analyze the effects of PE-MPs on soil aggregate properties.High throughput sequencing technique was used to analyze the microbial diversity in soil aggregates.The correlation between the change of soil aggregate properties and the change of microbial diversity was discussed by means of correlation analysis.The structural equation model was used to proposed the possible paths on the influence of soil aggregate properties on microbial diversity.Experimental results of this study are as follows:(1)PE-MPs reduced the content of soil pH,CEC,TOC,available phosphorus,DOC,DON,TN,TP,TK and other physical and chemical indexes in soil aggregates.The change trend of physical and chemical indexes was different with different particle sizes of aggregates.The decrease trend of physical and chemical indexes was more significant in the aggregates <53 ?m.The change of physical and chemical properties indicates that PE-MPs tend to acidify soil,change the aggregate structure of soil,reduce the content of nutrient elements such as carbon,nitrogen,phosphorus and potassium,and affect the level of soil function.(2)PE-MPs inhibited the activities of catalase,polyphenol oxidase,urease,manganese oxidase,laccase and ?-glucosidase in soil aggregates,and the degree of inhibition varied with the size of aggregates.Among them,catalase and urease activities were significantly inhibited in >250 ?m aggregates,polyphenol oxidase and ?-glucosidase activities were significantly inhibited in 53~250 ?m aggregates,and manganese oxidase and lassase activities were significantly inhibited in <53 ?m aggregates.The inhibition of soil enzyme activity suggests that PE-MPs may affect the enzymatic reaction in the soil system,further inhibit the circulation of soil nutrients C,N and P,and reduce soil fertility and soil quality.(3)PE-MPs significantly reduced the diversity and richness of bacteria in soil aggregates,changed the dominant bacteria in soil aggregates.Actinomycetes replaced proteobacteria to become the dominant bacteria.The relative abundance of actinomycetes increased significantly in >250 ?m aggregates.The relative abundance of acidobacteria increased significantly in 53~250 ?m aggregates.In <53 ?m aggregates,the relative abundance of proteobacteria decreased significantly.These results indicate that PE MPs are selective to different bacteria(phylum level),and the particle size of aggregates is also an important factor affecting the bacterial community structure.There was a significant correlation between bacteria(phylum level)and soil aggregates.Among the >250 ?m aggregates,bacteroidetes were significantly positively correlated with DOC and catalase,negatively correlated with DON and ?-glucosidase,and negatively correlated with actinomycetes and DOC.In 53~250 ?m aggregates,bacteroidetes were positively correlated with CEC,polyphenol oxidase,catalase and DOC,while proteobacteria were positively correlated with catalase.In <53 ?m aggregates,actinomycetes and bacteroidetes were positively correlated with DOC and available phosphorus.It indicates that the physicochemical indexes of soil aggregates can directly affect the bacteriophytes and indirectly affect the diversity of bacteria through the action on enzyme activity by the analysis of structural equation model.(4)PE-MPs significantly reduced the diversity and richness of fungi in soil aggregates and changed the community structure of fungi in soil aggregates.In >250 ?m and 53~250 ?m aggregates,the relative abundance of ascomycetes increased significantly,while in <53 ?m aggregates,the relative abundance of basidiomycetes decreased significantly.It indicates that the particle size of aggregates is also an important factor affecting the structure of fungal community.There was a significant correlation between fungi(phylum level)and soil physical and chemical properties and enzyme activity.Ascomycetes were negatively correlated with TN,catalase,polyphenol oxidase and laccase in >250 ?m aggregates.Ascomycetes were negatively correlated with DOC,available phosphorus and ?-glucosidase in 53~250 ?m aggregates.In <53 ?m aggregates,ascomycetes were positively correlated with DOC and polyphenol oxidase,and negatively correlated with CEC and TN.It indicates that the physicochemical indexes of soil aggregates can directly affect the fungi and indirectly affect the diversity of fungi through the action on enzyme activity by the analysis of structural equation model.