Effects Of Microplastics On Soil Soluble Organic Matter Composition And Structure

Microplastics are plastic particles with a size less than 5mm.Soil contains a large number of microplastics.However,the research on the influence of microplastic pollution on soil ecology has just getting started.Once microplastics entering the soil,which can change soil physical and chemical properties and further affect microbial community.Soluble organic matter（DOM）plays an important role in regulating soil nutrient cycles and contaminant behavior in soil,and the decomposition and transformation behavior of DOM in the environment is closely related to soil physical and chemical properties as well as microbial community.However,the effect of microplastics on soil DOM is not clear.Therefore,it is very important to explore the influence of microplastics on the structure and composition of soil DOM.In this study,we designed a 310-day soil incubation experiment to examine the effects of 7%and 14%（W/W）polyethylene（PE）,polystyrene（PS）,and polyvinyl chloride（PVC）MPs,to explore the influence of microplastics on soil physical and chemical properties and the change of microbial community on soil DOM structure.The results are as follows:（1）Microplastics increased the contents of soil organic matter（SOM）,and decreased the contents of total nitrogen（TN）and soluble organic carbon（DOC）.Microplastics had no significant effect on p H and reduced soil conductivity（EC）.With the increase of culture time,p H increased slowly,while the value of EC showed a downward trend.The findings revealed that all three kinds of MPs stimulated soil enzyme activities,with 14%PVC,7%PS,and 14%PE having the greatest impact on the activities of catalase,urease,and alkaline phosphatase.（2）MPs did not change the types but the relative abundance of these phyla in soil.MPs mainly increased the abundance of Proteobacteria,Actinobacteria,and Ascomycota as well as reduced the abundance of Acidobacteria,Basidiomycota,and Chytridiomycota.The response of fungi to MPs was stronger than that of bacteria,and the diversity of fungal communities was more sensitive to the impact of MPs than that of bacterial communities.PVC had the greatest impact on the diversity of microbial communities.PICRUSt analysis revealed that MPs mainly promoted the metabolic function of soil bacteria.Based on the FUNGuid tool,it was found that MPs had a significant effect on fungi,which were closely related to plant growth.（3）Ultraviolet and fluorescence spectra showed that microplastics significantly reduced the values of SUVA₂₅₄ and SUVA₂₆₀,as well as the fulminate-like substances.Microplastics were not conducive to the humification process of soil DOM.Fourier transform ion cyclotron resonance mass spectrometry revealed that soil DOM was mainly composed of CHO,CHON,CHOS and CHONS,and the abundance of CHO compounds was the lowest and CHON compounds was the highest in CK treatment.Microplastics significantly reduced the value of O/C,DBE and M/Z,and increased the soil H/C ratio.In addition,CK treatment had the largest relative abundance of lignin and aromatic compounds,and microplastics treatment had the largest relative abundance of proteins,amino acids and lipids.Microplastics may promote the degradation and transformation of soil DOM refractory organic compounds.The correlation analysis of soil physical and chemical factors,enzyme activities and microbial community diversity showed that microbial community diversity was significantly correlated with soil DOM composition and structural characteristics.Microplastics can change the physical and chemical properties as well as enzyme activities of soil,thus further affecting the composition and structure of soil microbial community,reducing the diversity of dominant bacteria community,and affecting the community functions of soil bacteria and fungi.Microplastics can change the composition and structure of soil DOM,reduce soil humus-like substances,and promote the transformation of soil DOM from non-degradable organic matter.Correlation analysis showed that microorganisms were the main factors affecting the composition and structure of DOM.More systematic and comprehensive studies are needed to explore the interaction between soil microbial community and DOM under microplastic exposure in the future.