Effects Of Mulch Film-derived Microplastics On The Structure And Function Of Soil And Earthworm Metaphire Guillelmi Gut Microbiota

Mulch film has been widely used in agricultural production because of its advantages of increasing temperature and preserving soil moisture,inhibiting weed growth,reducing the harm of pests and diseases,and improving crop yield.At present,mulch film is mainly made of polyethylene,which is chemically stable and difficult to degrade in the environment.In addition,due to lacking of efficient recycling technology and mechanism of residual mulch film,the mulch film residue in farmland soil is becoming more and more serious with extensively use of mulch film.The residual mulch film in farmland soil can fragment into microplastics under the combined action of mechanical crushing,ultraviolet irradiation,weathering and biodegradation,and then continue undergo a series of aging processes.These processes can result in significant changes in the structure and properties of microplastics,and the surface of microplastics can also be colonized by a variety of microorganisms and form biofilms,which will have an important impact on the soil ecosystem.However,previosu studies mainly focused on aquatic ecosystems and used virgin microplastics.Hence,further research into the impact of microplastics on soil ecosystems,especially microplastics aging in the actual environment is needed.In this study,farmland residual polyethylene mulch film(FRMF)and farmland soil(FS)were collected from peanut fields with mulch film planting history.The 16 S rRNA gene highthroughput sequencing technology was used to investigate the community structure and function of bacteria in FRMF and FS.At the same time,unused polyethylene mulch film and farmland residual polyethylene mulch film were used to prepare the unaged polyethylene mulch film-derived microplastics(MPs)and field-aged polyethylene mulch film-derived microplastics(MPs-aged).Earthowrms(Metaphire guillelmi)were exposed to soil amended with 0.25%(w/w)MPs or MPs-aged for 28 d to explore the effects of mulch film-derived microplastics on the community structure and function of soil and earthworm gut microbiota.This study will provide data support for ecological risk assessment of microplastics in soil environment.The main results of this study are as follows:(1)The community richness of bacteria attached on FRMF was significantly lower than that of FS,and the community structure of bactiera attached on FRMF was significantly different from that of FS,indicating that FRMF could provide a unique habitat for soil bacteria.The relative abundance of Nocardioidaceae,Clostridiaceae,Micrococcaceae and Mycobacteriaceaein in bacteria community attached on FRMF was obsviouly higher than that in FS,indicating that some potential plastic degrading bacteria and pathogenic bacteria enriched on the FRMF surface.The strength of the biological process related to aromatic compound degradation in bacteria community attached on FRMF was significantly higher than that in FS,while the strength of the biological processes related to the nitrogen cycle(such as nitrification,aerobic ammonia oxidation,nitrate reduction and aerobic nitrite oxidation)and plant pathogen was significantly lower than that in FS.(2)For soil samples,MPs and MP-aged exposure did not significantly affect the community diversity and richness of soil bacteria,but significantly changed the community structure of soil bacteria.For the earthworm gut content samples,MPs exposure significantly reduced the community richness of earthworm gut bateria compared with the control group,but did not significantly affect the community diversity and structure of earthworm gut bacteria.In contrast,MPs-aged exposure did not significantly affect the community diversity and richness of earthworm gut bacteria,but significantly changed the community structure of earthworm gut bacteria.(3)MPs and MPs-aged exposure resulted in the relative abundance of bacteria belonging to Proteobacteria,Bacteroidetes and Actinobacteria in soil and earthworm gut contents samples significantly changed compared with the control group.Specifically,MPs exposure resulted in a significant increase in the relative abundance of Methylophilaceae and a significant decrease in the relative abundance of Chitinophagaceae,Sinobacteraceae and Nitrospiraceae in the soil bacterial community compared with the control group.MPs-aged exposure led to a significant increase in the relative abundance of Aeromonadaceae and Pseudomonadaceae and a significant decrease in the relative abundance of Chitinophagaceae and Sinobacteraceae in the soil bacterial community compared with the control group.The relative abundance of Rhizobiaceae for the gut bacterial community in earthworm significantly increased after exposure to MPs,while the relative abundance of Gaiellaceae,Cytophagaceae,Pseudomonadaceae and Geodermatophilaceae significantly decreased.The relative abundance of Rhizobiaceae and Rhodospirillaceae for the gut bacterial community in earthworm significantly increased after exposure to MPs-aged.(4)Compared with the control group,the relative abundance of nitrate reduction,nitrate respiration,nitrogen fixation and nitrogen respiration in the soil bacterial community was significantly decreased after exposure to MPs,and the relative abundance of aerobic ammonia oxidation and nitrification in the soil bacterial community was significantly decreased after exposure to MPs-aged.These results indicated that MPs and MP-aged exposure had a potential inhibition effect on the soil biological processes related to nitrogen cycle.MPs and MP-aged exposure significantly reduced the relative abundance of aerobic ammonia oxidation and nitrification in the gut bacterial community of earthworm compared with the control group,and MPs exposure also significantly increased the relative abundance of aerobic nitrate oxidation,nitrate reduction and nitrogen fixation functions in the gut bacterial community of earthworm compared with the control group.These results indicate that MPs and MP-aged exposure have a potential inhibition effect on aerobic ammonia oxidation and nitrification in the biological processes related to the nitrogen cycle of earthworm gut,and MPs exposure also has a potential activation effect on aerobic nitrate oxidation,nitrate reduction and nitrogen fixation in the biological processes related to the nitrogen cycle of earthworm gut.