Study On The Distribution Characteristics Of Microplastics In Cotton Field Soil In Typical Plastic Film Covered Areas

A large number of plastic film fragments remain in the farmland soil in the long-term plastic film mulching planting area,and continue to be broken into microplastics through physical,chemical,and biological actions.Microplastics refers to plastic fragments with a particle size of<5 mm.As a persistent pollutant,they have a serious impact on soil ecosystems.The distribution characteristics of pollution risks in the terrestrial environment have not been clarified.Understanding the morphological characteristics and spatiotemporal distribution characteristics of farmland soil plastic fragments provides a theoretical basis for controlling the input of soil microplastics pollution at the source and proposing corresponding solutions for the prevention and control system.In this study,a typical long-term cotton field covered with plastic film in Xinjiang,northwest China,was selected as the research area,and farmland soils with different years of plastic film mulching were sampled for testing and analysis.The spatial distribution characteristics of mulch film residues and microplastics in 60 typical cotton fields were studied using a sample plot survey method.The experiment used saturated sodium chloride density separation method combined with a body microscope to extract and identify soil microplastics.Scanning soil microplastic samples using scanning electron microscopy（SEM）to study the morphology and surface structure of microplastics,and using Fourier transform near-infrared spectroscopy（μ-FTIR）method is used to identify the chemical composition of microplastics in soil,and the characterization and elemental composition of microplastics are analyzed using scanning electron microscopy energy X-ray spectroscopy（SEM-EDS）.The abundance,distribution,and surface characteristics of plastic fragments in farmland soil were studied.To determine the distribution and impact of long-term continuous plastic mulching residues,thereby providing a theoretical reference for plastic film pollution control and micro plastic pollution prevention and control.The main research contents and results are as follows:（1）The sample plot survey method was used to investigate the distribution characteristics of residual plastic film fragments in typical cotton fields of Xinjiang with different years of plastic film mulching.Incomplete recovery of plastic film mulching can cause a large amount of residual plastic film fragments to the soil.The content of plastic film residues in farmland soils with different film mulching years varies greatly.As the film mulching years increase,both the amount of plastic film residues and the amount of residual film fragments are significantly increasing,and the number of small quality residual film continues to increase.Covering planting for 5 to 30 years,the residual amount ranged from 40.219±5.527 to164.083±8.310 kg·ha^-1,and the<0.050 g fragments increased from 88.5×10⁴pieces·ha^-1to 216.5×10⁴pieces·ha^-1,indicating that the residual film degraded in the soil and became more broken.It is distributed at a depth of 0 to 60 cm in farmland soil,mainly in the 0 to 30 cm tillage layer.As the soil depth deepens,the amount of plastic film residue and the number and average size of residual film fragments show a significant downward trend as the soil depth increases.It can be seen that the more fragmented residual film in the soil is gradually infiltrating into the deep soil,causing small pieces of residual film to accumulate in the deeper soil layer.（2）The microplastics in cotton field soil were separated by density separation method,and the microplastics with a particle size of 0.05～5 mm were identified by visual inspection combined with a volumetric microscope.The distribution characteristics of plastics in cotton field soils with different mulching years have been explored.The presence of microplastics has been detected in all farmland samples,and the presence rate in the entire 0～60cm soil layer is 100%.The highest abundance of microplastics is in the 10～20cm depth of 30 years of mulching,reaching 114±3.06 pieces·100 g^-1.The minimum abundance value of microplastics appears in the 50～60cm soil layer of 5 years of mulching,reaching 13±2.13 pieces·100 g^-1.The average abundance of 0～60 cm microplastics in cotton field soil ranged from 36.83±3.24 to 78.51±2.57 pieces·100g^-1（soil）,and the abundance of microplastics increased with the increase of the number of years covered with plastic film,with a significant difference（P<0.05）.The abundance of microplastics gradually decreases with the depth of the soil layer,indicating that microplastics with smaller particle sizes are constantly migrating towards the depth of the soil.The study area is open air farmland,far from factories and cities,without other obvious pollution sources.The abundance of micro plastic is lower than that of suburban farmland and facility farmland.（3）Through SEM andμ-FTIR detected the composition and appearance characteristics of microplastics in cotton field soil.According to stereomicroscope and scanning electron microscope observations,cracks,bumps,and holes were observed on the surface of plastic film fragments remaining in the soil environment,indicating that oxidation of residual film had a certain contribution to the abundance of soil microplastics.Microplastics are mainly presented in the form of fragments,fibers,and particles in farmland soil,accounting for 48.39%,31.98%,and 19.63%,respectively.At the same time,it was observed that the surface of the microplastics had many cracks and depressions,fractures and twists,as well as multiple micropores,and the edges were damaged,showing continued oxidation;The cracking and porosity of microplastics lead to an increase in their specific surface area and enhanced adsorption capacity.EDS analysis showed that the surface of microplastics not only adsorbed common elements in soil,but also major elements and metal elements in fertilizers,resulting in a more serious soil composite pollution effect.adoptμ-FTIR spectral analysis and comparison results showed that the main component was polyethylene（PE）,indicating that it originated from the weathering and cracking of plastic film residues that were not completely left in the soil due to long-term mulching.