Distribution And Degradation Characteristics Of Residual Film In Farmland Soil And Its Influence On Soil Physical And Chemical Properties

China has a large area of mulched arable land,and the annual use of mulch is huge.As the use of mulch increases year by year,its recycling measures are not widely and compulsorily applied,resulting in increasingly prominent problems caused by the mulch residues on the mulched farmland.The residual film does not only affects the normal growth of crop roots but also adversely affects the physical and chemical properties of the soil.In recent years,the microplastics formed during the degradation of the residual film have become a new type of pollutant,which was once considered a hidden danger threatening the ecological safety of the earth.It is believed that its abundance in the soil may be higher than that in the ocean.This research adopted an experimental method which was carried out on a farm in Tumuji Town,Inner Mongolia Autonomous Region from 2018 to 2020.The distribution of macroscopic residual film fragments and film-type microplastics in the 0-40cm layer of the soil profile were studied respectively.Also,the changes in the distribution of the microplastics over time were investigated.By comparing the artificial landfill of ordinary polyethylene film with the collected residual film,the degradation characteristics of the residual film in the soil over time were studied.The experiment also studied the influence of the residual film and film-type microplastics on the physical and chemical properties of the soil.The results of the study are as follows:（1）The changes in the distribution characteristics of the residual film over time.In this experiment,in mid-April 2018,when the farmland was not ploughed in spring,1763samples of the residual film were collected from the soil in the 0.61km2 film-covered area.Thus,two sampling areas,5 sampling points in each sampling area,and 4 sampling depths in each sampling point（0-10,10-20,20-30,and 30-40）.The size of each residual film fragment was measured through digital images in the laboratory.The results of the study showed that film mulching for 8 years increased the number of residual film fragments in the soil,and these fragments can be distributed within the first 30cm depth of the farmland soil.The surface size of the residual film fragments was between 0.25cm²-109cm²,and the average size,number,and distribution density of the residual film fragments showed a decreasing trend with the increase in soil depth.After ending the film mulching,the overall density of residual film in the soil（0-40cm depth）decreased at an average annual rate of only 0.525kg·ha^-1.The decrease in the distribution density of the large residual film was also accompanied by a small residual film.The distribution density of small residual film showed an increasing trend,and the number of small residual film fragments showed an overall increasing trend during with an average annual growth rate of 2.13×105p·ha-1,which also illustrates the occurrence of the fragmentation and degradation process of residual film in the soil from another perspective.In addition,according to the analysis of the vertical distribution characteristics of the residual film over time in this experiment,it can be seen that the residual film accumulated significantly over time in the soil layer below 10 cm.Among the microplastics of all sizes collected,small pieces of residual membrane fragments（<1cm2 and 1-5cm2）were the most as they occupied a large proportion of the total sampled microplastics and were mainly accumulated in the deeper layer of the soil.In other words,after the mulching was completed,the residual film fragments in the soil became more dispersed over time,allowing small pieces of residual film to accumulate in deeper soil layers,making the recovery of the mulching film more challenging.From this study,the residual film distribution over time in the soil profile after long-term mulching was characterized as there are few related studies at present.（2）Distribution characteristics of microplastics over timeThe abundance of film-type microplastics in the mulched area was compared to its surrounding environment,and we found that the abundance of microplastics in each soil layer of the mulched area was significantly higher than that in the surrounding environment,which fully demonstrated that the film mulching operation brought extremely high levels of disturbance to the soil.Subsequent research results showed that the highest abundance of the film-type microplastics in the mulched area was recorded in the 0-10cm soil layer,while the richness of the microplastics were recorded in the 10-20cm and 20-30cm soil layers.The test results showed that with the degradation of the residual film in the soil,the film-type microplastics in the film-mulched area gradually accumulated in the soil profile.The film-type microplastics accumulated faster in the 0-10cm soil layer,and the accumulation rate was 1781p·kg-1.In addition,studies have shown that compared with film-type microplastics of 5-1mm and1-0.25mm,the number of film-type microplastics with a particle size of 0.25mm-0.05mm is more dominant in the soil at various depths.（3）Changes in the degradation characteristics of ordinary polyethylene film and residual film in soilThe test results showed that the surface characteristics of the ordinary polyethylene film had no obvious changes in the soil in the short term（0-150 days）,and the relative strength of its functional groups decreased at 2917cm-1.The tensile strength,elongation at break,weight,and average molecular weight all showed a significant decline in the short term（2018-2020）.The degradation rate of the residual film was very low especially the residual film distributed below the 20cm depth.This also indicates that once the residual film enters the deep soil,it will go through a longer-term degradation cycle.（4）The influence of residual film on the physical and chemical properties of soilThe test results showed that during the degradation of the residual film and the accumulation of film-type microplastics,at each depth of the film-covered area and the conventional farming area the soil p H and electrical conductivity did not vary significantly with time however the p H and electrical conductivity of the film-covered area were significantly different from those in conventional farming areas,especially in the 0-10cm and 10-20cm soil layers.Compared with the conventional farming area,the film-covered area recorded an increase in the amount of residual film over the years.Also,the film-covered area recorded an increase in the soil bulk density and a decrease in total soil porosity at each depth of the soil layer.The soil bulk density and total porosity of the 20-30cm varied the most.The water holding capacity of the 20-30cm soil layer in the mulched area was significantly lower than that of the30-40cm soil layer,and these were significantly different from the conventional farming area.The overall soil organic carbon in the 0-30cm soil layer of the film-mulched farming area decreased significantly（P<0.05）,while the soil organic carbon content of the conventional farming area did not increase or decrease significantly during the entire experimental period（P<0.05）.In summary,the residual film and the thin-film microplastics were widely distributed in the0-30cm soil layer,and the maximum distribution density of the two was concentrated in the0-20cm soil layer.Over time,the deeper soil layers accumulated small residual membranes and thin-film microplastics.From the perspective of the degradation characteristics of the residual film,although the newer mulch film shows a slower degradation rate,the results showed that if the residual film reaches the 20-30cm soil layer,the degradation rate is much lower than that of the 0-20cm soil layer.The existence of residual film within the soil profile will affect the physical properties of the soil and also create conditions that are not conducive to crop growth.Although film mulching has a positive effect on the accumulation of soil organic carbon,the negative impact of the residual membranes cannot be overlooked.Therefore,in extremely fragile farmland ecosystems,the long-term residual film residue and the gradual accumulation of microplastics will cause the deterioration of soil physical properties and the reduction of soil nutrients.Moreover,if the residual film is not recycled after long-term mulching,it may pollute the deeper horizons of the soil over time,which is not conducive to the sustainable development of agriculture.Hence more attention should be given to the film-mulching technology and how to improve its effectiveness.