Formation Of Microplastics During The Use Of Degradable Plastic Film And Its Effect On Soil-plant System

Microplastics(MPs)refer to plastics with particle size less than 5mm,which widely exist in the earth’s environment.One of the sources of microplastics is ageing and broken plastic products.During agricultural production in China,a large number of mulching films are used.With the ageing and breaking of mulching films in farmland,the residual films including microplastics may negatively affect the soil-plant system and even cause groundwater MPs pollution.This problem has attracted everyone’s attention.With the increasing serious problem of "white pollution" in China,the research on degradable plastic film is increasing,and it has been used in some areas.However,ageing and breaking of degradable plastic film is faster than non-degradable plastic film,which means that residual film and MPs may be formed within short time.They may change the environmental behaviors of present pollutants,and cause harm on soil and groundwater.However,presently little is known in this field.In this study,through indoor and field in-situ experiments,the degradation dynamics of a degradable plastic film and the possibility of micro-plastic formation were studied during it utilization,and the changes of surface morphology and film structure were also observed.The effects of residual film and MPs on water transportation and root distribution in soil were studied.Through large-scale soil column experiment,the MPs migration in soil and the related influencing factors were studied,and the risk of MPs to groundwater pollution was evaluated.The main experimental results are as follows:Experiment on of the MPs formation during the use of degradable plastic film: Field in-situ experiment showed that natural lighting in summer significantly increased ageing and breaking of degradable plastic film.MPs could be found after 120 days lighting without additional forces.If additional forces was used,such as rubbing,more than 95.5%residual films belonged to MPs.However,in the indoor simulated illumination experiment,degradable plastic film did not form MPs independent of additional forces in 120 days.Similar results were also observed in indoor and field in-situ experiment that studied the effects of soil microorganism on degradable plastic film degradation.It was pointed out that degradation and the changes of morphology and film structure were observed in these experiment.This indicated that MPs formation may occur during use of degradable plastic film in filed..(2)Experiment on effects of residual films/MPs on soil-plant system: Residual film /MPs caused negative effects on water transportation and root distribution in soil,and the more the area of residual film is,the larger the negative effects were.Residual film / MPs caused significant effect on community structure of soil bacteria.The abundance of Proteobacteria and Verrucomicrobia in soil bacteria increased with increasing contact time between residual films/MPs and soil.Residual film / MPs caused an increase in soil urease activity,and it showed complex changes with increasing contact time.(3)Experiment on MPs migration in soil: Among red soil,black soil and brown soil,MPs migration increased with increasing MPs amount in soil.Relative to neutral rainwater,acid rainwater decreased MPs migration in soil.MPs migration increased with increasing rainfall in brown soil.In terms of MPs size(<0.33 mm,0.33-0.46 mm,>0.46mm),>0.46 mm MPs had the longest distance migration,while 0.33-0.46 mm MPs had the least distance migration.In terms of total MPs migration in soil,black soil > brown soil > red soil.Moreover,there were the most MPs with <0.33 mm in brown soil leachate,and the most MPs with >0.46 mm in black soil leachate.This indicated that groundwater under red soil suffered the least possibility by MPs pollution.