Distribution Characteristics Of Residual Plastic Film And Microplastics In Farmland And Their Effects On Water And Soil Environment

"Drought"and"cold"are the bottlenecks that restrict the improvement of agricultural productivity in arid and semi-arid areas of northwest China.Plastic film mulching technology has effectively broken the restrictions of these two factors,and has become the main technology for drought resistance,water saving and grain increase in arid areas of north China.However,due to the low recovery rate of plastic film and the difficulty of polyethylene film degradation,the residual film continues to accumulate in the surface soil,posing a threat to farmland soil and water environment.A large amount of plastic film residues not only cause adverse effects on soil structure and crop growth,but also cause a series of ecological and environmental problems.In recent years,many studies have been carried out on the ecological and environmental problems derived from farmland plastic film residues.However,the distribution characteristics of farmland residual plastic film（RPF）and microplastics derived from RPF（RPF-MPs）,as well as the impact mechanism of dissolved organic carbon（DOC）and phthalate acid esters（PAEs）released on soil and water environment are still unclear.Therefore,through field sampling,artificial climate chamber cultivation,potted experiment and model simulation,this study explored the distribution characteristics of residual film and microplastics in farmland and their effects on soil and water environment,in order to provide scientific and technological support for agricultural sustainable development and food security.The main results are as follows:（1）The driving factors of RPF accumulation and its direct and indirect effects on maize yield were explored.The four main driving factors affecting the RPF content were plastic film mulching years,government recycling policies,spacing between rows and recycling methods.The contribution of plastic film thickness to the RPF content was relatively low（R²（M）=0.099）.RPF had direct and indirect effects on maize yield,with the direct effect value being-0.111 and the indirect effect value being-0.010.The indirect effect of RPF on maize yield was also positive and negative.The negative value mainly reflected that the RPF hindered the downward migration of water,increased the contents of available nitrogen,phosphorus and potassium in the surface soil,aggravated soil salinization,and reduced nutrient absorption of crops.The positive value mainly reflected that the RPF increased the content of nitrate nitrogen in soil,which promoted the growth and yield of maize.However,the total indirect effect value was still negative（-0.010）.（2）The distribution characteristics and correlation between RPF and RPF-MPs were clarified.The mulching years decrease or the soil depth increase would significantly reduce the content of RPF and RPF-MPs.The RPF mainly distributed in 0-10 cm soil layer,and the RPF-MPs were mainly distributed in 10-20 cm soil layer.The RPF area was mainly 1-5cm²,accounting for 45%of the total,and the RPF-MPs size was mainly 0.1-1 mm,accounting for 48%of the total.There was a significant positive correlation between the RPF and RPF-MPs,with the weight of RPF increasing by 1.00 kg ha^-1,and the weight of RPF-MPs increasing by 0.27 kg ha^-1.Both Scanning electron microscope and Fourier infrared spectroscopy of RPF-MPs revealed that the RPF-MPs may further break down and decompose into smaller particles,which may adversely affect crop production and human health.（3）The effects of RPF source DOC on soil carbon cycling were explored.Plastic film releases DOC into soil,which can be utilized by microorganisms and accelerates the decomposition of soil organic carbon（SOC）,leading to increased CO₂emissions.Both plastic C and SOC provided carbon sources for CO₂emissions.The addition of RPF to the soil at the early stage（15 days）slows down the decomposition of SOC,while at the later stage（60 days）it accelerates SOC decomposition.The microbial utilization rates of DOC released from polyethylene（PE）and biodegradable plastic（BP）RPF were 44-46%and81-82%,respectively.The release rate of DOC from RPF in soil is affected by many factors such as soil water content,organic carbon content,and microorganisms.RPF releases a large amount of DOC in the soil immediately and then gradually decreases.（4）The release regularity of PAEs from plastic film and their migration in soil-plant-atmosphere continuum（SPAC）system were analyzed.The release rate of PAEs from polyethylene（PE）and biodegradable plastic（BP）films increased with the increase of water content during the plastic film mulching（PFM）stage and the RPF stage.During the life cycle of plastic film（BP and PE）,about 50%of PAEs were released into the atmosphere,about 30%into the soil,and the remaining less than 20%were released slowly into the soil.The higher the soil PAEs concentration and plant transpiration,the higher the PAEs concentration in maize roots,stems and grains.The higher plant transpiration,the lower PAEs concentration in maize leaves.The higher the soil water content,the higher the transpiration rate,and the stronger the upward migration power of dibutyl phthalate（DBP）and di（2-ethylhexyl）phthalate（DEHP）.The overall upward migration ability of DBP was greater than that of DEHP.PAEs migrated easily from soil to plant roots（RCF>1）and weakly from roots to stems（IF<1）.During the maize growing period,the total amount of PAEs absorbed was 2%-4%of the total amount released by the RPF.（5）The weight of RPF and RPF-MPs,the carbon dioxide excess emissions of RPF farmland,and the accumulation of PAEs in the SPAC system in northwest China were estimated.In 2020,the weight of RPF and RPF-MPs in the six northwest provinces of China was 9.95?10⁵ and 2.68?10⁵ tons,respectively.The addition of PE plastics would increase soil CO₂ emission by 15%.The RPF and RPF-MPs would increase farmland CO₂emission by 1.77?10⁵ tons in the six provinces of Northwest China.Both soil PE plastic concentration and soil type significantly affected the CO₂ emission.If PE film was replaced by BP film in the six northwest provinces of China,the residual weight of BP film in the six northwest provinces in 2020 would be 5.15?10⁵ tons,and 3.38?10⁵ tons would be degraded,resulting in 1.05×10⁶ tons of farmland soil CO₂ excess emissions,which was about 6 times of PE film.In 2020,DBP and DEHP emissions from PE plastic film in six northwest provinces of China were 12.59 and 22.83 tons,respectively.After harvest,1.30tons of DBP and 4.46×10^-3 tons of DEHP were accumulated in the atmosphere,0.27 tons of DBP and 4.19 tons of DEHP in soil,and 5.35×10^-5 tons of DBP and 7.34×10^-9 tons of DEHP in plants.Most PAEs were eliminated through degradation.In summary,this study started from the correlation between RPF and RPF-MPs,estimated the excess CO₂ emission in farmland and the accumulation of PAEs in field crops in the six northwestern provinces of China based on the content of RPF and RPF-MPs.The RPF would lead to a large amount of CO₂ excess emission in farmland,but the released PAEs accumulate less in plant bodies.Considering the aspects of soil CO₂ emission and plant accumulation of PAEs,BP film is not suitable to replace PE film.This study provides a new perspective for exploring the environmental risks of plastic film mulching,and can provide important technological support for sustainable agricultural development,food security,and health.