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Abstract [Objectives] This study was conducted to explore the characteristics of soil residual film pollution load in cotton fields in the Yellow River Delta， so as to effectively control soil film pollution.
[Methods]Cotton field mulching film survey and residue monitoring were conducted in the Yellow River Delta area.
[Results] The amount of mulching film residues in cotton fields in the Yellow River Delta was 37.7-128.7 kg/hm which had a significant increase compared with 5 years ago， and the differences between plots were large; and after straw was returned to the field， the soil residual film obviously gathered in the 20-30 cm soil layer. The residual film blocks with a size of more than 25 cm2 accounted for 62.6%， which was a relatively high proportion， indicating that soil pollution can be caused easily， and it is difficult to control. During a certain period of time， the soil residual film pollution may have a tendency to aggravate， and the ecological risk is higher.
[Conclusions]This study has important theoretical and practical significance for improving soil quality in the region and ensuring the safety of cotton field ecosystem and environmental health.
Key words Yellow River Delta; Cotton field; Plastic film residue; Distribution characteristics; Control
Received： April 2 2021 Accepted： June 20， 2021
Supported by Coarse Cereals Innovation Team of Shandong Province Modern Agricultural Industry Technology System （SDAIT-15-11）; Cotton Innovation Team （SDAIT-15-07）.
Haiyun YAN （1965-）， female， P. R. China， senior engineer， devoted to research about saline land agriculture.
Lixia LI （1979-）， female， P. R. China， senior agronomist， PhD， devoted to research about soil environment and crop cultivation.
#These authors distributed equally to this work.
*Corresponding author. E-mail： lilixia123@163.com.
Mulching film is one of the important means of production in China’s agricultural production， which can significantly improve the water and heat conditions of the soil and enhance the ability of crops to resist adverse environments. After being introduced into China in the 1980s， it greatly promoted the increase in crop production and the expansion of suitable areas for crop production in China， and has gradually become an important means for increasing temperature and maintaining moisture in farmland， and for high and stable agricultural production. The application of mulching film has brought about major changes in agricultural production methods and significantly improved productivity. However， due to the large-scale and long-term application of mulching film， especially the application of ultra-thin mulching film， and the lack of residual film recycling technology and mechanism， the problem of the pollution of residual mulching film in farmland exists for a long time， which further causes uneven spatial distribution of soil moisture， poor soil structure， imbalance of soil beneficial microbial communities and relatively low quality of cultivated land， which affect the operation of agricultural machinery and the agricultural environment[1-7]. The impacts of residual plastic film on the ecological environment and soil quality， the risk value of plasticizers and microplastics on the soil ecosystem and human health， and the application and evaluation of new biofilms in agriculture are the hotspots and difficulties of future industry research. Shandong Province is a large agricultural province with a large area covered by plastic film and a wide variety of crops. According to the Shandong Provincial Agricultural Statistics Yearbook， in 2019， the province’s plastic film coverage area was 1 767 801 hm accounting for 16.2% of the province’s crop sown area; and in Dongying City and Binzhou City， the main cities in the Yellow River Delta， the plastic film coverage areas were 47 378 and 61 410 hm respectively， accounting for 16.08% and 9.26% of the crop sown areas in Dongying City and Binzhou City， respectively. The total amount of agricultural film used in the province was 267 113 t， and the total amount of mulching film used was 101 591 t， accounting for 38.03%; the application amounts of mulching film in Dongying City and Binzhou City accounted for 68.77% and 51.54% of the application amount of agricultural film， which far exceeded the average level of the province; and moreover， it is difficult to pick up and recycle the mulching film， which brings greater risks to the environment. Relying on the construction of comprehensive test stations for the modern agricultural industry technology system in Shandong Province， our research team carried out long-term positioned monitoring of soil mulching film residues， and studied the temporal and spatial distribution of soil residual film and the characteristics of residual film pollution load in cotton fields in the Yellow River Delta. Objectively and comprehensively understanding the status of residual film pollution and the characteristics and changing trends of residual film pollution in cotton fields in the region have important theoretical and practical significance for improving soil quality in the region and ensuring the safety of cotton field ecosystems and environmental health， and can also provide references for cotton production and soil ecosystem research in similar regions.
Materials and Research Methods
Genreal situation of experimental site
The monitoring points were located in the Lubei cotton area of the Yellow River Delta， where the annual average temperature is 13.2 ℃， the annual sunshine hours are 2 639.4 h， the frost-free period is 217 d， and the accumulated temperature of ≥10 ℃ is 4 612.4 ℃， which can meet the requirement of two crops a year. The annual precipitation is 540.3 mm， which is mostly concentrated in summer， accounting for 65% of the annual precipitation. The inter-annual variation of precipitation is large， and this region is prone to drought and waterlogging disasters. The soil at the monitoring site was salined flavo-aquic soil， belonging to coastal saline-alkali soil， which is suitable for planting cotton. The monitoring site belonged to the coastal cotton area in northern Shandong. Cotton planting is mainly based on film mulching， and the amount of mulching film is between 25-30 kg/hm2. The thickness of mulching film is generally between 0.003 and 0.006 mm. Collection and treatment of residual mulching film
Typical fields that had been mulched and planted with cotton for more than 20 years were selected， and soil film residues was collected before planting. The monitoring points were divided into long-term positioned monitoring points and ordinary monitoring points. For each monitoring field， five 100 cm×100 cm quadrats were selected， and the sampling depths were 0-10， 10-20 and 20-30 cm. After delimiting the sampling quadrats， the soil was dug and the remaining mulching film was picked up. In case of large clods， they were smashed， and sieved to the remaining mulching film. The collected residual film was brought back to the laboratory， the residual film was washed to remove the attached debris， and absorbent paper was used to absorb the water on the residual film. The curled residual film was carefully unfolded to prevent the residual film from rupturing， and naturally dried in the shade in a dry place. Then， according to the size of the residual film area （0-25， 25-100 and greater than 100 cm2）， the number and weight of the residual film were recorded， and weighing was performed with an electronic balance of 1/10 000.
Analysis methods
Excel 2010 was used for data processing， statistical analysis， and chart drawing.
Results and Analysis
Inter-annual variation of mulching film residues in cotton fields
Residual film accumulates in the soil for a long time， and due to the differences in mulching film characteristics， agronomic measures， planting methods， and mulching years， the residual characteristics of the film in the soil are also different. Studies have shown that the residual amount of mulching film in the cotton field soil of Handan in North China was 59.1-103.4 kg/hm2; the residual amount of mulching film in the soil of Xinjiang cotton area was 218.8-381.1 kg/hm2; and the residual amount of mulching film in the cotton fields of the Yellow River Delta was 37.7-128.7 kg/hm2. There were big differences between different regions， different fields， and different years. The Yellow River Delta is an important cotton production area in Shandong Province， and the proportion of cotton in crops was higher than the province’s level （Table 1）. In 2019， the cotton sown area in Dongying City was 19 606 hm accounting for 6.65% of the city’s crop sown area， and the cotton sown area in Binzhou City was 22 803 hm accounting for 3.44% of the city’s crop sown area. When calculating the amount of mulching film used in cotton fields according to 30 kg/hm the film used for cotton planting in Shandong Province was about 5 078 t. Dongying City and Binzhou City， the main cities in the Yellow River Delta， consumed about 1 272 t of mulching film， accounting for 25.05% of the film used in the cotton fields in the province. Moreover， cotton is extensively planted in the field， which makes the mulching film more likely to remain in the soil. Therefore， the environmental risk of mulching film residues in the Yellow River Delta is relatively high. In the early 1980s， the thickness of the mulching film used was generally 0.005-0.010 mm. Later， the ultra-thin mulching film was widely promoted. The thickness of the mulching film was mostly less than 0.008 mm， or even only 0.004 mm. The mulching film was ultra-thin， and had the characteristics of poor tensile strength and easy breaking. Later， the large-scale application and promotion of ultra-thin mulching film reduced the input cost of agricultural materials， and simultaneously increased the difficulty of mulching film recovery， and the amount of mulching film residues in the soil increased sharply. From the results of long-term positioned monitoring （Fig. 1）， at monitoring point No. the cumulative amount of soil residual film in the cultivated layer was 40.0 kg/hm2 in 2013， and 130.1 kg/hm2 in 2019， which increased by 90.1 kg/hm 2.25 times in 6 years; at monitoring point No. the cumulative amount of soil residual film in the cultivated layer was 37.7 kg/hm2 in 2013， and 103.3 kg/hm2 in 2019， which increased by 65.6 kg/hm 1.74 times in 6 years; and with the increase of cotton planting years， the accumulation of soil residual film showed an overall increasing trend.
Effects of cultivation methods on the soil layer distribution of residual film in cotton fields
The overall promotion of the rural revitalization plan in the Yellow River Delta has greatly improved the rural living environment， the project of natural gas entering villages and households has been fully implemented， and the use of cotton stalks as fuel in cotton areas has been greatly reduced. In order to avoid waste of resources and environmental pollution caused by unused cotton stalks piled up in the field， the technology of returning straw to the field has been promoted in cotton areas. In recent years， the technology of returning cotton stalks to the field has become more mature， and the machinery for returning straws to the field has been continuously updated and optimized， which provides the possibility for the resource utilization of cotton stalks and greatly reduces the pressure of banning straw burning in cotton areas. Returning straw to the field can not only reduce the use of inorganic fertilizers， but also increase soil organic matter， improve soil fertility， realize the recycling of soil nutrients， and improve saline-alkali cotton fields. However， the deep ploughing of the land corresponding to the return of cotton stalks to the field has greatly changed the spatial distribution of the residual film in cotton fields， and brought new risk points for the residual film in cotton fields. From the monitoring results （Fig. 2 and 3）， returning cotton stalks to the field could increase the accumulation of residual soil film to a certain extent and increase the migration of residual film to deep soil. Under the conventional planting mode， the cumulative amount of residual film in the 0-30 cm soil averaged 47.43 kg/hm and under the straw returning mode， the cumulative residual film in the 0-30 cm soil averaged 52.07 kg/hm which was significantly higher than the conventional planting model. Under the conventional planting mode， the cumulative amount of residual film in the 0-30 cm soil averaged 47.43 kg/hm and under the straw returning mode， the cumulative residual film in the 0-30 cm soil averaged 52.07 kg/hm which was significantly higher than the conventional planting model. In terms of spatial distribution， under the conventional planting mode， more than 91% of the residual soil film was concentrated in the 0-10 cm soil layer， and below the 20 cm soil layer， the residual film amount was very small， even zero. Under the straw returning mode， the residual soil film was concentrated in the 0-30 cm soil layer as the soil plow pan was at 25-30 cm， and there was no residual film accumulation below 30 cm; and the residual film of the 10-20 cm soil layer accounted for the largest proportion， accounting for 31.2%-44.9%， followed by the 0-10 cm soil layer and the 20-30 soil layer， in which the cumulative amounts of residual film accounted for 26.3%-44.2% and 22.6%-34.4%， respectively （Table 2）.
Effects of farming methods on the distribution of residual film density
The morphological characteristics and quantity of the soil residual film are important factors affecting the quality of farmland soil. The residual film in the soil is affected by natural conditions and human activities， and presents fragments of different shapes and sizes in the soil. This study conducted a detailed investigation on the residual status of the film in cotton fields in the Yellow River Delta. The results showed （Table 3） the residual density of the mulching film in sample fields could reach more than 2 million blocks/hm and the size of residual film ranged from a few square centimeters to thousands of square centimeter. Larger area fragments above 25 cm2 accounted for a larger proportion. In ordinary fields， the number of residual film blocks above 25 cm2 was more than 65%， and the proportion of residual film blocks of 25-100 cm2 was 55%; in the fields with straw returning， the number of residual film blocks above 25cm2 was slightly less than 60%， and residual film with a size of 25-100 cm2 accounted for 44%; and large residual film above 500 cm2 accounted for less than 5%. Studies have shown that for cotton fields covered with plastic films for ten years in Xinjiang， the density and quantity of residual soil film are 230.3 kg/hm2 and 5.0 million blocks/hm respectively. Cheng et al.[8-10] reported that when the residual film area was larger than 25cm it would affect the soil moisture and crop yield， and as the residual film area increased， the harm got greater， and the residual film larger than 100 cm2 would cause more significant effects on soil moisture and crop yield. Compared with cotton areas such as Xinjiang and Hebei， the amount of residual film in cotton fields in this region was relatively small， but the residual film above 25 cm2 accounted for a higher proportion， which is more likely to cause soil pollution and even decrease of crop yields. Meanwhile， the shape of the residual film in the soil was diversified， and had a certain correlation with the size of residual film. Smaller residual film was mostly sheet-shaped， and large residual film was mostly in rod， spherical and other irregular shapes. Results and Discussion
The Yellow River Delta is located in the Lubei Plain. It is an old cotton area and an advantageous cotton production area. The amount of residual mulching film per hectare of cotton field ranges from tens of kilograms to more than 100 kilograms， and the residual density is more than 2 million blocks/hm2. Compared with Xinjiang， Hebei and other places， the residual amount and residual density of soil mulching film in the cotton fields in this area were relatively small， and the number of residual film blocks above 25 cm2 was relatively high， which is likely to cause soil pollution and even decrease of crop yields. It is difficult to control the residual film pollution of the soil， which may have a tendency to aggravate within a certain period of time.
Local cotton production is extensive， with low intensification， and agricultural mulching film production standards are not clear. The treatment of residual pollution of mulching film in cotton fields is limited to manual picking and rake cleaning before planting， which has obvious shortcomings. The wide application and large-scale promotion of cotton stalk return technology has accelerated the migration and accumulation of residual film to deep soil. In addition， the residual film is extremely difficult to degrade in the soil. Once deep soil pollution is caused， the difficulty of restoration and treatment will become increasingly difficult. The negative impact of agricultural mulching film has become increasingly prominent.
Measures to Deal with Film Pollution
First， we should establish and improve the production standards of mulching film to improve the quality of mulching film， and promote the increase of mulching film recovery rate. The thickness of the currently widely used ultra-thin mulching film can be increased to more than 0.01 mm. It is necessary to increase the aging resistance of mulching film， and improve the warming and moisture retention effects of mulching film. Meanwhile， combining with actual production， mulching film recycling rate should be improved by improving agronomic technology and promoting optimization of film covering methods， film covering and recycling machinery， and film uncovering time and methods.
Second， we should speed up the development and application of new agricultural mulching films， especially biodegradable agricultural mulching films. Starting from source control， circular economy， and biodegradable material substitution， we should strive to reduce the generalized agricultural film pollution， soil microplastic accumulation pollution， and secondary pollution transformed from microplastic degradation from the source. Third， we should establish and improve the ecological compensation mechanism and establish a long-term mechanism for the use of standard mulching film and the recycling of waste mulching film， so as to promote the production， promotion and application of practical new mulching film and degradable mulching film， as well as recycling and reuse， by means of ecological compensation.
References
[1] YAN CR， HE WQ， MEI XR， et al. Residual characteristics and control status of agricultural mulching film in China[J]. 2006 Annual Conference of Chinese Agricultural Society： 480-483. （in Chinese）
[2] YAN CR， MEI XR， HE WQ， et al. Present situation of residue pollution of mulching plastic film and controlling measures[J]. Transactions of the Chinese Society of Agricultural Engineering， 2006， 22（11）： 269-272. （in Chinese）
[3] LIU JH， GUO XM， YU XK， et al. Present status and prospects of cotton production in the Yellow River Delta[J]. China Cotton， 2013， 40（1）： 1-2. （in Chinese）
[4] DONG HG， WANG D， WANG YT， et al. Spatial and temporal distribution characteristics of mulch residues in cotton field in Shihezi， Xinjiang[J]. Journal of Arid Land Resources and Environment 2013（9）： 182-186. （in Chinese）
[5] MA H， MEI XR， YAN CR， et al. The residue of mulching plastic film of cotton field in North China[J]. Journal of Agro-Environment Science， 2008， 27（2）： 570-573.（in Chinese）
[6] DONG HG， LIU T， LI YG， et al. Effect of residual film in Xinjiang cotton fields on cotton yield and soil physical and chemical properties[J]. Transactions of the Chinese Society of Agricultural Engineering， 2013（4）： 91-99. （in Chinese）
[7] LI LX， WANG ZH. Characteristics of mulching plastic film residue in cotton fields in the Yellow River Delta[J]. Shandong Agricultural Sciences， 2016， 48（6）： 84-87. （in Chinese）
[8] DU XM， XU G， XU DP， et al. Mulch film residue contamination in typical areas of North China and countermeasures[J]. Transactions of the Chinese Society of Agricultural Engineering， 2005， 21（13）： 225. （in Chinese）
[9] SUN XG， LIU WJ， GAN RW. The harm of residual film in Xinjiang cotton fields and its control countermeasures[J]. China Cotton， 2006， 33（2）： 7-9. （in Chinese）
[10] NAN DJ， XIE HE， GAO LS， et al. Study of the influence of the residue film on soil and cotton growth in the cotton fields[J]. Cotton Science， 1996， 8（1）： 50-54. （in Chinese）
[Methods]Cotton field mulching film survey and residue monitoring were conducted in the Yellow River Delta area.
[Results] The amount of mulching film residues in cotton fields in the Yellow River Delta was 37.7-128.7 kg/hm which had a significant increase compared with 5 years ago， and the differences between plots were large; and after straw was returned to the field， the soil residual film obviously gathered in the 20-30 cm soil layer. The residual film blocks with a size of more than 25 cm2 accounted for 62.6%， which was a relatively high proportion， indicating that soil pollution can be caused easily， and it is difficult to control. During a certain period of time， the soil residual film pollution may have a tendency to aggravate， and the ecological risk is higher.
[Conclusions]This study has important theoretical and practical significance for improving soil quality in the region and ensuring the safety of cotton field ecosystem and environmental health.
Key words Yellow River Delta; Cotton field; Plastic film residue; Distribution characteristics; Control
Received： April 2 2021 Accepted： June 20， 2021
Supported by Coarse Cereals Innovation Team of Shandong Province Modern Agricultural Industry Technology System （SDAIT-15-11）; Cotton Innovation Team （SDAIT-15-07）.
Haiyun YAN （1965-）， female， P. R. China， senior engineer， devoted to research about saline land agriculture.
Lixia LI （1979-）， female， P. R. China， senior agronomist， PhD， devoted to research about soil environment and crop cultivation.
#These authors distributed equally to this work.
*Corresponding author. E-mail： lilixia123@163.com.
Mulching film is one of the important means of production in China’s agricultural production， which can significantly improve the water and heat conditions of the soil and enhance the ability of crops to resist adverse environments. After being introduced into China in the 1980s， it greatly promoted the increase in crop production and the expansion of suitable areas for crop production in China， and has gradually become an important means for increasing temperature and maintaining moisture in farmland， and for high and stable agricultural production. The application of mulching film has brought about major changes in agricultural production methods and significantly improved productivity. However， due to the large-scale and long-term application of mulching film， especially the application of ultra-thin mulching film， and the lack of residual film recycling technology and mechanism， the problem of the pollution of residual mulching film in farmland exists for a long time， which further causes uneven spatial distribution of soil moisture， poor soil structure， imbalance of soil beneficial microbial communities and relatively low quality of cultivated land， which affect the operation of agricultural machinery and the agricultural environment[1-7]. The impacts of residual plastic film on the ecological environment and soil quality， the risk value of plasticizers and microplastics on the soil ecosystem and human health， and the application and evaluation of new biofilms in agriculture are the hotspots and difficulties of future industry research. Shandong Province is a large agricultural province with a large area covered by plastic film and a wide variety of crops. According to the Shandong Provincial Agricultural Statistics Yearbook， in 2019， the province’s plastic film coverage area was 1 767 801 hm accounting for 16.2% of the province’s crop sown area; and in Dongying City and Binzhou City， the main cities in the Yellow River Delta， the plastic film coverage areas were 47 378 and 61 410 hm respectively， accounting for 16.08% and 9.26% of the crop sown areas in Dongying City and Binzhou City， respectively. The total amount of agricultural film used in the province was 267 113 t， and the total amount of mulching film used was 101 591 t， accounting for 38.03%; the application amounts of mulching film in Dongying City and Binzhou City accounted for 68.77% and 51.54% of the application amount of agricultural film， which far exceeded the average level of the province; and moreover， it is difficult to pick up and recycle the mulching film， which brings greater risks to the environment. Relying on the construction of comprehensive test stations for the modern agricultural industry technology system in Shandong Province， our research team carried out long-term positioned monitoring of soil mulching film residues， and studied the temporal and spatial distribution of soil residual film and the characteristics of residual film pollution load in cotton fields in the Yellow River Delta. Objectively and comprehensively understanding the status of residual film pollution and the characteristics and changing trends of residual film pollution in cotton fields in the region have important theoretical and practical significance for improving soil quality in the region and ensuring the safety of cotton field ecosystems and environmental health， and can also provide references for cotton production and soil ecosystem research in similar regions.
Materials and Research Methods
Genreal situation of experimental site
The monitoring points were located in the Lubei cotton area of the Yellow River Delta， where the annual average temperature is 13.2 ℃， the annual sunshine hours are 2 639.4 h， the frost-free period is 217 d， and the accumulated temperature of ≥10 ℃ is 4 612.4 ℃， which can meet the requirement of two crops a year. The annual precipitation is 540.3 mm， which is mostly concentrated in summer， accounting for 65% of the annual precipitation. The inter-annual variation of precipitation is large， and this region is prone to drought and waterlogging disasters. The soil at the monitoring site was salined flavo-aquic soil， belonging to coastal saline-alkali soil， which is suitable for planting cotton. The monitoring site belonged to the coastal cotton area in northern Shandong. Cotton planting is mainly based on film mulching， and the amount of mulching film is between 25-30 kg/hm2. The thickness of mulching film is generally between 0.003 and 0.006 mm. Collection and treatment of residual mulching film
Typical fields that had been mulched and planted with cotton for more than 20 years were selected， and soil film residues was collected before planting. The monitoring points were divided into long-term positioned monitoring points and ordinary monitoring points. For each monitoring field， five 100 cm×100 cm quadrats were selected， and the sampling depths were 0-10， 10-20 and 20-30 cm. After delimiting the sampling quadrats， the soil was dug and the remaining mulching film was picked up. In case of large clods， they were smashed， and sieved to the remaining mulching film. The collected residual film was brought back to the laboratory， the residual film was washed to remove the attached debris， and absorbent paper was used to absorb the water on the residual film. The curled residual film was carefully unfolded to prevent the residual film from rupturing， and naturally dried in the shade in a dry place. Then， according to the size of the residual film area （0-25， 25-100 and greater than 100 cm2）， the number and weight of the residual film were recorded， and weighing was performed with an electronic balance of 1/10 000.
Analysis methods
Excel 2010 was used for data processing， statistical analysis， and chart drawing.
Results and Analysis
Inter-annual variation of mulching film residues in cotton fields
Residual film accumulates in the soil for a long time， and due to the differences in mulching film characteristics， agronomic measures， planting methods， and mulching years， the residual characteristics of the film in the soil are also different. Studies have shown that the residual amount of mulching film in the cotton field soil of Handan in North China was 59.1-103.4 kg/hm2; the residual amount of mulching film in the soil of Xinjiang cotton area was 218.8-381.1 kg/hm2; and the residual amount of mulching film in the cotton fields of the Yellow River Delta was 37.7-128.7 kg/hm2. There were big differences between different regions， different fields， and different years. The Yellow River Delta is an important cotton production area in Shandong Province， and the proportion of cotton in crops was higher than the province’s level （Table 1）. In 2019， the cotton sown area in Dongying City was 19 606 hm accounting for 6.65% of the city’s crop sown area， and the cotton sown area in Binzhou City was 22 803 hm accounting for 3.44% of the city’s crop sown area. When calculating the amount of mulching film used in cotton fields according to 30 kg/hm the film used for cotton planting in Shandong Province was about 5 078 t. Dongying City and Binzhou City， the main cities in the Yellow River Delta， consumed about 1 272 t of mulching film， accounting for 25.05% of the film used in the cotton fields in the province. Moreover， cotton is extensively planted in the field， which makes the mulching film more likely to remain in the soil. Therefore， the environmental risk of mulching film residues in the Yellow River Delta is relatively high. In the early 1980s， the thickness of the mulching film used was generally 0.005-0.010 mm. Later， the ultra-thin mulching film was widely promoted. The thickness of the mulching film was mostly less than 0.008 mm， or even only 0.004 mm. The mulching film was ultra-thin， and had the characteristics of poor tensile strength and easy breaking. Later， the large-scale application and promotion of ultra-thin mulching film reduced the input cost of agricultural materials， and simultaneously increased the difficulty of mulching film recovery， and the amount of mulching film residues in the soil increased sharply. From the results of long-term positioned monitoring （Fig. 1）， at monitoring point No. the cumulative amount of soil residual film in the cultivated layer was 40.0 kg/hm2 in 2013， and 130.1 kg/hm2 in 2019， which increased by 90.1 kg/hm 2.25 times in 6 years; at monitoring point No. the cumulative amount of soil residual film in the cultivated layer was 37.7 kg/hm2 in 2013， and 103.3 kg/hm2 in 2019， which increased by 65.6 kg/hm 1.74 times in 6 years; and with the increase of cotton planting years， the accumulation of soil residual film showed an overall increasing trend.
Effects of cultivation methods on the soil layer distribution of residual film in cotton fields
The overall promotion of the rural revitalization plan in the Yellow River Delta has greatly improved the rural living environment， the project of natural gas entering villages and households has been fully implemented， and the use of cotton stalks as fuel in cotton areas has been greatly reduced. In order to avoid waste of resources and environmental pollution caused by unused cotton stalks piled up in the field， the technology of returning straw to the field has been promoted in cotton areas. In recent years， the technology of returning cotton stalks to the field has become more mature， and the machinery for returning straws to the field has been continuously updated and optimized， which provides the possibility for the resource utilization of cotton stalks and greatly reduces the pressure of banning straw burning in cotton areas. Returning straw to the field can not only reduce the use of inorganic fertilizers， but also increase soil organic matter， improve soil fertility， realize the recycling of soil nutrients， and improve saline-alkali cotton fields. However， the deep ploughing of the land corresponding to the return of cotton stalks to the field has greatly changed the spatial distribution of the residual film in cotton fields， and brought new risk points for the residual film in cotton fields. From the monitoring results （Fig. 2 and 3）， returning cotton stalks to the field could increase the accumulation of residual soil film to a certain extent and increase the migration of residual film to deep soil. Under the conventional planting mode， the cumulative amount of residual film in the 0-30 cm soil averaged 47.43 kg/hm and under the straw returning mode， the cumulative residual film in the 0-30 cm soil averaged 52.07 kg/hm which was significantly higher than the conventional planting model. Under the conventional planting mode， the cumulative amount of residual film in the 0-30 cm soil averaged 47.43 kg/hm and under the straw returning mode， the cumulative residual film in the 0-30 cm soil averaged 52.07 kg/hm which was significantly higher than the conventional planting model. In terms of spatial distribution， under the conventional planting mode， more than 91% of the residual soil film was concentrated in the 0-10 cm soil layer， and below the 20 cm soil layer， the residual film amount was very small， even zero. Under the straw returning mode， the residual soil film was concentrated in the 0-30 cm soil layer as the soil plow pan was at 25-30 cm， and there was no residual film accumulation below 30 cm; and the residual film of the 10-20 cm soil layer accounted for the largest proportion， accounting for 31.2%-44.9%， followed by the 0-10 cm soil layer and the 20-30 soil layer， in which the cumulative amounts of residual film accounted for 26.3%-44.2% and 22.6%-34.4%， respectively （Table 2）.
Effects of farming methods on the distribution of residual film density
The morphological characteristics and quantity of the soil residual film are important factors affecting the quality of farmland soil. The residual film in the soil is affected by natural conditions and human activities， and presents fragments of different shapes and sizes in the soil. This study conducted a detailed investigation on the residual status of the film in cotton fields in the Yellow River Delta. The results showed （Table 3） the residual density of the mulching film in sample fields could reach more than 2 million blocks/hm and the size of residual film ranged from a few square centimeters to thousands of square centimeter. Larger area fragments above 25 cm2 accounted for a larger proportion. In ordinary fields， the number of residual film blocks above 25 cm2 was more than 65%， and the proportion of residual film blocks of 25-100 cm2 was 55%; in the fields with straw returning， the number of residual film blocks above 25cm2 was slightly less than 60%， and residual film with a size of 25-100 cm2 accounted for 44%; and large residual film above 500 cm2 accounted for less than 5%. Studies have shown that for cotton fields covered with plastic films for ten years in Xinjiang， the density and quantity of residual soil film are 230.3 kg/hm2 and 5.0 million blocks/hm respectively. Cheng et al.[8-10] reported that when the residual film area was larger than 25cm it would affect the soil moisture and crop yield， and as the residual film area increased， the harm got greater， and the residual film larger than 100 cm2 would cause more significant effects on soil moisture and crop yield. Compared with cotton areas such as Xinjiang and Hebei， the amount of residual film in cotton fields in this region was relatively small， but the residual film above 25 cm2 accounted for a higher proportion， which is more likely to cause soil pollution and even decrease of crop yields. Meanwhile， the shape of the residual film in the soil was diversified， and had a certain correlation with the size of residual film. Smaller residual film was mostly sheet-shaped， and large residual film was mostly in rod， spherical and other irregular shapes. Results and Discussion
The Yellow River Delta is located in the Lubei Plain. It is an old cotton area and an advantageous cotton production area. The amount of residual mulching film per hectare of cotton field ranges from tens of kilograms to more than 100 kilograms， and the residual density is more than 2 million blocks/hm2. Compared with Xinjiang， Hebei and other places， the residual amount and residual density of soil mulching film in the cotton fields in this area were relatively small， and the number of residual film blocks above 25 cm2 was relatively high， which is likely to cause soil pollution and even decrease of crop yields. It is difficult to control the residual film pollution of the soil， which may have a tendency to aggravate within a certain period of time.
Local cotton production is extensive， with low intensification， and agricultural mulching film production standards are not clear. The treatment of residual pollution of mulching film in cotton fields is limited to manual picking and rake cleaning before planting， which has obvious shortcomings. The wide application and large-scale promotion of cotton stalk return technology has accelerated the migration and accumulation of residual film to deep soil. In addition， the residual film is extremely difficult to degrade in the soil. Once deep soil pollution is caused， the difficulty of restoration and treatment will become increasingly difficult. The negative impact of agricultural mulching film has become increasingly prominent.
Measures to Deal with Film Pollution
First， we should establish and improve the production standards of mulching film to improve the quality of mulching film， and promote the increase of mulching film recovery rate. The thickness of the currently widely used ultra-thin mulching film can be increased to more than 0.01 mm. It is necessary to increase the aging resistance of mulching film， and improve the warming and moisture retention effects of mulching film. Meanwhile， combining with actual production， mulching film recycling rate should be improved by improving agronomic technology and promoting optimization of film covering methods， film covering and recycling machinery， and film uncovering time and methods.
Second， we should speed up the development and application of new agricultural mulching films， especially biodegradable agricultural mulching films. Starting from source control， circular economy， and biodegradable material substitution， we should strive to reduce the generalized agricultural film pollution， soil microplastic accumulation pollution， and secondary pollution transformed from microplastic degradation from the source. Third， we should establish and improve the ecological compensation mechanism and establish a long-term mechanism for the use of standard mulching film and the recycling of waste mulching film， so as to promote the production， promotion and application of practical new mulching film and degradable mulching film， as well as recycling and reuse， by means of ecological compensation.
References
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