Development and Research Status of New Environmental Films

Yumeng ZHANG Lixia SHEN

Abstract The development and utilization of new environmental film is an important measure to tackle the residual film pollution， protect the ecological environment and promote the sustainable development of agriculture. The types of new environmental film and their development status were reviewed from the aspects of photodegradable film， biodegradable film， photobiodegradable film， liquid film， paper film and bast fiber film. In terms of the experimental research status of environmental films in China， analysis and comparisons were made from three aspects including the different types， the same type and imported environmentfriendly film. Suggestions were proposed for the research and development prospect of environmental film.

Key words Environmentfriendly film; Ordinary plastic film; Residual film pollution; Degradation

With the wide application of film mulching cultivation technology， which has greatly promoted the development of agricultural production， its negative environmental effects have become more and more serious. Ordinary agricultural film are not easily degraded under natural conditions and can remain in the soil for 200-400 years[1]. With the increasing use and service life of mulching films， the positive significance of film mulching is being eroded by the damage caused by residual pollution. The residual films in farmland has seriously affected the reproduction capacity of the soil and seriously threatened the sustainable development of agriculture， natural environment and agricultural production， making it become a major hidden danger restricting the development of modern agriculture. Therefore， the development and utilization of environmental films is an inevitable measure to deal with "white pollution" and a strategic channel to solve the problems restricting the current sustainable development of agricultural production in China and even the whole world.

Types and Development Status of New Environmental Films

Photodegradable film

In 1971， DuPont of the United States developed the earliest synthetic photodegradable film[2].

The whole degradation process of photodegradable film can be divided into 3 stages： the first stage is the controllable induction period. At this stage， the properties of the degraded film are the same as those of the ordinary films， and the toughness and tensile strength， impact resistance， tear strength of the film are kept stable. The length of the induction period is related to the used amount of additives such as antioxidants and stabilizers， the thickness of the material and the local climate. The second is the end of the induction period， entering the photodegradation period. The polymer film rapidly undergoes photocatalysis， oxidation reaction， continuous embrittlement and fragmentation， and the effect is related to the time consuming for the transition from the induction period to the embrittlement， that is， the shorter the time， the better the result is. The third is the stage when the film quickly mix with the soil and eventually transforms into carbon dioxide and water under the combined action of biotic factors and abiotic factors in the presence of prooxyenic agents.

The photodegradable film is classified into 2 types according to the synthetic methods： synthetic type and additive type. A synthetic photodegradable film is a film formed by introducing photosensitive groups into the polymer chain. Among them， polyethylene （PE） photodegradable polymers have been studied the most. The polyethylene can be degraded into oligomers （molecular weight of less than 500）， which can be absorbed and degraded by microorganisms in the soil. The additive photodegradable film is to add photosensitizers to the common plastic raw material， and the polymers can be degraded into oligomers by the action of the photosensitizers under the action of ultraviolet light[3].

The problems of photodegradable film in practical application are as follows： the degradation performance is greatly affected by environmental conditions， and the degradation rate is difficult to control; the buried part of the films in field can not be degraded， and the fragments after degradation are not easy to continue to differentiate or be assimilated by the soil， and the problem of contaminated soil cannot be solved fundamentally; in addition， the production cost is high， which limits its promotion and application[4].

Biodegradable film

In 1973， the British scientist Griffin proposed the concept of "biodegradation"， which led to the development of biodegradable film[5].

The biodegradable film promotes the decomposition of the mulching film by utilizing the actions of microorganisms and enzymes under natural conditions. In biodegradation， bacteria， fungi and actinomycetes play a major role in degradation， and there are 3 forms of digestion and degradation： Biophysical action， which uses physical growth of microbial cells to physically destroy the film; Biochemical action， which uses certain substances produced by microorganisms to perform chemical actions on the film; Direct action of the enzyme， which uses microbial enzymes to act on certain components of the film， leading to its division or oxidative degradation[6]. The degradability of biodegradable film mainly depends on 3 factors： the structure and size of film molecules; the type of microorganisms; environmental factors， including humidity， temperature， pH and availability of nutrients. Generally， the biodegradation process is easier to carry out under the conditions of being away from light， high humidity， a large amount of inorganic salts and presence of effective carbon source[2].

Biodegradable film can be divided into complete biodegradable film and additive biodegradable film according to degradation mechanism and destructional form[7]. The additive biodegradable film is prepared by adding natural or synthetic polymers， which have the biodegradability， or the biodegradation accelerator， processing agents to the generalpurpose plastics which have no biodegradability. At present， the additive biodegradable film is mainly composed of generalpurpose plastics， starch， compatilizer， selfoxidizing agent and processing agents， and its typical variety is the polyethylene starch biodegradable film. The complete biodegradable film is a kind of plastic film composed of substances that can be completely decomposed by microorganisms. The substances are mainly derived from starch， cellulose， chitosan and other natural polysaccharide materials， and the final products of degradation are CO2 and H2O， which can be completely consumed by nature and pose no secondary pollution to the environment. The main varieties are polylactic acid （PLA）， polycaprolactone （PCL）， polyhydroxybutyrate （PHB）.

Photobiodegradable film

After 1975， British scientists such as Odie proposed the concept of "double degradable" on the basis of Griffins concept of "biodegradable"， which led to the generation of photobiodegradable film[8-9].

Photobiodegradable film is a kind of mulching film which undergoes photooxidative degradation under natural sunlight and can continue to be degraded by microorganisms after photodegradation reaches the decay period. Generally， the photobiodegradable film is prepared by adding appropriate amounts of photosensitizer， biodegradable agents， prooxidant and degradation control agents （including stabilizers， accelerators and biodegradable sensitizers） to the generalpurpose polymer materials （such as PE）. Beijing Plastics Research Institute has made the thickness controllable degradable film after extruding blow moulding by using polyethylene as the base material and adding the condensed masterbatch which contains the photodegradable system composed of photosensitizer and photooxidation stabilizer， and biodegradable system composed of N， P， K and various chemical elements. After field application， the degradable film not only has heat preservation， moisturizing and mechanical properties of the common films， but also has good controllability and stable induction period. Under the exposure conditions， it can be basically degraded into powder in the same year[10].

The photobiodegradable film has the synergistic effect of photodegradability and biodegradability. Photodegradation technology is introduced into the ground part， and biodegradation technology is introduced into the buried part under the soil. Therefore， both the buried part and the ground part can be degraded， making it an ideal complete degradable film. Compared with common films， the photobiodegradable film can eliminate the residual film damage. At the same time， the technology is in line with Chinas national conditions， and the cost is acceptable， so it has great potential for popularization and application. However， the photobiodegradable film is subject to the influences of agroclimate， natural conditions and crop diversity， which makes it more difficult to control the degradation.

Liquid film

The study of liquid film began in 1963 with the development of asphalt emulsifier by the joint efforts of Petrofina Petroleum and Ghent University of Belgium. The product can become film after spraying on the surface ground， and it has the soil moisture conservation and warming effect similar to the mulching film. Moreover， it can also improve soil structure， avoiding white pollution[11]. By the end of the 1970s， with the advent of liquid film products with more stable quality and performance， the application of liquid film in agriculture and other fields has been expanding， especially it plays an important part in dryland agricultural production， sand fixation and grass planting， tidal flat treatment and canal seepage prevention[12]. In 1990， a spraytype degradable film was developed in Italy. The film is made of natural polymer material. It is liquid before use， and can naturally become film after spraying evenly on the surface ground with the evaporation of water. At the end of the 20th century， this product was widely used in Italy， Germany and Sweden for tomato， asparagus， strawberry， potato and flower planting， achieving remarkable effects[13].

The research on liquid mulch film in China began in 1986. The Soil and Fertilizer Institute of the Chinese Academy of Agricultural Sciences took the lead in introducing asphalt emulsifier from Belgium to conduct agricultural research on liquid film. However， the product had poor environmental adaptability in the Chinese market， and it was expensive and inconvenient to transport. Thus， after learning from the foreign experience according to the application area， productivity level and economic conditions， the Institute conducted research on the domestication of liquid film， and the first liquid film was successfully developed in 2001 in China. Compared with similar foreign products， the liquid film developed in China has the features of low price， much more convenience in use， more significant effect， and it is more suitable for the production of farmland in China[14-16].

In recent years， research institutes such as the Soil and Fertilizer Institute of the Chinese Academy of Agricultural Sciences， Shandong University of Science and Technology have developed humic acid liquid films. Based on using organic biomass such as weathered coal and plant straw rich in humic acid as the main raw materials， the multifunctional black environmental liquid film is prepared by adding various additives， silicon fertilizers， trace elements， pesticides and herbicides to the modified papermaking black liquid， seaweed waste liquid， molasses waste liquid， brewing waste liquid or starch waste liquid. The liquid film not only has the functions of warming， moisturizing and seedling protection， but also has strong adhesion ability， which can combine soil particles into ideal agglomerates， thereby completely solving the pollution of waste liquid like papermaking black liquid and plastic films to land and environment. At the same time， it is also characterized with the integration of the features of pesticides， fertilizers and agricultural films. In other words， it can be the soil conditioner after plowing into soil， it can be the mulching film after spraying on the ground， and it can promote the natural emergence of seedlings. It also shows strong adaptability to topography[17-18].

Paper film

The paper mulch is made of plant pulp， which is used as the basic raw material. After adding moisturizing agent， preservative and transparent agent to the pulp， the conventional papermaking process is applied to manufacture the base paper， which is then processed to make the paper have the functions required for mulching films， such as mechanical strength， light transmission， water permeability， heat preservation， warming effect， moisture retention[19]. The paper film can be degraded within the required time and can also be buried in the soil for use as fertilizers.

Agricultural paper film has been successfully developed in Japan. The paper film mulch mainly contains chitosan and plant cellulose， which can promote the propagation of soil bacteria which are beneficial to crop growth. Its hear preservation performance is superior to that of plastic films， and it has good air permeability， high water absorption and strong evaporative heat loss features， making it have a wide range of applications[20].

The No.1 Paper Mill of Zhicheng City， Hubei Province， China has produced a multipurpose agricultural paper film， which has been put into production in batches. The paper film can be decomposed into fertilizers within 2 or 3 months， and it can control pests and diseases， eradicate weeds， increase soil nutrients and improve soil structure. Tianjin Horticultural Engineering Research Institute has also developed a multieffect agronomic nutrition paper film. It has been proved by experiments that the product can not only decompose by itself， but can also release a variety of nutrients， promote plant growth and development， and have certain effects on controlling plant insects and diseases[21].

At present， many paper materials have been applied to the manufacture of paper film， such as kraft paper， heavy paper， thick paper， wax paper， asphalt infiltration paper， broken newspaper and so on. Among them， kraft paper has been most commonly used for it can be used directly as a mulch film and as a raw material for mulching film. In addition， many coating materials are also applied to paper film， such as soybean oil， paraffin， polyethylene film， degradable polyethylene and so on. The reason is that the performance of the base paper material can be improved by coating these materials on the base paper material to prepare the paper film[22-23].

Although the paper film can be completely degraded， some chemical additives must be used in the paper film processing to enhance the mechanical properties， degradation properties and biological properties of the paper film， such as watersoluble lipids， alcohols， plants or animal fats and synthetic fibers used in coating aids. However， further study is needed to investigate the biodegradability of these chemical materials and their impact on the soil environmental system.

Bast film

The bast film is made of pure bast or bast which is mixed with other fibers by a nonwoven method. The Institute of Bast Fiber Crops of the Chinese Academy of Agricultural Sciences have been engaged in the research on the manufacture and application of environmentally friendly bast films from the late 1990s. The bast film products developed by the Institute are mainly made of natural bast fibers supplemented by other natural materials. They not only promote the growth and complete degradation of crops， but also improve soil， increase soil fertility and increase crop yield[24].

The bast film is generally cracked in the field for about 50-70 d. After being buried in the soil for more than 60 d， it can be broken into pieces or slices of 5 cm or less， which can be completely degraded in the field at last. The degraded bast fibers are pollutionfree， and can improve the physical properties of the soil， improve the soil structure， such as reducing the soil bulk density and increasing the porosity， improving the soil threephase ratio. In addition， the effect increases with the increase of the bast film residues. The degradants of the bast film can increase soil organic matter content， increase soil microbial quantity， enhance soil bioactivity， effectively accelerate soil nutrient conversion， facilitate crop nutrient absorption， increase soil fertility， promote crop growth and development， and significantly improve crop yields.

Since 2005， the bast film developed by the Institute of Bast Fiber Crops of the Chinese Academy of Agricultural Sciences has been put into production in enterprises. In 2010， a production line of bast film with an annual output of 10 000 t was built. Bast film products have been promoted and applied in Hunan， Hubei， Zhejiang， Jiangsu and other provinces， with an accumulated promotion area of 67 000 hm2， which has been well received by enterprises and farmers and achieved good results[25]. At the same time， the bast film has also achieved good results in coverage of vegetables through the application demonstration in Japan and Italy， which has aroused great interest from Japanese and Italian merchants[26].

Experimental Research Status of Environmental Films in China

The research and development of degradable films began in the 1970s from the photodegradable film in China. In the 1980s， the starchadditive degradable film was developed. In the 1990s， the development of degradable film boomed. With the maturity of production technology， the development of environmental degradable films has made gratifying progress， and gradually entered the field trial demonstration stage.

Comparative test of different types of environmental films

Wang et al.[27]studied the effects of covering 3 different kinds of degradable films on corn growth and soil environment， and the results showed that the film covering had significant effects on raising soil temperature but no significant effects on water conservation at the early and late stages of corn growth， while the soil water conservation effect was significant in the middle stage but the effect of raising soil temperature was not significant. The degradable films and ordinary film had the same effects on soil conservation and soil temperature， resulting in increasing crop yield， but the difference between the was not significant. The order of the degradable rates of degradable films was： biodegradable film > biophotodegradable film > photodegradable film.

Wang et al.[28]conducted an experiment to the Shaanxi Wolin biodegradable film， Qinfeng photobiodegradable film and Lyuye liquid degradable film， and the results showed that all the 3 kinds of degradable films had remarkable functions of warming， heat and humidity preservation in the early stage of corn jointing， but the humidity preservation effect was not significant in liquid film. The growth period corn was advanced with the application of mulching films over the open air one except with the application of liquid film， for which the growth period of corn was close to that in the open air. And the whole growth period was shortened. The application of all kinds of films had the corn yield increased compared with the openfield corn， and the increases were extremely significant in the application of biophotodegradable film and biodegradable film. In terms of environmental protection， the liquid film had the best performance， followed by the biodegradable film and the biophotodegradable film came the last. However， considering both the yield and environmental protection effect， biophotodegradable film was suitable for crops such as corn with a procreation age， the biodegradable film was suitable for crops with a long growing period， and the liquid mulch was more suitable for crops with short growth periods such as vegetables.

Zhang et al.[29]investigated the influences of different covering materials mulching on soil moisture and crop yield， and the results showed that under the conditions of planting of rainfall harvesting， the mulching of both the common film and biodegradable film increased the soil water preservation capacity at the soil layer of 0-60 cm in different growth periods of corn compared with the control of corn at open field， and the increase was significant between the group of liquid film and the openair control. Different mulching treatments showed the same spatial variation of soil moisture. The mulching of common plastic film and biodegradable film could effectively increase the soil moisture content and enhance the stability of soil moisture content in the 0-200 cm soil layer. Moreover， the grain yield of corn with common plastic film mulching and biodegradable film mulching increased by 19.96% and 19.67% from the control， respectively， and the water use efficiency increased by 32.08% and 31.81%， respectively， which were significantly different from the control. There was no significant difference in grain yield and water use efficiency between the liquid film treatment and the control.

Li et al.[30]studied the effects of degradable films （bast fiber film and liquid film） on growth of faba bean in high altitude area in Qinghai， finding that there were no obvious difference in the effects of moisture and temperature of soil and growth of faba bean between degradable films and common plastic films， and their soil moisture， temperature of surface soil （0-20 cm） and underground soil， the available N， available P， available K as well as the organic matter content and leaf chlorophyll content were all higher than the CK significantly. The both of the 2 degradable films could increase emergence rate， height， effective branches and pods per plant， and their yield was 21.65% higher than that of the CK. Liquid film had significant effects on the growth of faba bean at the early stage， but with the promotion of the growth process， the effect was not significantly different from that of the CK because of rapid degradation of liquid film influenced by the environmental factors， which resulted in poor heat and water preservation effects.

Li et al.[31]compared the effects of different degradable films （biodegradable film of 0.012 mm thick， biodegradable film of 0.008 cm thick and liquid film） mulching on soil water potential， temperature and sunflower growth. The results showed that there were no significant differences between the plastic film and biodegradable film treatments during early crop growing period （June and July）， whereas the soil temperature in 0-20 cm soil layer with plastic film mulching was significantly higher than that with biodegradable films mulching in late growth periods （especially September）， especially for the treatment of biodegradable film of 0.008 mm thick which was slightly lower than that with the thickness of 0.012 mm. However， the soil temperatures of liquid film and no film treatments were significantly lower than those of plastic film treatment and biodegradable film mulching. The results for soil water potential among different treatments were similar， and it was very close between plastic film and biodegradable treatments， but the maximum soil water potential was shown in 0-20 cm soil layer during the end of September， and there was significant difference between plastic film and biodegradable film treatments. Although the liquid film mulching showed some water preservation effect compared with the no film treatment， the difference was not significant. Moreover， no significant differences were found between plastic film and biodegradable film treatments for crop height， leaf area index （LAI）， dry matter of leaf， stem， flower dish and yield. However， both of these 2 treatments were clearly better than liquid film and no film treatments.

Comparative test of the same type of environmental films

Shen et al.[32]investigated the application of 4 different types of photobiodegradable films produced by Liaoning Huanrui Degradable Resin Co.， Ltd with the standardized induction periods of 30， 50， 60， 70 d， respectively. The results showed that all of the 4 types of photobiodegradable films could complete the degradation process （degraded by soil microbial） under different climatic conditions and different planting systems southern， northern and eastern Liaoning Province， and by the end of October， there was no large mulch film residue on the surface area of the plot. The films on both sides of the ridge could be broken after plowing， posing no effect on the growth of the nextstubble crops. During the observation period， the photobiodegradable film mulching treatment showed similar performances in water and heat preservation to the plastic film， and showed the trend of yield decrease in different areas， but the difference was not significant.

Wang et al.[33]studied the effects of 3 different types of biodegradable mulching film produced buy Jiangsu Changzhou Bailiji Biomaterials Co.， Ltd. With the standardized induction periods of 60， 90， 120 d， respectively. The results showed that the biodegradable mulching film and plastic films had the same effect on heat preservation and warming in tobacco ridges， which could improve fluecured tobacco growth and quality of tobacco leaf， especially the biodegradable film could preserve humidity in the middlelate period of tobacco field growth， which was benefit to the full maturity of upper leaves. The appearance quality and economic characters of the backing tobacco under biodegradable film treatment were better than that of the control. Compared with the control， the biodegradable film treatment saved 60 labor/hm2， and  the net income increased by 5 732.3-7 431.8 Yuan/hm2. Comprehensively considering the 3 types of biodegradable mulching films， the one started to degradation after 90 d had the best economic characters that the tobacco yield， production value， average price， net income and the appearance quality was better than those of the control， and therefore it was the suitable biodegradable mulching film for Yongzhou tobacco growing areas.

Zhao et al.[34]studied the biodegradable mulching film of Nanjing Huanlv. The results showed that the degradation of the film began from the edge， changing from small holes into big pieces， and finally completely decomposed at crop harvest. The buried pieces in soil and around the crop were also gradually degraded. Soil waterstoring capacities at seedling and jointing stages were 3 and 1 times higher under mulching condition than those without mulching， respectively. However， the effect of conserving water through mulching film decreased after silk stage. Soil waterstoring capacity fell below the level of the sowing stage. Compared with no film mulching， soil temperature under film mulching increased 0.4-0.9 at early stage of corn growth， but no difference afterwards. Similar to plastic film mulching， corn with the degradable film mulching had 10 d early of maturation stage and higher yield than that with no mulching cultivation， but the difference was not significant. Qiao et al.[35]conducted studies to the same biodegradation film， finding similar degradation conditions to that of Zhao et al.[34]. Its heat and water preservation effects were almost the same as those of common plastic films. It showed significant effects of heat and water preservation on corn in the early growth stage， but the warming effect was not significant in the middlelate stages. Both the biodegradable film and common plastic film mulching could advance the growth period of con， increased the plant height. On the other hand， mulching biodegradable film had significantly effect on increasing the yield， but the difference was not significant from that of the common plastic films.

In order to identify the heat preservation of bast fiber mulching film and its effects on growth and yield of crops in different seasons， Shang et al.[36]from the Institute of Bast Fiber Crops of the Chinese Academy of Agricultural Sciences conducted comparison tests on Chinese cabbage covered with different films in winter， spring and summer， respectively. The results showed that bast fiber mulching film could increase the soil temperature in different seasons and the effects were stable. In summer， the temperature under the bast fiber mulching film would not reach too high. With the covering of bast fiber mulching film， it could increase the rate of leaf emergence， plant height， leaf number and leaf area. The interaction between different seasons and different covering was significant. And the yieldincreasing effect in spring and summer were more remarkable than winter. The yield of Chinese cabbage covered with bast fiber mulching film in spring and summer were 45.97% and 48.89% higher than that of Chinese cabbage without film， respectively， which were also 6.66% and 51.05% higher than that of Chinese cabbage covered with plastic film， respectively. In addition， Yang et al.[37]from the same institute made investigation to the bast films of different colors. The results showed that bast film mulching could significantly increase the plant height and number of branches of pepper， and the dry matter accumulation of white bast fiber mulching film was the highest. The yield of white bast fiber mulching film and black bast fiber mulching film were significantly higher than the yield of the plastic film and no film mulching.

Comparative test of imported environmental film

Yang et al.[38]studied the biodegradable mulch film imported from France in tobacco production， finding that the film decomposed significantly after 15 d of mulching and its cumulative degradation rate was closed to 70% after covering 120 d. However， there were still some shortcomings such as reducing water retention capacity of soil， no promoting effects on tobacco seedlings quick growth， delaying tobacco growth period and restricting main agronomic traits growth of tobacco.

Shang[39]compared the degradable film and liquid film of China with the degradable film of Showa， Japan. The results showed that in terms of the impact on economic traits， the order from high to low was degradable film > common plastic film > liquid film > open field control; in terms of tobacco yield， the order was degradable film of China > imported degradable film from Japan; the output value was in the order of imported degradable film from Japan > domestic degradable film of China; the proportion of superior tobacco was in the order of imported degradable film from Japan > domestic degradable film of China.

Men et al.[40]studied 2 types of biodegradable mulch films of KANEKA in Japan， which showed that the 2 mulching films could significantly increase the soil temperature. Compared with the open field control， the soil temperature increased by 15.5% and 19.2% in the treatments with the 2 types of biodegradable films at the soil depth of 5 cm， respectively， and 16.1% and 17.5% at the depth of 10 cm; the yield of cucumber in the film mulching treatments increased by 12% and 22%， respectively， and the yield of lettuce increased by 0.8% and 15%， respectively， but the yield increase effect did not reach a significant difference. The degradation film No. 1 had low strength， which showed mechanical tears prematurely in bad weather such as strong winds. The film reached the induction period 21 d after filming， and strip cracks appeared. The film showed serious damage， and the field service period was less than 1 month. The degradation film No.2 showed high strength with no obvious mechanical damage under the external force such as strong wind， and it had round holes naturally formed on the surface. It reached the induction period 30-34 d after mulching， and it had relatively longer service life.

Wang et al.[41]investigated the Mitsubishi biodegradable films of Japan. The results showed that biodegradable film was observed to be decomposed about 30 d after mulching， and began to be rapidly decomposed after 40 d， with 85%-95% of it degraded until 80-90 d. the partially mulched film on ridge was basically the same as common plastic film remaining intact after harvesting， but the partly land filled in furrow was decomposed significantly. Waterretention of biodegradable film was 90.4%-95.4% of common plastic film. The average temperature in 0-25 cm soil of full biodegradable film mulching on double ridges and planting corn in furrows was increased by 1.91 in comparison with nonmulching， and was reduced by 0.85in comparison with full plastic film mulching. Emerging date was advanced 5-9 d and the whole growth period was shortened 11-12 d.

Summary and Prospect

For different types of environmental films， photodegradable， biodegradable and photobiodegradable films can achieve the effect of warming， preserving water and increasing yield of common mulch film， while the mulching effect of liquid film is only better than that of open field control， and still shows a certain distance with the common plastic films and other degradable films. From the perspective of degradation， biodegradable and photobiodegradable films have ideal and complete degradation， which is the direction of degradable mulching film development in the future. However， biodegradable mulching films have problems such as poor mechanical properties， weak water resistance and poor strength， which make it different for them to adapt to the large scale mechanized mulching. Moreover， further exploration is needed for the  thoroughness of photobiodegradable film and the safety of degradation products to the soil environment. For the same type of environmental film， the degradation performances and field effects of the degradable film are different due to the different standardized induction periods. For the natural conditions and soil environment of different regions， the research and development of degradable film with strong adaptability is the key in the promotion and application. As for degradable mulch films of special materials （such as bast fiber film）， attention should be paid to the material sources and production cost issues. For the imported mulch films of different source countries， consideration should be given to the applicability in different regions in China and the  economic cost.

To sum up， the field application of new environmental film is still in a smallscale experimental stage， and there is still a certain distance from the largescale promotion， which is not only related to the instability of the environmental protection film itself and the imperfection of the preparation process， but also the application environment that varies widely in the field. It is an important measure for solving "white pollution" to replace the common plastic film with environmental film with stable degradability and good agronomic traits in production. Therefore， agricultural workers engaged in field production research should actively cooperate with the producer engaged in the research and development of mulching film， make timely feedback information on the field application of environmental film， and promote the improvement of environmental film preparation process， so as to lay a solid foundation for largescale application of environmental film in fields.

References

[1]WANG P. Countermeasures and measures for residual film pollution control[J]. Journal of Agricultural Engineering[J]. 1998， 14（3）： 185-188.

[2]YANG HD. Mulching film and ecological environment protection[M]. Beijing： Chemical Industry Press， 2000.

[3]LI XF. Study status on degradable mulching film[J]. Plastics， 2004， 33（1）： 76-81.

[4]HE WQ， LIU Q， LI YQ， et al. Development and industrial prospect of new biodegradable film materials[J]. Bioyechnology and Business， 2017（2）：7-13.

[5]XU XC， WANG CY. The status and development trend of cultivation mulch film at home and abroad[J]. Plant Fibers and Products， 2006， 28（1）： 6-11.

[6]CHEN R， YU HH. Progress in research and development of degradable plastics[J]. Agroenvironmental Protection， 1998， 17（6）： 274-277.

[7]WEN YX. Functional mulching film[M]. Beijing： Mechanical Industry Press， 2005.

[8]LAMERS J， BRUENTRUP M， BUERKERT A. The profitability of traditional and innovative mulching techniques using millet crop residues in the West African Sahel[J]. Agriculture Ecosystems & Environment， 1998， 67（1）： 23-35.

[9]SONG ZZ， ZHAO MF. Production technique of degradable plastic[J]. Advances in Fine Petrochemicals， 2005， 6（3）： 13-20.

[10]HAN CT， ER Y， HU HC， et al. Research and application of nonstarch type controllable light and biodegradable mulch film[J]. Plastics Science and Technology， 1997（2）： 10-15.

[11]LI FY， FU BR， SHANG DL， et al. Environmental problems and its countermeasures for agriculture sustainable development in China[J]. Journal of Liaoning University （Natural Science Edition）， 1999，26（1）：78-83.

[12]LA XP， ZENG YT， WEI TM， et al. Development history and application of liquid film[J]. Gansu Agricultural Science and Technology， 2015（9）： 90-92.

[13]HAIRSTON JE， JONES WF， MACCONNAUGHEY PK， et al. Tillage and fertilizer management effects on soybean growth and yield on three Mississippi soils.[J]. Journal of Production Agriculture， 1990， 3（3）：317.

[14]LODSDON SD， KARLEN DL. Bulk density as a soil quality indicator during conversion to notillage[J]. Soil & Tillage Research， 2004， 78（2）： 143-149.

[15]WANG CH， ZHANG J， LI JH， et al. Research on the application technique of liquid plastic film[J]. Journal of Anhui Agricultural Sciences， 2006， 34（14）： 3289-3290.

[16]WANG HY， LI L， ZHAO YF. Research progress of liquid mulching film[J]. Jiangsu Agricultural Sciences， 2016， 44（7）： 413-415.

[17]HUANG W， TIAN YY， QIAO YY， et al. Preparation and application of multifunctional degradable black liquid overlay film on soil from humic acid and black liquor of papermaking[J]. Humic Acid， 2005（4）：21-26.

[18]TIAN YY， QIAO YY， HUANG W， et al. The white pollution vs humic acid multifunction degradable black liquid mulching film[J]. Humic Acid， 2006（2）： 19-23.

[19]LYU GH， BAI WB， GUO JY， et al. Analysis on the research and development tendency on mulching paper in China[J]. Journal of Agricultural Mechanization Research， 2012， 34（9）： 249-252.

[20]SHENG W， LUO J， GE MQ. Research and characterization of paper based cotton linter mulch film[J]. Technical Textiles， 2009， 27（3）：9-12.

[21]LU CP， TIAN YL， MA QB， et al. Research advance and development prospect of paper mulch[J]. Journal of Anhui Agricultural Sciences， 2016（5）： 95-96.

[22]COOLONG T. Performance of paper mulches using a mechanical plastic layer and water wheel transplanter for the production of summer squash[J]. Horttechnology， 2010， 20（2）： 319-324.

[23]HAAPALA T， PALONEN P， KORPELA A， et al. Feasibility of paper mulches in crop production： A review[J]. Agricultural & Food Science， 2014， 23（1）： 60-79.

[24]FU DQ. Water conservation and heat preservation properties of bast fiber mulch film and its impact on crops[D]. Beijing： Chinese Academy of Agricultural Sciences， 2008.

[25]YANG RL， WANG HY， YI YJ， et al. The quality parameters and determination method for agricultural bast fiber mulching film[J]. Academic Periodical of Farm Products Processing， 2015（12）： 55-57.

[26]WANG CY. Recent progress of research on environment friendly bast fiber mulch film[J]. Plant Fiber Sciences in China， 2009， 31（Z1）： 98-100.

[27]WANG X， LYU JL， SUN BH. Effects of covering degradable films on corn growth and soil environment[J]. Journal of AgroEnvironment Science， 2003， 22（4）： 397-401.

[28]WANG X， XU GB， REN ZG， et al. Effects of environmentfriendly degradable films on corn growth and soil environment[J]. Chinese Journal of EcoAgriculture， 2007， 15（1）： 78-81.

[29]ZHANG J， REN XL， LUO SF， et al. Influences of different covering materials mulching on soil moisture and corn yield[J]. Transactions of the Chinese Society for Agricultural Engineering， 2010， 26（6）： 14-19.

[30]LI P， LIU YJ， ZHANG YC， et al. Effects of degradable films on growth of faba bean in high altitude area in Qinghai[J]. Guizhou Agricultural Sciences， 2014（12）：92-97.

[31]LI XY， PENG ZY， SHI HB， et al. Effects of different degradable films mulching on soil water potential， temperature and sunflower growth[J]. Transactions of the Chinese Society for Agricultural Machinery， 2015， 46（2）： 97-103.

[32]SHEN H， GUI T， CHEN DW， et al. Application of photo& biodegradable mulching film in agricultural production[J]. Journal of Liaoning Agricultural College， 2004， 6（1）： 1-3.

[33]WANG XC， JIN ZL， ZHOU XP， et al. Effects of biodegradable mulch film on soil temperature and moisture and yield and quality of fluecured tobacco[J]. Chinese Agricultural Science Bulletin， 2016， 32（24）： 146-152.

[34]ZHAO AQ， LI ZZ， GONG YS. Effect of biodegradable mulch film on corn growth and its degradation in field[J]. Journal of China Agricultural University， 2005，10（2）： 74-78.

[35]QIAO HJ， HUANG GB， FENG FX， et al. Degradation and its effect on corn growth of biodegradable mulch film[J]. Journal of Gansu Agricultural University， 2008， 43（5）： 71-75.

[36]SHANG RG， WANG CY， YI YJ， et al. Effects of bast fiber mulching film on heat preservation and growth and yield of Chinese cabbage[J]. Chinese Agricultural Science Bulletin， 2012， 28（16）： 255-260.

[37]YANG YR， WANG CY， YI YJ， et al. Effects of bast fiber mulching on growth and yield of pepper in the greenhouse[J]. Chinese Agricultural Science Bulletin， 2014， 30（16）： 203-206.

[38]YANG LX， XU XY， LIU Y， et al. Study on effect of application of biolice film imported from France in tobacco production[J]. Journal of Kunming University， 2012， 34（6）： 32-35.

[39]SHANG ZQ. Effect of degradable films on growth， yield and quality of fluecured tobacco[J]. Inner Mongolia Agricultural Science and Technology， 2012（6）：29-31.

[40]MEN LZ， LIU ZY， YANG YT， et al. Applicability and evaluation of two controllable degradable mulch films[J]. Agricultural Engineering Technology， 2015（28）： 42-45.

[41]WANG SY， PAN TL， LI SZ， et al. Property of biodegradable film degradation， waterretention and increasing soft temperature and its impact on maize growth and development process[J]. Agriculture Research in the Arid Areas， 2016， 34（1）： 127-133.