The effects of biodegradable mulch film on the growth, yield, and water use efficiency of cotton and maize in an arid region

Plastic residual film pollution in China is serious, and the use of degradable mulch film instead of plastic mulch can effectively alleviate this situation. The substitution of common polyethylene plastic mulch film with biodegradable mulch film in the agricultural production of cotton and maize in an arid region was investigated in the present study. Using bare soil as the control, we compared the effects of common polyethylene plastic film and biodegradable mulch film on crop growth, yield, and water use efficiency (WUE) in maize and cotton. The results indicated that: (1) the biodegradable mulch film in this region remained intact for 60 days after being laid down, significantly degrading after 120 days, and was associated with increased soil temperature, moisture conservation, and degradability in comparison to a bare soil control. (2) Both the biodegradable mulch film and the polyethylene plastic film significantly increased various physiological parameters, such as crop height, stalk diameter, and leaf area. (3) The biodegradable mulch film significantly increased maize and cotton crop yield by 69.4–76.2% and 65.2–71.9%, respectively, compared to the bare soil control. (4) Compared to the bare soil control, the biodegradable mulch film effectively increased WUE in the crops by 64.5–73.1%. In summary, biodegradable mulch film had comparable results to the common polyethylene plastic film in increasing soil temperature, moisture conservation, crop growth, yield, and WUE. As the biodegradable mulch film causes no residual pollution, it is thus preferable to common plastic mulch film for agricultural applications in arid regions and supports the sustainable development of agroecosystems.

http://www.stats.gov.cn/).Xinjiang is a typical arid region where plastic film is widely used in agricultural production [3] for increasing soil temperature and moisture conservation, as well as for increasing crop yield [4][5][6][7][8]. Polyethylene plastic degrades poorly and leaves residual film for a long period of time, causing severe pollution [9][10][11]. Xinjiang is one of the major regions in China experiencing major residual film pollution. The area has a residual film content of 262-597 kg/hm 2 , which has seriously impacted on the sustainable development of local agriculture.
Using environmentally-friendly, controllable biodegradable mulch film to replace common plastic film can increase crop yield, reduce agricultural water consumption, and effectively resolve the issues related to residual film pollution [3]. It is one of the technologies that possess the most potential for promoting the sustainable development of agriculture. Han et al. [12], Chen et al. [13] and Ren et al. [4,5] found that in the Loess Plateau, China, biodegradable mulch film could improve maize yield and water use efficiency (WUE) compared to no film. Moreno et al. [14] found that biodegradable mulch film increased tomato yield in comparison to no film in Central Spain. Yao et al. [15] showed that in Hubei Province, biodegradable mulch film could improve rice production and reduce the emissions of the greenhouse gases CH 4 and N 2 O. However, the above-mentioned studies have some shortcomings. For instance, the effectiveness of biodegradable mulch film is closely correlated with environmental conditions (temperature, precipitation, etc.) and management practices. The degree of degradation and degradation period have been found to differ in different areas, and thus the ultimate effectiveness of biodegradable mulch film varies [3]. Most studies on biodegradable mulch film have focused on semi-arid regions where the annual precipitation and temperature vary from 435-550 mm and 3.6-14.3°C, respectively. However, arid regions experience more extreme environmental conditions, including major differences in day and night temperature (daily mean temperature between −25°C to 30°C), low precipitation (annual precipitation < 200 mm), and strong solar radiation.
Very few studies have evaluated the efficacy of biodegradable mulch film in arid regions.
Additionally, most of the studies mentioned above only focused on the final crop yield, rather than tracking the entire crop growth process. These studies also only focused on the impact of biodegradable mulch film on a single crop and thus lack a comparative assessment of the effects in different crops. Finally, these studies only assessed the effects of biodegradable mulch film on crop yield and the external environment, rather than focusing on the degradation of the biodegradable mulch film itself.
To address the inadequacies mentioned above, and to further provide a theoretical basis and technical support for the application and promotion of biodegradable mulch film in agricultural practices, the present study comparatively analyzed the differences in the effects of biodegradable mulch film and common polyethylene plastic film on the growth characteristics, yield increase, and WUE of maize and cotton in an arid region in Northwestern China. The major aims of this study were to: (1) investigate whether the positive impacts of biodegradable mulch film in the arid region on crop physiological characteristics could match those of common plastic film; (2) investigate whether biodegradable mulch film in the arid region could significantly increase crop biomass and yield, and whether this differed from common plastic film; (3) comparatively analyze the effect of biodegradable mulch film and common plastic film on WUE; (4) comparatively analyze the degradation of biodegradable mulch film and common plastic film.

Site description
The experimental site was located in Shangsanqi Village in Erliugong Town, Changji City, Xinjiang Uygur Autonomous Region, China (87°13′E, 44°02′N; altitude 574 m) between 2015 and 2017. The location is shown in detail in Fig. 1. The experimental site has a typical continental temperate arid climate, with cold winters and hot summers, as well as large variations in day and night temperatures. The soil mellowing level is relatively poor in the experimental site, with low fertility levels. At the 0-60 cm soil layer, the soil organic matter content is 3.07-9.68 g·kg −1 and the total nitrogen content is 0.15-0.30 g·kg −1 .
From 2015 to 2017, the annual precipitation was 174.5 mm, 187.6 mm, and 97.5 mm, and the mean temperature was 7.8°C, 7.1°C, and 7.6°C. Precipitation and temperature at the experimental site are shown in Fig. 2. Low temperatures during early growth stages and lack of sufficient precipitation during the entire growth period are the major factors that limit crop growth in this region.

Experimental materials
Xinjiang's main crops -maize and cotton, were selected as research objects. The mulch films were obtained from Xinjiang Blue Ridge Tunhe Chemical Industry Joint Stock Co., Ltd.

Management measures
For maize and cotton, the four treatments (A, B, C, and D) mentioned above were established in the experimental site. Each treatment plot was 30 m in length and 20 m in width.
This study was a completely randomized controlled study with six replicates for each treatment.
The plots were randomly arranged. The experiments lasted for three years, and the sowing and harvesting dates for each year are shown in Table 1. For the cotton treatment, 80 kg P ha −1 diammonium phosphate was applied to the experimental plots as the base fertilizer. The films were laid out and seeds were sown using an integrative machine. Cotton seeds were sown with a row interval of 60 cm, with 10 cm spacing between each plant. From around April 20 to September 1, water was supplied every 15 d (10 times total). In the early and later stages, 25 mm water was applied; in the middle stage, 50 mm of water was applied. From April 20 to July 30, 80 kg N ha −1 urea was applied with the irrigation a total of four times. During the growth period, the field management measures remained the same for all four treatments.

Parameters tested and methods
In the middle of each growth stage, the growth and development of the maize and cotton in the different treatments were observed and recorded, and the emergence rate was calculated. The plant height, stalk diameter, and leaf area of the crops under the different treatments were measured. In addition, at the boll-opening stage, the boll number per plant and boll weight were recorded. Furthermore, the aboveground and underground parts of the maize and cotton plants in six areas of 1 m 2 were randomly collected. After removing impurities such as soil and sand, the samples were chopped, dried in an oven at 105 ° C for 2 hours, then dried at 80 ° C to constant weight, and weighed to obtain the total biomass in 1 m 2 , as well as the weight of the maize kernel and cotton. From this, we calculated the biomass and yield in 1 hectare.
The degradation of the mulch was graded according to Yang H.D.'s research [16]. The degradation of the mulch film was observed every 10 d after being installed. A 0-5 grade rating was used to define the level of mulch film degradation. Grade 0 represents intact mulch film with no cracks. Grade 1 represents the appearance of the first crack. Grade 2 represents the appearance of small cracks in 25% of the field. Grade 3 represents the appearance of 2-2.5cm-long cracks. Grade 4 represents the appearance of evenly distributed, network-like cracks.
Grade 5 represents the breakage and degradation of the pieces into fragments smaller than 4 cm × 4 cm.
Water use efficiency (kg·hm −2 ·mm −1 ) = grain yield (kg·hm −2 )/total water consumption (mm). Total water consumption was the sum of precipitation and irrigation water usage.

Effect of different mulch film treatments on crop physiological characteristics
Compared to the bare soil, the three mulch films greatly affected the emergence rate and growth progression of the maize and cotton ( Table 2). The emergence rate of both crops was similar in the three mulch film treatments, all of which were higher than in the bare soil (P < 0.05). The emergence rate of the maize and cotton was in the following order: A > B > C > D.
The emergence rate of the maize and cotton under the mulch film treatments was 56.1% and 62.4% higher, respectively, than that under the bare soil. For maize, the emergence times in treatment A and B were 11.3 d and 0.7 d earlier than in treatment C, and the emergence time was significantly shorter in the three mulch film treatments than that under bare soil (P < 0.05 treatment A with B and C, the differences in the time required for growth became increasingly significant. Furthermore, the time needed for growth to progress in B and C became increasingly similar to D, particularly in treatment C. It is possible that in the early stages, the biodegradable mulch films were intact and had a similar effect as the common plastic film, but as the biodegradable mulch film gradually degraded, the moisture conservation function decreased, and thus the effect became increasingly similar to that of the bare soil. Similarly, compared to the bare soil, the three mulch films also had a large impact on the physiological characteristics of the maize and cotton (Tables 3 and 4 Boll number per plant Note: Different letters in lower case represent significant differences among treatments at the same time (P < 0.05). A: common plastic mulch film. B: clear biodegradable mulch film. C: black biodegradable mulch film.
D: bare soil control.
(2) For maize, we found that at the jointing stage, heading stage, and harvest stage, treatment A, B, and C increased plant height by 36.8%, 6.5%, and 8.2%, respectively, compared to treatment D, suggesting that film mulching had the greatest impact on maize plant height at the jointing stage. Treatment A, B, and C increased stalk diameter by 13.2%, 14.2%, and 14.9% at the jointing stage, heading stage, and harvest stage, respectively, compared to treatment D.
This indicated that mulch films had a similar effect on maize stalk diameter at each stage.
Treatment A, B, and C increased leaf area by 38.3%, 7.8%, and 9.5% at the jointing stage, heading stage, and harvest stage, respectively, compared to D, indicating that film mulching had the most significant impact on maize leaf area during the jointing stage.

Effect of different mulch films on crop biomass
The maize biomass under different treatments during the experiment is shown in Fig. 3. The results showed that: (1) in the three years, during the entire growth period, treatment A increased the maize biomass by 27% compared to treatment D; B increased maize biomass by 21% compared to treatment D; and C increased maize biomass by 20.1% compared to treatment D.
This indicated that under all three mulch film treatments, maize biomass was significantly higher than the bare soil treatment. Of these treatments, the common plastic film increased the maize biomass most significantly, while the two biodegradable mulch films did not differ significantly from each other. (2) During the three years, treatment A increased the maize biomass by 6 showed that the changes in maize biomass during the growth period fitted a logistic shape growth curve, with the film mulching having a greater impact on maize biomass in the later growth stages than in the early growth stages.  (2) During the three years, treatment A increased cotton biomass by 16 showed that the changes in cotton biomass during the growth period fitted a logistic shape growth curve, with the effect of mulch films being greater in the later growth stages than in the early stages.
The interannual variations in cotton biomass showed that the mulch film treatments had a similar effect on biomass accumulation in cotton and maize. Specifically, during the entire growth period, the three mulch film treatments and the unmulched treatment all exhibited a logistic shape growth curve, with biomass being relatively low before the end of May and rapidly increasing thereafter, and then changing slowly after September. All three mulch film treatments significantly increased cotton biomass (P < 0.05), but there was no significant difference among the three treatments, although the differences gradually increased during the later growth stages.
The final crop yield during the experiment is shown in Fig. 5. As shown in Fig. 5, the mulch film treatments significantly increased the yield of maize and cotton (P < 0.05), but there was no significant difference among the three mulch film treatments (P > 0.05). In terms of maize yield, treatment A had the highest average yield of 6,473.6 kg/ha, which was 76.2% higher than in treatment D, and the yield of treatment B was 69.4% higher than treatment D. In treatment C, the yield was 72.6% higher than that in treatment D. Treatment A, B, and C did not differ significantly in average yield over the three years (P > 0.05). Of these treatments, treatment A had the highest average yield, followed by treatment C and then treatment B. The variations in cotton yield followed a similar pattern as in maize. Treatment A was associated with the highest cotton yield of 6,575.2 kg/ha, which was 71.9% higher than the yield in treatment D, while the yield of treatment B was 65.2% higher than treatment D. In treatment C, the yield was 69.2% higher than in treatment D. Treatment A, B, and C did not differ significantly in terms of the average cotton yield over the three years (P > 0.05). Overall, the different mulch film treatments increased maize yield by 72.7% on average and increased the cotton yield by 68.8% on average.
This indicated that the mulch films significantly increased the yield of maize and cotton at the experimental site, and the increase in maize yield was higher than that of cotton.

WUE
As shown in Table 5, during the three-year experimental period, the mulch film treatments significantly increased the WUE in maize and cotton (P < 0.05), but there were no significant differences in the WUE among the three mulch film treatments (P > 0.05). In the maize treatment, treatment A had the highest WUE of 10.0 kg·hm −2 /mm, which was 76.2% higher than treatment D. The WUE of treatment B was 69.5% higher than that in treatment D, and treatment C was 73.1% higher than treatment D. The variations in WUE for cotton exhibited a similar pattern as for maize. Treatment A had the highest WUE of 31.9 kg·hm −2 /mm, which was 71.1% higher than treatment D. Treatment B and C had 64.5% and 68.4% higher WUE, respectively, than treatment D. No significant differences in WUE were observed among the mulch treatments during the three years. On average, the three mulch film treatments increased WUE in the maize and cotton by 73.0% and 68.0%, respectively. This indicates that the mulch films significantly increased WUE in maize and cotton at the experimental sites, and the increase in maize was greater than that in cotton.

The degradation rate of the different biodegradable mulch films
The biodegradable mulch films exhibited similar degradation patterns in the cotton and maize treatments. The details are shown in Table 6. Table 6 Degradation rate of the different mulch films used in maize and cotton.
Year Treatment Days after the film was laid down   40 50 60 70 80 90 100 110 120 130 140   A  0  0  0  0  0  1  1  1  1  2  2   B  0  0  1  1  2  2  3  3  3  3  4   2015   C  0  0  1  1  2  3  3  3  3  4 [20] found that the effect of film mulching was significantly better in northern China than in southern China, and the effect was largely dependent on the local climate conditions and management measures. The temperatures of the area of the present study are relatively low in late April, which is unfavorable for maize and cotton germination. However, we found that film mulching drastically increased the crop germination rate, and during the entire growth period, mulch film effectively reduced the time required for crop development. However, as time progressed, the effect of the biodegradable mulch film gradually diminished, which was possibly a result of the high level of degradation at the later stages. In the entire growth period, the plant height, stalk diameter, and leaf area of maize in the different treatments were higher than those in the bare soil, with no significant differences detected among the three films. In cotton, the plant height, leaf area, boll number per plant, and boll weight in the different film mulch treatments were also higher than those in the bare soil, with no significant differences observed among the film types. From the data above, we concluded that the positive effect of biodegradable mulch film on crop plant height, stalk diameter, and leaf area in this region was comparable to that of the common plastic film.
In this study, film mulching significantly increased crop biomass, yield, and WUE.
Although there were some differences among the three mulch films in terms of crop yield, these differences were not significant. Due to the stable properties of the common plastic film, it was associated with the greatest increase in crop yield. The black biodegradable mulch film had the second greatest effect due to the longer growth period, while the effects of the clear biodegradable mulch film were slightly smaller than that of the black biodegradable film. Thus, from the long-term perspective of resolving white pollution and reducing the impacts on the environment, these two biodegradable mulch films both have high application value in this region. The increase in WUE from the mulch films was 5% higher in maize than in cotton, which suggests that the applicability of film mulching in maize is slightly better than that in cotton in this region. This might be due to the nature of the plant itself. Xinjiang is a typical arid region, and thus water scarcity is the most important factor limiting the development of local agriculture. Due to the limitations in production materials and preparation technologies, biodegradable mulch films often degrade too rapidly, thus having inconsistent effects on crop yield [21,22].
Thus, before biodegradable mulch films can be promoted on a large scale, it is necessary that field experiments are conducted to understand the degradation characteristics of mulch films and whether they are applicable under the local settings. Over a three-year period, we found that both clear and black biodegradable mulch film could retain a relatively intact shape within 60 d after being laid down in the field. Furthermore, their effect on increasing crop yield and WUE was very similar to that of the common plastic film. The replacement of common plastic film with biodegradable mulch films is thus very much applicable in this region.
Based on the agricultural characteristics of this region, the amount of mulch film used in one hectare of soil is 25 kg. When the residual mulch film exceeds 240 kg hm −2 , it can significantly affect yield [20]. This implies that after nine years of using plastic film, it could negatively impact on crop yield. In addition to the effect of plastic film on crop yield, it can also degrade and produce microplastics [23] which are likely to be transferred into the human body through the food cycle [24], resulting in potential health risks. In this study, after 140 d, the biodegradable mulch films exhibited a significantly higher level of degradation than the plastic film. The ultimate degradation products of the biodegradable film were H 2 O and CO 2 .
Through a three-year field experiment, our study showed that in the short-term, the cultivation of maize and cotton in an arid region using biodegradable mulch film could increase the crop yield and WUE to a similar level as that when using the common plastic film.
Considering the environmental benefits and the negative effect of residual film on crop yield, we concluded that biodegradable mulch film was favorable to common plastic mulch film. The effect of biodegradable mulch film under different management conditions was not explored in this study, which is an aspect that could be explored in future research.

CONCLUSIONS
Through field experiments, we found that film mulching in an arid region was extremely beneficial for crop growth, with the biodegradable mulch films having a similar effect as the common plastic film in the short-term. The three mulch films could significantly increase plant height, stalk diameter, leaf area, crop biomass, and crop yield, and the effects of the biodegradable mulch films were comparable to that of the common plastic film. Additionally, film mulching could significantly increase the WUE in the crops grown in this region, with similar effects observed between the biodegradable mulch film and common plastic film. The degradation properties of the biodegradable mulch films greatly exceeded those of the common plastic film. In addition to increasing crop yield, the biodegradable mulch films are also environmentally friendly. Thus, from a long-term sustainability perspective, the benefits of the biodegradable film outweigh those of the common plastic film.