Assessing the aging and environmental implications of polyethylene mulch films in agricultural land

Abstract

Polyethylene mulch films (MFs) are widely employed in agricultural land to enhance crop yield and quality, but the MF residue causes significant environmental concerns. To promote the sustainable application of MFs, it is essential to assess their fate throughout their service life and understand the underlying degradation mechanisms. In this study, surface-exposed and soil-buried MFs were separately collected from agricultural land in Inner Mongolia, China. The variations in aging performance and corresponding property alterations of MF were thoroughly examined. The results indicated that sunlight exposure considerably hastens MF degradation, whereas buried MFs experience a more moderate aging process due to the inhibitory effects of the dark and anaerobic environment on oxidation. Surface cracking was observed in MF-Light samples as a result of photodegradation, while chemical and moisture interactions with soil caused partial perforation in MF-Soil samples. Relative to the pristine MF, the oxidation, unsaturation, and hydroxylation levels of MF-Light increased to 0.88, 0.35, and 0.73, respectively, with corresponding values for MF-Soil at 0.44, 0.13, and 0.24. The generated oxygen-containing functional groups lead to a decrease in contact angles of MF-Light and MF-Soil, enhancing their hydrophilicity. The aging process of MFs led to a decline in mechanical properties, posing challenges for recycling. Moreover, nearly all phthalate esters (PAEs) were released from MFs, regardless of sunlight exposure or soil burial. The use of MFs also impacted the abundance of soil microbial communities. Specifically, the selected polyethylene MF enriched Actinobacteriota by 75%, while reducing Chloroflexi and Firmicutes by 27% and 45%, respectively.

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Environmental significance

Polyethylene mulch films (MFs) are widely employed in agricultural land, however, the MF residue causes significant environmental concerns. To promote the sustainable application of MFs, it is essential to evaluate their fate throughout their service life and understand the underlying degradation mechanisms. The aging process of MFs during their service life is a complex process. In this study, we aimed to: (1) explore the different aging properties of PE MFs under various conditions in a real agricultural environment; (2) reveal the interactions between the aging of MFs and their recycling properties, as well as their potential environmental pollution. This work is critical to understanding and controlling the plastic pollution caused by degradation of MFs and providing suggestions for their recycling.

1. Introduction

Mulch films (MFs) have been widely used in agriculture due to their ability to regulate soil temperature, reduce water loss, suppress weed growth, and protect soil from erosion, which ultimately leads to an increase in crop yield. The global agricultural MF market is expected to reach 9.14 million tons by 2030,¹ with China being the largest contributor.² However, the recycling rate of MFs in China is less than two-thirds annually,³ resulting in the accumulation of MF fragments in soil that seriously degrades soil quality. The residual MFs can lead to soil compaction and reduced water and nutrient penetration. Furthermore, chemicals released from MFs can lead to soil acidification, thereby reducing soil fertility and plant growth. Additionally, the substantial quantity of MFs remaining in farmland soil is a crucial source of microplastics⁴ that can harm wildlife and ecosystems.⁵ Yang et al. (2022) detected 147 particles per cm² of microplastics derived from the polyethylene (PE) black MF after 28 days of UV irradiation.⁶ Proper installation and recycling of MFs from fields, as well as understanding their potential pollution, are essential for mitigating the negative impacts of MFs on soil quality and the environment.

The utilization of MFs in China was not uniformly distributed, and some regions, such as Inner Mongolia, had a higher proportion of usage due to their cold and arid climatic conditions.⁷ The abundance of microplastics found in Inner Mongolia soil was 77.6 kg ha⁻¹, which was considerably higher than that in other provinces, except Xinjiang.⁸ Additionally, Inner Mongolia experiences high levels of UV radiation,⁹ which is a critical factor that influences the aging process of MFs and affects their mechanical and physicochemical properties, making their recycling challenging.¹⁰ An in-depth comprehension of the mechanisms involved in the aging process and their impact on MF recycling can aid in the development of strategies to prolong the lifespan of MFs and reduce their environmental impact.

Current research on the aging and recycling of MFs showed mixed results, as the aging process of MFs during their service life is a complex process that involves several factors, such as UV radiation, temperature fluctuations, moisture, and chemical degradation. Aging conditions can have a significant impact on the quality and feasibility of MF recycling. Thermal oxidation reactions due to UV radiation and high-temperature conditions can cause polymer chain scission, resulting in a reduction in the film's tensile strength, elongation, and tear resistance.¹¹ Exposure to chemicals, such as fertilizers, pesticides, and herbicides, can cause chemical degradation of MFs, leading to a decrease in their mechanical properties.¹² In farmland, the edges of the MFs are secured with a generous amount of soil to fix the MF, and both abiotic and biotic factors affect the aging process of MFs. Photodegradation is the primary abiotic factor causing a loss of mechanical properties. In contrast, the MF buried in the soil undergoes a distinct aging process due to anaerobic conditions, absence of light radiation, and varying moisture content, temperature conditions, and microbiological environments. After harvesting, the MF should be removed from the land, while the aging and mechanical properties of the buried MF may be different from those of the sunlight-exposed MF, which should be considered during MF recycling. However, most studies on MF aging are conducted under laboratory conditions in simulated aging environments that may differ significantly from real-farmland conditions, and the buried MF, which has a significant contribution to plastic residue in the soil, has been ignored in previous studies.

In this study, MFs that were sunlight-exposed and soil-buried were separately collected from the farmland in Inner Mongolia after completing their service life. The morphology, chemical, thermal and mechanical properties of MFs have been studied extensively. We aimed to: (1) understand the aging mechanism of MFs exposed to sunlight and buried in soil in a real agricultural environment; (2) explore the property change of corresponding MFs under different aging conditions; (3) reveal the interactions between the aging of MFs and their recycling properties, as well as their potential environmental pollution.

2. Materials and experiments

2.1 Materials

The raw polyethylene (PE) MF with a thickness of 0.01 mm was produced on 15th February 2021 according to the standard of GB-13735-2017. The field experiments were conducted by The Bureau of Agriculture and Livestock, Wongniute, Inner Mongolia in the cornfield located at 119°E, 42°N. The width of the MF is 800 mm. During MF installation, 200 mm on each side of the MF was buried underground to fix the MF. The MF cover time was from mid-April to mid-September 2022. The maximum temperature of the soil under the MF was about 40 °C, and the ambient temperature varies with the seasons, within the range of 5–30 °C. The soil humidity was about 16%. After harvesting, the buried and sunlight-exposed MF with a length of 10 m were manually collected separately, labeled as MF-Soil and MF-Light, respectively, and then mailed to the lab with the pristine MF for further analysis. The pristine MF was labeled as MF-Raw. After receiving in the lab, all the MF samples were rinsed with deionized water to remove the soil and organic residue, and then stored at room temperature in the dark. For each analysis, five small squares with a side length of 5 cm were randomly selected from the received samples for the test, ensuring the accuracy of the data.

Besides the MF, the soil with and without MF cover was collected in this study to investigate the effect of the MF on the soil environment. The cultivation, irrigation, and fertilization conditions, as well as the climatic conditions for all the soil samples, are the same except for the laying of the MF. The cultivated soil layer (0–20 cm) has the ability to supply nutrients and affect crop productivity, and the cultivated layer soil quality is directly affected by crop roots and farming conditions.¹³ In addition, Ma et al. (2018) found that mulching promoted soil moisture and nitrate concentrations in topsoil (0–20 cm), but had little effect in the deeper soil layer.¹⁴ So, the soil samples were collected from the cultivated soil layer with a depth of 0–15 cm in this study. Three soil samples with MF cover were randomly collected from different sites, and labeled as PT1, PT2, and PT3. The soil without MF cover was labeled as NONE. Soil samples were sealed and stored at −18 °C after receiving until further analysis.

2.2 Characterization methods

2.2.1 Physicochemical characterization of the MF. The morphology of the MF was observed via scanning electron microscopy (SEM, TM 4000plus, Hitachi, Japan). Samples were sputter-coated with a thin gold layer before SEM analysis. Fourier transform infrared-attenuated total refraction (FTIR-ATR) was performed to determine the functional groups of the MF by using an FTIR spectrometer (IRTracer-100, Shimadzu, Japan) over a range of 400–4000 cm⁻¹. Each sample was tested five times at different positions. The carbonyl index (C.I.), vinyl index (V.I.), and hydroxyl/hydroxyperoxide index (H.H.I.) were calculated according to the following equations:^6,11,15where A_1650–1850, A_1620–1660 and A_3150–3650 are the area of peaks attributed to carbonyl, vinyl and hydroxyl groups; A_1420–1500 is the area of the reference peak. The area of peaks was calculated by using the origin software (Origin 2020b) and the standard deviation is less than 2%.

In addition, the chemical structure of the MF was determined using a Raman microscope (287 Q00, Renishaw, UK) that coupled a Raman spectrometer to an optical microscope (Leica DM 2500 M). The hydrophily of the MF was characterized by using a contact angle measurement instrument (ThetaLite, Biolin Scientific, Sweden).

Thermogravimetric analysis (TGA) was conducted using a TGA-50 analyzer (Shimadzu, Japan) to determine the thermal stability of the different MF samples. Samples of 5–10 mg were placed in aluminum oxide crucibles and heated from room temperature to 800 °C at a rate of 10 °C min⁻¹ in the presence of nitrogen (10 mL min⁻¹). Thermal analysis of the MF was performed using a differential scanning calorimeter (DSC-60 Plus, Shimadzu, Japan). Approximately 6 mg of the MF sample was sealed in an aluminum pan, then heated from room temperature to 150 °C with a heating rate of 10 °C min⁻¹, and held at this temperature for 2 min to erase any thermal history. After that, the sample was cooled down to 35 °C with a cooling rate of 10 °C min⁻¹, and then heated again to 150 °C again with the same heating rate. The melting temperature was measured during the second scan. The crystallinity of MF-Raw, Light and Soil was calculated from the DSC test according to eqn (4).

where ΔH_m is the melting enthalpy of the samples (J g⁻¹); ΔH⁰_m is the melting enthalpy of the standard PE with 100% of crystallization (287.3 J g⁻¹).¹⁶

2.2.2 Plasticizer extraction and characterization. The prepared MF samples were cut into squares with a side length of less than 5 mm. Subsequently, 0.2 g of the samples were placed in a glass centrifuge tube, and mixed with 10 mL of ethyl acetate for ultrasonic extraction. All tubes were sealed with Teflon screw caps and sonicated for 15 minutes. After ultrasonic extraction, the extraction solution was transferred to a volumetric flask and the previous extraction was repeated three times to make sure the plasticizer had been extracted well. Then, all the extraction solutions were mixed and their composition was analyzed using a GC-MS (7890A-5975C, Agilent Technologies, USA) equipped with a HP-5 capillary column (30 × 0.25 mm × 0.25 μm i.d.). Helium (99.99% purity) was adopted as the carrier gas at a flow rate of 1 mL min⁻¹. The inlet and detector temperatures were 250 °C and 280 °C, respectively. The GC oven temperature was maintained at 50 °C for 5 min followed by a ramp at 5°C min⁻¹ to 260 °C and held for 5 min. Three empty groups (the operating procedures were similar to the experimental group, except without the MF) were conducted as controls. Each sample was analyzed twice.

2.2.3 Mechanical property analysis. The mechanical properties were analyzed using a micro-electromagnetic type dynamic mechanical experimental system (CARE, M-100, CARE Measurement & Control, Tianjin, China). Samples were placed carefully on the grips aligned and tight before testing. Each sample was tested five times.

2.2.4 Soil microbial community analysis. The microbial communities of soil samples with and without MF cover were analyzed by Majorbio (Shanghai, China) using high-throughput 16S rRNA gene sequencing. The analysis was completed within one month of receiving the samples. It was reported that storage at −18 °C for one month will not affect the microbial community structure.¹⁷ Genomic DNA was extracted from soil samples and prepared by using the TruSeqTM DNA Sample Prep Kit. Sequencing was performed on the Illumina platform (Illumina, San Diego, CA, USA). Each sample has been run in triplicate. UPARSE (7.0.1090) was used to realize operation taxonomic unit (OTU) clustering. The OTU species taxonomy was analyzed using the ribosomal database program (RDP) classifier with the Bayesian algorithm. A principal coordinate analysis (PCoA) was done to analyze the microbial community variety of different samples.

3. Results and discussion

3.1 Physicochemical aging process of the MF

MFs collected from different conditions have undergone different aging processes that can affect their surface morphology during their service life. As shown in Fig. 1, The surface of MF-Raw was smooth and uniform. On the other hand, the surface of MF-Light became faded after exposure to sunlight. The discoloration may be caused by the breakdown of the PE polymer chains under UV irradiation and the release of the pigment during the aging process. Light-induced photobleaching can result in uneven discoloration and chalking of the MF surface.¹⁸ The breakdown of the polymer chains may produce free radicals that induce depolymerization and accelerate the carbon black release. In addition, the surface of MF-Light is rougher than that of other samples. It is reported that the sunlight exposure resulted in an increase of crystallinity and surface cracking.¹⁹ The stress concentration at surface cracks led to further crack propagation and polymer degradation. The development of cracks and fissures on the MF-Light surface may result in a loss of mechanical strength. Different from the rough surface of MF-Light, pits were observed on the surface of MF-Soil. The anaerobic conditions in the soil layer may have inhibited the photo and thermal degradation, but the MF-Soil was exposed to chemicals such as fertilizers, pesticides, and herbicides, which could promote the chemical breakdown of the polymer chains, resulting in the formation of craters. It is reported that the aging process of the PE films starts from dot cracks, and then gradually leads to block damage and flaky damage.²⁰ Comparing the different morphologies of the samples of MF-Raw, MF-Light and MF-Soil, it could be concluded that the MF-Light degraded more rapidly than MF-Soil and ultraviolet light may play a key role in the aging process of the MF.

Fig. 1 Pictures and SEM images of MF-Raw, MF-Light and MF-Soil.

FTIR-ATR was employed to examine the functional groups of the MF under different aging conditions (Fig. 2). The characteristic bands of PE were observed in all spectra of MF samples: the two representative peaks at 2914 and 2847 cm⁻¹ are attributed to asymmetric and symmetric absorption of CH₂ groups; the band located at 1462 cm⁻¹ corresponds to bending and stretching of CH₂ groups; the band at 719 cm⁻¹ is due to the occurrence of C–H bending. The intensity of these bands decreased in both MF-Light and MF-Soil, with a more pronounced decrease observed in MF-Light. The same phenomenon was observed by Xiong et al., (2023),²¹ who attributed the decrease in peak intensity to the aging process and the oxidation reaction of the films. Compared to MF-Raw, an extra band at 1027 cm⁻¹ corresponding to C=O groups was shown for MF-Light and MF-Soil, indicating the generation of carbonyl groups. The bands between 1610–1800 cm⁻¹ are attributed to –C=C–, –C=O of acids, esters, and ketones. The peak intensity at this region was stronger for MF-Light. The border bands between 1620–1660 cm⁻¹ for MF-Light illustrated the development of unsaturation.¹¹ This result was confirmed by the stronger peaks of C=C wagging vibrations at 874 and 911 cm⁻¹ for MF-Light.²²

Fig. 2 FTIR spectra of MF samples (A) and the corresponding contact angles (B).

For MF-Light, the absorbed UV radiation led to the formation of free radicals due to the breakage of chemical bonds in the MF. The generated free radicals then react with oxygen in the air to form peroxyl radicals, generating –OH, C=O, COOH, and COO– groups on the main chain of the MF matrix. In addition, the formed reactive intermediates such as free radicals and carbonyl groups can react with each other to form double bonds, leading to the development of unsaturation in the polymer. Hydroxylation could be caused by hydroxy and peroxyl radicals. All these reactions resulted in the chain scission of the MF. In this study, C.I., V.I. and H.H.I. as measurements of the oxidation level, unsaturation level, and hydroxylation level were calculated based on the FTIR spectra (Table 1). MF-Light had the highest C.I. and H.H.I. value because the free radical generation and oxidation degradation rate of the polymer were determined by the UV irradiation and the content of oxygen. MF-Soil was buried under the soil, where the dark and anaerobic environments inhibited the oxidation process, resulting in a much lower intensity of the peak at 1027 cm⁻¹ compared to that of MF-Light. Soil inhibits the photoaging, while chemicals, metals, enzymes and microorganisms can cause gradual oxidation and degradation.²³ PE is highly hydrophobic because of the presence of the linear backbone of carbon atoms, its degree of crystallinity and its high molecular weight.²³ The generation of oxygen-containing functional groups and hydroxylation altered the hydrophobic nature of the MF surface. As shown in Fig. 2B, the contact angle of MF-Raw was 95.07°, while after use in the farmland, the contact angles of MF-Light and MF-Soil decreased to 73.31° and 86.82°, respectively. This finding is consistent with other research.²⁴ In addition, the generation of oxygen-containing functional groups could make the MF adsorb organic pollutants and nutrients more easily,²⁵ increasing the risk of environmental pollution. UV irradiation also resulted in the highest V.I. value of MF-Light. It was reported that the unsaturated MF is more susceptible to oxidative degradation than the saturated one. In addition, the unsatured MF is more brittle and exhibits a decrease in mechanical strength.

Table 1 The C.I., V.I., and H.H.I. values of MF samples calculated based on FTIR spectra

Sample	A 1650–1850	A 1420–1500	A 3150–3650	A 1620–1660	C.I.	V.I.	H.H.I.
MF-Raw	0.26	2.49	0.45	0.08	0.10	0.03	0.18
MF-Light	1.83	2.08	1.52	0.73	0.88	0.35	0.73
MF-Soil	1.06	2.37	0.58	0.33	0.44	0.13	0.24

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FTIR-ATR probes the near-surface region of a sample only, and Raman spectra are not influenced by the thickness of samples. It could provide additional information about contained pigments.²⁶ As shown in Fig. 3, besides the typical PE bands at 1061, 1127 and 1300 cm⁻¹, the band at 1530 cm⁻¹ which corresponds to the pigment²⁶ was clearly present in the Raman spectra of MF-Raw. After the aging process, the peak intensity at 1530 cm⁻¹ decreased, indicating the release of the additives. The two Raman peaks at 2846 and 2881 cm⁻¹ were the identification peaks of PE. Different from FTIR results, the aged MF, including MF-Light and MF-Soil had a stronger intensity at 2846 and 2881 cm⁻¹ which corresponds to CH₂ groups. This may be caused by the loss of additives. In addition, Raman spectra provided the crystallization structure of different MF samples. The bands at 1440 and 1460 cm⁻¹ are assigned to the bending modes of the amorphous trans chains and amorphous chains, respectively.¹⁶ Compared to MF-Raw, MF-Light had a stronger intensity at 1440 cm⁻¹, while a weaker intensity at 1460 cm⁻¹ (Fig. 3C), suggesting the formation of trans chains in the amorphous phase, and the recrystallization occurred during the MF aging under light irradiation. This result could be confirmed from the DSC results in Fig. 4(B).

Fig. 3 The whole Raman spectra of different MF samples (A) and the Raman spectra in the wavelength ranges of 3000–2750 cm⁻¹ (B) and 1800–900 cm⁻¹ (C).

Fig. 4 Thermograms (A) and DSC curves (B) for different MF samples.

3.2 Thermal-mechanical behavior of MFs

TG analysis was employed to analyze the thermal stability of the MF samples (Fig. 4A). The degradation of MF samples occurred in the temperature range of 400–550 °C with one step. The degradation temperatures of various MF samples followed the trend of MF-Raw < MF-Soil < MF-Light, indicating that the MF-Light contains more stable PE chains. The aging process increased the thermal stability by around 5 °C, while having little effect on degradation mass difference. These results are probably attributed to side polymer chain cleavage or the release of additives.

DSC provided information about the melting and crystallization behavior of MF samples (Fig. 4B). Upon heating, MF-Raw showed melting peaks at 125.45 °C. After recycling from the farmland, the melting temperatures of MF-Light and MF-Soil were slightly increased to 128.05 and 127.79 °C. The crystallinity of various MF samples was determined using eqn (4). Studies about the PE aging process performed in the lab showed that the crystallinity decreases under UV irradiation, while our results showed that the aged MF in natural environments had a higher crystallinity than the pristine one, no matter under light or soil conditions (Table 2). It was probably caused by the loss of small-sized microplastics during the aging process. Chain scission during the aging process promoted the release of the entangled polymer chains in the amorphous regions, leading to an increase in crystallinity.¹⁰

Table 2 Thermal data for MFs extracted from DSC analyses

Sample	T m (°C)	ΔHm (J g^−1)	T c (°C)	X c (%)
MF-Raw	125.45	55.95	107.07	19.47
MF-Light	128.05	64.96	108.25	22.61
MF-Soil	127.79	61.83	109.03	21.52

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Compared to the crystallization temperature (T_c) of MF-Raw, the value of MF-Light and MF-Soil increased to 108.25 and 109.03 °C, respectively, during the DSC cooling process. The chain scission of MF amorphous during the aging process allowed crystalline phases to form at higher temperatures.¹⁰ These results suggested that the MF underwent chain scission, but the aging extent was not yet serious enough to cause crystallinity loss.

Mechanical properties are critical to predicting the service life of the MF and providing suggestions for MF recycling. Tensile testing is the dominant measurement employed to assess the ability of films to withstand external forces resulting from film recycling and weed growth. The tensile strength tests were performed to study the mechanical properties of the MF samples. As shown in Fig. 5, all the MF samples displayed a linear elastic behavior at first, after yield initiation, a highly none nonlinear elastic behavior started. The yield point is defined as the point where the strain starts to increase significantly for a small increase in tensile strength.²⁷ The aged MF showed a lower yield than the pristine one. MF-Soil lost the mechanical strength most rapidly and had the minimum elongation. The higher moisture content condition in the soil environment may accelerate the dissolution of additives, leading to a deterioration of the mechanical strength. For MF-Light, photodegradation caused the polymer chains to break down, leading to film embrittlement and losses in mechanical integrity with aging. The MF needs to be recycled after harvesting to avoid the accumulation of plastics in soils. MF collection is difficult and time-consuming because the aging process decreases mechanical integrity. Mechanical collection may break the MF and result in a low recycling rate. Additionally, the MF that is partially buried in the soil is more difficult to recycle because it has lower mechanical integrity, as shown in Fig. 5. This should be considered when optimizing the recycling methods.

Fig. 5 The dynamic mechanical experimental system and the mechanical properties of different MF samples.

3.3 Potential effect of MF aging on the environment

3.3.1 Plasticizer release. Plasticizers, common toxic substances, are important additives in the production process of MFs. However, plasticizers such as phthalate esters (PAEs) can easily migrate into the environment from MFs because they are only physically, not chemically, bonded to the polymeric products.²⁸ The aging process may accelerate the release of PAEs. Emissions and pollution caused by PAEs from MFs are related to the regional climate conditions. The total emissions of dibutyl phthalate (DBP) and bis(2-ethylhexyl) phthalate (DEHP) in Inner Mongolia were estimated at 1.48 and 3.31 tons in 2017, respectively, higher than that of most other regions of China.⁷ The potential environmental pollution caused by PAEs from MFs should be a concern.

In this study, DEHP was the main PAE detected in MF-Raw (peak observed at the retention time of 45.058 min, as shown in Fig. 6), while little DBP was detected. The difference between our result and Zhang's data⁷ may be due to the different origins of the MF materials. DEHP was not detected in MF-Light and MF-Soil, suggesting that almost all PAEs were released during the MF's service life. The PAEs in MF-Soil would finally be released into the soil, while most of the PAEs in MF-Light would be released into the air.²⁹ It has been reported that the release of PAEs can reach 84% after 150 days of exposure under air conditions. The PAE content in film-covered soils was reported to be up to 5 times higher than that in soil without film cover.³⁰ The release rate of PAEs may be related to their composition, concentration and the MF aging process. Understanding their release performance is important to control the plasticizer pollution. In future studies, the plasticizer release rate under varied conditions should be considered. It should be noted that a peak at a retention time of 43.089 min was shown for all the samples, including the blank, the MF-Soil, MF-Light and MF-Raw. This peak may be attributed to the interferences introduced during the extraction process. We did not discuss it in this study.

Fig. 6 GC-MS spectra of extractions from MF samples. The DEHP was shown at the retention time of 45.058 min as peak II.

3.3.2 Effect of MFs on the change of the soil microbial environment. The effect of MFs on soil microbial communities is an important concern, as they can affect nutrient cycling and other soil processes.³¹ Studies have reported that the residual MF has little effect on the soil physicochemical properties in the short term but significantly changes the soil microbial community composition.^5,32 Additionally, the effect of MFs on the soil microbial communities is varied and depends on the plastic type, the crop in the farmland, and the environmental conditions. The MF residue may affect the ratio of carbon and nitrogen, frequently accompanied by the change of the microbial communities and the soil properties. Therefore, it is essential to explore the different responses of soil bacterial community assembly to the usage of MFs.

In this study, the microbial community composition and abundance at the phylum levels of bacteria were analyzed to investigate the effect of the MF on soil microbial communities. The MF covering has little effect on the composition of soil microorganisms. Actinobacteriota, Chloroflexi, Firmicutes, Proteobacteria, Acidobacteria and Genmmatimonadetes are the most abundant communities at the phylum level for all the soil samples, accounting for more than 90% of the microorganism sequences (Fig. 7A). This finding is consistent with the other research.³³ However, the PCoA revealed that soil samples PT1, PT2, and PT3 grouped tightly in PC1 (64.77%) but had a big distance with sample NONE, indicating that all the MF-covered soil samples share a high similarity bacterial structure, and show a distinct clustering from the soil without MF covering (Fig. 7B). After MF covering, the abundance of Actinobacteriota in soil was increased by 75%; this result was consistent with current findings, which found that the accumulation of microplastics, especially those derived from PE, in soils could increase the abundance of Actinobacteriota.^34,35 On the other hand, the abundance of Chloroflexi and Firmicutes dropped by 27% and 45%, respectively for the MF-covered soil. Chloroflexi tends to dominate in oligotrophic environments where N availability is low. The decrease of Chloroflexi suggested that the MF has altered the N cycling in the soil environment. Firmicutes are generally associated with natural organic polymers, and they would be also involved in the degradation of synthetic polymers such as PE.³⁶

Fig. 7 Effect of MFs on the soil microbial community at the phylum level (A) and the PCoA on the operational taxonomic unit (OTU) level (B).

Currently, most studies focus on the microplastic pollution of terrestrial soil in mulch film research. The results indicated that microplastics may affect the soil properties, as well as the soil microbial community.³¹ While PE can remain relatively stable in soils, and the field environment was open and had a relatively strong buffering capacity, the microbial community change caused by the microplastic derived from MF aging needs a long time to be observed. Wang et al. (2023) indicated that, compared to concentration, the shape of the MF residue displays a more pronounced effect on the soil bacterial community.³³ The larger-sized MF, including the soil-buried MF during service life and MF residues after harvesting, may increase the possibility of damaging the structure of soil aggregates,³⁷ resulting in the nutrient release. The bacterial community structure may be affected accordingly.

The microbial community results indicated that usage of MFs changes the soil bacterial communities, thus affecting the biogeochemical processes of the soil ecosystem. The results in Fig. 7 are only a rough estimate of the actual soil microbial diversity. The enzyme, fungi of the soil, and the microbial community on the MF residue surface were not detected. A long-time examination of the MF effect on the soil properties, as well as the microbial environment, should be conducted in further study.

4. Conclusion

In this study, polyethylene MFs covered and buried in the soil were collected from the farmland in Inner Mongolia after their service life to explore the different aging properties. Exposure to UV radiation and elevated temperatures resulted in a rougher MF surface, while chemicals and moisture in the soil led to pitting. In addition, the generation of oxygen-containing functional groups and hydroxylation caused by photodegradation alter the hydrophobic nature of the MF surface. The aging process facilitated the release of additives and the chain scission, resulting in a reduction in the film's tensile strength and elongation. The release of PAEs from the MF during service life raises concerns regarding environmental pollution. In addition, MF usage altered the soil microbial community composition. Its effect on the soil features and nutrients should be further investigated in the future.

Author contributions

Chao Zhang: data analysis, original draft; Xingyu Liu: performed the experiments and data analysis; Li Zhang: mulch film and soil collection; Qindong Chen: Raman analysis; Qiyong Xu: review and editing, funding acquisition, project administration and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Shenzhen Fundamental Research Program (No. GXWD20201231165807007-20220724202837001). Thanks to the Bureau of Agriculture and Livestock, Wongniute, Inner Mongolia for providing the mulch film and soil samples.

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