Impacts of Biodegradable Mulch: Benefits, Degradation, and Residue Effects on Soil Properties and Plant Growth
2024 Volume 12 Pages 262-280
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2024 Volume 12 Pages 262-280
Biodegradable mulch is gaining popularity as a sustainable replacement for conventional plastic mulch because it can fully degrade in the soil, making it more environmentally friendly. However, uncertainties remain regarding its in-situ degradation under diverse natural conditions and over prolonged durations. Furthermore, questions persist about the long-term environmental impact of biodegradable mulch usage on plant–soil health. This review focuses on the use of plastic mulch in agriculture, including its benefits, application across climates, degradation characteristics, and the impact of plastic mulch residues on soil properties and plant growth.
Plastic is an integral part of our lives. Its production steadily increases with the rapid economic growth, yielding growing amounts of plastic waste [1]. This issue poses a potential threat of significant plastic pollution. For instance, plastic pollution in the ocean has a detrimental effect on marine ecosystems and wildlife [2, 3, 4]. The most significant impact occurs when animals ingest or become entangled in plastic, leading to suffocation, entanglement, lacerations, infections, and internal injuries [4]. These can result in lethality, altered reproduction, changes in feeding and swimming behavior, immune function alterations, and stress in aquatic organisms [3]. Similar to water bodies, plastic pollution in soil disturbs the soil ecosystem and negatively affects the survival and growth of plants by altering the soil’s chemistry [5]. Furthermore, many animals in terrestrial environments unintentionally eat the plastic from the soil, leading to damage in their digestive systems and reducing their survival chances, especially earthworms [6, 7, 8]. Finally, they can enter human bodies through the food supply chain, inhalation, and direct contact with harmful chemicals leached from plastic which pose a serious risk to human health [6, 8, 9, 10, 11, 12].
Mulching refers to the agronomic practice of placing mulch on the soil surface to provide favorable conditions for plant growth and development [13]. Polyethylene (PE) film was the earliest form of mulch utilized in the late 1950s [14] in the United States for high-value crops, such as flower and potted plants. Low-density polyethylene (LDPE) was widely applied because it is affordable, easy to process, flexible, and highly durable [15]. Global consumption of LDPE mulches has been steadily rising since the 1960s, with an increase of 35% between 2006 and 2017, reaching over 2 million tons [16, 17]. However, recycling rates for used mulch are generally low [18], primarily because of poor management of plastic mulch removal, difficulty in collecting fragments remaining after mulch use, and low value of recovered mulch fragments [19]. The inability to recover all mulch fragments from the soil after usage results in their release into the agricultural soil. They remain in the soil for long periods, harming the environment and affecting soil quality and crop yield [20, 21].
Biodegradable plastics have emerged as a sustainable alternative to LDPE plastics. Figure 1 shows the rising global production of biobased polymers. The actual production of biobased polymers reached 4.5 million tons in 2022 [22]. Based on observed trends, their production will reach 6.7 million tons in 2025 [23]. Despite its promise as a solution to the issue of conventional plastic accumulation in soil, the challenges of applying biodegradable mulch in agriculture remain ongoing. While some research indicates that biodegradable mulches can improve soil quality, other studies urge caution in their widespread adoption. More information is needed about their possible environmental impact. Therefore, this study comprehensively reviews the development of plastic mulch and its application in agriculture, summarizing the latest research trends on the impact of biodegradable plastic mulch residues on soil’s physical and chemical properties and growth performance. Additionally, it explores the impact mechanism of both pristine and aged biodegradable plastic mulch residues on soil, an area that has not been extensively studied. This study thus provides an essential reference for further sustainable agricultural development.
Mulching plays a crucial role in conserving soil moisture by reducing evaporation losses and enhancing water retention in the soil [15, 25]. This makes plastic mulch an important technology, especially in regions with low precipitation or hot and dry climates [26]. Furthermore, black or clear plastic mulches absorb sunlight and warm the soil underneath, allowing for earlier planting of warm season crops and potentially higher yields by promoting faster growth [27, 28]. They also help control weeds by creating a physical barrier that blocks sunlight from reaching the soil [28]. This weeds suppression ability reduces the need for labor-intensive weeding and herbicide use. Additionally, plastic mulch is widely used in typically high-value vegetable crops such as tomatoes, peppers, eggplant, broccoli [29], and strawberries [30] because it acts as a barrier between the soil and crops. This barrier prevents soil from splashing vegetables during rain or irrigation, resulting in cleaner produce at harvest [28] and protecting them from soil-borne plant diseases [31, 32].
The fact that conventional petroleum-derived plastics are not readily biodegradable because of their resistance to microbial degradation stimulates interest in biodegradable polymers. However, only a few studies have examined the benefits of biodegradable plastic mulch on soil health (Table 1). These studies found that biodegradable mulch improved soil nutrient levels, reduced soil compaction (bulk density), stimulated microbial activity, and increased urease and catalase activities, promoting a healthier soil ecosystem [33, 34, 35]. Additionally, they can significantly reduce labor and disposal costs through incorporation via soil tillage operations rather than disposal in landfills [15, 36].
| Plastic mulch experiment | Effect | Remark | Ref. | ||
|---|---|---|---|---|---|
| Type | Management practice | Experiment design | |||
| Starch-based raw material (Mater-Bi) | Strawberry plant cultivation | Collected soil samples after 9 months of contact with plastic mulch in the strawberry plots | positive | No evidence of ecotoxicity during the biodegradation process | [33] |
| positive | The diversity of ammonia-oxidizing bacteria, which is an indicator of soil health, remained unchanged | ||||
| PLA and cellulose-based mulch | High tunnel and open field production system | SQI (soil quality index) were assessed over an 18-month period | positive | Soil microbial biomass increased. | [34] |
| positive | Soil β-glucosidase activity was higher in the high tunnel system compared to the open field system. | ||||
| BDF | Garlic–maize rotation | Crop planting system conducted for six years (2013–2019) | positive | The soil’s total nitrogen, available phosphorus, and available potassium content increased. | [35] |
| positive | Enhanced microbial activity and enzymatic activity in the soil | ||||
| positive | Soil bulk density was significantly lower by 12–17% compared to PE mulch. | ||||
PLA: polylactic acid, BDF: unspecific biodegradable plastic film
Despite its mentioned benefits and environmentally friendly reputation, biodegradable mulch film may not always result in lower environmental impacts when considering its entire life cycle. In fact, when analyzed comprehensively, it often has more significant environmental impacts compared to non-biodegradable mulch film across various categories [36]. In a recent case study in Nordic conditions, the use of biodegradable plastic mulch may result in increased environmental impacts due to factors like energy use and acid production compared to non-biodegradable mulch if the collection after use is inadequate [36]. Furthermore, the degradation rates of biodegradable plastics can vary based on climatic conditions, such as Nordic countries where low temperatures in soils may lead to longer degradation periods, raising concerns about the effectiveness of biodegradable mulch films in such environments [36]. There is still uncertainty surrounding the use of biodegradable plastics. Therefore, more information is needed, especially in terms of in-field degradation rates differing from laboratory tests due to environmental conditions like temperature and moisture content and their possible environmental impact in different conditions.
Regional climate conditions are a key factor in agricultural productivity, as plant metabolic processes are influenced by variables like temperature, solar radiation, carbon dioxide (CO2), and water availability, thereby determining the types of crops that can be cultivated and the overall success of crop production [37, 38]. For instance, in arid and semi-arid regions, the rainfall is low and erratic, leading to frequent droughts and poorly structured, infertile soils [39]. In this case, mulching has important roles in increasing water infiltration, reducing evaporation [40], controlling weeds, and enhancing soil biological activities [41]. It also helps modify soil temperatures, increase nutrient availability, and improve soil organic matter levels, ultimately leading to increased crop yields [42]. Furthermore, mulching enhances the formation of a thin air-dry layer on the soil surface, which hampers capillary rise and slows down the evaporation process [43]. This is particularly beneficial during dry periods in arid and semi-arid regions where soil moisture deficiency is a common issue.
Table 2 shows the effects of plastic mulching on crop yield production. In most cases, applying plastic mulch increased crop yield across different climates. However, its impact was less pronounced in arid regions experiencing abundant rainfall during the growing season [41]. Plastic mulch has different roles depending on the climate condition. For example, the main function of plastic mulch in temperate and cool climates is to act as a greenhouse by absorbing and transmitting daytime solar radiation, which in turn raises the air temperature underneath the plastic, warms the soil, and reduces heat loss at night during the initial 30–35 days of the growing season [44, 45]. This accelerates the emergence and growth rate and protects seedlings from frost, allowing for early growth and development of crops like corn in regions with low temperatures [45].
| Climates | Plastic mulch for the experiment | Remark | Ref. | |
|---|---|---|---|---|
| Mulch type | Design experiment | |||
| Tropical (Nigeria, Bangladesh) | PE | Colocasia esculenta Schott harvested at 270 days with tillage and no tillage treatment | Corm yield was higher in tilled plastic-film mulched plots (104% to 180 %) than in tilled no-mulch. | [46] |
| Non-biodegradable | Chili pepper cultivated with 3 mulching treatments | Black plastic mulch produced a higher fruit yield per hectare (58%) compared to the no mulch treatment. | [47] | |
| Arid and semi-arid (Jordan, China) | PE | Okra and summer squash under rain-fed condition | Late yields of both vegetable crops are significantly higher compared to bare soil, with okra increasing by 140% and squash by 61%. | [48] |
| Non-biodegradable | Maize with 4 plastic mulch patterns. | Plastic film mulching can increase yield, but the effect may be insignificant with abundant rainfall during the growing season. | [41] | |
| Temperate (China) | Paper and BDF | Cucumber under 5 mulching treatments | Using biodegradable mulch (49% to 111%) resulted in higher plant yield compared to bare soil. | [44] |
| Non-biodegrdable | Maize with 4 treatments | Plastic mulch consistently increased grain yield by 8–24% over multiple seasons compared to traditional practices. | [49] | |
| Mediterranean (United States, Spain) | BDF and PE | Sweet corn under 6 mulching treatments | Plant growth and yield are generally greater in plastic mulch treatments compared to bare ground. | [50] |
| PE and BDF | Tomato with 7 mulching treatments | Plastic mulch treatment significantly increased marketable yield compared to control treatment. | [51] | |
| Continental (China) | PE and BDF | Maize under 5 mulching treatments | Plastic mulching did not increase maize yield in a humid climate with effective herbicide weed control. | [52] |
| Non-biodegradable | Maize under the asymmetric ridge and furrow system (DRF) with different fractions of film cover. | Full film cover resulted in the highest yield increase compared to partial film cover or no film cover. | [53] | |
| Subtropical highland (Nepal) | BDF | Broccoli with different mulching material | The higher yield (123%) was recorded with black plastic mulch compared to the no mulch treatment. | [54] |
| Cool (Canada) | Photodegradable plastic | Maize with covered and non-covered treatment | Plastic mulch increased total dry matter yield by about 22% in the first year and 14% in the second year. | [45] |
Conventional plastic mulches are nonbiodegradable and tend to persist in the environment [55]. The most used conventional mulches are mainly made of PE, polyvinyl chloride (PVC), and ethylene vinyl acetate. PE comes in various types, including high-density PE, LDPE, very low-density PE, and ultrahigh-molecular-weight PE [56].
| Type polymer |
Degradation characteristics | Predicted degradation period | Derived harmful substances | Ref. |
|---|---|---|---|---|
| PE | Combination photo- and thermos-oxidation and biological process | Over 300 years | Aldehydes and ketones | [57] |
| PVC | Thermal and photodegradation | Approximately 100 years | PAEs | [58, 59] |
| PLA | Hydrolysis and biological process | >3 years | - | [60, 61] |
| PBAT | Hydrolysis or oxidation and biological process | 2–5 years | TPA | [62, 63] |
| PBSA | Biological process | At least 1 year | - | [64, 65] |
PBAT: poly (butylene-adipate-co-terephthalate); PBSA: poly (butylene succinate adipate); PAEs: phthalate esters; TPA: terephthalic acid
Currently, the most widely used PE mulches are made of LDPE, with a density of 0.91–0.94 g/cm3 [56]. LDPE is a white material with a highly branched structure, forming a low-density polymer with excellent properties. It exhibits good thermal and chemical resistance, excellent tensile strength, high durability, and flexibility [66]. Table 3 shows that PE polymers degrade through a combination of photo- and thermos-oxidation and biological processes [67]. The decomposition process in the soil may span more than 300 years [57]. During this time, harmful substances, such as aldehydes and ketones, may degrade but not completely disappear. Consequently, 5%–10% of the mulch residues may remain [57].
PVC is the second most used mulch film material after PE. PVC polymer manufacturing involves combining acetylene with hydrochloric acid under high temperatures and pressure. Pure PVC is a very rigid plastic material that can be made more flexible by adding plasticizers such as dioctyl phthalate or tricresyl phosphate [57]. Soft PVC uses approximately 87% of the phthalate esters (PAEs) produced [68]. PVC plastic mulch residues in the soil may generate PAEs, which are carcinogenic and mutagenic [69, 70]. PAEs are highly hydrophobic and easily adsorbed by soil organic matter. Their residues may remain in the soil for a long time, probably affecting the soil microbial populations and enzyme activities [71, 72].
Some reports have been published over the past decade on the development of biopolymer mulch films using materials such as starch, cellulose, lignin, fruit by-products, soy protein, pectin, and chitosan [73, 74]. These films are designed to possess properties similar to those of synthetic polymers but with the added advantage of rapid and complete biodegradation into water and carbon dioxide. This trend may reduce the negative environmental impact of synthetic polymers [57].
Biodegradable plastic feedstock can be obtained from biobased sources, fossil fuels, or their combination. The most common biobased feedstocks used to make biodegradable mulches are starch, PLA, and polyhydroxyalkanoates (PHA) [75]. PHA is polyester that is naturally synthesized through a single-step process of bacterial fermentation using plant sugars, lipids, or both [76]. PLA is a thermoplastic polyester that ferments starch with yeast or other microorganisms to produce lactic acid. It is subsequently synthesized through a series of reaction steps. Starch and PLA have different degradation rates. In addition, starch degrades faster than PLA in soil. Starch degradation usually takes only a few months, whereas PLA degradation generally takes several years [60].
PBAT, PBSA, poly (butylene succinate) (PBS), and poly(ε-caprolactone) (PCL) are the most common fossil fuel-based polymers used to manufacture biodegradable plastic mulch. PCL, which has a relatively low melting point of 60°C, is frequently mixed with starch to enhance biodegradability. In addition, PBAT is completely degradable under composting conditions and possesses high elasticity, wear, fracture resistance, and water and oil resistance. During PBAT film degradation, molecular chain ruptures occur due to hydrolysis or oxidation, resulting in micropores and radical cracks on its surface. Terephthalic acid (TPA) is a mild-to-moderate environmental toxin because it has a special inhibitory effect on microorganisms [63].
PBS is a thermoplastic polyester with physicochemical characteristics similar to those of polypropylene. These synthetic polymers provide the functionality, flexibility, and affordability of plastic films, which are degraded by bacteria and fungi commonly found in soil [77, 78]. PBS is generally combined with other compounds, such as starch and adipate copolymer, to form PBSA [79]. PBSA film degradation occurs primarily because of the polyester-degrading activity of enzymes produced by soil microorganisms rather than nonenzymatic chemical hydrolysis [64].
Although biodegradable films hold potential as an alternative solution to plastic pollution, their widespread adoption cannot currently be promoted because of unresolved concerns regarding their in-situ degradation under various natural climates and over extended periods [80]. Furthermore, uncertainty arises regarding its long-term effects on plant-soil health [81].
Figure 2 shows that plastic mulch residues can affect the physicochemical and biological properties of the soil and disturb plant growth and performance through various pathways. First, the presence of plastic mulch residues as physical contaminants can directly impede root growth and prevent roots from absorbing available nutrients in the soil, such as nitrogen and water, by creating physical obstacles (illustrated by the brown arrow) [82]. Second, microplastics (MPs) can act as carriers of PAEs and can release them directly into the environment (green arrow) [83]. PAEs are additives detected in plastic mulch that are detrimental to the diversity, activity, and community structure of soil microbes [84]. Finally, plastic mulch residues and MPs/NPs (nanoplastics) can alter bulk density, porosity, soil moisture, and air diffusion (orange arrows) [85, 86]. These changes indirectly affect the chemical properties and nutrient availability (blue arrow). Consequently, plastic mulch residues can lead to osmotic stress in soil microorganisms because they influence soil water dynamics and nutrient availability. This stress disrupts nutrient cycling, ultimately hindering plant growth.
Biodegradable plastic mulch is expected to degrade completely after tilling into the soil. However, only 90% of bioplastic biodegrades in soil within 2 years [75]. When plastic mulch residues decompose or remain in the soil, they potentially affect the chemical and physical properties of the soil. In addition, harmful additives may be released into the environment during degradation. The effects of plastic mulch residues on soil physicochemical properties are complex. The interaction between soil and plastic mulch film residues is influenced by the type of polymer, whether pristine or aged, size, content (dose), and term duration of the mulch film used.
Table 4 shows that the presence of pristine biodegradable plastic mulch residues in soil mostly had a positive effect on soil physicochemical properties, including increased field capacity [85], soil organic matter [87], significantly increased soil urase and catalase activity [35], and higher abundance of Nitrobacter compared to control and PE treatments [88]. In contrast, with aged mulch film residues (mulching film practices for more than 2 years), the PE plastisphere shared a higher number of bacterial species with the soil than the PBAT/PLA plastisphere. PBAT or PLA plastisphere networks were less complex and less modular and had more competing interactions [89].
| Plastic mulch for the experiment | Effect | Remarks | Ref. | |||
|---|---|---|---|---|---|---|
| Type | Size | Concentration | Experiment design | |||
| BDF | Macro: 5 × 5 mm Micro: 50–250 µm, 250–500 µm, 500–1,000 µm |
0.5%, 1%, 2% | Pristine mulch film in the mesocosm experiment: 20°C, 35% humidity for 30 days | Positive | Increased field capacity. | [85] |
| PLA, STP | 1.14 mm thick with an area density of 298 g m-2 0.05 mm thick with an area density of 20 g m-2 |
- | Pristine mulch film in the crop planting system experiment: sweet corn (Zea mays) and cabbage (Brassica oleracea) for 22 months | Negative | Immobilized short-term nitrogen in low-fertility soil. | [90] |
| PBAT, PLA | Thickness of 0.012 mm | - | Pristine mulch film in the crop planting system experiment: potato and rice as rotated plants for 2 years | Positive | Increased soil organic matter. | [87] |
| Positive | Increase in the activities of β-glucosidase (bg) and β-1,4-n-acetylgluco-amidase (nag), two nutrient cycling-related enzymes. | |||||
| PBAT, PLA | 2,000 mm wide; colorless; the thickness of 0.008 mm during 2013–2016 and 0.006 mm during 2016–2019 | - | Pristine plastic mulch residues in the crop planting system experiment: garlic growing season for 6 years (2013–2019) | Positive | Microbial activity in each soil layer showed the following trend for both years: biofilm > PE film > no mulch. | [35] |
| Positive | Increased soil urase and catalase activity. | |||||
| PBAT, PLA | - | - | Aged mulch film residues in the sampling site with mulching film practices for more than 2 years | Negative | Compared to PE plastisphere, PBAT or PLA plastisphere networks are less complex and less modular and have more competing interactions. | [89] |
| BDF | Black and white with 0.008 mm thickness | - | Pristine plastic mulch residues in the sampling site located in the potato planting area | Positive | Nitrobacteria were more abundant in BDF than in the control and PE treatments. | [91] |
| Positive | Microbial species diversity and richness were higher compared with the control treatment. | |||||
STP: starch polyester bioplastic.
Plastic mulch residues may affect the physical, chemical, and biological properties of the soil. This impact may be caused by the plastic polymer itself or additives in the plastic. Soil structure pertains to the arrangement and size of soil particles, along with the pore spaces that allow the presence of water, nutrients, gases, and soil organisms [88, 92]. First, pristine and aged biodegradable physical materials in the soil can alter bulk density and soil porosity [87]. Consequently, the rate of soil water penetration slows down, adversely impacting air diffusion [93] and changing soil aeration [94] and moisture [95].
Second, the adsorption capability of plastic residues in the soil may be a significant factor because it can affect soil nutrient availability and water mobility. Previous studies have reported that PVC MPs can adsorb labile nitrogen from the soil, obstructing its transfer to the water surface [96]. In addition, Lozano et al. (2021) [98] stated that MPs can adsorb minerals and organic molecules from soil, reducing their availability to microorganisms. Furthermore, the nature of plastic mulch residues can affect water mobility in the soil. A previous study [85] demonstrated that LDPE residues reduced field capacity, whereas bio mulch residues increased it. This explanation clarifies that LDPE residues have a lower water-holding capacity than bioplastic residues. Consequently, these factors affect water and air movement, thereby restricting the availability of nutrients in the soil. In general, pristine MPs have a lower adsorption capacity than aged MPs because aging processes can increase the specific surface area and porosity and introduce oxygen-containing functional groups on the plastic surface, which enhances its adsorption potential [99, 100, 101].
Several additives are used in plastics to improve performance and increase versatility [102], including stabilizers, plasticizers, flame retardants, and monomers [103], which is the third factor affecting changes in soil properties. PAEs are the most widely used plasticizers across all polymer types because of their low cost, low volatility, elastic nature, and durability [104]. This additive can leach out during the product’s life cycle, particularly in the soil environment, and interact with soil microorganisms. PAEs negatively affect soil microbial activity, diversity, and community structure, as well as metabolic activity, because they can destroy cell membrane fluidity [105] and significantly inhibit the respiration of the microbial community [92]. According to previous studies, the presence of PAEs can also enhance the activities of dehydrogenase, catalase, proteinase, and phosphatase enzymes in soil; however, they hinder the activities of urase, cell enzymes, and beta glucose [72, 84, 105]. The impact of PAEs on enzyme activities and microbial functional diversity in the soil can vary depending on environmental conditions and the types and concentrations of PAEs in plastic film residues.
According to previous studies, PAEs can be released not only by residual PE films [102] but also by bioplastics during the degradation process, which may negatively impact soil quality and crop yield [106]. Uzamurera (2023) [107] noted that the amount of PAEs released in the bioplastic film was significantly higher than that in the PE film residue. Furthermore, aged plastics release more additives than pristine plastics. During the aging process, physical and chemical changes occur in the plastic material. For example, ultraviolet radiation from sunlight can break down polymer chains, creating pathways for additives to migrate out of the plastic. Mechanical stress and weathering can accelerate plastic degradation, resulting in the release of additives. The physical and chemical changes that occur during the aging process can facilitate the migration of additives, such as PAEs and organophosphate esters, from the plastic surface or interior into the surrounding environment [73, 74].
Finally, biofilm formation in plastic can create microenvironments with distinct microbial activities, potentially influencing microbial diversity and abundance in the surrounding environment [108]. During a 10-week field exposure period, it was observed that aged plastic residues had more biofilm on their surfaces compared to pristine plastic residues [109]. The search results indicate that the aging process, such as exposure to ultraviolet light, can increase the surface free energy and change the surface chemistry of plastics, making them more prone to biofilm formation compared with pristine plastics [110].
5.2 Effect of biodegradable plastic mulch residues on plant growth and productivityTable 5 shows that pristine biodegradable plastic mulch residues negatively affected plant growth. In addition, Qi (2018) [111] reported that macro- and microplastics of pristine biodegradable mulch residues showed significantly smaller leaf areas of wheat than the control treatment, as well as reduced total biomass. Furthermore, the biodegradable macro- and microplastic treatment led to the smallest stem diameter when compared to the control and LDPE treatments on wheat. The negative effect on other plants was also observed in rice, sweet corn, and red cherry/tomato in which the presence of pristine biodegradable mulch film residues inhibited plant growth [90, 112, 113]. Conversely, the aged biodegradable mulch plastic film showed no negative effects on plant growth. Gao (2021) [87] observed residual biodegradable films did not cause any statistically significant yield loss of rice as compared to the treatment with no residual film.
| Plastic mulch for the experiment | Effect | Remarks | Ref. | |||
|---|---|---|---|---|---|---|
| Type | Size | Concentration | Experiment design | |||
| BDF | Bio: macro (Ma): 6.98 × 6.01 mm Micro (Mi): 1 mm, 500 µm, 250 µm, 50 µm | 1% (w/w) | Pristine plastic film in wheat planting system (Earthworm treatment) for 2 months | Negative | Bio-Ma and Bio-Mi significantly reduced leaf areas. | [111] |
| Negative | Bio-Mi wheat plants had the thinnest stems. | |||||
| BDF | Macro: 5 mm × 5 mm Micro: 50 µm–1 mm |
1% (w/w) | Pristine plastic film in wheat planting system, 3-week experiment with fungal pathogen F. culmorum | Negative | Bio-Mi decreased plant shoot biomass. | [114] |
| BDF | Macro: 4–10 mm Micro: 50 µm–1 mm |
1% (w/w) | Pristine plastic film in wheat planting system harvested at 61 and 139 days | Negative | Bio-Ma and Bio-Mi treatments showed the most significant adverse effect on total plant biomass. | [115] |
| PBAT | 50 µm | 1% (w/w) | Pristine plastic film in rice planting system, sampled at 2 months after 4 months | Negative | Decreased the height of rice plants (2-month planting), rice shoot and root dry weight (4-month planting) | [112] |
| PLA STP |
PLA: 1.14 mm thick with an area density of 298 g m−2 STP: 0.05 thick and an area density of 20 g m−2 |
- | Pristine plastic film in sweet corn (Zea mays) and cabbage (Brassica oleracea) planting system, 6, 12, 18, and 22 months | Negative | Sweet corn yield was decreased. | [90] |
| PBAT, PLA, PHB | Thickness 15–17 µm | 4.5% (w/w) | Pristine plastic film and field-weathered plastic film in red cherry/tomato and Trocadero Ribera/lettuce planting system at 5 and 7 weeks | Negative | Residues of PHB-based biodegradable mulch film hindered tomato and lettuce plants. | [113] |
| Negative | Growth of lettuce plants was significantly retarded by all pristine and field-weathered biodegradable mulch film residues. | |||||
| PBAT/PLA | PBAT/PLA: thickness of 0.012 mm | - | Pristine plastic film in potato planting and aged plastic film in rice planting, 2-year experiment | Positive | No significant impact on rice yield. | [87] |
| Positive | The effect on potato yield was slight. | |||||
PHB: polyhydroxy butyrate.
Plastic mulch residues can directly and indirectly affect plant growth. They may directly interfere with root development and function as a physical barrier. In addition, they can be adsorbed onto the surface of plant roots, thereby affecting nutrient absorption [116]. A recent study revealed that biodegradable and nonbiodegradable plastic mulch residues significantly reduced total nitrogen uptake by plants [117]. This effect was attributed to the use of square-shaped plastic mulch residues, which create physical barriers in the soil, impacting soil moisture and nutrient availability [117]. Furthermore, smaller nanoplastics can be directly absorbed by roots [118], consequently disrupting plant development.
Furthermore, the presence of plastic mulch residues in the soil indirectly affects plant growth and productivity. As mentioned in Section 4.1, they increase bulk density and decrease porosity [87], resulting in changes in soil moisture, soil water retention, and distribution. These changes can lead to plant water stress. Furthermore, pristine and aged mulch can release hazardous additives during degradation, which may significantly increase ecological toxicity [119]. Alteration of the physical properties of the soil or the additive released into the soil can change the community structure and interfere with the complexity of the symbiotic community network [120]. Microbial communities play essential roles in nutrient cycling. In addition, changes in microbial communities alter nutrient cycling and availability [121], thereby affecting plant growth.
5.3 Impact mechanism of pristine and aged plastic on agricultural soilPrevious studies have frequently investigated the impact of “unaged” plastic mulch residues, commonly known as “pristine” plastic, on soil ecosystems (Tables 4 and 5) [122]. However, almost all MPs detected in agricultural soils have been considered “aged” [116]. Visible cracks in field-collected plastic support this finding [123, 124]. Research on the harmful effects of aged plastics in soil ecosystems is limited, raising doubts about whether the damage results from the plastic itself or leachate additives. Figure 3 shows some possible mechanisms by which aged and pristine plastic affects the agricultural soil.
Lan et al. (2022) [127] noted that the characteristic differences in morphology, color, and chemical properties between pristine and aged plastic particles in soil could affect their adsorption behavior and interaction with pollutants. For example, aged MPs have a rougher surface, more cracks, and oxygen-containing functional groups. Accordingly, aged MPs can easily adsorb organic pollutants, such as pesticides, compared with pristine MPs. This difference in adsorption behavior indicates that aged MPs may pose a greater risk to human beings because they are present in agricultural soil films closer to our living environment [126].
Additionally, manufacturers often keep the chemical compositions of BDP confidential. However, evidence suggests that these plastics can release toxic additives harmful to soil biota during degradation [128, 129]. These additives have adverse effects on soil ecosystems. The aging process of plastic, which includes exposure to environmental factors, such as sunlight, heat, and microbial activity, can lead to the breakdown of the plastic and the release of PAEs. Aged plastic, whether biodegradable or non-BDP, releases higher amounts of PAEs than pristine plastic [125].
Furthermore, biodegradable MPs affect soil organic carbon fractions. Aged biodegradable MPs can reduce particulate organic carbon (POC) content. This decline is attributed to increased demand for microbial carbon caused by the presence of microplastics in the soil, which results in decreased POC content detrimental to carbon sequestration [125]. Moreover, microbial decomposition of POC may release CO2, exacerbating greenhouse gas emissions. These effects on soil organic carbon fractions highlight the potential environmental impact of biodegradable MPs in agricultural soils [125].
In this review, the impact of biodegradable mulch residues on plant growth mostly led to negative effects but can have positive effects on soil properties. The disturbance of plant growth might be due to plastic mulch residues acting as physical barriers to the absorption of water and nutrients by plants, serving as carriers by adsorbing organic pollutants, and releasing PAEs, which affect microbial activity in the soil. Hopefully, this study will enhance understanding of BDM’s potential impacts, helping to formulate strategies to mitigate risks and fully harness the benefits of biodegradable mulch materials in sustainable agriculture.
List of Abbreviations
