ReviewThe long-term uncertainty of biodegradable mulch film residues and associated microplastics pollution on plant-soil health
Graphical Abstract
Introduction
Plastics provide a vital role within almost all agronomic management regimes, however, their use is receiving increased attention and scrutiny due to their potential to contaminate and pollute land, and migrate to freshwater and marine habitats (Maraveas, 2020a). Plastic mulch films are one of the most important plastic products used in agriculture and are widely used to suppress weeds and reduce water usage in crop production (Kasirajan and Ngouajio, 2012). The global mulch films market is currently valued at US$ 5 × 109 and is expected to have a compound annual growth rate of 5.9% over the next 5 years (TMR, 2022). Although values vary widely, it is currently estimated that 2.5 × 106 tonnes (t) of plastic film are used annually within greenhouses and for mulching (ca. 0.5% of global plastic production) and that this covers an area of land ca. 25 × 106 ha (Qadeer et al., 2021, OECD, 2022). These films, however, have a limited lifespan (<1 y for mulches and ca. 5 y for greenhouses) due to progressive chemical, physical and biological degradation. Removing the plastic mulch films from soil in the agroecosystem is time-consuming (estimated ∼42 h per ha; Velandia et al., 2019), and expensive (estimated to be €176.5, €186 and €192 per ha for removal, landfill and recycling, respectively) (Marí et al., 2019). At the end of their working lifetime, complete removal from fields is often therefore unviable economically, if not impossible, leading to a substantial accumulated legacy of both macro- and microplastics (MPs, diameter < 5 mm) in agroecosystems (Rillig et al., 2012). The detection of MPs in edible vegetables (e.g. carrots, lettuce, broccoli, potatoes) and fruits (e.g. apples and pears) suggests that MPs also transfer into the food chain and may pose a threat to human health (Li et al., 2020, Conti et al., 2020, Wang et al., 2022a). This is supported by recent evidence showing that human exposure to MPs can lead to intestinal inflammation, as well as accumulation in organs (including the kidney, liver, gut and placenta) resulting in metabolic changes (Kannan and Vimalkumar, 2021, Schwarzfischer and Rogler, 2022, Shi et al., 2022a), although a causal link to more serious medical conditions has yet to be established. It is therefore clear that strategies are needed to minimize the formation, persistence, movement, and toxicity of MPs within agriculture.
One way to lessen plastic pollution in agroecosystems is by the substitution of petroleum-based plastic with biobased plastics (examples include polyhydroxyalkanoates (PHAs), polyhydroxybutyrate (PHB), polybutylene succinate (PBS) and polylactic acid (PLA)). Biodegradable film are progressively replacing conventional polyethylene (PE)-, polypropylene (PP)- and polyvinyl chloride (PVC)-based films, and their use is predicted to increase from 1.5 × 106 t in 2021 to 5.3 × 106 t by 2026 (Plastics Europe, 2020). Although biobased films are typically more expensive than their petroleum-derived counterparts (Jogi and Bhat, 2020), the cost is often offset by the absence of removal and disposal costs (Malinconico et al., 2008). Biodegradable plastic mulch films have also shown equivalent agronomic performance compared with conventional polyethylene mulches in many cases (Fig. 1). A meta-analysis conducted in China by Liu et al. (2021) found that the performance of biodegradable mulch film differed little from that of conventional mulch films, increasing the water use efficiency of crops by 19–25% and soil temperature by 0.9 °C, consequently leading to an enhancement in crop yields (i.e., maize, wheat, cotton, and potato) by 18–26%. However, it is still too early to promote the use of biodegradable films on a large scale, due to the questions about in situ degradation particularly in different (natural) climates and over longer temporal periods, as well as the uncertainties about the long-term impacts on plant-soil health and ecosystem multifunctionality in agroecosystems (Fig. 1, Fig. 2). It has been widely accepted that biodegradable plastic mulch films are designed to break down into carbon dioxide (CO2) and water (usually by hydrolysis) with some carbon (C) incorporated into the soil microbial biomass within less than one year, according to the standard laboratory analysis at temperatures from 20 °C to 28 °C (Liwarska-Bizukojc, 2021). However, it cannot be guaranteed that biodegradable mulch films will indeed degrade in the field within a 24-month time frame as environmental conditions, such as the microbial community present, temperature, and moisture content, vary from soil to soil, and are also dependent on climatic conditions (Sintim et al., 2020). Therefore, biodegradation is most likely much slower as average soil temperatures and moisture contents seldom reach those used in laboratory test conditions (Tabasi and Ajji, 2015). For example, Liao and Chen (2021) found that the weight loss was only 1.1–8.0% for PLA, and 0.8–6.8% for poly(butylene adipate-co-terephthalate) (PBAT) after a 180 day incubation in soil. Of note, numerous MPs were formed after poly(p-dioxanone) degradation, leading to the presence of 2103 plastic items/g in the soil (Liao and Chen, 2021). Thus, the question as to whether biodegradable mulch films offer a promising alternative to solve the conventional plastic accumulation/legacy problem in soil over the long term, remains unclear. Therefore, investigation and assessment of the degradation processes, as well as the timescale of biodegradable mulch films degradation in the natural environment are required. This is of particular importance given biodegradable mulch films are designed to be tilled into the soil and not removed after use.
Equally, while the main plastic biopolymer in the film may be classified as environmentally benign (Maraveas, 2020a, Maraveas, 2020b), mulch films also contain a multitude of undeclared additives (e.g. metals, volatile organic chemicals) that are used to provide additional functionality (e.g. UV resistance, flexibility, improved tensile strength) (Marra et al., 2016, Arza et al., 2018). While some of these are added to accelerate degradation, the short- and long-term impact of these on plant and soil health remains virtually unknown (Fig. 2). Since biodegradable mulch films are designed to degrade within a few years, it is highly likely that more MPs will be generated over short periods in comparison to conventional plastics (Liao and Chen, 2021). Consequently, this may lead to even more serious bio-MPs pollution in soils with implications for nutrient (i.e., N and P) cycling processes, with consequences for soil quality, ecosystem functions and multifunctionality (Ma et al., 2022), and therefore on the food security (Beltrán-Sanahuja et al., 2021). In this perspective piece, we therefore discuss the potential effect of bio-MPs from the decomposition of biodegradable mulch films with a focus on: C storage, nutrient (i.e., N and P) cycling, greenhouse gas emission, soil biology (microorganisms and mesofauna), and plant health, as these are crucial to agroecosystem functions and the delivery of key ecosystem services.
Section snippets
Soil carbon storage
Biodegradable mulch films are C-rich (typically around 60–80%), and may influence soil organic matter (SOM), and biogeochemical cycling. Assuming an average film thickness of 25 µm and an average density of 1.2 g cm-³ , we calculate that biodegradable mulch films will typically contribute 0.30 t C ha−1 y−1. For context, annual inputs from cereal root systems and crop residues (e.g. maize or wheat) can exceed 5 t ha−1 y−1, even excluding C inputs from root exudation (Zhang et al., 2013, Komainda
Influence of bio-MPs on nutrient cycling and GHG emissions
Since bioavailable C is the dominant element in bio-MPs, their degradation could supply C for microbial communities, which potentially induces the cycling of other macro and micro nutrients (Sun et al., 2022a). N turnover was significantly altered by bio-MPs addition, documented by the changes in NH4+, and reduction in NO3- (Table 1). The accumulation of NH4+ is dependent on its balance between production (e.g. ammonia oxidization and mineralization) and depletion (e.g. nitrification and
Influence of bio-MPs on the microbial community
The zone of soil surrounding bio-MPs is likely to induce a shift in the microbial community leading to the formation of microbial hotspots (so termed the microplastisphere; Zhou et al., 2021b). As soil contains numerous spatial niches (i.e., rhizosphere, fertisphere, detritusphere, drillosphere), a key question is whether this shift in soil community has any major influence on soil biodiversity and function. Compared to conventional MPs, bio-MPs disintegration is more rapid and formation of
Influence of bio-MPs on mesofauna
Agroecosystem sustainability is supported by an ecological network comprised of a diversity of organisms. Therefore, understanding the impact of bio-MPs on a range of trophic levels is required to gain a holistic view of soil biology change. Bio-MPs share some common features with MPs, from which some parallels may be drawn. That is, bio-MPs can cause weight loss, reduce growth rates and casting yield, increase mortality, decrease reproduction rate, as well as induce DNA damage and oxidative
Influence of bio-MPs on plant health
Plants are fundamental to terrestrial ecosystem functioning as well as agroecosystem service provision, and therefore a fuller understanding of MPs-plant interaction is needed. Like conventional MPs, toxic effects of bio-MPs on plants have also been recorded (as reviewed by Zhou et al., 2021a). It is unclear, however, if these are direct effects on key plant processes (e.g. signaling, cell expansion) or indirect effects (e.g. nutrient deficiency due to microbial immobilization of N and P, or
Uncertainty 1
Although biodegradable mulch films are designed to break down into CO2 and water in the field, a paucity of information exists about their physical disintegration and subsequent degradation under realistic field conditions. If the biodegradation is much slower under cooler, drier conditions, then more bio-MPs (and nanoplastics) may accumulate in the soil in comparison to conventional plastics. Hence, it is still unclear whether current biodegradable plastic mulches will solve the problem of
Environmental Implications
Bioplastics potentially offer an encouraging alternative to conventional (petroleum-based) plastics. Although bioplastics are designed to degrade within a few years, it is highly likely that more MPs will be generated over short periods in comparison to conventional plastics. Consequently, this may lead to even more serious bio-MPs pollution in soils with implications for soil and plant health. However, the short- and long-term impact of bio-MPs on plant-soil health remains virtually unknown.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was supported by the Fellowship of China Postdoctoral Science Foundation (2022M713397) and the Young Elite Scientists Sponsorship Program funded by China Association for Science and Technology (2020QNRC001). D.L.J. acknowledges the support of the UK Natural Environment Research Council Global Challenges Research Fund programme on Reducing the Impacts of Plastic Waste in Developing Countries (NE/V005871/1), and the Centre for Environmental Biotechnology Project, part-funded by the
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