ReviewUnraveling consequences of soil micro- and nano-plastic pollution on soil-plant system: Implications for nitrogen (N) cycling and soil microbial activity
Graphical abstract
Introduction
Periodic monitoring of key ecosystem functions is a common snapshot indicator of environmental management (Bellino et al., 2020) and is often used to assess the ecological feed-back responses against a broad range of environmental challenges (Turunen et al., 2019; Chen et al., 2020a). Micro- (100 nm–5 mm) as well as nano-plastic (<100 nm) pollution has emerged as a major global environmental threat (Klaine et al., 2012; Verschoor, 2015; Mattsson et al., 2015; da Costa et al., 2017). Given the wide distribution pattern (de Souza Machado et al., 2018a), these emerging pollutants have become a menace for biodiversity and essential ecosystem functions across a range of environmental systems (Eerkes-Medrano et al., 2015; da Costa et al., 2016; He et al., 2018a; Boots et al., 2019; Chen et al., 2020b). In terrestrial ecosystems, soil is a vital natural resource base that grapples with the onslaught of emerging contaminants, including plastic particles (Hodson et al., 2017; Wang et al., 2019; Hurley and Nizzetto, 2018). Micro- and nano-plastic contamination is not a monolithic issue, since it arises from wide ranging plastic polymers (e.g., polyethylene, polystyrene, polyurethane, polypropylene, polyvinylchloride, polyurethane, fibers and polyethylenterephthalat) varying in chemical composition and properties (Awet et al., 2018; Rillig et al., 2019). Some of the agricultural lands in America, Europe and Australia have now become microplastic hotspots receiving an annual microplastic loadings of ∼44,000–300,000 metric tons, ∼63,000–430,000 tons and ∼2800–19,000 tons, respectively. The analytical methods of microplastic detection in water and sediments are commonly followed for soil as well, since no standard microplastic detection methods in soil are available. First microplastic is visually identified under an optical microscope, followed by micro-Fourier transformed infrared (m-FT-IR) and Raman spectroscopy confirmations (Liu et al., 2018; Peng et al., 2018). At first, a flotation-based method has been developed to differentiate and quantify certain types of microplastics in soil (Zhang et al., 2018). However, measuring the trace concentration of nano-plastic particularly in soil still await development of analytical methods (Koelmans et al., 2015), which is a significant limitation for assessing the impacts of micro as well as nano plastic pollution in real time at ecosystem scale. Approximately 72% of these particles can become trapped within soil aggregates, while the remainder are dispersed (Zhang and Liu, 2018). Much of the recent knowledge on microplastic pollution comes from aquatic environment; however, large quantities of aquatic microplastics are originally sourced from terrestrial ecosystems and related anthropogenic activities (Alimi et al., 2018; Malizia and Monmany-Garzia, 2019). Almost 16–38% of microplastics, specifically heavier than water, are thought to have local deposition history (Nizzetto et al., 2016b). At present, terrestrial ecosystems are the greater recipient (∼4–23%) of microplastic waste than that of found in aquatic systems (de Souza Machado et al., 2017; Horton et al., 2017). Each year, almost 80% of aquatic microplastic, ∼4.8 to 12.7 million tons, is driven out of terrestrial land surfaces and discharged into marine environments (Sheavly and Register, 2007; Andrady, 2011; Jambeck et al., 2015). Therefore, plastic contamination in terrestrial environments presents as a potentially larger problem than in aquatic environments.
Global plastic production has seen massive rise of approximately 359 million metric tons (Geyer et al., 2017), and of these, ∼96% of increase has occurred since 2000 (Garside, 2019, Fig. 1). Micro- and nano-plastics can enter terrestrial environments through different pathways (Wang et al., 2018, Fig. 2). If the current trend of improper plastic use without an effective management strategy continues, our terrestrial environment may end up overloaded with plastics. It is projected that microplastic debris will reach close to 12,000 million metric tons in next three decades (Geyer et al., 2017). Identified direct sources of micro- and nano-plastic are as follows: nano-coated seeds with fertilizers/pesticides (Heuchan et al., 2019); soil amendments such as compost (Zhang and Liu, 2018; Piehl et al., 2018; Weithmann et al., 2018) and sewage sludge (Nizzetto et al., 2016b, c; Mahon et al., 2017; J Li et al., 2018a, 2018b; Corradini et al., 2019); and irrigation using lake water and wastewater (Bläsing and Amelung, 2018; Zhang and Liu, 2018). Additionally, indirect sources such as mulching (Farmer et al., 2017; Sintim and Flury, 2017; Qi et al., 2018; Weithmann et al., 2018) tenting, underground troughs, and littering also introduce plastic debris into the environment (Weithmann et al., 2018; Corradini et al., 2019). These materials fragment into micro and/or nano-particles through UV, temperature, microbial and faunal processing (Rillig et al., 2017; He et al., 2018a). Soil mulching and sewage sludge applications are large contributors of micro-plastics in agricultural landscape (Nizzetto et al., 2016b, c; Liu et al., 2018). In agricultural soils, polyethylene, polystyrene, and polypropylene are the most abundantly found microplastic polymers (Piehl et al., 2018). Hence, it can be inferred that agricultural practices are the major contributor of plastic particles accumulation into the soil. Entry of plastics into the soils causes environmental problems, particularly those related to biophysical properties (Liu et al., 2017). Nonetheless, this issue has remained widely unexplored, especially regarding the prospects of N cycling. One potential explanation for the lack of research attention is the prevailing notion that all types of plastics are durable, persistent, and less reactive to soil environments (Rillig, 2012; Rillig et al., 2019).
As micro- and nano-plastic contamination is an emerging problem for soil ecosystems, many fundamental research questions still need to be addressed. Although studies on nutrient cycling are not available, it is important to discuss possible soil fertility problems that could be encountered in contaminated agricultural soils. These pollutants could present new set of issues. Much of the available research is confined to the physical properties of soil (de Souza Machado et al., 2018b, 2019; Boots et al., 2019) and plant growth (Qi et al., 2018; Bosker et al., 2019; de Souza Machado et al., 2019). Indeed, these studies provide good foundations for further investigating its impact on soil microbial processes involved in nutrient cycling. In the present review, ecological implication of micro-nano plastic pollution on soil microbes, N cycling process and plant performance will be elucidated.
Section snippets
Micro-nano plastic interplay with soil and microbes
Microbial communities and their activities are key indicators to assess contamination effects in soil (Bergkemper et al., 2016; Delgado-Baquerizo et al., 2016). To date, only handful of studies has revealed that both abundance and activity of microbes such as bacteria and fungi can be negatively affected by micro- and nano-plastic pollution (Table 1). Effects of micro/nano-plastic can be variable depending on particles shape, size, and also the soil type (Seeley et al., 2020; Chen et al., 2020b
Microplastics effects on soil N cycling
Soil reactions determine the functioning of agroecosystems and environmental health. Our assumption is that micro and nano-plastics can affect many of these key processes (Fig. 3). Nitrogen (N) is one of the most essential nutrient element required in abundance for greater ecosystem productivity. In anthropocene, understanding of the N cycle is essential to manage and predict key ecosystem processes and mechanisms. There is paucity of data describing the direct impact of microplastic on soil N
Effects of micro- and nano-plastics on crop plants
A vigorous root system is essential for the uniform plant growth and development, because it performs a broad range of physiological and mechanical functions, including the supply of water, nutrients, hormone secretions and plant anchorage (Logsdon, 2013; Li et al., 2016).
We can now find increasing amount of scientific evidence that micro-plastics influence plant root development and biomass production (Qi et al., 2018; de Souza Machado et al., 2019; Bosker et al., 2019). In spring onion,
Plant toxicity and competition with N and other nutrients
As we live in an era of pervasive plastic use, a relatively less-observed but dangerous threat could be their ability to traverse food-web. The major concern lies in its small size, which may end up in humans and animals once the plant has absorbed these micro-particles through absorption mechanism (Navarro et al., 2008; Bouwmeester et al., 2015). Plants have a special system that regulates uptake of different elements and water (Trapp, 2000), and there is growing anticipation that plants can
Conclusions
Micro- and nanoplastic resulting from a range of anthropogenic activities is now seen as an emerging global problem across agroecosystems. At present, the impacts of these micro-nano pollutants on soil nutrient cycling are still unknown; however, we have identified a number potential ecological risks associated with N cycling. There are physical and chemical mechanisms as well as direct toxicities that can underpin shifts in soil microbial activity upon exposure. Some physical and chemical
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
We acknowledge funding from Yunnan Human Resource and Social Security Department (under the postdoctoral research grant), the Key Research Program of Frontier Sciences, CAS (Grant no. QYZDY-SSW-SMC014 and 2017CASSEABRIZD003) and the National Science Foundation of China (NSFC) (Y4ZK111B01, 41661144001, 2017YFC0505101, 31861143002, 31650410651, and 31550110215) for this study. Dr. Sehroon Khan thanks the Chinese Academy of Sciences for the President’s International Fellowship Initiative (CASPIFI)
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