Microplastic contamination is ubiquitous in riparian soils and strongly related to elevation, precipitation and population density
Graphical Abstract
Introduction
Plastic products are ubiquitously used; in 2017 alone, 348 million metric tons of plastic was produced worldwide, with this amount growing by ~5% each year (Geyer et al., 2017, Statista, 2018). A large proportion of these products end up as plastic waste, as even with immediate and concerted collection strategies, a total of 710 million metric tons of plastic accumulates in aquatic and terrestrial environments (Lau et al., 2020). Small plastic debris cumulatively appear in marine (Kane et al., 2020, Peeken et al., 2018), freshwater (Corcoran et al., 2020, Mintenig et al., 2020), atmospheric (Brahney et al., 2020, Evangeliou et al., 2020), and terrestrial ecosystems (Rillig and Lehmann, 2020, Scheurer and Bigalke, 2018), following the continuous physical and/or photochemical degradation they are exposed to in the environment. Microplastics (MPs), which are defined as small plastic debris (<5 mm), have recently been found widely distributed in soils as they did in aquatic environment, e.g., in east China coastal soils with 571 p kg1 and in central China suburban soils with 2020 p kg1 (Chen et al., 2020, Zhou et al., 2020). Primary and secondary MPs are often addressed in relevant reports (Brahney et al., 2020, Kane et al., 2020). Plastic beads or pellets, i.e., those manufactured at a micron size range and incorporated into, for example, cosmetics and shampoos, are classified as primary microplastics (Gunay et al., 2017, Habib et al., 2020), whereas small fragments or fibers resulting from the breakdown of large plastic products are defined as secondary microplastics (Waldman and Rillig, 2020). Recent years have witnessed mounting concerns about how MPs contamination affects different environmental compartments and human health (Campanale et al., 2020). Researchers have also began to focus on the prevalence of MPs in terrestrial ecosystems during the last few years, with studies representing this biome lagging behind research in aquatic ecosystems by nearly a decade (Rillig, 2012, Rillig and Lehmann, 2020, Thompson et al., 2004).
Determining the fate of microplastics, as well as the extent of contamination, in terrestrial environments is essential for realistic assessments of their ecotoxicity and ecosystem effects. Owing to their small size, MPs can be ingested directly or indirectly by a wide range of organisms from all trophic levels, which makes them a potential carrier for the transfer of harmful substances (Chae and An, 2020, da Costa Araujo and Malafaia, 2021, Le Guen et al., 2020). In terrestrial environments, soils have been theorized to play a vital role in the storage and transfer of MPs (Nizzetto et al., 2016a, Scheurer and Bigalke, 2018). According to recent research, the application of sewage sludge (Su et al., 2019), industrial production (Fuller and Gautam, 2016), plastic waste (Bläsing and Amelung, 2018), road dust (Yukioka et al., 2020), street runoff or irrigation (Bläsing and Amelung, 2018), and atmospheric deposition (Brahney et al., 2020, Evangeliou et al., 2020) are the major routes through which microplastics enter soil environments. It was reported that the application or deposition of sewage sludge onto land could cause annual soil MPs loads that exceed the amount entering marine environments (Nizzetto et al., 2016b). High MPs concentrations, up to 91,000 particles kg1 (particles per kg dry soil, hereafter denoted as p kg1), have recently been recorded from MPs hotspots at Chinese landfills (Su et al., 2019). Furthermore, it seems that no place is safe from MPs contamination, even remote regions with low population densities, as average MPs concentrations of 593 p kg1 were observed in Swiss floodplain soil samples (Scheurer and Bigalke, 2018). Therefore, determining which factors exert the most influence on the accumulation and distribution of MPs is crucial to determining the threat that these contaminants pose to terrestrial systems.
The spatial distribution and mobility of microplastics in soils depend largely on population density, precipitation patterns, and the average slope at a site (Nizzetto et al., 2016a, Scheurer and Bigalke, 2018, Yonkos et al., 2014). Two theoretical assessments (the INCA-Contaminants model and modified INCA-Contaminants model), identified precipitation and average land slope as factors controlling the transfer and distribution of microplastics in soils (Lazar et al., 2010, Nizzetto et al., 2016a). However, what should be pointed out is that one of these models is purely theoretical, and does not include any empirical data. It has already been established that microplastic abundance in river sediments (estuarine rivers in the USA and Brisbane River) is greatly impacted by the population of the study area (He et al., 2020, Yonkos et al., 2014), and microplastic concentrations in Swiss floodplain soils are positively correlated with the number of inhabitants (Scheurer and Bigalke, 2018). Furthermore, there is some evidence that the ecotoxicity and ecosystem effects of microplastics appear to be mediated by polymer type, shape and size (Rillig and Lehmann, 2020). This relationship also seems to hold true for the distribution of MPs in soils. For example, MPs with a density marginally greater than that of water and a size > 0.2 mm are likely to be retained in soils (Nizzetto et al., 2016a). Thus, these MPs can directly interact with soil aggregates, which will further impact their distribution and mobility in soils (Rillig and Lehmann, 2020). However, there is currently only scant data available regarding the concentrations and compositions of MPs within soil, while little has been published about which factors most influence the soil distribution of MPs.
Yangtze River, the third longest river in the world, flows 6418 kilometers from glaciers on the Qinghai-Tibet Plateau into the East China Sea at Shanghai. The topography, hydrological conditions and population density along this river vary greatly. In the present study, we investigated MPs distribution and composition in riparian soils from twenty nine locations along the Yangtze River to assess whether topsoils and subsoils differ in these two aspects. Furthermore, we were interested in determining whether elevation, precipitation, and population influence the extent of MPs contamination. This, to the best of our knowledge, is the first work to characterize the spatial distribution of microplastics, along with influencing factors, in riparian soils along the Yangtze River on a large scale.
Section snippets
Study sites and sampling
During October 10–November 25, 2019, field sampling was conducted at 29 riparian sites along the Yangtze River, China. Pictures of representative sampling sites are depicted in Fig. 1a and b. At each sampling site, a composite soil sample consisting of fifteen soil cores (3.2 cm diameter and 15 cm depth) was collected from areas that were 0.5–1.5 m above the river water level and at least 10 m from the water (Fig. 1c). The applied sampling method was largely based on the method presented by
Global patterns of microplastics contamination in soils and beach sediments
A global MPs contamination map, along with an associated graph of concentrations (Fig. 2a and b, respectively) from various soils and beaches around the world demonstrates that terrestrial environments have been widely contaminated by microplastics, and that there is considerable variation between sites around the world. Until now, investigations about the MPs contamination of terrestrial environments have mainly focused on low-and middle-latitudes regions. This is because there is scarce
Microplastics occurrence and composition
We quantified MPs abundance and composition in soil samples from 29 locations along the Yangtze River (Fig. 3). MPs was detected in all of the samples from our study area, with an average abundance of 3877 ± 2356 p kg1 (Fig. 3), which is relatively higher than what has previously been reported for soils and beach sediments (i.e., in comparison to results reported as p kg1) (Fig. 2b). In particular, the average MPs abundance reported in this study is four orders of magnitude higher than what was
Conclusions
In this work, we quantified MPs abundance in riparian soils along the Yangtze River, as well as investigated the factors influencing MPs contamination. Subsoils showed higher MPs levels, as well as a larger share of small particles (< 200 µm), than topsoils, which primarily retain larger MPs particles. The size of a MPs particle appears to have a more significant influence on MPs distribution in the soil environment than the type of synthetic polymer. The population density of a study site is
CRediT authorship contribution statement
Yanfei Zhou: Methodology, Investigation, Formal analysis, Writing - original draft. Gang He: Methodology, Investigation, Writing - original draft. Xiaoliang Jiang: Investigation, Resources. Lunguang Yao: Investigation, Resources. Lu Ouyang: Resources, Writing - review & editing. Xiaoyan Liu: Investigation, Resources. Wenzhi Liu: Conceptualization, Writing - review & editing, Funding acquisition. Yi Liu: Conceptualization, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare no conflicts of interest.
Acknowledgements
This work was supported by the Natural Science Foundation of Hubei Province, China (2020CFA108), the Youth Innovation Promotion Association of the Chinese Academy of Sciences, China (2017388), and the National Natural Science Foundation of China (31971532 and 32022051).
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The first two authors contributed equally to this work.