Focus topics on microplastics in soil: Analytical methods, occurrence, transport, and ecological risks

Focus topics on microplastics in soil: Analytical methods, occurrence, transport, and ecological risks

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Journal Pre-proof Focus topics on microplastics in soil: Analytical methods, occurrence, transport, and ecological risks Jia Li, Yang Song, Yongbing Cai PII:

S0269-7491(19)33588-2

DOI:

https://doi.org/10.1016/j.envpol.2019.113570

Reference:

ENPO 113570

To appear in:

Environmental Pollution

Received Date: 4 July 2019 Revised Date:

1 November 2019

Accepted Date: 3 November 2019

Please cite this article as: Li, J., Song, Y., Cai, Y., Focus topics on microplastics in soil: Analytical methods, occurrence, transport, and ecological risks, Environmental Pollution (2019), doi: https:// doi.org/10.1016/j.envpol.2019.113570. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Retention mechanisms

Potential sources, transport pathways, and ecological risks

1

Focus topics on microplastics in soil: Analytical methods, occurrence,

2

transport, and ecological risks

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Jia Lia,*, Yang Songb, Yongbing Caic

4

a

5

China

6

b

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Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, PR China

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c

9

China

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School of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127,

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,

College of Resource and Environment, Anhui Science and Technology University, Anhui 233100,

Abstract

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Microplastics with extremely high abundances are universally detected in marine and

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terrestrial systems. Microplastic pollution in the aquatic environment, especially in ocean, has

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become a hot topic and raised global attention. However, microplastics in soils has been largely

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overlooked. In this paper, the analytical methods, occurrence, transport, and potential ecological

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risks of microplastics in soil environments have been reviewed. Although several analytical

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methods have been established, a universal, efficient, faster, and low-cost analytical method is still

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not available. The absence of a suitable analytical method is one of the biggest obstacles to study

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microplastics in soils. Current data on abundance and distribution of microplastics in soils are still

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limited, and results obtained from different studies differ significantly. Once entering into surface

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soil, microplastics can migrate to deep soil through different processes, e.g. leaching, bioturbation,

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and farming activities. Presence of microplastics with high abundance in soils can alter

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fundamental properties of soils. But current conclusions on microplastics on soil organisms are

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still conflicting. Overall, research on microplastics pollution in soils is still in its infancy and there

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are gaps in the knowledge of microplastics pollution in soil environments. Many questions such as

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pollution level, ecological risks, transport behaviors and the control mechanisms are still unclear,

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which needs further systematical study.

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Keywords: Microplastics; Soil pollution; Analytical method; Transport; Ecological risks

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Introduction

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Plastics, as one kind of synthetic polymer materials with high chemical stability and strong 1

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plasticity, are widely used in packaging, construction, textile, pharmaceutical, agricultural

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production, and electronics manufacturing industries (Thompson et al., 2009; Andrady, 2011). The

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global plastic production reached to 348 million tons in 2017 (Plastics, 2018). The high

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consumption of plastics is accompanied by large amounts of plastic wastes. However, only a small

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fraction (6–26%) of plastic wastes is recycled (Alimi et al., 2018). Depending on the particle size,

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plastic wastes in environments can be divided into large plastic (>5 mm), microplastic (0.1 µm–5

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mm), and nanoplastic (<0.1 µm) (Barnes et al., 2009; Anderson et al., 2016; Alimi et al., 2018).

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Compared with large debris, microplastics may be more harmful due to their high abundances,

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smaller particle size, and long rang transport (Andrady, 2011; Law and Thompson, 2014).

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Microplastics are universally detected in marine and terrestrial systems in recent decades

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(Thompson et al., 2004; Barnes et al., 2009; Cole et al., 2011; Lee et al., 2013; Cozar et al., 2014;

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Auta et al., 2017; Zhang and Liu, 2018). The published studies showed that microplastics could

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pose threats to the whole ecosystem (Andrady, 2011; Peng et al., 2017; de Souza Machado et al.,

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2018b). For instance, microplastics are considered as vectors for various toxins such as heavy

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metals, hydrophobic organic pollutants, and pharmaceutical and personal care products (Guo et al.,

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2012; Turner and Holmes, 2015; Wu et al., 2016; Li et al., 2018). Due to their sizes similar to

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algae or mineral grain, microplastics can be easily ingested by organisms with different trophic

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level, and accumulate along the food webs (Lee et al., 2013; Wright et al., 2013; Huerta Lwanga et

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al., 2016, 2017). After ingestion, the adsorbed pollutants and/or the toxic additives (e.g.

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plasticizers, organotin compounds, alkylphenols, nonylphenol, bisphenol A) contained in the

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polymer can be transfered to organisms (Teuten et al., 2009, Bakir et al., 2014; Koelmans et al.,

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2014), and then causing negative effects to organisms (Lusher et al., 2017; Lo and Chan, 2018).

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Microplastic pollution has received increasing attention and become a hotspot in the field of

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ecological and environmental science research.

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Currently, research on microplastic pollution is overwhelmingly focused on the marine

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system (Rillig, 2012). As early as 1974, a study has reported the presence of microplastic particles

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(0.2–3.4 mm) in the surface waters of the Atlantic Ocean (Colton et al., 1974). In 2004, Thompson

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et al. (2004) called for attention to marine microplastics contamination again. Subsequently, more

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and more literatures about the source, analytical method, abundance, spatial and temporal 2

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distribution, transport behavior and ecological effects of microplastics in the marine environment

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were published (Barnes and Milner, 2005; Bhattacharya et al., 2010; Moret-Ferguson et al., 2010;

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Browne et al., 2011; Wang et al., 2016; Zhang, 2017). Compared with ocean, terrestrial

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environment is a more significant “sink” for microplastics. It is estimated that annual plastics

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released to land were 4–23 times higher than that released to oceans (Horton et al., 2017).

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However, microplastic pollution in soils has been largely overlooked. One key reason is believed

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that a suitable analytical method for microplastics in soils is still unavailable (Rillig, 2012;

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Scheurer and Bigalke, 2018).

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Actually, soil pollution caused by the large plastic debris is nothing new. “White pollution” in

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soils caused by plastic bag or film mulch is well known (Liu et al., 2014). Many studies have

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proved the limited degradation of plastics in soils (Albertsson, 1980; Arkatkar et al., 2009). But

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these large plastics persisting for decades in soils can break into smaller plastic residues (Krueger

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et al., 2015; Briassoulis et al., 2015). Nowadays, only few studies have reported the occurrence of

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microplastics in soil environments (David et al., 2018; Liu et al., 2018; Scheurer and Bigalke,

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2018; Zhang and Liu, 2018; Zhou et al., 2018; Lv et al., 2019). Nizzetto et al. (2016b) estimated

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that more than 700,000 tons of microplastics entered into soil annually in Europe and North

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America, which was more than the global burden of microplastics in oceanic surface water

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(93,000–236,000 tons). Once entering into soils, a complex and heterogeneous system,

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microplastics may undergo different environmental processes and cause various ecological risks

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(de Souza Machado et al., 2018a; He et al., 2018; Hurley and Nizzetto, 2018).

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This paper provided a review of the existing literatures reporting microplastic pollution in

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soils, focusing on analytical methods, occurrence, transport, and ecological risks. We discussed the

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advantages and constraints of available analytical methods for the extraction-identification of

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microplastics in soils. Then, we reviewed current reports on the occurrence, distribution, transport

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process, and ecological risks of microplastics in soils. Lastly, we discussed current gaps in

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knowledge regarding understanding of microplastic pollution in soils and proposed several

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perspectives for future studies.

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2 Analytical methods of microplastics in soils

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An accurately analytical method is the foundation of research on microplastics. Generally, 3

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analytical methods of microplastics in soils contain four steps, i.e. extraction, clean-up,

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identification, and quantification. Recently, several new methods without extraction and clean-up

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can directly detect microplastics in soils. The available analytical methods were summarized in

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Table 1. The advantages and limitations of each analytical method were evaluated.

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2.1 Extraction

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The density values of frequently detected microplastics ranging from 0.8 to 1.4 g cm−3

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(Hidalgo-Ruz et al. 2012), which are smaller than soils (2.6–2.7 g cm−3) (Suthar and Aggarwal,

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2016). Therefore, density fractionation methods were widely used to extract microplastics from

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complex soil matrix. However, microplastics can be strongly absorbed or embedded by soil

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aggregates (Zhang and Liu, 2018), thus decreasing the extraction efficiencies of microplastics. To

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overcome this problem, several procedures including ultrasonic treatment, stirring, aeration, and

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continuous flow were conducted to destroy those attachments during extraction (Table 1).

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Currently, different density solutions have been used including water, NaCl, CaCl2, ZnCl2, and

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NaI (Table 1). Among them, water is harmless and easily available, however it could be just used

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for separating microplastics with density < 1.0 g cm−3. NaCl is also easily available, and Na

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benefits the dispersion of particles, but the maximum solution density of NaCl is still low (1.2 g

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cm−3). The concentrated ZnCl2 has a density of 1.55 g cm−3, however this solution is corrosive and

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toxic. The solution density of NaI is high enough (1.8 g cm−3), but NaI is expensive. It seems that

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CaCl2 solution is relatively suitable to separate microplastics from soils. But the divalent Ca ions

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would have bridged the negative charges of the organic molecules which may promote the

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extraction of soil organic material (Scheurer and Bigalke, 2018). That is, all the commonly used

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density solutions have their limitations. As shown in Table 1, the recovery rates of various

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microplastics by using density separation method were higher than 90%, indicating that this

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method was efficient. Density separation method was simple and widely used, however it may be

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not suitable to separate those more smaller plastic particles (< 10 µm) (Claessens et al., 2013).

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Recently, Fuller and Gautam (2016) developed a method based on pressurized fluid

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extraction (PFE), which can extract microplastics from solid matrix (e.g. municipal waste and soil).

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The PFE based extraction method has several benefits including fully automation, low cost, and

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high efficiency. In addition, this method can efficiently extract plastic particles less than 30 µm. 4

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The limitation of this method is sensitivity. It is a challenge for quantifying microplastic samples

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accurately due to small extracted sample amounts.

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Table 1. Available analytical methods of microplastics in soils. Extracting solution NaCl (1.19 g cm-3)

Separation method Stir for 30 min and treat by ultrasonic for 2 min, then settling for 24 h.

Repeat Three times

Extraction Density fractionation

Clean-up H2O2 (30%)

Identification method Visual identification using microscopy, µ-FT-IR spectroscopy

Quantification Counting

NaCl (1.19 g cm-3)

Stir for 30 min, then settling for 24 h.

Three times

Density fractionation

H2O2 (30%)

Visual identification using microscopy, µ-FT-IR spectroscopy

NaI (1.8 g cm-3)

Treat by ultrasonic for 20 min, then centrifuging for 10 min at 2300 rpm. Use a continuous flow and floating separation apparatus

At least two times

Density fractionation

H2O2 (35%), NaOH (0.5M)

Two times

Density fractionation



Distilled water NaCl (1.20 g cm-3) ZnCl2 (1.55 g cm-3)

Stir and centrifuge

Three times

Density fractionation

Distilled water

Shake and settling

Two times

Density fractionation

Distilled water

Stir and treat by ultrasonic for 2 h, then settling for a night.

At least four times

Density fractionation

NaCl (1.2 g cm-3) NaI (1.6 g cm-3)

Reference Liu et al., 2018

Counting

Method validation 50 g clean soils were spiked with 20 items of 9 different types of microplastic particles (1-5 mm). Except for PET and PVC, mean recoveries of other polymers were >90%. NM

Visual identification using microscopy

Counting

NM

Zhang and Liu, 2018

Visual identification using microscopy, FT-IR spectroscopy

Counting

Zhou et al., 2018

Visual identification using a stereo microscope

Counting



Visual identification using polarized light microscopy

Counting

Commercial polypropylene or polyethylene particles (0.2-5 mm) were mixed with field-cleaned sands. Recoveries were 97%. Microplastics (0.5-4.1 mm) were added to 10 different soil samples. Recovery of acrylic fibers was 49%, other polymers (Polyester, Nylon, polyethylene, and polyvinyl chloride) were >77%. NM



Heating and visual identification using microscopy

Counting, Weighing

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Polypropylene (<400 µm) and polyethylene (<150 µm) particles were added to three different soil samples at five concentration gradients (0.05%, 0.1%, 0.2%,

Lv et al., 2019

Corradini et al., 2019b

Zubris and Richards, 2005 Zhang et al., 2018

0.5% and 1.0%, w/w). The mean recoveries were 86%.

NaCl (1.2 g cm-3) CaCl2 (1.5 g cm-3)

a) sedimentation cylinder method; b) use a self-constructed MP separator; c) Stir for 10 min, then centrifuging for 30 min at 3450 G; —

Three or four times

Density fractionation

KClO (13%), NaOH (50%), H2SO4 (96%), HNO3 (65%), H2O2 (30%)

Raman spectroscopy, FT-IR spectroscopy

Weighing

10 Polypropylene particles (0.5-1 mm) were added to 50 g of sand. Recoveries ranged from 93% to 98%.

Scheurer and Bigalke, 2018

One

Pressurized fluid extraction



FT-IR spectroscopy

Weighing

Fuller and Gautam, 2016













TGA−MS

10-50 mg selected microplastics particles (1 mm) were added to municipal waste material. Average recoveries ranged from 845% to 94%. NM













vis-NIR Spectroscopy

Methanol, Hexane, Dichloromethane

David et al., 2018 Corradini et al., 2019a

Note: “—” mean “Not Conducted. “NM” mean “Not Mentioned”. µ-FT-IR: Micro-Transformed Infrared Spectroscope; FT-IR: Transformed Infrared Spectroscope; TGA-MS: Thermogravimetry-Mass Spectrometry; NIR: near-infrared.

7

143 144

2.2 Clean-up Soil is a complex and heterogeneous system. Some components (e.g. SOM and organic fibers)

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in soils and microplastics have similar densities (Brady and Weil, 2000; Zhang et al., 2018). These

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components also can be extracted by density solution, so there is difficulties in separating

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microplastics from soil matrix (Hidalgo-Ruz et al., 2012; Scheurer and Bigalke, 2018). In addition,

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microplastics in soils can be surrounded by an ecocorona (Galloway et al., 2017), consisting of

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microbes and various organic deposits. These attachments could substantially influence the

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characterization of microplastics (e.g. shape, density, and size) (Chubarenko et al., 2016).

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Therefore, a clean-up procedure to remove SOM and/or other organic attachments is frequently

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used. Currently, peroxide digestion (H2O2), alkaline digestion (NaOH), and acid digestion (HNO3,

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H2SO4) are the dominant clean-up procedures (Table 1). The removal rates of SOM by diverse

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digestion methods are different. Scheurer and Bigalke (2018) tested different chemicals (HNO3,

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H2SO4, H2O2, NaOH, KClO) for removing SOM and found that most organic matter were

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removed in a short time by HNO3 than the other reagents. However, HNO3 treatment caused

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several plastic materials (e.g. acrylonitrile butadiene styrene, polyamide (PA), and polyethylene

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terephthalate (PET)) to decompose or disintegrate into smaller debris. For those easily degradable

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plastic materials, a 1:1 mixture of KOH and NaClO was recommended (Enders et al., 2017). Thus,

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it is advisable to choose an appropriate digestion method for the targeted microplastics.

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2.3 Identification

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Identification of microplastics is usually based on the physical and chemical characterizations

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of isolated particles in mixtures after the extraction and clean-up steps. Therefore, the commonly

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used identification methods consist of physical identification (i.e. visual sorting) and chemical

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identification (e.g. spectral analysis and mass spectrometry) (Table 1).

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Based on the specific properties (e.g. color, shape or surface texture), microplastics can be

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identified by naked eyes (Nor and Obbard, 2014; Peng et al., 2017). The commonly used criterias

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to sorting microplastics were 1) particles that cannot be torn apart; 2) particles that have

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distinguishable colors; and 3) no visible cellular or organic structures (Nor and Obbard, 2014).

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Visual sorting of relatively larger microplastics (1–5 mm) offers a simple and fast method for both

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experts and the non-professional volunteers (Shim et al., 2017). For the identification of smaller 8

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microplastics (i.e. <1 mm) in soils, stereoscopic or dissecting microscopy with professional image

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software were widely used (Liu et al., 2018; Zhang and Liu, 2018). However, some smaller

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particles (<100 µm) with no color or typical shape were difficult to be characterized with

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confidence as plastics by visual or microscopy identification (Song et al., 2015). According to the

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changes of physical properties (e.g. shape, transparency) of plastics before and after heating,

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Zhang et al. (2018) recently established a simple and cost-saving method which could identify

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polyethylene (PE) and polypropylene (PP) microplastics from soils. This heating method was not

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affected by the presence of SOM. With the help of the microscope and image software, particles

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size, shape, and number of microplastics could be determined visually. More importantly, heating

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method could be used to identify smaller particles (<100 µm). Currently, heating method is only

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suitable for PE and PP, and its applicability for other plastics still needs confirming. In addition,

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Zubris and Richards (2005) used polarized light microscopy to identify synthetic fibers in soils.

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Indeed, this is also a visual identity method based on the different physical characterizations of

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synthetic and natural fibers under polarized light.

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Visual sorting was considered to be questionable because it exhibited error rates of 20–70%

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(Eriksen et al., 2013). In most cases, suspected microplastics were usually picked out for further

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confirmation by chemical characterization analysis (Liu et al., 2018). Fourier Transform Infrared

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(FTIR) Spectroscopy is a reliable identification method because it can record the specific chemical

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bonds of chemicals. Through comparing the obtained spectrums of the targeted polymers with the

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standard database provided by spectrum library, it enables not only confirmation of plastics, but

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also identification of plastic types. FTIR spectroscopy and its optimized technology (i.e.

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micro-FTIR) have been applied to microplastics identification in soils (Fuller and Gautam, 2016;

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Liu et al., 2018). However, it remains a challenge to apply FTIR in analyzing ultra-fine plastic

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particles (<1 µm). More importantly, success rate of this method once applied to soil still depend

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on the effectiveness of removing interfering SOM. Corradini et al. (2019a) explored the

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possibilities of using the vis-NIR spectra to rapidly evaluate microplastics concentrations in the

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soil without extraction. Their results showed that vis-NIR technique was suitable to quantify PET,

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low-density polyethylene (LDPE), and polyvinyl chloride (PVC) in soils, with a 10 g kg−1

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accuracy and a detection limit ≈ 15 g kg−1. Although the vis-NIR technique is faster and simpler, it 9

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seems to be useful only for pollution hotspots due to its low accuracy. Furthermore, the same

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authors ignored the impacts of adsorption and biofouling. Because they mixed the tested

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microplastics with dry soil samples and recorded the spectra immediately. David et al. (2018)

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applied Thermogravimetry-Mass Spectrometry (TGA-MS) to develop a more simple and accurate

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method for the direct quantitative analysis of PET in soils without further sample pretreatment.

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This method is not affected by SOM, but it cannot provide characterizations (e.g. shape, size, and

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color) of microplastics besides concentration because microplastics are pyrolyzed. Furthermore,

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this method is just used for analyzing one type of microplastics. That is, TGA-MS cannot

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simultaneously analyze various kinds of microplastics in soils.

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2.4 Quantification

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According to published papers, quantification of microplastics in soils include counting,

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weighing, mathematical calculation, and instrumental analysis (Table 1). Among them, counting is

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the most commonly used quantitative method, and the corresponding unit is N kg-1 or N m-2.

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Counting is a huge workload, but application of professional image software significantly

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improves the working efficiency (Li et al., 2018). Compared with counting, weighing seems to be

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simpler and its corresponding unit is mg kg-1. Nevertheless, weighing is more suitable for soil

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samples contain high microplastics concentrations. Zhang et al. (2018) found a good linear

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relationship (R2=0.99, p < 0.001) between microplastics weight and particle volume after heating.

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They created a mathematical model to roughly calculate the mass of microplastics in the field.

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Furthermore, several studies directly measured microplastic concentration in soils using

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instrument (e.g. TGA-MS, vis-NIR) (David et al., 2018; Corradini et al., 2019a). Direct

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quantification means no extraction procedure is required, but it cannot provide data of physical

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properties (e.g. shape, size, and color) of microplastics.

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2.5 Method validation

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To test the reliability of the analytical methods, researchers usually carried out recovery

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experiments (Table 1). That is, microplastics with known amount or weight were added to clean

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soil or sand samples. These samples were treated using corresponding extraction methods. Then,

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the recovery rates of microplastics could be calculated based on the initial amount and the final

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extraction amount. As shown in Table 1, the reported recoveries of current methods were relatively 10

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high, and all the researchers supposed that their analytical methods were perfect enough. However,

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it is not hard to find that tested microplastics used in current recovery experiments are easy to be

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extracted and identified. Because most of these tested microplastics are unaged plastics (Liu et al.,

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2018; Zhang et al., 2018; Zhou et al., 2018), and/or relatively large particles in millimeter range

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(Fuller and Gautam, 2016; Liu et al., 2018; Scheurer and Bigalke, 2018; Zhou et al., 2018).

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Meanwhile, mediums used in current recovery experiments are sand (Scheurer and Bigalke, 2018;

236

Zhou et al., 2018) or only one kind of soil (Liu et al., 2018). These mediums, in a way, were not

237

representative. Recently, Corradini et al. (2019b) reported that predicated recovery rates decreased

238

with increasing of soil organic matter. Therefore, although the recoveries of microplastics based

239

on current analytical methods were high enough, the recovery experiments may be questionable

240

and the tested microplastics could not replace environmental microplastics. If possible, some kinds

241

of standard surrogates should be developed in future studies and adding them to each soil samples

242

to be analyzed.

243

3 Sources and concentrations of microplastics in soils

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The possible sources of plastics in soils were recently reviewed by several studies (Nizzetto

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et al., 2016c; Blasing and Amelung, 2018; Chae and An, 2018; Hurley, et al., 2018; Rochman et al.,

246

2019). Plastic film mulching, sewage sludge landfill, application of compost, irrigation and

247

flooding of waste waters, car tires debris, and atmospheric deposition were considered as major

248

contributors of microplastics in soil environments (Fig. 1). atmospheric deposition UV irrigation runoff

fragmentation

ingestion adhere

root decompose

soil cracks leaching

249

mulching

sewage sludge compost landfill

egestion

groundwater

11

plowing bioturbation by plant root

250

Fig. 1. Sources and transport of microplastics in soils.

251

Currently, only few studies reported the occurrence and abundance of microplastics in soil

252

environments (Table 2). In industrial soils from Sydney, Australia, concentrations of microplastics

253

ranged from 300 to 67,500 mg kg−1 (Fuller and Gautam, 2016). Scheurer and Bigalke (2018)

254

found microplastics at concentrations of up to 55.5 mg kg−1 (593 N kg−1) in soil samples from 26

255

floodplain sites in Switzerland. In Chile, microplastics concentrations in agricultural field applied

256

with sludge ranged from 0.57-12.9 mg kg−1 (Corradini et al., 2019b). Zhang and Liu (2018)

257

investigated the concentration of plastics in four croplands and one riparian forest buffer zone in

258

Yunnan Province, China. They found that the concentration of plastic particles (0.05–10 mm)

259

ranged from 7100 to 42,960 N kg−1 (mean value was 18,760 N kg−1). Among them, 95% of these

260

sampled plastics were in the microplastics size range (0.05–1 mm). Liu et al. (2018) studied

261

microplastics in farmland soils from twenty vegetable fields around the suburbs of Shanghai,

262

China. They reported that the abundance of microplastics was 78.00 ± 12.91 and 62.50 ± 12.97 N

263

kg−1 in shallow (0–3 cm) and deep soils (3–6 cm), respectively. Another investigation conducted in

264

Shanghai showed the same order of magnitude for microplastics in rice soils (16.1 ± 3.5 N kg−1),

265

but a low concentration in aquaculture soils (4.5 ±1.2 N kg−1) (Lv et al., 2019). Zhang et al. (2018)

266

reported that mean concentrations of microplastics in agricultural field, fruit field, green house

267

field were 140, 440, and 180 N kg−1 respectively. In the study of Zhou et al. (2018), concentrations

268

ranging from 1.3 to 14,712.5 N kg−1 (dry weight) of microplastics were found in 120 soil samples

269

collected from coastline in Shandong province, China. Comparing the studies using the same units

270

of measurement, abundances of microplastics in soils differed significantly (Table 2). Actually,

271

those areas with high microplastics concentrations usually have typical pollution sources. For

272

instance, the highest concentrations in Yunnan Province is related to application of more sewage

273

sludge and irrigation with wastewater (Zhang and Liu, 2018). Another possible reason is related to

274

the different analytical methods. Although recovery experiments were performed in each study to

275

verify the extraction procedure (Table 1), the recovery experiments were questionable due to using

276

unaged and relatively larger plastic particles. Thus, differences caused by various analytical

277

methods should not be ignored.

278

Table 2. Investigation on microplastics abundances in soils. 12

Location Sydney,

Soil type Industrial soil

Size < 1 mm

Abundance (depth)

Ref.

−1

300–67500 mg kg

Fuller and

Australia Switzerland

Gautam, 2016 Floodplain soil

< 2 mm

−1

Scheurer and

55.5 mg kg (0–5 cm) −1

593 N kg (0–5 cm) Mellipilla, Chile

Agricultural field

< 1 mm

Bigalke, 2018

−1

0.57-12.9 mg kg (0-25 cm)

Corradini et al., 2019b

Shanghai, China

Aquaculture soils

20 µm–5

−1

4.5 ±1.2 N kg

Lv et al., 2019

40±126–320±329 N kg−1 (0–10 cm)

Zhang et al.,

8±25–540±603 mg kg−1 (0–10 cm)

2018

mm Loess plateau,

Agricultural field

China

Fruit field

< 5 mm

−1

Green house field

80±193–120±169 N kg (10–30 cm) 24±51–460±735 mg kg−1 (10–30 cm)

Shandong

Coastal soil

< 5 mm

1.3–14,712.5 N kg−1 (0–2 cm)

Province, China

279

Zhou et al., 2018

Yunnan

Farmland,

0.05–10

Province, China

Forest buffer zone

mm

−1

7100–42960 N kg (0–10 cm)

Zhang and Liu, 2018

4 Transport of microplastics in soils

280

As shown in Fig. 1, transport behaviors of microplastics in soils are complex. It has been

281

supposed that microplastics on the surface soils may be lost by surface runoff or wind (Nizzetto et

282

al., 2016a). Nevertheless, Zubris and Richards (2005) found evidence for downward translocation

283

of fibers by unknown mechanisms. Recent researchers have detected microplastics in both topsoil

284

and deep soil (Table 2). These results indicate that microplastics could move vertically in soils.

285

4.1 Transport pathways of microplastics in soils

286

Soil is a porous media with macro-pores and meso-pores in the µm range (Blasing and

287

Amelung, 2018), which makes the migration of dissolved chemicals or small particles in soils

288

possible. Several studies have demonstrated that the small particles can transport along soil pores

289

through leaching. For example, Grayling et al. (2018) reported that particles with a size range of

290

0.1–6.0 µm in diameter can move vertically in soil column. For those relatively larger microplastic

291

particles, soils will presumably retain them and act as a sink. However, the presence of external

292

forces (e.g. bioturbation and farming activity) may contribute to larger microplastic particles

293

movement in soils. Recent research has reported that microplastic particles can be moved and

294

distributed by two collembola species (i.e. Folsomia candida and Proisotoma minuta) in a

295

laboratory arena (Maaß et al., 2017). Zhu et al. (2018a) showed that mite (i.e. Hypoaspis

296

aculeifermoved) can also move and disperse the commercial PVC particles (80–250 µm) in the 13

297

plates. Rillig et al. (2017b) observed microplastics could stick to the earthworms. So, they

298

supposed that attachment to the outside of the earthworm was a possible transport mechanism.

299

Huerta Lwanga et al. (2017) also proved earthworm can contribute to microplastics movement in

300

soils, but they attributed this mechanism to the ingestion/excretion by earthworms. Bioturbation

301

by plant roots (e.g. root movement, root expansion, water extraction by roots) has a significant

302

impact on soil particle transport (Gabet et al., 2003). Similarly, the transport of microplastics could

303

also be influenced by plant roots. Furthermore, when the root decomposes, it leaves macropores

304

approximately the size of the root, which will facilitate the transport of microplastics in soils.

305

However, this is just an inference, and future studies should be conducted to reveal the effects of

306

plant root on microplastics transport. Farming activity such as plowing will bring about an

307

inversion of the surface soil and subsurface soil (Rillig et al., 2017a). Accordingly, microplastics

308

in surface soil will easily be brought to deep soil. In addition, harvesting of rhizome (e.g. potatoes,

309

carrots) may also facilitate the downward movement of microplastics. Lastly, it is known that dry

310

climate will lead to the appearance of soil cracks, which could open entryways for microplastics to

311

reaching deep soils. A recent study proved that wet-dry circles could accelerate microplastics

312

downward movement (O'Connor et al., 2019). Undoubtedly, above-mentioned external forces can

313

also promote transport of small microplastic particles. However, these external forces usually have

314

limited auxiliary effects on vertical transport of microplastics. For instance, conventional tillage

315

practices affect only the topsoil (20–30 cm) (Rillig et al., 2017a). By contrast, leaching, which is

316

defined as infiltration of water contained suspended or dissolved topsoil materials into the deepsoil,

317

has more significant facilitation for microplastics transport vertically in soils. As reported by Cey

318

et al. (2009), microplastics with average diameter of 3.7 µm could move downward to over 70 cm

319

deepsoil through leaching. It has even been predicted that microplastics may end up in shallow

320

groundwater with the help of leaching (Blasing and Amelung, 2018).

321

4.2 Influencing factors and retention mechanisms

322

An essential requirement for downward leaching of microplastics is that their sizes are

323

smaller than the diameter of soil pores, otherwise microplastics will be captured by soil. Therefore,

324

leaching of microplastics in soils with higher porosity, especially more macropores, is more likely

325

to happen. Soil texture has been experimentally shown to directly affect transport of microplastics 14

326

(Bradford et al., 2002; Cey et al., 2009; Rahmatpour et al., 2018). Because soil texture (grain size)

327

could influence its pore size. Studies showed that increasing ionic strength could significantly

328

promote the retention of microplastics in quartz sand media (Pelley and Tufenkji, 2008; Treumann

329

et al., 2014). This can be attributed to compression of the double layer thickness under high ionic

330

strength condition which produces a lower energy barrier and greater depths in the primary and

331

secondary minima (Bradford and Torkzaban, 2012). Similar impacts can also be expected for ionic

332

strength of soil pore water, even though there is no experimental evidence yet. Further, many

333

studies have shown that surface roughness of medium, biofouling, organic matter, saturation, and

334

hydrodynamic condition can affect transport and retention of microplastics in quartz sand (Pelley

335

and Tufenkji, 2008; Majumdar et al., 2014; Treumann et al., 2014; Mitzel et al., 2016). Compared

336

with homogeneous quartz sand, soil is a complex and heterogeneous medium. How does

337

physicochemical properties of soils and leaching condition affect the transport and retention of

338

microplastics? These questions have not been well solved, which creates obstacles for fully

339

understanding the transport of microplastics in soils. Future studies should be conducted from

340

simple medium to complex medium, that is, from pure quartz sand to sea sand, to sandy soil, then

341

to clay.

342

The movement of microplastics in soils is largely dependent on their properties (e.g. size,

343

shape, density). Previous studies have shown that the size and hydrophobicity of microplastics can

344

affect their transport in soil (Pelley and Tufenkji, 2008; O'Connor et al., 2019). Rillig et al. (2017b)

345

stated that the smallest plastic particles moved downward the most since that the small particles

346

could pass through soil pores and eventually reach deep soil. At present, the shape of most

347

commonly used microplastics in relevant experiments were sphere and particle (Zhuang et al.,

348

2005; Treumann et al., 2014; Huerta Lwanga et al., 2017; Rillig et al., 2017b). These studies

349

showed that microplastics with the two shapes could easily move to deep soils. Rillig et al. (2017a)

350

supposed that other shapes (e.g. fiber and film) would behave differently from microsphere. A

351

recent study indicated that microfibers could help them to entangle soil particles more efficiently

352

to form clods (Zhang et al., 2019). In addition, O'Connor et al. (2019) indicated that microplastics

353

with low density were difficult to leach downward. Currently, no studies have been able to explain

354

the effect of shape on microplastics migration in soils. The effects of the type and surface structure 15

355

of microplastics on their migration and retention in soil require further investigation.

356

After entering into soils, microplastics may undergo many processes such as attachment,

357

detachment, sedimentation, or incorporation into soil aggregates (Treumann et al., 2014; Rillig et

358

al., 2017a; Zhang and Liu, 2018), which can restrict the movement of microplastics. Previous

359

studies on colloid transport in quartz sand and glass bead revealed that straining and

360

physicochemical deposition (including collision and attachment) were the key processes

361

controlling the transport and retention of microplastics (Wan and Tokunaga, 1997; Gamerdinger

362

and Kaplan, 2001; Bradford et al., 2002; Zhuang et al., 2005; Bradford and Torkzaban, 2012).

363

These studies supposed that attachment at solid-liquid interface, film straining, air-water

364

interfacial capture, and pore exclusion were the dominant mechanisms (Fig. 2). Contribution of

365

these mechanisms depend on microplastics properties (e.g. particle size and surface structure) and

366

environmental factors (e.g. pore size, ionic strength, and saturation). For instance, straining is

367

more important for relatively larger microplastics, while attachment is more significant for smaller

368

microplastics (Bradford et al., 2002). Above-mentioned studies could provide implications for

369

exploring the transport mechanism of microplastics in soils. However, the physicochemical

370

properties of quartz sand were quite different from soils, such as particle size distribution, surface

371

charge, surface roughness, mineral composition and pore size. The controlling mechanisms of the

372

migration and retention of microplastics in heterogeneous soils may be more complex. It is

373

necessary to combine column experiments with numerical simulation as well as microscopic

374

imaging technology in future studies. Pore exclusion Straining

Physicochemical deposition Film straining

375 376 377

Fig. 2. Retention mechanisms of microplastics in sand. 5 Ecological risks of microplastics on soil ecosystem 16

378

With the concern raised by many researchers regarding the risks posed by microplastics in

379

aquatic environments (Bhattacharya et al., 2010; Setala et al., 2014; Bouwmeester et al., 2015;

380

Batel et al., 2016; Green et al., 2016), some studies also focused on ecological risks of

381

microplastics on soil ecosystem. However, recent studies on the toxic effects of microplastics on

382

soil ecosystem were still in the early stage. That is, a scientific conclusion about whether

383

microplastics could contaminant soil ecosystem is still not available.

384

5.1 Effects of microplastics on soil properties

385

As a kind of solid pollutants, microplastics could alter fundamental properties of soils. Liu et

386

al. (2017) studied the response of soil dissolved organic matter to PP microplastic addition in

387

Chinese loess soil. They found that the lower level (7% W/W) of microplastic addition had a

388

negligible effect on the nutrient contents (e.g. DOC, DON, DOP, NH+4 , NO-3 , PO3-4 ) in DOM

389

solution, while the higher level (28% W/W) of microplastic addition significantly increased the

390

nutrient contents. de Souza Machado et al. (2018b) studied the potential of microplastics to disturb

391

soil structure. They exposed a loamy sand soil to environmentally relevant nominal concentrations

392

(up to 2% W/W) of four common microplastic types (polyacrylic fibers, PA beads, polyester fibers,

393

and PE fragments) for 5 weeks. Their results showed that microplastics affect the bulk density,

394

water holding capacity, and water stable aggregates of soils. However, different microplastics

395

showed different impacts on these indicators. For instance, soils contaminated with polyester

396

fibers showed a significant decrease in bulk density and water stable aggregates with increasing

397

polyester concentrations, while none of the other microplastics elicited similar effects. Meanwhile,

398

the same authors also noted that microplastics in soils may pose further effects due to these tested

399

indicators (i.e. bulk density, water holding capacity, and water stable aggregates) correlates with

400

soil physical quality and rootability. However, Zhang et al. (2019) reported that polyester

401

microfibers ( 0.3% W/W) did not alter soil bulk density and saturated hydraulic conductivity. The

402

different results in two studies may be attributed to the different test concentrations of

403

microplastics. Furthermore, Zhang et al. (2019) also found that polyester microfibers reduced the

404

volume of <30 µm pores, while increased the volume of >30 µm pores. Microfibers could enter

405

micropores and then occupied the space of micropores. The linear shape of polyester microfibers

406

can help them to entangle soil particles more efficiently to form clods. Therefore, the increase in 17

407

clods caused by polyester microfibers can also make more soil macropores. Recently, Rillig (2018)

408

argued that microplastics in soils make a hidden contribution to soil carbon storage. Because

409

plastics are mostly carbon (e.g. PS or PE are almost 90% carbon). It should be noted that this

410

fraction of carbon may interact differently with soil microbes because they are likely not

411

functionally similar to natural soil organic matters.

412

5.2 Ecological risks of microplastics on soil microbial community and plants

413

Previous study has shown that microplastics in aquatic environments are a distinct microbial

414

habitat and may be a novel vector for the transport of unique bacterial assemblages (McCormick et

415

al., 2014). Although there is no research on reaction of microplastics and microbes in soils yet, we

416

could suggest that microplastics may change soil microbial community during their transport in

417

soils. Microplastics showed high adsorption capacity for antibiotics, heavy metals, and other toxic

418

pollutants (Turner and Holmes, 2015; Wu et al., 2016; Li et al., 2018). These contaminants will

419

affect microbes adhered to microplastics. Further, Sun et al. (2018) reported that the existence of

420

microplastics

421

bacteria/phage-harbored resistance genes (ARGs). Awet et al. (2018) documented short-term

422

detrimental impacts of PS nano-plastics on soil microbe. de Souza Machado et al. (2018b, 2019)

423

studied the effects of various microplastics (PA, polyester, PE, PP, PS, PET, polyacrylic) on soil

424

microbial activity. They found that soil microbial activity varied among microplastic types. For

425

instance, the general microbial metabolic activity was increased by PA, PE, and polyester while

426

decreased by PS and PET. However, Judy et al. (2019) reported that there was little evidence the

427

microplastics (PE, PVC, and PET) affected soil microbial community diversity. Obviously,

428

researches on effects of microplastics on soil microbial community are still in the early stage.

(polyolefin

film)

inhibited

the

dissipation

of

soil

antibiotics

and

429

Plant performances depend significantly on root colonizing microbes, including N-fixers,

430

pathogens and mycorrhizal fungi (Powell and Rillig, 2018). Thus, microplastics could influence

431

plant growth via affecting soil microbes. Likewise, microplastics may influence plant growth

432

directly or indirectly. As reported by de Souza Machado et al. (2019), microplastics could affect

433

plant (Allium fistulosum) root traits, leaf traits, and total biomass, but the positive and negative

434

effects varied among microplastics types. Liao et al. (2019) demonstrated the toxic impacts of PS

435

(5 µm) on the growth of wheat (Triticum aestivum). Qi et al. (2018) reported that microplastic 18

436

residues affected the wheat (Triticum aestivum) during both vegetative and reproductive growth.

437

They also found the biodegradable plastic mulch had stronger negative effects as compared to PE.

438

Recently, Rillig et al. (2019) proposed several potential mechanistic pathways through which

439

microplastics could affect plant performance. Their paper provided guidelines for future studies on

440

this topic.

441

5.3 Ecological risks of microplastics on soil animals

442

5.3.1 Direct ecological risks of microplastics on soil animals

443

Like to other contaminants, microplastics may have a direct toxic effect on soil animals

444

(Table 3). Huerta Lwanga et al. (2016) studied the survival of the earthworm (Lumbricus terrestris)

445

exposed to LDPE microplastics (<400 µm) in sandy soil at different concentrations (0, 7, 28, 45,

446

60% W/W). They found that small plastic particles (<50 µm) can be easily ingested by

447

earthworms. Mortality was higher at 28, 45, and 60% W/W than at 7% W/W and in the control

448

(0%). Growth rate was significantly reduced at high microplastic concentrations (>28% W/W).

449

They supposed that the ecological effect mechanisms of microplastics on earthworms were

450

dilution of ingested food and changing food quality. It should be noted that microplastics

451

concentration used in their study was very high. Their conclusions may not be suitable for actual

452

environmental concentration. Other studies also demonstrated that microplastics have no

453

significant impacts on mortality and growth of earthworms at relatively lower concentrations

454

(<20%) (Hodson et al., 2017; Rodriguez-Seijo et al., 2017; Wang et al., 2019). In a recent study,

455

Zhu et al. (2018b) reported that PVC microplastics exposure (0.1% W/W) could alter feeding

456

behavior of soil collembolan (Folsomia candida), and then inhibited their growth and reproduction.

457

Lei et al. (2018) showed that PS microplastics (1 mg L-1) could accumulate in the intestine of

458

nematodes, then resulted in decreasing of survival rate, body length, and reproduction and caused

459

intestinal damages and oxidative damages. They also emphasized that there was strong association

460

between microplastic particle size and its toxicity. These are important researches indicating that

461

lower concentration of microplastics exposure will cause adverse effects on growth of soil animals.

19

462

Table 3. Recent studies on ecological risks of microplastics on soil animals Microplastics

Toxin chemicals Polybrominated diphenyl ether (PBDE)

Test organisms

Test soil

Exposure concentration MPs: 1:2000 (W/W) PBDEs: 83 mg kg-1 MPs: 0, 7, 28, 45, 60% (W/W)

Exposure time 7, 14 and 28 d

Evaluating indicator Bioaccumulation

Conclusions

Ref.

Earthworm, Eisenia fetida

Artificial soil

PBDEs accumulate in organisms ingesting soils containing biosolids or waste plastics.

Gaylor et al., 2013

LDPE (<400 µm) particles

-

Earthworm, Lumbricus terrestris

Sandy soil

60 d

Mortality; growth; activity; ingestion

Huerta Lwanga et al., 2016

Clay loam

MPs: 1 g kg-1

28 and 56 d

Growth; reproduction; isotope composition; gut microbiota.

Caenorhabditis elegans

-

MPs: 1 mg L-1

3d

-

Snails, Achatina fulica

Cultivation soils

MFs: 0.014, 0.14 and 0.71 g kg-1

28 d

Growth; motor behavior; Oxidative damage Food intake; excretion; histopathology; oxidative stress

PVC particle

arsenic

Earthworm, Metaphire californica

Farmland soil

28 d

Bioaccumulation gut microbiome

High-density

Zn

Earthworm,

Woodland

MPs: 2000 mg kg-1 As(V): 40 mg kg-1 Zn-bearing

28 d

Growth;

PE particles can be ingested by earthworms; Mortality was higher at 28, 45, and 60% W/W than at 7% W/W and in the control (0%); Growth rate was significantly reduced at 28, 45, and 60% W/W. Micro-PVC altered gut microbiota and increased bacterial diversity; Collembolan growth and reproduction were inhibited; Micro-PVC enhanced δ15N and δ13C values of collembolan tissues. PS microplastics decreased survival rate, body length and reproduction of nematodes and caused intestinal damages and oxidative damages. MFs were uptake and depurated by the digestive tract; MFs inhibited food intake and excretion; 0.71 g kg-1 MFs induced villi damages in walls of gastrointestinal tract; MFs could affect oxidative stress. PVC reduced arsenic accumulation in gut and body tissues. PVC alleviated the effect of arsenic on the gut microbiota. There was no evidence of Zn accumulation,

PVC (80-250 µm) particles

-

Collembolans, Folsomia candida

PS (0.1-500 µm) particles

-

PET (1257.8µm) fibers

Polyurethane foam (PUF, < 75 µm) particles

20

Zhu et 2018b

al.,

Lei et 2018

al.,

Song et 2019

al.;

Wang et al., 2019

Hodson et al.,

Polyethylene (HDPE, <400 µm) pieces PE (250 µm-1 mm)

Lumbricus terrestris

soil

MPs: (W/W)

0.35%

Earthworm, Eisenia andrei

OECD artificial soil

MPs: 62.5, 125, 250, 500, 1000 mg kg-1

28 and 56 d

bioaccumulation; mortality

mortality, or weight change.

2017

Survival, number of juveniles; weight; histopathological analysis; damages; immune system response Ingestion; antioxidant defense system

No effect on survival, number of juveniles and, in the final weight of adult earthworms, but damages and immune system responses were confirmed.

RodriguezSeijo et al., 2017

MPs could be ingested by earthworms; Exposure to PE or PS particles (20%) significantly influenced enzyme activity of E. fetida, while no discernible effect was detected at low rates ≤10%. Microplastic decreased bioaccumulation of PAHs and PCBs in E. fetida.

Wang et al., 2019

LDPE (≤300 µm) particles PS (≤300 µm) particles

-

Earthworm, Eisenia fetida

Sandy loam

MPs: 1, 5, 10, 20% (W/W)

14 d

LDPE (≤300 µm) particle (PS (≤300 µm) particles

phenanthrene, fluoranthene, pyrene, benzo[α] pyrene, PCB52, PCB70, and PCB153 Chlorpyrifos

Earthworm, Eisenia fetida

Sandy loam

MPs: 1, 5, 10% (W/W)

28 d

Bioaccumulation

Earthworm, Eisenia fetida

OECD artificial soil

MPs(5mm): 16 N kg-1 MPs (250 µm-1 mm): 360-400 N kg-1

14 d

Ingestion; neurological; response

LDPE (5 mm and 250 µm-1 mm) pellets

21

LDPE (5 mm) cannot be ingested by earthworms. Data obtained from this study cannot provide a precise answer to whether LDPE were carriers of pesticides to biota.

Wang et al., 2019

RodriguezSeijo et al., 2019

463

Although lower concentration of microplastics have no effect on mortality and growth of

464

earthworms, histopathological and immune system responses have already been confirmed. For

465

instance, Rodriguez-Seijo et al. (2017) reported that PE pellets (125-1000 mg kg-1) caused tissue

466

and immune system damages of earthworms (E. andrei). Wang et al. (2019) found that exposure to

467

PE or PS particles (20% W/W) significantly influenced enzyme activity of earthworms (E. fetida),

468

while no discernible effect was detected at low rates ≤10% (W/W). Except for earthworms, recent

469

studies also demonstrated the adverse effects of microplastics on immune system of other soil

470

organisms. Zhu et al. (2018b) indicated that PVC particles (1 g kg-1) enhanced δ15N and δ13C

471

values of collembolan tissues. Song et al. (2019) studied the toxic effects of PET fibers on

472

terrestrial snails (Achatina fulica) after 28 d at concentrations of 0.014-0.71 g kg-1 (dry soil

473

weight). They found that PET fibers were uptake and depurated by the digestive tract, and PET

474

fibers could decrease food intake and excretion, induce villi damages in walls of gastrointestinal

475

tract, and influence oxidative stress. This study implied that 0.14 g kg-1 PET fibers caused adverse

476

effects on snails.

477

5.3.2 Indirect ecological risks of microplastics on soil animals

478

Microplastics can accumulate contaminants from soil environments and then may act as

479

vectors to increase pollutants exposure in animals. Several studies have been conducted about this

480

topic, but different results are got. Gaylor et al. (2013) showed that PBDEs leached from

481

polyurethane foam (<75 µm) could be accumulated by earthworms (Eisenia fetida). They also

482

supposed that such earthworms might transfer PBDEs to predators or translocate them from the

483

application site. Adsorption and desorption of Zn on fragmented HDPE bags (<400 µm) were

484

studied by Hodson et al. (2017). However, they reported that there was no evidence of Zn

485

accumulation in earthworms. Similar result was also reported by Rodriguez-Seijo et al. (2019),

486

they cannot be sure whether LDPE were carriers of pesticides to soil biota. Meanwhile, two recent

487

studies demonstrated that microplastics reduced As(V), PAHs, and PCBs accumulation in gut and

488

body tissues of earthworms (Wang et al., 2019; Wang et al., 2019). This is an important finding

489

because it overturns the traditional view that microplastics can increase toxic pollutants

490

bioavailability. Furthermore, Hüffer et al. (2019) studied the impacts of PE microplastics on the

491

transport of atrazine and 4-(2,4-dichlorophenoxy) butyric acid in soils. Their results implied that 22

492

the presence of microplastics in soils could increase the mobility of organic contaminants by

493

reducing the sorption capacity of natural soils. As a result, these organic contaminants may seep

494

into groundwater or other surrounding water sources, and then pose great threats to humans.

495

5.4 Potential human health risks caused by microplastics

496

As mentioned in section 4, microplastics can transport vertically in soils. Thus, researchers

497

supposed that microplastics especially those in micron range might reach groundwater (Rillig et

498

al., 2017a), and then increased the possibility of entering the body. Meanwhile, a published study

499

has provided field evidence for transfer of microplastics along a terrestrial food chain

500

(soil-earthworm-chicken) (Huerta Lwanga et al., 2017). The authors supposed that microplastics

501

accumulated in chicken has potentially negative consequences for human health. Besides, a recent

502

study indicated that PS microplastics can be absorbed by the roots of lettuce and then transport to

503

stems and leaves (Li et al., 2019). This will absolutely facilitate microplastics to enter human. To

504

date, we still have no evidence that microplastics are harmful to human. All we know is just that

505

soil microplastics may accumulate in body via drinking groundwater or food chain.

506

6 Future perspectives

507

Microplastic pollution in aquatic environments (especially the oceans) has garnered a global

508

concern, while soil systems have received far less scientific attention. There are still gaps in the

509

knowledge of soil microplastic pollution and many questions still remain unclear. That is, before

510

comprehensively revealing soil microplastic contamination, much more studies should be

511

conducted. Here, we highlighted several key gaps in understanding of microplastic pollution in

512

soils based on the published literatures. And then, we proposed several perspectives for future

513

studies.

514

6.1 Major gaps of current studies

515



The methods used to extract, quantify and characterize microplastics from water or sediment

516

samples were adjusted and then used for soil samples (Zubris and Richards, 2005; Liu et al.,

517

2018; Scheurer and Bigalke, 2018; Zhang and Liu, 2018; Zhou et al., 2018). Soil is a

518

complex and heterogeneous media which makes the identification of microplastics from it

519

extremely challenging. Although several analytical methods have been established and

520

proven have their own advantages, a universal, efficient, faster, and low-cost analytical 23

521

method is still not available. Furthermore, the absence of standardized methods will hinder

522

evaluating soil microplastics contamination due to the errors between different analytical

523

methods.

524



Currently, field data on measured microplastic concentration in soil systems are still not

525

widely available, which will limit our understanding of the current state of microplastic

526

pollution in soils. The physicochemical properties of most abundant microplastics in soils has

527

not been reported. Due to the lack of quantitative data of environmental concentrations, it is

528

difficult to assess the ecological risks posed by microplastics in soil system under realistic

529

exposure conditions. Several studies have summarized the possible sources of microplastics

530

in soils (Horton et al., 2017; Alimi et al., 2018; Chae and An, 2018), but the contribution of

531

each source and the total flux of microplastics released into soils remain unknown.

532



To scientifically evaluate the effects of soil microplastic pollution, understanding the

533

transport processes of microplastics in the soil environment is undoubtedly an important task.

534

However, transport behaviors and dominant mechanisms of various kinds of microplastics in

535

the soil environment remain unclear. Recent studies about transport simulation have focused

536

on polystyrene (PS) spheres but abundant PP and PE with different shapes (e.g. fragment,

537

fiber, and film) were detected in soils (Gamerdinger and Kaplan, 2001; Li et al., 2004; Liu et

538

al., 2018). Due to well-defined size and surface charge of spherical microplastics, using PS

539

spheres in experiments is convenient, but results obtained from these simulation experiments

540

are not applicable to real environment.

541



Although several studies have been conducted to reveal the ecological effects of

542

microplastics on soil ecosystems, the risk evaluation system has not been developed. We are

543

not even sure whether variety microplastics (especially the aged microplastics) at

544

environmental concentration have adverse effects on soil organisms, because current

545

conclusions are conflicting. Recent studies have demonstrated that microplastics can alter soil

546

structure and properties (Liu et al., 2017; de Souza Machado et al., 2018b). So, what is the

547

subsequent effect on soil biota? Microplastics can enter to body through soil food chain,

548

however the amount of entering body need to be estimated. Furthermore, the question of

549

whether microplastics have adverse impacts on human need also be answered. 24

550

6.2 Perspectives for future studies

551

Future studies on soil microplastics research still need to focus on four aspects (Fig. 3).

552

Analytical method is the foundation of soil microplastics research. A standard and accurate

553

method will absolutely facilitate investigation of occurrence, distribution, and transport behaviors

554

of various microplastics in soils. Data of abundance, distribution, and fate of microplastics in soils

555

will provide the basis for their ecological risk assessment. However, recent studies on these topics

556

are still limited. Based on the published researches, future studies should be conducted from the

557

following topics.

Ecological risks analysis Abundance and distribution

Fate and transport

Optimization of analytical methods

558 559 560

Fig. 3. Topics for future studies.



Developing and optimizing extraction methods to improve extraction efficiency. If possible,

561

establishing a standard extraction method for soil microplastics. Recovery experiments

562

should be optimized and the standard surrogates should be developed. Meanwhile, a database

563

of microplastics characteristics including morphology, chemical component, thermology,

564

mechanical property, and electromagnetism should be built. Research and development of

565

analytical instruments are also an important topic. The combined use of different analytical

566

instruments

567

Chromatography Mass Spectrum) are recommended.

568



(e.g.

Thermogravimetric

Analysis,

Micro-IR

spectroscopy,

and

Gas

Abundance and distribution of microplastics in global soils should be investigated.

569

Developing mathematical models to evaluate the amount of microplastics released into soils

570

from different sources. 25

571



Performing column (i.e. packed-column and undisturbed-column) experiments as well as

572

field experiments to simulate transport behaviors of various kinds of microplastics (different

573

types, shapes, sizes, and surface morphology) in various soil environments, and to determine

574

the key mechanisms and influencing factors. Future studies should be conducted from simple

575

medium to complex medium, that is, from sea sand to sandy soil, then to clay.

576



Assessing the ecological risks posed by various kinds of microplastics (especially those

577

collected from soil environment) in soil ecosystems under realistic exposure conditions. The

578

test species should include microorganism, edaphon, and plant. Lastly, assessment system

579

should be developed to evaluate soil microplastics for human.

580 581

Acknowledgement

582

This study was financially supported by Shandong Key Laboratory of Coastal Environmental

583

Processes, YICCAS (2019SDHADKFJJ12), Research Startup Project of Yangzhou University

584

(137011522), the National Natural Science Foundation of China (41877032).

585 586

References

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Advantages and limitations of current analytical methods of soil microplastics were evaluated. Microplastics could move vertically to deep soils through multiple processes. Whether microplastics have adverse effects on soil systems or humans remain unclear. Several perspectives for future studies on microplastic pollution in soils were proposed.