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. 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Retention mechanisms Potential sources, transport pathways, and ecological risks 1 Focus topics on microplastics in soil: Analytical methods, occurrence, 2 transport, and ecological risks 3 Jia Lia,*, Yang Songb, Yongbing Caic 4 a 5 China 6 b 7 Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, PR China 8 c 9 China 10 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 11 Microplastics with extremely high abundances are universally detected in marine and 12 terrestrial systems. Microplastic pollution in the aquatic environment, especially in ocean, has 13 become a hot topic and raised global attention. However, microplastics in soils has been largely 14 overlooked. In this paper, the analytical methods, occurrence, transport, and potential ecological 15 risks of microplastics in soil environments have been reviewed. Although several analytical 16 methods have been established, a universal, efficient, faster, and low-cost analytical method is still 17 not available. The absence of a suitable analytical method is one of the biggest obstacles to study 18 microplastics in soils. Current data on abundance and distribution of microplastics in soils are still 19 limited, and results obtained from different studies differ significantly. Once entering into surface 20 soil, microplastics can migrate to deep soil through different processes, e.g. leaching, bioturbation, 21 and farming activities. Presence of microplastics with high abundance in soils can alter 22 fundamental properties of soils. But current conclusions on microplastics on soil organisms are 23 still conflicting. Overall, research on microplastics pollution in soils is still in its infancy and there 24 are gaps in the knowledge of microplastics pollution in soil environments. Many questions such as 25 pollution level, ecological risks, transport behaviors and the control mechanisms are still unclear, 26 which needs further systematical study. 27 Keywords: Microplastics; Soil pollution; Analytical method; Transport; Ecological risks 28 Introduction 29 Plastics, as one kind of synthetic polymer materials with high chemical stability and strong 1 30 plasticity, are widely used in packaging, construction, textile, pharmaceutical, agricultural 31 production, and electronics manufacturing industries (Thompson et al., 2009; Andrady, 2011). The 32 global plastic production reached to 348 million tons in 2017 (Plastics, 2018). The high 33 consumption of plastics is accompanied by large amounts of plastic wastes. However, only a small 34 fraction (6–26%) of plastic wastes is recycled (Alimi et al., 2018). Depending on the particle size, 35 plastic wastes in environments can be divided into large plastic (>5 mm), microplastic (0.1 µm–5 36 mm), and nanoplastic (<0.1 µm) (Barnes et al., 2009; Anderson et al., 2016; Alimi et al., 2018). 37 Compared with large debris, microplastics may be more harmful due to their high abundances, 38 smaller particle size, and long rang transport (Andrady, 2011; Law and Thompson, 2014). 39 Microplastics are universally detected in marine and terrestrial systems in recent decades 40 (Thompson et al., 2004; Barnes et al., 2009; Cole et al., 2011; Lee et al., 2013; Cozar et al., 2014; 41 Auta et al., 2017; Zhang and Liu, 2018). The published studies showed that microplastics could 42 pose threats to the whole ecosystem (Andrady, 2011; Peng et al., 2017; de Souza Machado et al., 43 2018b). For instance, microplastics are considered as vectors for various toxins such as heavy 44 metals, hydrophobic organic pollutants, and pharmaceutical and personal care products (Guo et al., 45 2012; Turner and Holmes, 2015; Wu et al., 2016; Li et al., 2018). Due to their sizes similar to 46 algae or mineral grain, microplastics can be easily ingested by organisms with different trophic 47 level, and accumulate along the food webs (Lee et al., 2013; Wright et al., 2013; Huerta Lwanga et 48 al., 2016, 2017). After ingestion, the adsorbed pollutants and/or the toxic additives (e.g. 49 plasticizers, organotin compounds, alkylphenols, nonylphenol, bisphenol A) contained in the 50 polymer can be transfered to organisms (Teuten et al., 2009, Bakir et al., 2014; Koelmans et al., 51 2014), and then causing negative effects to organisms (Lusher et al., 2017; Lo and Chan, 2018). 52 Microplastic pollution has received increasing attention and become a hotspot in the field of 53 ecological and environmental science research. 54 Currently, research on microplastic pollution is overwhelmingly focused on the marine 55 system (Rillig, 2012). As early as 1974, a study has reported the presence of microplastic particles 56 (0.2–3.4 mm) in the surface waters of the Atlantic Ocean (Colton et al., 1974). In 2004, Thompson 57 et al. (2004) called for attention to marine microplastics contamination again. Subsequently, more 58 and more literatures about the source, analytical method, abundance, spatial and temporal 2 59 distribution, transport behavior and ecological effects of microplastics in the marine environment 60 were published (Barnes and Milner, 2005; Bhattacharya et al., 2010; Moret-Ferguson et al., 2010; 61 Browne et al., 2011; Wang et al., 2016; Zhang, 2017). Compared with ocean, terrestrial 62 environment is a more significant “sink” for microplastics. It is estimated that annual plastics 63 released to land were 4–23 times higher than that released to oceans (Horton et al., 2017). 64 However, microplastic pollution in soils has been largely overlooked. One key reason is believed 65 that a suitable analytical method for microplastics in soils is still unavailable (Rillig, 2012; 66 Scheurer and Bigalke, 2018). 67 Actually, soil pollution caused by the large plastic debris is nothing new. “White pollution” in 68 soils caused by plastic bag or film mulch is well known (Liu et al., 2014). Many studies have 69 proved the limited degradation of plastics in soils (Albertsson, 1980; Arkatkar et al., 2009). But 70 these large plastics persisting for decades in soils can break into smaller plastic residues (Krueger 71 et al., 2015; Briassoulis et al., 2015). Nowadays, only few studies have reported the occurrence of 72 microplastics in soil environments (David et al., 2018; Liu et al., 2018; Scheurer and Bigalke, 73 2018; Zhang and Liu, 2018; Zhou et al., 2018; Lv et al., 2019). Nizzetto et al. (2016b) estimated 74 that more than 700,000 tons of microplastics entered into soil annually in Europe and North 75 America, which was more than the global burden of microplastics in oceanic surface water 76 (93,000–236,000 tons). Once entering into soils, a complex and heterogeneous system, 77 microplastics may undergo different environmental processes and cause various ecological risks 78 (de Souza Machado et al., 2018a; He et al., 2018; Hurley and Nizzetto, 2018). 79 This paper provided a review of the existing literatures reporting microplastic pollution in 80 soils, focusing on analytical methods, occurrence, transport, and ecological risks. We discussed the 81 advantages and constraints of available analytical methods for the extraction-identification of 82 microplastics in soils. Then, we reviewed current reports on the occurrence, distribution, transport 83 process, and ecological risks of microplastics in soils. Lastly, we discussed current gaps in 84 knowledge regarding understanding of microplastic pollution in soils and proposed several 85 perspectives for future studies. 86 2 Analytical methods of microplastics in soils 87 An accurately analytical method is the foundation of research on microplastics. Generally, 3 88 analytical methods of microplastics in soils contain four steps, i.e. extraction, clean-up, 89 identification, and quantification. Recently, several new methods without extraction and clean-up 90 can directly detect microplastics in soils. The available analytical methods were summarized in 91 Table 1. The advantages and limitations of each analytical method were evaluated. 92 2.1 Extraction 93 The density values of frequently detected microplastics ranging from 0.8 to 1.4 g cm−3 94 (Hidalgo-Ruz et al. 2012), which are smaller than soils (2.6–2.7 g cm−3) (Suthar and Aggarwal, 95 2016). Therefore, density fractionation methods were widely used to extract microplastics from 96 complex soil matrix. However, microplastics can be strongly absorbed or embedded by soil 97 aggregates (Zhang and Liu, 2018), thus decreasing the extraction efficiencies of microplastics. To 98 overcome this problem, several procedures including ultrasonic treatment, stirring, aeration, and 99 continuous flow were conducted to destroy those attachments during extraction (Table 1). 100 Currently, different density solutions have been used including water, NaCl, CaCl2, ZnCl2, and 101 NaI (Table 1). Among them, water is harmless and easily available, however it could be just used 102 for separating microplastics with density < 1.0 g cm−3. NaCl is also easily available, and Na 103 benefits the dispersion of particles, but the maximum solution density of NaCl is still low (1.2 g 104 cm−3). The concentrated ZnCl2 has a density of 1.55 g cm−3, however this solution is corrosive and 105 toxic. The solution density of NaI is high enough (1.8 g cm−3), but NaI is expensive. It seems that 106 CaCl2 solution is relatively suitable to separate microplastics from soils. But the divalent Ca ions 107 would have bridged the negative charges of the organic molecules which may promote the 108 extraction of soil organic material (Scheurer and Bigalke, 2018). That is, all the commonly used 109 density solutions have their limitations. As shown in Table 1, the recovery rates of various 110 microplastics by using density separation method were higher than 90%, indicating that this 111 method was efficient. Density separation method was simple and widely used, however it may be 112 not suitable to separate those more smaller plastic particles (< 10 µm) (Claessens et al., 2013). 113 Recently, Fuller and Gautam (2016) developed a method based on pressurized fluid 114 extraction (PFE), which can extract microplastics from solid matrix (e.g. municipal waste and soil). 115 The PFE based extraction method has several benefits including fully automation, low cost, and 116 high efficiency. In addition, this method can efficiently extract plastic particles less than 30 µm. 4 117 The limitation of this method is sensitivity. It is a challenge for quantifying microplastic samples 118 accurately due to small extracted sample amounts. 5 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 6 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) 145 in soils and microplastics have similar densities (Brady and Weil, 2000; Zhang et al., 2018). These 146 components also can be extracted by density solution, so there is difficulties in separating 147 microplastics from soil matrix (Hidalgo-Ruz et al., 2012; Scheurer and Bigalke, 2018). In addition, 148 microplastics in soils can be surrounded by an ecocorona (Galloway et al., 2017), consisting of 149 microbes and various organic deposits. These attachments could substantially influence the 150 characterization of microplastics (e.g. shape, density, and size) (Chubarenko et al., 2016). 151 Therefore, a clean-up procedure to remove SOM and/or other organic attachments is frequently 152 used. Currently, peroxide digestion (H2O2), alkaline digestion (NaOH), and acid digestion (HNO3, 153 H2SO4) are the dominant clean-up procedures (Table 1). The removal rates of SOM by diverse 154 digestion methods are different. Scheurer and Bigalke (2018) tested different chemicals (HNO3, 155 H2SO4, H2O2, NaOH, KClO) for removing SOM and found that most organic matter were 156 removed in a short time by HNO3 than the other reagents. However, HNO3 treatment caused 157 several plastic materials (e.g. acrylonitrile butadiene styrene, polyamide (PA), and polyethylene 158 terephthalate (PET)) to decompose or disintegrate into smaller debris. For those easily degradable 159 plastic materials, a 1:1 mixture of KOH and NaClO was recommended (Enders et al., 2017). Thus, 160 it is advisable to choose an appropriate digestion method for the targeted microplastics. 161 2.3 Identification 162 Identification of microplastics is usually based on the physical and chemical characterizations 163 of isolated particles in mixtures after the extraction and clean-up steps. Therefore, the commonly 164 used identification methods consist of physical identification (i.e. visual sorting) and chemical 165 identification (e.g. spectral analysis and mass spectrometry) (Table 1). 166 Based on the specific properties (e.g. color, shape or surface texture), microplastics can be 167 identified by naked eyes (Nor and Obbard, 2014; Peng et al., 2017). The commonly used criterias 168 to sorting microplastics were 1) particles that cannot be torn apart; 2) particles that have 169 distinguishable colors; and 3) no visible cellular or organic structures (Nor and Obbard, 2014). 170 Visual sorting of relatively larger microplastics (1–5 mm) offers a simple and fast method for both 171 experts and the non-professional volunteers (Shim et al., 2017). For the identification of smaller 8 172 microplastics (i.e. <1 mm) in soils, stereoscopic or dissecting microscopy with professional image 173 software were widely used (Liu et al., 2018; Zhang and Liu, 2018). However, some smaller 174 particles (<100 µm) with no color or typical shape were difficult to be characterized with 175 confidence as plastics by visual or microscopy identification (Song et al., 2015). According to the 176 changes of physical properties (e.g. shape, transparency) of plastics before and after heating, 177 Zhang et al. (2018) recently established a simple and cost-saving method which could identify 178 polyethylene (PE) and polypropylene (PP) microplastics from soils. This heating method was not 179 affected by the presence of SOM. With the help of the microscope and image software, particles 180 size, shape, and number of microplastics could be determined visually. More importantly, heating 181 method could be used to identify smaller particles (<100 µm). Currently, heating method is only 182 suitable for PE and PP, and its applicability for other plastics still needs confirming. In addition, 183 Zubris and Richards (2005) used polarized light microscopy to identify synthetic fibers in soils. 184 Indeed, this is also a visual identity method based on the different physical characterizations of 185 synthetic and natural fibers under polarized light. 186 Visual sorting was considered to be questionable because it exhibited error rates of 20–70% 187 (Eriksen et al., 2013). In most cases, suspected microplastics were usually picked out for further 188 confirmation by chemical characterization analysis (Liu et al., 2018). Fourier Transform Infrared 189 (FTIR) Spectroscopy is a reliable identification method because it can record the specific chemical 190 bonds of chemicals. Through comparing the obtained spectrums of the targeted polymers with the 191 standard database provided by spectrum library, it enables not only confirmation of plastics, but 192 also identification of plastic types. FTIR spectroscopy and its optimized technology (i.e. 193 micro-FTIR) have been applied to microplastics identification in soils (Fuller and Gautam, 2016; 194 Liu et al., 2018). However, it remains a challenge to apply FTIR in analyzing ultra-fine plastic 195 particles (<1 µm). More importantly, success rate of this method once applied to soil still depend 196 on the effectiveness of removing interfering SOM. Corradini et al. (2019a) explored the 197 possibilities of using the vis-NIR spectra to rapidly evaluate microplastics concentrations in the 198 soil without extraction. Their results showed that vis-NIR technique was suitable to quantify PET, 199 low-density polyethylene (LDPE), and polyvinyl chloride (PVC) in soils, with a 10 g kg−1 200 accuracy and a detection limit ≈ 15 g kg−1. Although the vis-NIR technique is faster and simpler, it 9 201 seems to be useful only for pollution hotspots due to its low accuracy. Furthermore, the same 202 authors ignored the impacts of adsorption and biofouling. Because they mixed the tested 203 microplastics with dry soil samples and recorded the spectra immediately. David et al. (2018) 204 applied Thermogravimetry-Mass Spectrometry (TGA-MS) to develop a more simple and accurate 205 method for the direct quantitative analysis of PET in soils without further sample pretreatment. 206 This method is not affected by SOM, but it cannot provide characterizations (e.g. shape, size, and 207 color) of microplastics besides concentration because microplastics are pyrolyzed. Furthermore, 208 this method is just used for analyzing one type of microplastics. That is, TGA-MS cannot 209 simultaneously analyze various kinds of microplastics in soils. 210 2.4 Quantification 211 According to published papers, quantification of microplastics in soils include counting, 212 weighing, mathematical calculation, and instrumental analysis (Table 1). Among them, counting is 213 the most commonly used quantitative method, and the corresponding unit is N kg-1 or N m-2. 214 Counting is a huge workload, but application of professional image software significantly 215 improves the working efficiency (Li et al., 2018). Compared with counting, weighing seems to be 216 simpler and its corresponding unit is mg kg-1. Nevertheless, weighing is more suitable for soil 217 samples contain high microplastics concentrations. Zhang et al. (2018) found a good linear 218 relationship (R2=0.99, p < 0.001) between microplastics weight and particle volume after heating. 219 They created a mathematical model to roughly calculate the mass of microplastics in the field. 220 Furthermore, several studies directly measured microplastic concentration in soils using 221 instrument (e.g. TGA-MS, vis-NIR) (David et al., 2018; Corradini et al., 2019a). Direct 222 quantification means no extraction procedure is required, but it cannot provide data of physical 223 properties (e.g. shape, size, and color) of microplastics. 224 2.5 Method validation 225 To test the reliability of the analytical methods, researchers usually carried out recovery 226 experiments (Table 1). That is, microplastics with known amount or weight were added to clean 227 soil or sand samples. These samples were treated using corresponding extraction methods. Then, 228 the recovery rates of microplastics could be calculated based on the initial amount and the final 229 extraction amount. As shown in Table 1, the reported recoveries of current methods were relatively 10 230 high, and all the researchers supposed that their analytical methods were perfect enough. However, 231 it is not hard to find that tested microplastics used in current recovery experiments are easy to be 232 extracted and identified. Because most of these tested microplastics are unaged plastics (Liu et al., 233 2018; Zhang et al., 2018; Zhou et al., 2018), and/or relatively large particles in millimeter range 234 (Fuller and Gautam, 2016; Liu et al., 2018; Scheurer and Bigalke, 2018; Zhou et al., 2018). 235 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 244 The possible sources of plastics in soils were recently reviewed by several studies (Nizzetto 245 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. 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