A comparative study of soil microplastic pollution sources: a review

ABSTRACT

Recently, the use of plastics has become more widespread due to the vital role they play in daily life and human health. Plastic decomposes into microplastic (MP) in the soil. Owing to the negative consequences of soil MPs’ growth on both soil and human health, several studies conducted over the past 10 years have concentrated on identifying and tracking the origins of MPs in soil. These studies have demonstrated that soil MPs originate from a range of sources, including plastic films, fertilizers, runoff, sewage sludge, irrigation, and the atmosphere. Despite the efforts made to identify specific sources of soil MPs, a thorough analysis and comparison of the primary sources is still absent. By assembling the results of earlier research that demonstrate how irrigation, the atmosphere, and sewage sludge all contribute to the richness of soil MP, this review will aid in filling this knowledge gap. The MP abundance from each of these sources was compared using an analysis of variance (ANOVA). The results of this investigation showed that plastic films are the largest and most significant contributors to soil microplastic contamination, owing to human overdependence on plastic films . Air deposition is the least common source of MPs . The amount of MP did not differ noticeably across irrigation, runoff, sewage sludge, and plastic film. This is because no nterference occurs when MPs from these sources interact with the soil. The findings of this study will be useful for choosing effective management and control techniques for sources of soil MP.

KEYWORDS:

• Atmospheric deposit
• plastic film
• irrigation
• runoff
• soil
• sewage sludge

1. Introduction

For a very long time, the human population has widely relied on plastic [1,2]. Plastics play a critical role in sustaining human survival and activity [3]. For instance, plastics are used to manufacture home items and packaging [4,5]. Furthermore, they have been extensively used in crop cultivation as mulch to create climatic conditions that benefit plant growth by reducing excessive soil evaporation, shortening the growing season, and protecting crops from environmental hazards [6,7]. In the crop cultivation industry, plastics have been used in the construction of greenhouses, which are useful for cultivating crops in harsh or pest-infested environments, especially those that experience moisture changes due to land-use changes [8,9].

Apart from the agricultural sector, plastics are also used in the production of clothes, dining utensils, parks, and even car tires, which are essential for human lives [10,11]. Although plastics are widely used today, they are often produced only for single use and have limited recycling capabilities [12,13]. The accumulation of plastic waste has increased dramatically in recent decades, resulting in plastic pollution [1,14,15]. Plastic waste discharged into the terrestrial environment after a long period can fragment into microscopic sizes, known as microplastic (MP), which end up in a variety of environmental compartments, including soil and groundwater [16,17]. For example, Zhou et al. [18] showed that the use of plastic for mulching contributes to approximately 86 particles/kg of MP in soils. Crossman et al. [19] showed that plastics from industrial waste in Canada accounted for approximately 104 particles/kg of soil.

Since 2017, there has been an increase in articles published on soil MP contamination [15] (Figure 1). These studies revealed an increasing trend in soil MP contamination, which is a significant global concern for researchers [20–24]. MP pollution of the soil environment is a major concern because the presence of this pollutant in the soil can adversely affect soil and/or human health [25–27]. To regulate, relieve, and/or restrict soil MP contamination, soil MP sources must be acknowledged and discussed, as shown in Figure 2.

Figure 1. Temporal distribution of soil microplastic contamination articles published from 2014 to 2023 based on the Web of Science database.

Figure 2. Sources of microplastics in direct (ex. runoff, sewage, plastic film, and irrigation) and indirect (ex. atmospheric deposit) contact with soil.

In addition, knowledge of the classification of various soil MP source entry points is important for decision-makers, land managers, and stakeholders to develop sustainable management practices, guidelines, and rules to reduce soil MP pollution. However, among the multiple studies on soil MP documented thus far, there is still a limitation regarding which soil MP source or entry point is significantly the highest or lowest. For example, according to Yang et al. [21], there is insufficient information on the extent to which plastic mulching contributes to soil MP sources. To be able to recommend methods to mitigate the deleterious impacts of soil MP contamination on soil and human health, it is necessary to evaluate, and compare soil MP contamination sources and their related mechanisms identified thus far. Therefore, this study aimed to (a) The results of this study contribute to the management and control of MP contamination.

2. Material and methods

2.1. Data sampling and selection

To examine and contrast the origin of MP in soils, original research papers from PubMed, Scopus, Google Scholar, and Web of Science were chosen and summarized (Figure 3) [28]. A cross-sampling approach was used to select recent original research papers that showed the contributions of different sources of soil MP occurrence sources [29]. Not all the papers suggested by the search engines were considered for discussion in this study [25]. Original research papers without a clear explanation of the source of MP were excluded. These protocols ensured that the aim of the study was met with limited bias by ensuring fair representation and accessibility of the selected papers [25].

Figure 3. The systematic technique employed in this study to choose scientific publications from the PubMed, Scopus, Google Scholar, and Web of Science.

Papers from 2017 to 2023 were selected using key phrases such as ‘plastic film and soil microplastic pollution’, “fertilizer and soil microplastic pollution’,’ ‘irrigation and soil microplastic pollution’, ‘runoff and soil microplastic pollution’, ‘atmospheric deposition and soil microplastic pollution’, and ‘sewage sludge application and soil microplastic pollution’. A total of 284 articles were identified. [111 articles (search term: plastic film and soil microplastic pollution’{excluded correction papers, meeting papers, letters, proceeding papers, and early access papers}) + 39 articles (search term: fertilizer and soil microplastic pollution) + 25 articles (search term: irrigation and soil microplastic pollution) + 28 articles (search term: runoff and soil microplastic pollution) + 24 articles (search term: atmospheric deposition and soil microplastic pollution) + 33 articles (search term: sewage sludge application and soil microplastic pollution) + 48 articles (search term: soil microplastic and greenhouse)].

To ensure the accuracy of the 284 papers gathered, all studies underwent a thorough screening process to exclude everything except original research articles with a degree of inaccuracy. Furthermore, studies that failed to identify the source of soil contamination with microplastics were often excluded. Original studies that did not consider the layers of soil sampled or various forms of land use were also disregarded. 87 papers were selected as the main dataset to investigate the sources of MP contamination in soils after filtering 284 publications (Figure 3).

Of the 87 original research publications analyzed, 30 studies using stereomicroscopy to estimate MP concentrations and micro-infrared spectroscopy (μ-FTIR) to identify MP components were selected to compare which soil MP contamination source was the least and the most. This was done to eliminate the possibility of the methods employed for MP detection having a significant impact on the MP concentration. So, six studies providing information on each source of MP contamination were considered for the statistical analysis [30 = 6 MP source (film) + 6 MP source (sewage) + 6 MP source (irrigation) + 6 MP source (runoff) + 6 MP source (atmospheric deposits)].

2.2. Statistical analysis

The normalcy of each distinct source of soil MP abundance was originally established using the Shapiro-Wilk test [25]. At a 5% significance level, a one-way analysis of variance (ANOVA) was employed to assess statistical variations by soil MP source. Before attempting a one-way ANOVA, the presumptions of violation of homoscedasticity for each soil layer (as determined by the Durbin-Watson statistic) and constancy of variance (as determined by the Spearman rank correlation) were verified [30,31]. The abundance data for each source of soil MPs indicate that the assumption of normality failed. Consequently, the Kruskal-Wallis non-parametric analytic test was developed [32]. The investigation was performed using SigmaPlot (version 12.0; Systat Software Inc., San Jose, U.S.A.).

3. Results and discussion

Currently, plastic films are being increasingly used for effective and sustainable agricultural farming to address the issue of sparse arable land caused by the burgeoning global population [23,33] (Table 1). Several studies have shown that the use of plastic films contributes to MP contamination of soils. In some cases, sewage sludge is used as an organic fertilizer to increase agricultural production and contribute to soil MP contamination [43,44] (Table 2). At the same time irrigation, which is used in some parts of the world to improve crop cultivation, contributes to the production of soil MPs [54,55] (Table 3). Agriculture in other parts of the world relies on rainfall to produce crops [61–63]. As a result of these rainfall events, crops sometimes undergo runoff or erosion when rain or ice melts [63,64]. These measures have been used by farms in various parts of the world to improve crop yields, and some studies have shown that runoff, as a byproduct of rain-off, can contribute to soil MP contamination (Table 4). In addition, some studies have shown that high human dependence on plastic use may lead to by-products, such as atmospheric deposition, which is a source of soil MP pollution (Table 5). By comparing these sources of MPs, we found that the use of plastic films to increase crop yields was a significantly higher source of MP pollution in the soil than in atmospheric sediments (p = 0.040) (Figure 4). This result is similar to previous studies where plastic cover was a significantly higher source of MP than atmospheric sources [41,76].

Figure 4. Box plot of soil microplastic (MP) abundance from different sources, such as: plastic films, sewage, irrigation, runoff, and atmospheric sediments. Identical capital letters (AB, AB) indicate no statistical difference in MP abundance between microplastic sources.

Table 1. A summary of plastic film and fertilizer contribution to soil microplastic pollution. NM means not mentioned in the manuscript. μ-FTIR Fourier-transform infrared spectroscopy and means micro-infrared spectroscopy and FTIR.

Land use	Source of MP	Location	Microplastic detection method	Quantity (particles/kg)	References
Cropland	Plastic film	China	Stereomicroscope, μ-FTIR	571	[34]
Cropland	Plastic film	Chile	Stereomicroscope, μ-FTIR	306	[35]
Orchard	Plastic film	Korea, Yeoju	Stereomicroscope, FTIR	664	[36]
Greenhouse	Plastic ropes	Korea	FTIR microscope	221.4	[6]
Cropland	Plastic film	Korea	FTIR microscope	356.8	[6]
Paddy field	Plastic film,	China	Stereomicroscope, Raman spectroscopy	15,461.52	[37]
Cotton land	Plastic film,	China	Stereomicroscop, μ-FTIR	80.3	[38]
Copland	Plastic film,	China	μ-FTIR	10,586	[39]
Cropland	Plastic film	China	Stereomicroscope, μ-FTIR	525	[40]
Green house	fertilizer	China	NM	4,496	[41]
Greenhouse	fertilizer	China	FTIR	1,246.39	[42]

Table 2. Sewage sludge application contribution to soil microplastic contamination.NM means not mentioned in the manuscript. μFTIR and FTIR – ATR means micro-infrared spectroscopy and attenuated total reflection Fourier transform infrared.

Source of MP	Land use	Region	Microplastic detection method	Quantity (particles/kg)	References
Sewage sludge	Agriculture field	Spain	μ-FTIR	5,190	[45]
Sewage sludge	NM	Ontario, Canada	Stereomicroscope, μ-FTIR	1.4 × 10^4	[19]
Sewage sludge	farmland	China	Stereomicroscope, μ-FTIR	149.2	[21]
Sewage sludge	farmland	China	Stereomicroscope, μ-FTIR	68.6	[21]
Sewage sludge	farmland	China	Stereomicroscope, μ-FTIR	73.1	[21]
Sewage sludge	farmland	Germany	Raman spectroscopy	97.66	[46]
Sewage sludge	farmland	China	NM	8,452	[47]
Sewage sludge	Farmland	Bangladesh	Stereomicroscope, μ-FTIR	2.92 × 10^4	[48]
Sewage sludge	Farmland	Mauritius	FTIR – ATR	420.0	[49]
Sewage sludge	Farmland	Germany	μ-FTIR spectrometer	31.80	[50]
Sewage sludge	Farmland	China	NM	87.6	[51]
Sewage sludge	Farmland	China	NM	545.9	[51]
Sewage sludge	paddy farm	China	Stereomicroscope, μ-FTIR	2,522	[52]
Sludge	Farmland	China	Stereomicroscope, μ-FTIR	2,733	[53]

Table 3. A summary of soil microplastic abundance from irrigation. μFTIR means micro-infrared spectroscopy.

Source	Land use type	Region	Microplastic detection method	Quantity (particles/kg)	References
Irrigation	Paddy fields	Yongin, Korea	Stereomicroscope, μ-FTIR	160	[56]
Irrigation	Paddy fields	Japan	NM	48	[57]
Irrigation	Paddy fields	Iran	Micro-Raman microscope	49	[58]
irrigation	Cropland	Yongin, Korea	Stereomicroscope, μ-FTIR	81	[56]
irrigation	Paddy rice farm	China	Stereomicroscope, μ-FTIR	481.62	[59]
Irrigation	Paddy seedling farm	China	FTIR	3,805	[42]
Irrigation	Paddy fields	China	Stereomicroscope, μ-FTIR	6,390	[60]

Table 4. Runoff and erosion contribution to soil microplastic contamination. μFTIR means micro-infrared spectroscopy.

Source	Land use	Region	Microplastic detection method	Quantity (particles/kg)	References
Runoff	Mangrove forest	Ghana	Fluorescent microscope	467	[65]
Runoff	Experimental farm	Spain	Stereomicroscope, μ-FTIR	226–412	[66]
Runoff	Urban area	Iran	Raman spectroscopy	619	[67]
Runoff	Open cropland	China	Stereomicroscope, μ-FTIR	1,444	[68]
Runoff	Cropland	China	Stereomicroscope, μ-FTIR	78.181	[52]
Runoff	Cropland	China	Stereomicroscope, μ-FTIR	10.3	[69]

Table 5. Atmospheric deposition contribution to soil microplastic contamination. NM means not mentioned in the manuscript. μFTIR means micro-infrared spectroscopy.

Source	Land use	Region	Microplastic detection method	Quantity (particles/m^2)	References
Atmospheric disposal	Natural forest	Oaxaca, Mexico	Stereoscopic microscope	1.49	[70]
Atmospheric disposal	Beech/oak forest	Germany	Fluorescence microscope	70.9	[71]
Atmospheric disposal	Desert	China	NM	6.0	[41]
Atmospheric disposal	Urban Area	Canada	Stereomicroscope, Raman spectroscopy.	57.0	[72]
Atmospheric disposal	Recreational trail surfaces	Australia	Nikon SMZ745T microscope	162.5	[73]
Atmospheric disposal	Forest	China	Stereomicroscope, μ-FTIR	67.06	[74]
Atmospheric disposal	Primary Forest	China	Stereomicroscope, μ-FTIR	637.5	[75]

This can be attributed to the following reasons: First, a large load of MPs released from plastic films directly contacts the soil following degradation during field application. This is because there are no filtering, dilution, interception, and/or screening processes before application [77]. However, not all MPs from atmospheric deposition come into direct contact with soil because of dilution and interception by trees and houses. This is evidenced by the fact that most studies to date on soil MP pollution from atmospheric sediments have been conducted in forestland areas (Table 5). These studies have shown that MPs from atmospheric deposits can be captured and/or retained by trees and plants, which function as filters before reaching the soil at the study site [78–80]. Second, the plastic materials used for the production of plastic films are often strong and resistant to deterioration [81]. This implies that they accumulate in the soil environment following the deposition of fragmented plastics that have been used for several growing seasons. However, MPs from atmospheric deposits can be composed of materials that do not have cumulative accumulation properties and can easily be blown away from the soil once deposited. For example, according to [82], breaking plastic films and cumulative properties are major drivers of high MP occurrence in agricultural soil. Third, plastic films, even after being released into soils after breakage over time, can be further broken down into more MPs and redistributed to a wider range by crop cultivation activities, such as plowing, tilling, and other agricultural practices [83,84]. Atmospheric deposits of MPs are mostly redistributed only by natural factors such as wind, or, on very rare occasions, by human activities. For example, according to Weber et al. [50], anthropogenic plowing is mainly responsible for the presence and redistribution of MPs rather than natural transport processes, such as wind.

In documented studies on soil MP abundance to date, plastic films, runoff, irrigation, and sewage have generally been reported as independent entry points for MPs into the soil. However, no significant differences in MP were observed between the sources (p < 0.05) (Figure 4). This result is consistent with the study by Wei et al. [85], which showed that sewage and runoff contributed equally to soil MP abundance. In addition, the results of this study are similar to those of Lang et al. [86], who showed that plastic films from mulching, sewage, and irrigation equally contribute to soil MP contamination. This could be due to the following reasons. First, MPs released from these sources directly interact with soils once released without interruption or interception and can spread across a wide border. For example, irrigation, runoff, sewage, and plastic film water can spread MPs more equally across concentrated locations, causing more localized and intensive MP contamination in the soil [84]. Second, the influence of human activity on MPs from these sources renders it almost impossible to detect any significant differences in the contribution of MPs to soils from these sources. For example, the application of sewage sludge as a fertilizer to the soil environment leads to MP contamination by MP synthetic fibers, which are mobile and persistent [51]. In addition, human irrigation of crop fields contributes to secondary MP pollution in soils [84].

According to the findings of this study, MPs from plastic films, runoff, irrigation, and sewage are extensively distributed and account for the bulk of soil MP pollution. Although air deposition is a small source of soil MP contamination, it should be considered in soil MP research. This highlights the importance of reducing the long-term use of plastic sheets in agricultural production. Owing to the dearth of research on non-forested or vegetated regions, determining the contribution of air deposition to soil MP pollution and developing relevant mitigation measures and legislative acts is challenging. Furthermore, more studies are needed to determine which soil MP sources, such as plastic films, runoff, irrigation, and sewage, contribute the most or least to soil MP contamination. This will be critical for successfully resolving the issue of soil MP contamination.

4. Conclusion

This study assessed and contrasted soil MP sources because the present strains of MP pollution are expected to remain in the future owing to increases in general plastic consumption and inadequate plastic waste management capabilities. Previous studies have indicated that the most prevalent sources of soil MP contamination are fertilizers, sewage sludge, runoff, irrigation, and air deposition. When these sources were evaluated, the results showed that MPs from plastic films were much greater than those from atmospheric deposits, whereas MPs from other sources, such as runoff, sewage, and irrigation, did not differ significantly. Plastic film, sewage sludge, runoff, and irrigation are by far the most significant sources of soil MP because of the direct interaction of plastic film with soil after the release of MP into the soil. In contrast, MPs deposited in the atmosphere may come into contact with plants before reaching the soil. To make effective soil MP contamination management choices, such as economizing the use of soil plastic films and sustainable irrigation, and controlling soil MP contamination at the source, it is necessary to first know which soil MP source(s) contaminate the soil.

Disclosure statement

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Additional information

Funding

This research was financially supported by Korea Environmental Industry Technology Institute (KEITI) through Measurement and Risk assessment Program for Management of Microplastics Program, funded by Korea Ministry of Environment (MOE) (2020003110010). Also this research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2019R1A6A1A03033167).