Terrestrial Microplastic Pollution: Occurrence, Fate, and Ecological Effects on Soil Systems

Abstract

Terrestrial environments function as major sinks and dynamic sources of microplastics. Land use strongly influences inputs, accumulation, and transport pathways of these contaminants in the environment. Despite the extensive literature, few reviews have compared contamination levels and the potential impacting factors across land uses. To fill this gap, this review synthesizes current knowledge on the origins, occurrence, pathways, and ecological effects of microplastics across diverse land uses. The review revealed multiple interconnected pathways that drive microplastic contamination in terrestrial systems. Abundances are consistently higher in intensively managed croplands, urban areas and industrial vicinities. However, their detection in remote environments underscores the critical role of diffuse inputs and long-range atmospheric transport. Vertically, microplastics are enriched in topsoils, and their concentrations declines with depth. Horizontally, concentration declines with increasing distance from major hotspots like agricultural fields, industrial facilities, and road networks. Ecologically, microplastics alter soil physical properties, modify chemical conditions, and shift microbial community composition and enzyme activities. Furthermore, they stress soil fauna and plants through ingestion, toxicity, and physical blockage, with impacts contingent on polymer type, particle morphology, and concentration. Collectively, this review reveals consistent spatial patterns and widespread adverse ecological impacts, highlighting the clear need for integrated management strategies to mitigate terrestrial microplastic pollution.

1. Introduction

1.1. The Plastic Age and the Rising Issue of Microplastics

Plastic production and application have expanded dramatically since the 1950s. Plastics have become a necessity in daily life and an essential material for industries, leading to the “Plastic Age” [1,2,3]. This growth is attributed to their advantages, such as low production costs, high molding flexibility, and extended durability [1,4]. Global plastic production exceeded 414 million metric tons in 2023 [5], and is projected to reach 1231 Mt by 2060 [6]. Despite these advantages, a major challenge arises at the end of the plastic lifecycle; the same durability that underpins the utility of plastics complicates their disposal, since resistance to degradation leads to long-term accumulation in ecosystems. Some plastics can persist for hundreds of years and become prevalent pollutants under normal environmental conditions [7,8].

Effective plastic recycling and reuse are critical to reducing environmental consequences. However, only 15% of plastic waste worldwide is collected for recycling. In comparison, 46% is landfilled, 17% is incinerated, and 22% is mismanaged and vulnerable to environmental leakage, making soils the primary sink for this degrading debris [6]. Once plastics enter the environment, they undergo physical, chemical, and biological degradation, becoming brittle and fragmenting into smaller particles [9,10]. In addition to environmental weathering, everyday human activities, such as laundry, agricultural practices, and tire wear on roads, further contribute to the generation of microplastics [11,12,13,14,15].

The occurrence of microplastics was first reported in the surface waters of the Sargasso Sea in 1974 by Carpenter and Smith [16], who studied polystyrene (PS) spherules with diameters < 5 mm. However, the term microplastics was introduced only in 2004 by Thompson, et al. [17]. Since then, attention to microplastics has accelerated, and they are now recognized as emerging contaminants detected in diverse ecosystems worldwide [18,19].

1.2. Microplastics in the Terrestrial Environment and Knowledge Gaps

Despite being recognized as a critical component of the global plastic cycle, terrestrial environments have received less attention than aquatic ecosystems and the ocean [4,20]. Estimates indicate that terrestrial environments receive up to 23 times more plastic than that directly released into the oceans [19,21]. In addition, most plastics entering the oceans originate from terrestrial environments [22,23,24,25]. This land–sea connection is particularly pronounced in densely populated coastal regions, where high plastic waste loads and proximity to the ocean facilitate efficient transport to the oceans, primarily via rivers and surface runoff [26]. Consequently, terrestrial systems represent an essential hub in the global plastic cycle. Understanding the occurrence, fate, and impacts of microplastics in the terrestrial environment is essential for closing major knowledge gaps in global plastic budgets and environmental risk assessments.

Beyond their ubiquity, microplastics in the terrestrial environment pose multiple ecological and health concerns. They can act as vectors for chemical contaminants [27], provide substrates for microbial colonization and biofilm formation [28], and alter soil physical structure, nutrient cycling, and biological functioning [29,30]. Furthermore, microplastics can enter food webs, raising concerns about long-term risks to human health [31]. Despite growing awareness, the fundamental processes governing microplastic inputs, transport, retention, and accumulation in terrestrial ecosystems remain poorly understood [19,32].

1.3. Land Use Impacts Microplastic Inputs, Transport, and Retention

Microplastics enter terrestrial environments through various sources, many of which are closely linked to land use practices. The main inputs originate from agricultural activities, as well as domestic and industrial waste generated in urban and residential areas. Agricultural systems receive substantial microplastic inputs through plastic mulching, greenhouse films, irrigation with treated wastewater, and the land application of biosolids, compost, and manure [33,34,35,36]. In urban and industrial environments, dominant sources include atmospheric deposition, wastewater and stormwater discharge, construction activities, textile fibers from laundering, and tire and road-wear particles [36,37,38,39,40].

Once introduced into the terrestrial environment, microplastics can be transported vertically and laterally within soils by infiltration, bioturbation, agricultural tillage, erosion, and surface runoff. Their small size and light weight also facilitate transport through wind and water, and they can reach natural landscapes such as forests, wetlands, and reserved areas, as well as the most remote areas, such as the Himalayan mountains [41] and polar marine environments [42]. Soil properties, such as texture, organic matter content, and aggregation, and landscape features, such as slope, vegetation cover, and hydrology, interact with land use to regulate microplastic retention, redistribution, and accumulation patterns. Land use, therefore, impacts microplastic dynamics by regulating potential sources, transport pathways, and environmental retention.

1.4. The Need for a Comparative Synthesis of the Land Use Impacts

Despite rapid expansion in research on microplastics, wide gaps remain in their definition, analytical methodologies, source attribution, transport processes, land use impacts of, and long-term ecological and health impacts; for example, the size range used to define microplastics has been debated and comprehensively reviewed. The proposed size ranges have varied considerably, from 1 to 20 μm at the lower end to 1 to 5 mm at the upper end [43]. The current broad consensus defines microplastics as plastic particles with a longest dimension of <5 mm [44,45,46,47]. While this upper limit is widely accepted, the lower limit of size remains under discussion, with the term “nanoplastics” increasingly applied to particles below the micrometer scale [48]. Classification of microplastics also varies based on physical morphology into fibers, beads (pellets), fragments, films, and foams [49,50]; sources into primary and secondary types [50,51,52]; and polymer types into polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), polyurethane (PUR), polypropylene (PP), and polyvinyl chloride (PVC) [4].

With the increasing documentation of microplastic occurrence in different terrestrial settings [53,54], recent reviews have also summarized the occurrence, sources, fate, and analytical techniques of microplastics in soil and the broader terrestrial environment [55,56]. However, limited reviews have compared contamination levels between land uses or systematically examined how landscape factors, such as mulching, biosolid application, irrigation, or proximity to roads, influence the distribution of microplastics. Therefore, a comprehensive assessment of terrestrial microplastics is critically needed that examines their sources, transport pathways, and potential risks to these ecosystems in the context of land use practices. This review aims to narrow the broad knowledge gap by focusing on microplastics in terrestrial environments, which are now recognized as a critical yet understudied component of the global plastic cycle.

1.5. Objectives, Scope, and Methodology of This Review

In this review, we analyze studies reporting microplastic abundance, polymer composition, morphology, and associated factors across major terrestrial land uses, including agricultural, urban, industrial, roadside, landfill, and natural systems. By providing an updated comparative synthesis, we aim to elucidate spatial patterns of contamination, identify key drivers of land use and management, and highlight research gaps relevant to mitigating microplastic pollution in terrestrial ecosystems. To construct this synthesis, we employed a systematic literature search and structured screening process. Specifically, a search of the Web of Science database was conducted using Boolean combinations that paired the term “microplastics” with a range of terrestrial keywords (soil, agricultural, urban, industrial, roadside, landfill, biosolids/sludge, compost, irrigation, wastewater/stormwater, and atmosphere). Peer-reviewed studies were included if they (i) examined at least one land use category in the terrestrial environments, (ii) provided quantitative microplastic abundances, and (iii) reported at least one of the characteristics, including polymer composition, morphology, and size distribution. Studies focusing on marine environments and reviews were excluded. After removing duplicates, titles and abstracts were screened, followed by full-text assessment of potentially eligible articles. For each included study, information was extracted on land use, microplastic abundance, polymer composition, morphology, and relevant covariates; e.g., mulching, soil amendments, irrigation source, roadside distance, and season. The main literature search was completed in October 2025. The complete search string, specific restrictions applied, and a detailed breakdown of title and abstract exclusions are provided in Supplementary Information A. The overall study selection process, including identification, de-duplication, screening, and eligibility assessment, is summarized in the PRISMA flow diagram (Figure 1). This structured screening and data-extraction process enabled a systematic compilation of existing evidence, comparative mapping of microplastic contamination across terrestrial land uses, and identification of key knowledge gaps concerning the behavior and impacts of microplastics in terrestrial environments.

Figure 1. PRISMA 2020 flow diagram for the systematic literature search, screening, and selection process. The diagram details the number of records identified via the Web of Science database, the records excluded at each screening and eligibility phase, and the final number of studies included (n = 99) in the quantitative comparative synthesis.

The final screening resulted in 99 studies being ultimately included in the quantitative review. While 103 articles reported quantitative abundance data suitable for inclusion in comparative analysis, 4 of these articles reported concentrations exclusively in mass units (mg kg⁻¹) rather than particle counts (Table S2). They were therefore excluded from the quantitative synthesis and comparative figures (Section 3) to ensure consistency, though their significant findings remain incorporated qualitatively within the narrative review (Section 2). While necessary for comparability, excluding mass-based studies introduces a slight bias; our quantitative synthesis is likely to underestimate contamination in urban and industrial zones, where mass reporting is standard for high-emission sources.

Within the narrative text of this review, abundance values are reported using the original units provided by the respective authors to maintain fidelity to the source literature. However, for all quantitative syntheses and comparative visualizations, all abundance values were harmonized to a common scale of particles kg⁻¹ of dry soil (particles kg⁻¹ hereafter), converting from different reported units (e.g., particles/g, particles/5 g, or particles/100 g) or descriptors (e.g., items, MPs, n) prior to analysis. Each sampling site was categorized into five primary land use groups (Agricultural, Industrial, Landfill, Natural, and Urban). Where original authors provided explicit land use labels, these were directly adopted; otherwise, sites were mapped based on the specific settings identified (e.g., residential areas, petrochemical vicinities, or mulch-treated fields). Priority was given to the land use descriptor most directly linked to contaminant sources emphasized in the source publication, with all original author-defined settings preserved to ensure transparency.

Data extraction was performed using a standardized framework to ensure the consistency of the compiled dataset. Information was extracted on land use, microplastic abundance, polymer types, and particle sizes. Additionally, relevant covariates, such as mulching practices, soil amendments, irrigation sources, roadside distance, and sampling season, were recorded where available in the source literature. For studies reporting data at multiple locations or depths, we utilized the primary representative values provided by the original authors to maintain fidelity to the source data. A summary of the compiled data is presented in Table 1, and the complete dataset is available in Table S1.

Table 1. Summary of soil microplastic contamination parameters across five major land use types. Abundance values are reported as medians and ranges in particles kg⁻¹ dry soil. The table identifies dominant polymer types, predominant particle morphologies, and primary anthropogenic input sources for each category. PE: Polyethylene; PP: Polypropylene; PA: Polyamide; PET: Polyethylene terephthalate; PES: Polyester; PS: Polystyrene; PVC: Polyvinyl chloride; ABS: Acrylonitrile butadiene styrene; n: number of study sites.

Land Use	Number of Sites (n) *	Median (Particles kg−1)	Range (Min–Max)	Dominant Polymers	Dominant Shapes	Major Input Sources
Agricultural	79	1137	0.34–412,000	PE, PP (secondary: PA, PET, PES, PS)	Fragments, Films, Fibers	Mulching films, greenhouse plastics, biosolids, compost
Urban	52	1612	0.9–26,850,000	PE, PP (secondary: PS)	Fragments, Fibers (Secondary: Films, Pellets, Other)	Textiles, household waste, stormwater, tire wear
Industrial	12	2823	103–127,000	PE, PP (secondary: PET, PVC, ABS)	Fibers, Pellets, Fragments (Secondary: Films)	Industrial emissions, process losses
Landfill	30	5438	85.6–150,000	PE, PP (secondary: PET, PS)	Fragments, Films, Fibers (Secondary: Foam)	Waste degradation, leachate, wind dispersal
Natural	22	2122	160–99,000	PE, PP (secondary: PET, PS)	Fragments, Pellets, Films (Secondary: Fibers)	Atmospheric deposition, long-range transport, surface runoff

* n represents the number of independent, spatially distinct site concentrations extracted from the reviewed literature.

2. Occurrence, Abundance, and Distribution in Terrestrial Environments

Microplastics are increasingly recognized as persistent and ubiquitous in terrestrial environments, contributing to long-term soil pollution and ecosystem disruption [57,58]. However, their concentrations and impacts vary significantly depending on localized land use activities and environmental factors [59]. As illustrated in the conceptual model in Figure 2, terrestrial systems act as both sinks and sources of microplastics, driven by distinct input routes, such as agricultural amendments and atmospheric deposition, and redistribution pathways, such as surface runoff and wind dispersal. While particles are trapped and accumulate in soil, they also migrate into aquatic systems via leaching, erosion, and surface runoff [60,61]. Recent estimates suggest that approximately 79 million metric tons of land-generated plastic waste are either mismanaged or released into aquatic systems [62].

Figure 2. Conceptual framework of the major sources and pathways of microplastics entering terrestrial environments across land uses. The diagram illustrates primary input routes (e.g., mulch films, biosolids, tire wear, atmospheric deposition) and major transfer processes (e.g., surface runoff, vertical leaching, wind dispersal) within and between agricultural, urban, industrial, roadside, waste management, and remote/natural systems.

To better conceptualize the flux of microplastics in terrestrial environments and guide the interpretation of major drivers across land uses, transport pathways can be categorized into primary inputs and redistribution/output processes. However, the evidence base for these pathways is notably uneven. Generally, primary input pathways are frequently supported by directly quantified field measurements of soil abundance. In contrast, redistribution and output processes are often semi-quantitative or indirectly inferred, relying on mathematical modeling, laboratory simulations, or mass-balance estimates rather than directly quantified field fluxes. In this review, most of the current literature relies on static abundance sampling (particles/weight) rather than dynamic mass-flux monitoring; therefore, evaluating all these pathways on a strictly quantitative basis remains highly challenging. Accordingly, a key priority for future research is the development of a more explicit mass-balance framework to better quantify terrestrial microplastic inputs, internal transfers, and losses.

2.1. Primary Agricultural Inputs

Global use of agricultural plastics exceeded 6 million metric tons in 2019 and is projected to increase by 50%, reaching 9 million metric tons by 2030 [63]. Under continuous solar radiation and mechanical stress, these plastics break down into micro-sized particles, making agriculture a major source of microplastics pollution in terrestrial environments [64,65]. The distribution and prevalence of these microplastics in agricultural lands vary with seasonal conditions, agricultural practices, and cropping systems [13,14,66].

Plastic mulching is widely used for moisture conservation, temperature regulation, and weed control [67]. It is also considered a major source of microplastic pollution in agricultural soils [35,68]. Mulch films are often left in fields due to the difficulty and cost of removal and are later burned or incorporated into the soil through tillage [69,70,71]. Consequently, microplastics accumulate in agricultural soils at elevated concentrations, often reaching thousands of particles per kilogram [36,68,72,73]; for example, higher microplastics concentrations in mulched farmland (5386 particles kg⁻¹) and greenhouses (5124 particles kg⁻¹) compared to other farming systems (3386 to 3910 particles kg⁻¹) within the same region were observed in China [74].

Cropping systems also influence microplastics abundance through crop type, intensity, and associated management practices [75]; for example, a study in Chile found that 57% of samples collected from croplands contained microplastics, compared with 3% from unmanaged rangelands, highlighting the role of agricultural cropping in microplastics accumulation [32]. Studies showed crop-dependent differences in microplastics accumulation; for example, in arid northwestern China, soils cultivated with tall crops such as sunflower and maize contained higher microplastic abundances (72,173 and 57,351 particles kg⁻¹, respectively) than soils under short crops such as potato (26,721 particles kg⁻¹) [75]. Similar crop-related variability has been observed in Bangladesh, where diversified systems including paddy, potato, and vegetables exhibited variations in microplastic concentrations ranging from 3.33 to 46.67 particles kg⁻¹ soil [76]. Moreover, these practices influence not only quantity but also polymer diversity. For instance, conventionally managed vineyard soils exhibited a greater variety of polymers than organically managed ones, even when overall abundance was similar [77].

Soil amendments, such as biosolids and compost, represent another major pathway for microplastics accumulation. Long-term studies consistently show higher levels of microplastics in amended soils than in untreated fields [34,78,79,80,81,82]. Different amendments sources, such as compost from municipal waste or poultry litter, introduce diverse polymer types and morphologies, often contributing thousands of particles per kilogram [72,80,83,84,85].

Irrigation introduces additional microplastics via contaminated water and degradation of irrigation systems [86,87]. While concentrations in irrigation water may be low, cumulative irrigation can result in substantial accumulation of microplastics over time [88,89,90]. Soils irrigated with different water sources, including canals, recycled wastewater, and mixed irrigation systems, typically show higher microplastic concentrations than rain-fed farms or uncultivated soils [88,89,91]. A study in Mediterranean orchards reported that irrigation accounted for around 56% of annual microplastic inputs [86].

Beyond these traditional sources, polymer-coated seeds and slow-release fertilizers, are increasingly recognized as direct sources of microplastics in agricultural soil [92,93,94,95]. Plastics intentionally incorporated into seed coatings represent a frequently overlooked source of microplastics in soil [96]. Furthermore, many slow-release fertilizers rely on mineral granules encapsulated in polymer shells made of PE, ACR, and PU [97]. These polymers are recalcitrant; therefore, their widespread application drives a continuous and long-term input of persistent synthetic particles into agricultural soils [97].

Polymer composition in agricultural soils typically reflects farming practices. For instance, PE and PP dominate most studies due to mulching and greenhouse farming, often accounting for more than 80% of particles, with fragments and films as the main shapes [13,33,36,76,79,98,99,100,101]. Other polymers regularly detected in agricultural soils include PS, PET, PVA, PVC, PU, and PA, originating from irrigation systems, seed coating, plastic trays, and other plastic equipment and materials [33,74,102,103,104].

Microplastic morphology also varies with agricultural practices. Fragments dominate most systems, followed by films and fibers [98]. Film particles are common where mulch is widely used [76]. Continuous mulching correlates with higher abundance and smaller particle sizes, confirming in situ fragmentation [105,106]. Fiber abundance increases in irrigated systems; for example, on Tanzanian farms, fibers accounted for 40–70% of particles [91].

While the use of agricultural plastics and organic amendments has been shown to increase microplastic loads in soils, existing studies rarely allow quantification of accumulation rates or differentiation between legacy contamination and ongoing inputs. In addition, soil redistribution processes associated with tillage, erosion, and field management can redistribute microplastics within and beyond agricultural fields, complicating source attribution and mass-balance assessments.

Although agricultural soils have been investigated across a wide range of spatial scales, most studies rely on single-time-point measurements, and very few were conducted long enough to capture seasonal variations, which may be a significant factor considering that plastic or plastic-rich amendments are applied during specific seasons. The lack of long-term studies limits understanding of how microplastic abundance responds to seasonal management practices, crop rotations, and repeated inputs. Given the temporal variability in agricultural activities, it is essential to integrate it into assessments of accumulation patterns and the environmental impacts of microplastics in agricultural soils.

2.2. Domestic Sources and Urban Environments

Domestic sources and urban environments are major contributors to terrestrial microplastic pollution, functioning as diffusive hubs for microplastics derived from packaging, textiles, personal care products, and household goods [107,108]. Consequently, soil in these areas is often dominated by fibers and fragments that reflect these sources [109].

Personal care and cosmetic products (PCCPs) remain significant domestic sources of primary microplastics [110,111]. Recent investigations of products in Ho Chi Minh City found that 66% of examined samples contained microplastics, primarily in granular form [112]. Despite the partial bans in some countries, microbeads in exfoliating and cleansing products and “leave-on” cosmetics continue to enter wastewater collection systems [113,114]. These products can contain high levels of microplastic particles; for instance, certain toothpastes have been found to contain up to 25.77% microplastics by weight, corresponding to an estimated 337,579 particles per gram [112]. Furthermore, specific exfoliating products can contain microplastics at levels of 3.36 ± 0.20 g per product [115]. The European Union estimated that 42,000 metric tons of intentionally added microplastics, largely originating from personal care products, are released into the environment annually [116].

Synthetic textiles are another major source of microplastics. Microfibers, primarily polyester (PET/PES) and polyamide (PA), are released during daily use and laundering [11,12,117,118]. While wastewater treatment plants capture a significant portion of these fibers, where removal is incomplete, treatment plants may collectively discharge millions of particles per day [119,120]. On the other hand, retained particles accumulate in sewage sludge and are subsequently transferred to agricultural soils through sludge application [11,12,117,119,121]. However, precise quantification of domestic textile emissions remains challenging due to methodological inconsistencies and incomplete emission accounting [12,122].

Atmospheric deposition and indoor dust provide additional pathways from domestic sources. Indoor environments generate microplastic fallout containing diverse polymer types influenced by flooring materials, building components, and household activities such as dryers [11,108,117,123]; for example, it was found that indoor settleable microplastics in residential environments can reach thousands per square meter per day, with PET fibers dominating [107]. Outdoor sources, such as TWPs, contribute to airborne deposition near residential areas, with elevated emissions promoted by high-temperature urban surfaces [124].

These overlapping inputs create spatially heterogeneous contamination in urban sinks, including densely populated areas, urban wetlands, parks, and economic centers [38,125,126]. In cities with major petrochemical industries, the combined influence of domestic and industrial emissions significantly elevates soil microplastic concentrations and increases potential exposure [127]. Localized hotspots include playgrounds, artificial turf fields, and areas irrigated with reclaimed water [32,33,38,79]. Urban parks in Coimbra, for example, recorded some of the highest soil loads within the city domain [38].

Polymer compositions in these areas reflect the diversity of contributing sources [36,38,128,129]. Common polymers found in urban areas include PP, PE, PET, and PVC, with fragments and fibers representing the predominant morphologies [38,130,131]. Roadside soils and high-traffic zones typically contain higher levels of rubber-based polymers, such as styrene–butadiene rubber (SBR), and polyacrylates [15,40]. Meanwhile, urban green spaces, parks, and roadside verges tend to accumulate a mix of PE, PP, and PET fragments [132,133]; for example, green space soils in Nanjing, China contained notable levels of PETG and PP (>50%), with fibers and fragments accounting for approximately 80% of detected particles [130]. Similarly, roadside soils in the Beijing region were dominated by PP (about 64%), with fragments comprising around 63% and granules nearly 20% of the total microplastics [134].

Despite the well-known role of domestic and urban environments as major microplastic hubs, source distribution remains a critical knowledge gap. While qualitative links between household activities and soil contamination are well documented, quantifying the contributions of specific sources and pathways remain challenging due to overlapping inputs and methodological inconsistencies; for example, high organic matter content in samples can hinder microplastic detection and yield inconsistent results in the absence of standardized pretreatment protocols [135]. This lack of integrated source and pathway assessment constrains accurate evaluation of environmental exposure and the long-term risks of microplastics in urban environments.

2.3. Industrial Point Sources and Inputs

Industrial activities contribute substantially to microplastic pollution in terrestrial environments through multiple pathways, including industrial effluents and sludge, atmospheric fallout, and material losses during handling, processing, and production [127,136]. Microplastic abundance in industrial soil ranges widely (0–34,100 particles kg⁻¹), with peak concentrations significantly exceeding those in residential and recreational land uses. The composition, size, and morphology of these particles in industrial zones reflect the diverse material uses and waste streams of the surrounding sectors, such as plastic manufacturing, textiles, and construction [137].

Petrochemical industries release microplastics during drilling, cleaning, and maintenance operations. Microbeads made of melamine, PS, acrylic, and PE are widely used as abrasives [138,139,140]. Soils, sludges, and effluents associated with petroleum-related sites often exhibit elevated levels of complex microplastics, including ABS, PETG, PVC, and others, reflecting the use of these materials and effluent discharges [127,136]; for example, soils from the Daqing petrochemical region in China contained over 11,000 particles kg⁻¹, dominated by PP and PE fibers [127]. Similarly, petrochemical sites in Iran exhibited high polymer concentrations ranging from 1483 to 4620 particles kg⁻¹, with fibers accounting for 68.3% of the detected microplastics [136]. Inadequate containment allows these materials to leak, undergo weathering, and persist in the surrounding environment [136,141].

Textile manufacturing represents another major emitter of microplastic fibers (MPFs). During fiber production operations such as cutting, wet-processing, and fabric finishing, large quantities of fibers, typically smaller than 1 mm, are released [142,143,144,145]. These fibers readily become airborne and contribute to indoor fallout; for example, textile factories in Bangladesh have reported elevated indoor microplastic deposition, dominated by fibers of polymers such as polyester and nylon [40]. In addition, textile mill effluents may contain fibers ranging from hundreds to tens of thousands per liter, highlighting the variability in industrial wastewater control across facilities [142,145]. Consequently, both airborne and wastewater pathways must be considered when assessing the environmental impacts of the textile industry.

Construction activities represent an additional industrial source of terrestrial microplastics. Plastics such as EPS, PVC, and HDPE are widely used in insulation, piping, and flooring, accounting for 19.8% of total plastic consumption in Europe and 26% in Canada [146,147]. Improper handling and disposal of construction plastics can lead to their entry into surrounding soil, contributing to further localized pollution. In Scotland, PVC fragments have been detected in river sediments related to construction waste [148]. Although the reuse of plastic materials in construction, such as pavements and playground surfacing, supports circularity, these materials may still release microplastics through abrasion and weathering over time [149].

Plastic production and recycling facilities directly release microplastics into the surrounding environments via manufacturing processes, material losses, and atmospheric emissions; for example, soils around an insulation board manufacturing plant in Belarus exhibited high concentrations of PS particles [150]. Atmospheric deposition near a plastic production complex in Colombia showed increased concentrations of PS, PP, PE, and PVC particles in the surrounding area [151]. Dustfall within recycling facilities can contain airborne microplastics up to 100 times higher than those in nearby soils, underscoring occupational and environmental exposure risks [152].

Industrial wastewater and sludge reuse constitute additional pathways for the transfer of microplastics to the terrestrial environment; for example, in a WWTP in China, industrial effluents contained approximately twice the microplastic concentrations and higher discharge fluxes than domestic wastewater [153]. Industrial wastewaters include diverse polymer types, such as PE, PET, and PP [153,154,155]. Sludge produced by industrial WWTPs may contain very high microplastic loads, exceeding 4700 particles g⁻¹, posing substantial risks when land-applied [156]. Similarly, industrial composting processes may also fragment plastic residues, with mature compost containing notable concentrations of PE, PET, and other polyolefins [85].

Industrial zones consistently exhibit the highest microplastic abundances and polymer diversity due to concentrated manufacturing, recycling, and waste-handling activities. Some sites reported concentrations up to 5600 particles kg⁻¹ [36], while areas associated with chlorinated-plastics production facilities contained 7765 ± 15,918 mg kg⁻¹ [157]. In China, soil samples from the Guiyu e-waste industrial area contained 60 distinct microplastic types, many of which were engineered or modified for specialized industrial applications [158].

Beyond chronic emissions from industrial sites, accidental releases also cause acute contamination events. The MV X-Press Pearl incident in 2021, for example, discharged large quantities of nurdles into the marine and coastal environment, with LDPE fragments reaching 13.37 g kg⁻¹ on Sarakkuwa Beach in Sri Lanka [159].

Overall, industrial areas represent some of the most heavily contaminated terrestrial land uses for microplastics, due to emissions from production processes, waste handling, wastewater discharge, and atmospheric deposition. These environments exhibit high diversity in polymer types and a wide range of particle morphologies, reflecting differences among industrial sectors and production activities. While localized hotspots often show extremely high contamination levels, data on microplastic transport beyond industrial zones and long-term accumulation remain scarce. This knowledge gap constrains the ability to distinguish long-term industrial pollution and limits comprehensive assessments of environmental exposure and risks associated with industrial activities.

2.4. Road-Traffic Sources and Runoff Inputs

Roads and urban runoff are major pathways for microplastics entry into terrestrial environments, with inputs dominated by tire wear particles (TWPs) [15,160,161]. TWPs are chemically and physically complex mixtures composed of synthetic rubbers, carbon black, and additives such as oleamide and 6PPD [162,163,164,165]. They are continuously generated through tire abrasion, weathering of road-marking paints, and the degradation of polymer-modified bitumen in road surfaces [166,167,168,169,170].

Mass-flow assessments indicate that thousands of tons of TWPs are generated and redistributed among environmental compartments annually. In the UK, modeled mass-flow assessments estimated that approximately 79.5 kt of TWPs are released annually, with an estimated 29–43% transported to surface waters, 23–38% retained in soils, 2–8% emitted to the atmosphere, and 8–37% captured by management systems [171]. Comparable patterns have been reported in China, with emission hotspots releasing 4241 t year⁻¹ to air, 3140 t year⁻¹ to water, and 20,602 t year⁻¹ to soils [172]. At the global scale, estimated per capita annual TWP emissions strongly correlate with the number of vehicles in use, ranging from an estimated <1.5 kg year⁻¹ person⁻¹ in most Asian, African, and South American countries to 3.5 kg year⁻¹ person⁻¹ in Luxembourg and 3.1 kg year⁻¹ person⁻¹ in the United States [161]. Air monitoring studies further confirm elevated synthetic rubber levels near busy roads, with levels ranging from 1.6 to 42.5 ng m⁻³, and concentrations 2.8–4.6 times higher near stop-and-go traffic zones than in urban parks [173].

The generation rate and composition of TWPs are influenced by tire formulation, driving behavior, and road surface characteristics [15,174,175]. Rougher road surfaces produce more particles than smoother pavements, while low-speed, stop-and-go driving increases particle release by ~24% due to repeated braking and enhanced friction [15]. Pavement materials can also affect the short-term retention and release of particles; for example, bituminous mixtures can temporarily trap microplastics when temperatures increase and subsequently re-release them [176].

Once generated, TWPs accumulate on road surfaces, verges, and drainage ditches, and are dispersed by vehicle turbulence, wind, and runoff into nearby soils [165,166,167,168]. Tire-derived additives, such as benzothiazoles (BTHs) and 6PPD, serve as effective chemical tracers of traffic-related microplastics and are frequently detected in urban air, dust, and roadside soils [129,163,177]. While TWPs are readily detected in road drainage at milligram-per-liter levels, episodic events such as snowmelt can deliver much higher pulses to verge soils and stormwater detention systems, underscoring strong seasonal controls on lateral transport and accumulation [178,179].

2.5. Waste Management Reservoirs and Secondary Point Sources

Landfills and wastewater treatment plants (WWTPs) are major repositories of waste produced from domestic, industrial, and agricultural sources. They serve as long-term microplastic reservoirs and secondary sources that gradually release particles into surrounding soils and adjacent environments [180,181,182,183]. Plastic waste is projected to reach ~12,000 Mt globally by 2050, with the majority disposed of in landfills [4,184].

Plastic materials in landfills undergo physical, chemical, and mechanical degradation driven by heat, UV radiation, and mechanical stress, leading to the formation, leakage, and release of microplastics into the surrounding environments [7,9]. These particles may migrate with leachate, be transported downward through soil profiles, or become airborne through wind erosion. Leachate from landfills contains variable amounts of microplastics but may reach thousands of particles per liter. Reported levels range from 34 to 53 particles L⁻¹ in Indian landfills [185], 712 particles L⁻¹ in the North China Plain [186], and >33,000 particles L⁻¹ in Guangzhou, China [183]. These particles are typically <300 μm, dominated by PE, PP, PET, PU, and PS, and often appear as fibers or fragments [180,182,183]. Airborne microplastics, mostly PE and PET fibers, have also been detected around landfill operations [187]. Containment materials were typically used to prevent leachate and gas from landfills, but they may also contribute to microplastics pollution. Commonly used HDPE geomembrane liners have been shown in laboratory aging simulations to release 475–1689 particles m⁻² under long-term chemical and mechanical stress [188]. Once released, microplastics may migrate downward through soil profiles, increasing the risk of contamination of deeper soil layers and potentially groundwater systems [181,189].

WWTPs are another major point source of microplastics to terrestrial environments. They receive diverse microplastic inputs from household laundry, personal care products, textile processing, and industrial waste streams [190,191]. While overall removal efficiencies often exceed 70%, fine particles (<0.75 mm) frequently bypass treatment processes and contribute to downstream contamination [192,193]. Microplastics retained during treatment do not disappear but instead accumulate in sewage sludge, which may contain more than 6 × 10⁶ particles kg⁻¹, depending on the treatment configuration [156,194]. Anaerobic digestion cannot eliminate microplastics; digestates may still contain 7200 ± 1100 particles g⁻¹ dry weight (>20 µm), with the majority of these particles partitioned into the solid fraction intended for land application [195]. When applied to agricultural soils, these materials may deliver thousands of microplastic particles [195,196].

Advances in landfill management and sludge treatment create opportunities to reduce microplastic emissions. Gamma irradiation, for example, has been shown to achieve degradation efficiency of almost 70% for microplastics in sewage sludge [121], while improved mechanical recycling and sorting can reduce plastic load entering landfills [197]. Emerging bioremediation approaches are also promising, as newly identified bacterial isolates have demonstrated the ability to degrade PP microplastics, resulting in up to 25% mass loss within 30 days [198]. Despite these advances, informal or poorly regulated landfills continue to exhibit the co-occurrence of microplastics, pathogens, and antibiotic resistance genes, indicating complex contamination profiles that require further assessment [199].

In summary, waste management sites, including landfills and WWTPs, serve as both sinks and secondary sources of microplastics. Although treatment plants remove large quantities of microplastics, the remaining fraction still accumulates in sludge or escapes via effluents, creating indirect routes for microplastics to transfer to soils and downstream water bodies. The observed variability in concentrations reflects differences in treatment technologies, waste composition, and management processes. However, the long-term release and transfer rates of microplastics from these sites into soils and groundwater remain understudied. Addressing these knowledge gaps is essential for accurately assessing environmental risks and for evaluating the sustainability of sludge reuse and other circular waste-management practices.

2.6. Remote Sinks and Long-Range Transport Pathways

Remote and protected landscapes consistently exhibit measurable microplastic contamination despite minimal direct plastic inputs, highlighting the global reach of atmospheric microplastic transport and deposition [200,201]. Wind-driven dispersal enables lightweight particles to travel long distances and deposit in remote and isolated environments [202]; for example, snow in the Apennine Mountains, Italy, contained up to 74.69 ± 71.43 particles L⁻¹ of melted snow, comprising polymers such as PA, PET, PES, PP, PE, PTFE, and ABS, suggesting that these particles may be deposited by the air from regional sources in addition to local recreational activity [203]. Floodplain soils in Swiss nature reserves contained nearly 600 particles kg⁻¹, primarily PE, while beach sediments in the Po River Delta, Italy, contained up to 23 particles kg⁻¹, primarily PE and PS [204,205]. Grassland soils in China contained ~1200 particles kg⁻¹, with PET, PS, and PE prevalent [206]. Desert and woodland soils in the Ebinur Lake Basin also showed detectable loads, with concentrations of 29.15 ± 1.89 mg kg⁻¹ and 34.17 ± 3.21 mg kg⁻¹, respectively [207]. Similarly, sediments from mangroves in South Africa contained up to 100 particles kg⁻¹, and preliminary reports from forests and savannas in Mexico have indicated microplastic concentrations as high as 1500 particles kg⁻¹. [208,209]. These variations in microplastic concentrations are attributed to both atmospheric transport and regional land use interactions.

Forested ecosystems, typically serving as protected areas, act as effective interceptors of airborne microplastics. Soil horizons and throughfall samples indicate that deposition-driven inputs lead to enrichment in litter layers, which are subsequently transferred to soil through litter turnover processes [201]. Forest age also influences accumulation dynamics; for instance, Pinus massoniana forests contained 600–3858 particles kg⁻¹, with older stands showing higher microplastic loads [210]. Bamboo forests near urban and agricultural areas also accumulate microplastics, particularly PMMA and PET, which are strongly shaped by proximity to urban and agricultural activities [211].

High-elevation and highland regions such as the northeastern Qinghai–Tibetan Plateau also display notable contamination despite minimal direct inputs, with 200 to 3640 particles kg⁻¹ in grazing soils and 280 to 973 particles kg⁻¹ in farmlands, dominated by PP and PE fragments and films [212]. These findings further support the role of atmospheric deposition, supplemented by diffuse human activities such as grazing and tourism, as primary contributors to microplastic accumulation in remote environments.

Collectively, these findings demonstrate the presence of microplastics in remote and natural habitats, driven by both long-range transport and localized human activity. Although microplastic concentrations are generally lower in remote areas, natural habitats located near human activities or adjacent land uses often exhibit elevated levels due to indirect and diffusive inputs. Protected areas bordering agricultural, industrial, or urban areas show higher contamination associated with diffuse transport processes. As attention to microplastics in remote and minimally disturbed environments grows, understanding their long-term accumulation and ecological sensitivity remains an emerging priority.

3. Comparative Assessment of Microplastics Contamination Levels

A comparative analysis of microplastic abundance and distribution across land use types was conducted. As detailed in Table 1, the compiled data reveal significant variability in concentrations, polymer composition, and particle morphology, reflecting differences in input intensity, management practices, and environmental exposure. Specifically, while all environments share PE and PP as dominant polymers, their secondary polymers, particle shapes, and median abundances strongly reflect the differences in input intensity and specific management practices. The highest median abundances were observed in landfill, industrial, and urban soils, while agricultural soils also showed substantial contamination. Notably, natural systems also contain considerable microplastic loads, confirming the pervasiveness of these contaminants across terrestrial environments.

3.1. Abundance and Distribution Patterns

Microplastics abundance varies significantly across land use types. As illustrated by the abundance distributions in Figure 3, while the median microplastic concentrations (indicated by black circles) are relatively comparable, falling within the order of 10³ particles kg⁻¹ across agricultural, urban, industrial, and landfill soils, the interquartile ranges and extreme values highlight highly distinct, localized contaminant profiles. While urban soils exhibited the most extreme maximum values, likely due to hotspots, agricultural soils displayed substantial variability and elevated accumulation levels, which may be attributed to the extensive use of plastic mulch and continuous biosolid application. Landfill soils also exhibited a wide range, reflecting the heterogeneity of disposed waste. Although natural areas are not typically expected to contain high levels of microplastics, they nevertheless harbor substantial loads, suggesting local contamination from surrounding areas and direct human activities.

Figure 3. Distribution of soil microplastic abundances (particles kg⁻¹ dry soil) across five land use categories. Data are plotted on a log₁₀ scale to illustrate the high degree of heterogeneity across literature sources. Individual colored points represent observed values from independent studies (sample size n for each category is indicated at the top). Black circles indicate the median value for each group, and the vertical black bars represent the interquartile range (IQR).

The interconnected and overlapping abundances among land uses suggest that microplastic accumulation is determined by a spectrum of complex factors rather than by distinct barriers between land use types. This pattern underscores the need to adopt strategies that address both point and nonpoint sources, as well as the dynamics of anthropogenic practices and hydrological events.

3.2. Analysis of Polymer Composition and Morphology

Across all land use categories, PE and PP consistently represent the dominant polymers. As illustrated in the relative composition plots in Figure 4, the group medians (indicated by black circles) for PE and PP are the highest across the board. However, the widespread observations of individual studies (colored points) highlight significant site-specific variability. Secondary polymers, such as PET, PS, PVC, and PA, vary with land use activities and specific emission sources. Consequently, polymer-type analysis provides a valuable complementary indicator for inferring microplastic sources and transport pathways.

Figure 4. Relative polymer composition (%) of microplastics detected in soil across five land use categories. Colored points represent relative percentages from individual study observations, while black circles and vertical bars denote group medians with interquartile ranges (IQR). Polymer types are grouped into PE (Polyethylene), PP (Polypropylene), PET/PES (Polyesters), PS (Polystyrene), PVC (Polyvinyl chloride), and PA (Polyamide), with all other reported types consolidated into “Other”.

Specifically, agricultural soils exhibit a pronounced dominance of PE and PP, reflecting the extensive use of agricultural plastics and mulching materials. Although PE and PP are also prevalent in industrial and urban soils, these environments are more strongly characterized by PVC dominance, indicative of its widespread use in construction, healthcare, electronics, flooring, and related sectors. Furthermore, industrial, urban, and landfill soils exhibit more compositionally complex structures and generally high polymer diversity, including PU, PMMA, and engineered microplastics (classified as “other” in Figure 4). This complexity reflects the multiple input pathways connected to industrial processes and urban infrastructure. In contrast, soils in natural areas show relatively less variation in polymer types.

Microplastic morphology provides further insights into sources and transformation processes in soils (Table 1). Fragments and fibers dominate all land use categories, followed by films, although their relative rankings differ across land use categories. Agricultural soils, for example, reflect the influence of mulching films and organic amendments, whereas urban soils include higher proportions of fragments and fibers associated with TWPs and synthetic textiles.

The widespread distribution of microplastics in soils and the diversity of their sources highlight the role of terrestrial systems as integral components of the “plastic cycle” in the environment. Microplastics in soils result from interconnected systems of inputs and outputs across soil, water, and air reservoirs. Microplastics from households and textiles enter wastewater streams and are redistributed into soil via treated effluent discharge and sludge application. In contrast, particles accumulated in soils may be remobilized by erosion and surface runoff, facilitating their transport to aquatic systems. Although recycling reduces plastic disposal volumes, the weathering and abrasion of reused materials may still generate secondary microplastics, emphasizing the need to address the full life cycle of plastics rather than focusing on isolated pathways.

Overall, the wide interquartile ranges and extreme values observed across land use types reflect environmental variability and significant methodological heterogeneity across the compiled literature. Analytical variables, such as extraction protocols, polymer identification methods, and especially lower size detection limits, are not standardized across studies. Variations in these size limits, largely determined by the analytical instruments used, introduce inherent variability into reported abundances. While advanced spectroscopic techniques allow some recent studies to detect particles down to the 1 μm threshold, which is the conventional boundary between micro- and nanoplastics, other studies, or those relying on visual identification, often have much higher functional cutoffs. Furthermore, because most synthesized field studies rely on single-time-point grab sampling, the current data largely fail to capture seasonal dynamics, episodic redistribution events, or long-term accumulation rates. Consequently, all quantitative summaries presented here should be interpreted with an understanding of these underlying methodological and temporal uncertainties, as cross-study comparability remains inherently constrained.

4. Fate and Transport in Terrestrial Environments

Microplastic fate and transport in soils determine how particles persist, accumulate, and move into other environmental compartments. Their behavior in soil is complex and depends on polymer type, morphology, soil characteristics, and other environmental conditions, all of which affect degradation and mobility. Once in the environment, microplastics may persist for decades to centuries through slow biodegradation and weathering, including photodegradation, fragmentation, and abrasion. Soil salinity, organic matter, and microbial interactions can modify particle aggregation and transport efficiency, thereby influencing the long-term fate of particles [213,214].

4.1. Transport Pathways and Landscape Redistribution

Microplastic redistribution in terrestrial environments occurs through interacting pathways, including vertical migration via infiltration, lateral movement via runoff and erosion, atmospheric transport, and mobility via human and biological activities.

Microplastics are typically enriched in the top layer due to direct deposition, amendments, and surface disturbance, but may migrate to deeper layers over time [105,215]. However, this vertical distribution is not uniform and is heavily influenced by sampling depth intervals (e.g., 0–5, 0–20, or >20 cm) and localized disturbance regimes (Table 2). In undisturbed natural systems, such as forests and grasslands, microplastics are enriched in the uppermost layer (e.g., 0–5 cm) but still exhibit substantial downward migration into the subsoil (e.g., 5–10 cm) [216]; for example, while forest topsoils may hold roughly double the concentration of the immediate subsurface, grassland profiles can exhibit a much more gradual decline with depth. Similar concentration gradients are observed in other unmanaged or semi-managed environments, ranging from coastal mangroves [217] to municipal landfills [218]. In agricultural systems, mechanical management such as tillage mixes microplastics downward, resulting in a more homogenized distribution throughout the active plow layer (typically 0–30 cm) before concentrations drop off significantly in deeper subsoils [219]. Biological and physical factors also shape these profiles; bioturbation by soil fauna acts as a biological plow, continuously transporting particles into deeper burrows [217], while infiltration transfers microplastics downward based on soil texture, pore networks, and preferential flow paths [220,221,222].

Table 2. Overview of vertical microplastic distribution across different land uses and specific depth intervals.

Land Use	Location	Depth	Abundance (Particles kg−1)	Ref.
Agricultural	Shanghai, China	0–3 cm	78.00 ± 12.91	[101]
		3–6 cm	62.50 ± 12.97
Agricultural	Xinjiang, China	0–30 cm	1615 ± 52	[219]
		40–80 cm	112 ± 11
Natural (Forest)	London, UK	0–5 cm	2937	[216]
		5–10 cm	1300
Natural (Grassland)	London, UK	0–5 cm	540	[216]
		5–10 cm	410
Natural (Mangrove)	China	0–20 cm	6516 ± 1725	[217]
		20–40 cm	2246 ± 497
Landfill	Tehran, Iran	0–3 cm	863 ± 681	[218]
		3–6 cm	225 ± 138

Vertical migration via infiltration transfers microplastics downward through the soil profile, providing a pathway for contaminating deeper soil layers and underlying groundwater systems; for example, field measurements around municipal solid waste dumpsites in South India detected 2–80 particles L⁻¹ in the underlying groundwater [223]. Modeling further indicates that irrigation flux and dispersion mechanisms accelerate the transport of particles into deep saturated aquifers, with migration efficiency heavily influenced by particle size and polymer density [224]. Studies show that the vertical migration of microplastics in soil is influenced by pore structure, water flux, and microplastic particle properties such as size and density [220,221,222]; for example, a study reported that microplastics were concentrated in the top 30 cm of soil (~1160 particles kg⁻¹), and declined by one to two orders of magnitude in depths below that (~110 particles kg⁻¹), primarily influenced by particle size, as smaller particles facilitate easier migration to deeper layers [219]. Biological factors also shape these profiles; bioturbation by soil fauna acts as a biological plow, continuously transporting particles into deeper burrows and blurring distinct depth gradients [225]. The upper layers of soil under landfills are often hotspots of microplastic accumulation. Subsurface conditions, such as elevated temperatures and high salinity in landfill soils, constrain the mobility of microplastics. These factors reduce soil porosity and promote particle aggregation, leading to increased physical straining and attachment, which enhance microplastic retention in the topsoil [213,226]. However, the high variability in sampling depths across studies, often determined by local conditions rather than standardized protocols, makes direct cross-study comparisons of vertical profiles highly challenging. Thus, evaluating vertical gradients requires explicit contextualization of depth bins, land management, and soil biota rather than assuming universal surface accumulation.

Horizontal transport is primarily driven by surface runoff and erosion, which are major routes for moving microplastics from terrestrial to aquatic environments [227]. Stormwater runoff mobilizes accumulated particles and transports them through drainage networks, facilitating long-range terrestrial dispersal [170,171,174]. Laboratory and field studies show that both low- and high-density microplastics are exported during storm events, often in association with sediment fractions, though vegetation can enhance retention [228,229]. Extreme events, such as catchment-scale flooding, rapidly redistribute microplastic particles from riparian soils into reservoirs, leading to a sharp decrease in soil concentrations following peak flood discharge [230]. These transport mechanisms create distinct spatial patterns; for example, roadside soils display a clear gradient, with microplastic concentrations decreasing with increasing distance from the road, suggesting that runoff and wind dispersal from vehicular traffic contribute to the spread of microplastics into adjacent soils [125]. However, most transport estimates are derived from short-term experiments or event-based observations, and field-scale flux quantification across seasons and land uses remains limited.

Atmospheric transport is a main pathway for microplastic transfer and redistribution in terrestrial ecosystems. Wind erosion mobilizes microplastics, transports them over long distances, and disperses them across large landscapes [231,232]. Atmospheric deposition from industrial and urban sources contributes fibers and fragments to soils across land use types, with accumulated concentrations in urban soils reaching up to 5600 particles kg⁻¹ [36]. In semi-arid croplands in China with loose and erodible soils, fibers accounted for 70.6% of atmospheric dustfall [233]. Furthermore, wind-tunnel physical modeling shows that sub-micron PP fragments can aerosolize during UV-induced degradation and circulate within simulated urban canyon airflow systems during accident scenarios [234].

Airborne microplastics originate from household activities, urban processes, and industrial operations, including synthetic textile fibers, tires and brake pads fragments, and resuspended urban dust; for example, specific indoor micro-environments show varied microplastic abundances, with median concentrations of 528 particles m⁻³ in residential apartments and 2238 particles m⁻³ in car cabins. These airborne particles were predominantly fragments and fibers smaller than 10 µm (PM_1–10), dominated by PE, PA, and PP [235]. Textile industry areas exhibit extremely high atmospheric fallout, reaching up to 2.45 × 10⁵ particles m⁻² day⁻¹ indoors and 7.27 × 10⁴ particles m⁻² day⁻¹ outdoors [40]. High concentrations of airborne TWPs were also captured near busy roads and airports [129]. These results from various environmental systems indicate that atmospheric inputs can be as important as direct local sources in shaping soil microplastic contamination.

Atmospheric events, such as dust storms, further accelerate long-range microplastic redistribution and transport microplastics across regional scales [133,236]; for example, rainfall events enhanced scavenging and deposition efficiency, particularly near urban emission sources [237]. Field measurements further show correlations between precipitation, PM_2.5, and soil microplastic accumulation [126]. Another study showed that micro- and nanoplastics contribute an average of 12.33% of PM_2.5 mass, with PE and PVC nanoplastics widely dispersed in urban air in China [132]. These results demonstrate interactions between air quality, weather events, and terrestrial microplastic loading, highlighting atmospheric deposition as a critical component in the terrestrial microplastic cycle and emphasizing the need for a multi-pathway framework that integrates outdoor deposition, indoor emissions, long-range atmospheric transport, and climate-airflow interactions [132,234,235].

Human and biological activities also influence both vertical and horizontal distribution of microplastics. Earthworms ingest, egest, and displace microplastics, moving them vertically and horizontally via casts and burrows [225]. Vertebrates, such as amphibians, may serve as potential vectors, transferring microplastics from aquatic habitats into soils during migration and metamorphosis [238]. Human tillage practices, such as intense and deep plowing, result in a more mixed vertical profile, whereas no-till systems tend to retain microplastics closer to the surface [34,239,240]. Additionally, sewage sludge often leads to deeper microplastic deposition by incorporating it into the soil matrix [79,241]. Hydrology and land management largely determine which pathway is dominant, and the interactions among soil processes help explain variations in transport outcomes reported across studies [242].

4.2. Persistence and Residence Time

Microplastics in terrestrial environments change over time through degradation, weathering, transport, and long-term retention. The rate and direction of these processes depend on polymer type, additives, soil conditions, land management, and climate.

Photodegradation by UV radiation is a major abiotic driver of breakdown, weakening polymer chains and leading to fragmentation, surface oxidation, and cracking [105,243]. This process is critical, especially when plastics are exposed at the soil surface, as residual plastics exposed to photothermal degradation tend to generate significantly more and smaller microplastics under mechanical stress [243]. In soil, this mechanical stress is sustained by abrasion from soil particles and by agricultural activities such as tillage [240,244].

Weathering processes vary with environmental exposure; for example, mulch-derived microplastics aged in soil exhibit greater surface erosion and form microbial interactions, increasing bacterial network complexity and shifting communities toward more stochastic assembly processes [214]. Processing of sewage sludge can modify microplastic surfaces and influence microplastic aging. Hydrothermal treatment during sludge treatment reduces the release of additives such as BPA and phenols and alters adsorption behavior [245]. Additives, such as plasticizers and stabilizers, may slow down polymer breakdown, but their loss during weathering can accelerate cracking and increase surface reactivity. However, other additives also enhance the biodegradation of plastics, thereby improving the biodegradability of microplastics [246].

Despite these fragmentation processes, conventional microplastics are generally resistant to biodegradation and can persist in the environment for extended periods [247]. Persistence is supported by long-term field investigations showing substantial accumulation of microplastics after more than 30 years of mulching, with residues concentrated in the plow layer and detectable deeper in the profile [248]. Even after long-term sewage sludge application, microplastics remained spatially stable, suggesting slow turnover [249].

Overall, common plastic residues and their derived microplastic particles have long residence times, reaching decades. Depending on environmental conditions, biodegradable films can persist long enough to generate transient microplastic loads before further decomposition. Persistence is shaped by interactions among polymer properties, soil chemistry, hydrology, and biological activity, with many factors producing nonlinear and sometimes contradictory outcomes [242]. Across pathways including wastewater reuse, sludge and compost application, and landfill leachates, microplastics can accumulate in soils, with emerging evidence suggesting they may be taken up by plants and translocated through terrestrial food chains via trophic transfer, emphasizing the need for harmonized analytical approaches and improved management strategies [250].

5. Effects of Microplastics on Terrestrial Ecosystems

Microplastics affect terrestrial systems directly and indirectly by altering soil properties, plant performance, and ecosystem processes. These effects vary with soil type, land use, polymer type, particle size, and co-occurring contaminants.

5.1. Impact on Soil Properties and Function

Microplastic accumulation fundamentally alters soil physical structure, chemical properties, and biological activity, which in turn impact soil functions, productivity, and ultimately plant growth.

5.1.1. Soil Physical Properties

Soil structure and aggregation, bulk density, porosity, and water retention are impacted by microplastics. Bulk density generally decreases with microplastic contamination; for example, at a 2% (w/w) concentration of PE films, bulk density decreased from 1.03 to 0.77 g cm⁻³, while microaggregates decreased from 22% to 17% [251]. Microplastic shape modifies these effects, as coarser microplastics tend to disrupt soil aggregate stability more than finer particles [252,253].

Microplastics also alter pore-size distribution and hydraulic properties, reducing saturated hydraulic conductivity, infiltration, total porosity, and soil water-holding capacity [254,255,256]; for example, LDPE hydrophobicity decreases water availability in soil [257]. However, these changes are both size- and concentration-dependent [258,259].

Changes in pore structure can modify pore networks and accelerate pollutant movement [256]. Microplastics can also act as carriers for co-occurring contaminants, thus further enhancing their transport and environmental dispersion [260,261,262]; for example, laboratory microcosm results show that PE microparticles can release PAEs into the surrounding soil at concentrations up to ~0.2 mg kg⁻¹ [263]. Another study further illustrates soil-type dependence in 59 grassland soils, where TWP effects on aggregation, respiration, and decomposition varied with soil texture, grazing frequency, and land use intensity [264].

5.1.2. Soil Chemical Properties and Biogeochemical Cycling

Microplastics alter fundamental soil chemical properties, including pH, electrical conductivity (EC), and cation exchange capacity (CEC); for example, the accumulation of microplastics increases soil pH, as observed with PES in laboratory-spiked soils [265,266] and PE in mountainous forest soils [259]. Furthermore, under heavy contamination, these particles may affect soil fertility; for example, at a 10% (w/w) concentration, mixtures of PP, PES, and PA reduce EC by almost 67% [265], and fine PE particles (<125 μm) decrease CEC by almost 13% [263].

Microplastics also affect nutrient cycles, particularly nitrogen cycling. Microplastics stimulate N-acquisition enzyme activity [267], modify gross N-transformation rates [268], and alter N-cycling gene expression [269]. However, these shifts do not always improve plant-available N; for example, PE reduces nitrate (NO₃⁻) availability without impact on ammonium (NH₄⁺) availability [270]. Carbon cycling is also altered through priming effects and changes in microbial turnover; for example, PLA enhances carbon loss in acidic soils but promotes sequestration in alkaline soils [271,272]. The priming direction depends on mineral N and soil C:N ratios [273]. Microplastics also disrupt stable carbon pools, with PE and mulch-derived biodegradable particles reducing microbial necromass by up to 56% during early decomposition before aged particles later increase SOC through nutrient release and altered enzyme activity [274]. Wetland mangrove soils with high microplastic loads show the potential for enhanced methane production linked to increasingly complex methanogenic networks [217]. Additionally, remote mountain soils containing PE microplastics exhibit increased ecosystem multifunctionality linked to shifts in DOM chemistry [259].

As with physical properties, polymer type also affects soil chemical properties. Biodegradable microplastics such as PLA and PBAT often increase DOC and DON and release degradation products that further reshape soil biogeochemistry [267,272,275,276].

Co-contamination often amplifies these effects; for example, mixed microplastic–metal systems can reduce metal availability but still strongly alter soil biochemistry [258]. Microplastics also sorb heavy metals, such as cadmium (Cd) and lead (Pb), potentially increasing their mobility [277].

5.1.3. Soil Microbial Communities

Soil microbial communities are sensitive to microplastic contamination. Altering physical properties will change habitat and resource availability; for example, fibrous microplastics create favorable microenvironments [278,279,280]. Changes in porosity and moisture also alter oxygen diffusion, shifting the balance between aerobic and anaerobic microbes [20,281]. Microplastics also provide surfaces for microorganisms to colonize and partially utilize some polymers as carbon sources [280,282,283]. However, additives and sorbed pollutants can have toxic effects, inhibiting enzyme activity and microbial growth [284,285].

Both short- and long-term studies show that microplastics induce shifts in microbial community composition, functional pathways, and exoenzyme activity, often depending on polymer type [259,275,276,286,287]. Conventional microplastics, such as PVC, PET, PS, PE, and PP, suppress microbial and enzyme activities, including β-glucosidase [288,289,290,291]; for example, low-density polyethylene mulch (LDPEM) residues have disrupted fungal community networks, suggesting that fungi can be more sensitive to microplastic contamination [292]. The long-term biological impact is profound, as field evidence from greenhouse soil cultivated 10 to 30 years shows a reduction of around 47% in total microbial biomass [291]. Microplastics also inhibit symbiotic mycorrhizal fungi or immobilize nutrients as microbes attempt to degrade biodegradable fragments, reducing plant access to nutrients [293].

5.2. Effects on Soil Biota and Food Webs

Soil fauna ingest and transport microplastics, with consequences for health and food-web structure. Earthworms and other terrestrial insects take up microplastics and excrete them in casts, confirming their ingestion and contributing to vertical and horizontal redistribution [225,294,295]. However, microplastics can cause gut blockages, reduced growth, lower reproduction rates, and inflammation [296,297,298]. Microplastics exert toxicological effects across terrestrial fauna, altering physiology, immunity, metabolism, and survival [238,299,300,301]. However, responses are species- and polymer-specific. Smaller TWPs (0–75 μm) reduce springtail (Sinella curviseta) reproduction more than larger particles [302]. LDPE reduces nematode reproduction, and ingestion of PE, PA, and PS microplastics reduces reproduction in snails by up to 61% [303]. Microplastic mixtures affect species differently. Earthworms show minimal reproductive impacts, while springtails exhibit reduced reproductive output [304]. Co-contamination with metals or tire-derived chemicals increases toxicity, causing mortality, reproductive failure, DNA damage, and disruption of the gut microbiome [305,306]. Interestingly, experimental microcosm studies suggest that earthworms can partially buffer the negative impacts of microplastics on soil processes by enhancing soil structure, improving nutrient utilization efficiency, and promoting the recovery of microbial decomposition and metabolic functions [290,307,308]. These effects are particularly evident in the observed ability of earthworms to mitigate microplastic-induced inhibition of pollutant degradation and to alleviate cumulative stress from environmental factors, such as drought, under controlled conditions [290,307,308]. Wildlife exposure and trophic transfer are also documented; for instance, TWPs have been detected in 57% of endangered Okinawa rail (Hypotaenidia okinawae), indicating exposure via ground-feeding in road-adjacent environments, where roadside soils and sediments act as reservoirs for traffic-derived contaminants [309].

5.3. Impacts on Plant Development

Microplastics influence plants both indirectly, through altering soil structure and chemistry, and directly, by affecting plant development from germination to yield. Plant responses are mixed and context-dependent. Changes in aggregation and porosity affect root penetration, water availability, and nutrient transport, sometimes improving aeration but often restricting access to water and nutrients [278,310]. Laboratory studies report neutral, negative, or occasionally positive effects on growth depending on polymer type, particle size, and soil conditions [311,312]. Plants in their early stages are particularly vulnerable to microplastics. High levels of microplastics could physically block seedling emergence, shorten roots, and reduce germination velocity and synchrony, with native species more affected than some invasive plants [313,314,315,316,317]. Furthermore, microplastics can be toxic due to chemicals leached from them, as shown in wheat germination [316], or due to oxidative stress, as shown in rice seedling growth [317]. In contrast, some studies show minor effects of microplastics; for example, LDPE microplastics have minimal impact on white mustard germination but slightly stimulated shoot length at 5% w/w, demonstrating species- and context-dependent outcomes [318].

Accumulation in edible tissues raises food safety concerns. Studies using compost-amended substrates have reported microplastic levels in various crops, such as lettuce and radish, at up to 140 particles g⁻¹ [319,320]. Furthermore, field and experimental evidence demonstrate that atmospheric microplastics, such as PET and PS, can accumulate directly in leaves at levels up to 10⁴ ng g⁻¹, entering via stomata and translocating to vascular tissues [321]. Similarly, microplastics in soil can be taken up by roots and translocated to stems and leaves with the smallest particles, associated with significant yield losses [320]. Many studies report oxidative stress, disrupted nutrient uptake, decreased chlorophyll levels, and reduced biomass in various crops, particularly with prolonged exposure to photoaged particles [322,323,324].

Nutrient status and co-contaminants further shape outcomes, with microplastics interacting with N and P availability and with pesticides and plasticizers such as imidacloprid and di-2-ethylhexyl phthalate (DEHP) [325,326,327].

5.4. Microplastics and Co-Contaminants Interactions

Microplastics frequently co-occur with metals, pesticides, plasticizers, and other contaminants, and modify their mobility, persistence, and biological effects on plants, microbes, and soil fauna [328,329,330]. Acting as carriers and reactive surfaces, microplastics sorb and release dissolved organic carbon (DOC), heavy metals, and organic pollutants, subsequently releasing them into the rhizosphere, creating new exposure pathways for soil biota and crops [275,327,331,332]. Chemical co-contaminants can further alter microplastic behavior by modifying surface charge and interfacial interactions; for instance, benzethonium chloride dosage and ionic strength alter zeta potential and control polystyrene transport in sandy porous media [333]. DOC can further enhance the mobilization of pollutants, such as Sb and Zn, and alter chemical speciation [331].

Microplastics also modify the behavior and degradation of pesticides and plasticizers in soil–plant systems. Hydroponic and microcosm studies indicate that microplastics can increase pesticide toxicity by acting as “pesticide carriers”, thereby enhancing their accumulation in roots and potentially suppressing auxin signaling pathways [329,330]. Furthermore, the presence of microplastics has been shown to hinder the degradation of plasticizers like DEHP by interfering with microbial and faunal decomposition processes [308]. These interactions extend to multi-stressor scenarios involving invasive plants and pathogens, where microplastics can intensify oxidative stress while simultaneously triggering compensatory antioxidant responses [334].

These pollutant interactions collectively affect biotic networks. Microplastics restructure rhizosphere microbial communities, suppress nutrient-cycling enzymes, and destabilize fungal networks critical for nutrient uptake [275,292]. Co-contaminant stress intensifies oxidative damage in fauna and plants, while nutrient management (e.g., phosphorus availability) can partially regulate plasticizer uptake through root transport pathways [327].

Overall, microplastics act as a cross-cutting stressor in terrestrial systems. They modify soil structure and chemistry, reshape microbial and faunal communities, alter nutrient and contaminant dynamics, and ultimately affect plant performance and food quality. Evidence from multi-stressor studies shows that these components rarely act in isolation. Root traits, rhizosphere communities, and nutrient status interact with microplastics contamination to produce complex, context-dependent outcomes [293,327]. Future work should place greater emphasis on these integrated pathways in long-term, field-based settings rather than focusing solely on single pollutants or short-term laboratory exposures.

Furthermore, a critical distinction must be made between laboratory-observed toxicity thresholds and actual environmental concentrations when interpreting these ecological effects. Many studies demonstrate disruptions to soil properties and plant growth inhibition at high microplastic doses, typically 1% to 10% by soil weight. However, current real-world concentrations in most agricultural and urban soils are typically orders of magnitude lower. Consequently, acute toxicity or immediate collapse of ecosystem services may not be widespread under current background conditions. Nevertheless, the persistence of plastics means that continuous accumulation in hotspots could eventually exceed these effect thresholds. Furthermore, the confirmed translocation of microplastics into edible plant tissues, even at lower exposure levels, provides an immediate, real-world pathway into terrestrial food webs, underscoring the urgent need to establish environmentally relevant toxicity thresholds to assess safety risks accurately.

5.5. Biodegradable Plastics: Trade-Offs and Uncertainties

Biodegradable plastics are increasingly discussed as an alternative to conventional polymers in terrestrial systems, but their environmental performance involves clear trade-offs and substantial uncertainties. In agricultural contexts, bio-based coatings offer a sustainable alternative for reducing microplastic inputs from both fertilizers and seed treatments [92,95,96]. However, the environmental performance of biodegradable plastics remains conditional, and their net benefits in terrestrial systems are not assured across settings.

Evidence further indicates that biodegradable and bio-based polymers can modify soil physical and hydrological properties; for example, hydrophilic starch-based microplastics and PBAT enhance the microstructure, thereby improving water retention; in sandy-loam soils at a 1% concentration, these polymers were found to increase water availability by approximately 5% [257]. These outcomes emphasize that soil responses depend on polymer chemistry and particle properties. That improved degradability may coincide with meaningful, and sometimes prolonged, functional changes in soil structure and water dynamics.

Biodegradable plastics can also influence soil chemistry and biogeochemical cycling through both their presence as physical particles and the release of their active degradation products; for example, biodegradable materials such as PLA and biodegradable mulch-film (BDMs) residuals have been shown to elevate soil pH [276,292]. Furthermore, biodegradable microplastics, such as PLA and PBAT, often increase dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) pools [267,272,275,276]. While these changes may temporarily enhance microbial substrate availability, they also imply altered nutrient and carbon processing, with consequences that may vary across soils, climates, and management regimes.

Biological responses to biodegradable microplastics remain inconsistent and can vary between microbial groups and trophic levels. These particles induce more positive community changes than conventional polymers, likely due to the release of low-molecular-weight oligomers [275]. However, much like conventional mulch, residues from biodegradable mulch films (BDMs) negatively impacted fungal populations by destabilizing fungal networks, indicating that fungi are highly susceptible to agricultural plastic contamination regardless of their degradability [292]. Thus, even when biodegradable plastics ultimately mineralize, intermediate stage fragmentation, aging, and additive release may have biological effects.

Given these trade-offs, evaluating biodegradable plastics as a mitigation strategy requires field-realistic, life-cycle-informed assessment rather than assuming inherently lower environmental risk.

6. Conclusions

Terrestrial ecosystems function as both a fundamental sink and a dynamic secondary source in the global microplastic cycle, receiving microplastic loads that far exceed those entering the oceans directly. Through the comparative synthesis, a clear paradigm emerges: terrestrial microplastic contamination is not a uniform background issue but, rather, a highly heterogeneous form of pollution governed by land use intensity and anthropogenic activities. Agricultural practices (e.g., plastic mulching, biosolid amendments) and industrial/urban emissions (e.g., PVC, TWPs) create distinct localized hotspots of polymer diversity and abundance. Once deposited, these persistent, slowly weathering polymers are redistributed vertically through infiltration and bioturbation and laterally via runoff and erosion, essentially transforming soils to reservoirs that slowly release particles into groundwater and aquatic ecosystems.

The implications for soil health are multifaceted and concerning. Physically, microplastics alter bulk density, porosity, and water-holding capacity, disrupting the soil structure essential for plant growth. Chemically, they influence nutrient cycling, specifically carbon and nitrogen dynamics, and modify pH. Biologically, microplastics induce dysbiosis in microbial communities, impair the growth and reproduction of soil fauna such as earthworms, and act as vectors for heavy metals and organic pollutants. These combined stressors threaten ecosystem multifunctionality and raise concerns regarding food safety through trophic transfer and crop accumulation.

Despite the growing body of knowledge, critical gaps persist. The lack of standardized detection methods, compounded by varying size cut-offs, differing extraction protocols, inconsistent soil sampling depths, and varied reporting units, hampers the ability to conduct meta-analyses and to quantify global stocks accurately. Future research priorities must include: (1) methodological harmonization to capture both mass and particle number, particularly for the nano- and small micro-fraction; (2) long-term field monitoring that moves beyond short-term laboratory exposures to assess realistic polymer mixtures and their interactions with soil biota; (3) flux quantification to refine global mass-flow models between the atmosphere, soil, and water; (4) sustainability assessments of circular economy practices, e.g., sludge reuse and biodegradable plastic application, to prevent the unintended accumulation of this contaminant of emerging concern in agricultural soils; and (5) the development of quantitative ecological risk assessment frameworks to translate observed microplastic pollution data into risk metrics for terrestrial ecosystems.

In summary, terrestrial microplastic pollution is a pervasive and persistent global change driver. Addressing this challenge requires an integrated approach that connects point and non-point source control, waste management policy, agricultural innovation, and rigorous environmental monitoring to safeguard soil resources and ecosystem health.