Microplastics and soil microbiomes

Abstract

Microplastics, small particles that are released from plastics as they degrade, are ubiquitous and increasing in amount in most environments, including the soil. Here, we review the impacts of microplastics on the structure and activity of soil microbiomes and their key ecosystem functions. We then discuss how soil microbiomes regulate the environmental behavior of microplastics, such as enhancing pollutant adsorption and promoting degradation. Finally, we describe knowledge gaps and future priorities in understanding the ecological risks and potential mitigation strategies for microplastic pollution.

Microplastics are pervasive contaminants with potential risks to soil microbiomes

By 2050, it is projected that the global annual plastic production will reach 292 billion tons, with the annual generation of plastic waste amounting to 24.5 million tons [1]. In 2015, 79% of plastic waste ended up in landfills or the natural environment [2, 3]. From landfills, plastics can fragment and transport into the environment via leachate leakage, surface runoff, and wind dispersal, exacerbating environmental accumulation. Inadequate plastic recycling, coupled with long-term exposure to ultraviolet radiation, mechanical abrasion, chemical hydrolysis, and microbial degradation, leads to the formation of smaller polymer particles or fragments with size < 5 mm, termed as microplastics (MPs) [4, 5]. Common types of MPs in the environments include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyamide (PA), and polyurethane (PU) [6,7,8,9]. These particles span sizes ranging from nanometers to millimeters and exhibit diverse shapes such as fibrous, granular, and fragmented forms [10]. Once released into the environment, these MPs tend to persist for extended periods [11, 12]. For instance, a study on MPs in agricultural soils following sewage sludge applications showed that MPs accumulated rapidly and persisted at relatively constant levels for at least 25 years after application [13]. The persistence of MPs not only poses potential risks to human and animal health by bioaccumulating in the food chains and releasing toxic substances but also disrupts soil ecosystems by altering soil structure, biochemical properties, and microbial community dynamics, ultimately impacting agricultural productivity and overall soil health [14]. 

Soil serves as a significant sink for MPs, with the annual input of plastic waste into terrestrial ecosystems exceeding that into the oceans by 4 to 23 times, and this trend is on the rise [15]. Human activities are the primary driver of MPs accumulation in soils, with impacts categorized as intentional or unintentional [16, 17]. The unintentional impacts stem mainly from applying soil amendments (e.g., compost, sewage sludge) and irrigating with contaminated water [18]. Intentional impacts derive directly from anthropogenic practices such as littering, plastic mulching, using plastic-containing fertilizers, and landfilling [18]. Additionally, processes including flooding, atmospheric deposition, tire wear, and industrial/consumer waste disposal may also contribute to soil microplastic contamination [19, 20] (Fig. 1a). Film mulching is the most common pathway for soil MPs input, especially in arid and semiarid regions where films are intensively used. In soils with long-term plastic mulching, the concentration of MPs can even exceed 10,000 particles/kg, among which PE and PP are the main types of soil MPs [17, 21].

Fig. 1

The main sources of MPs in soil (a) and the influence pathways of soil MPs on soil microorganisms (b). MPs enter the soil ecosystem through a variety of interrelated pathways, including agricultural activities such as plastic film mulching, sewage and sludge irrigation, and composting, as well as nonhuman activities such as atmospheric deposition and soil runoff

Microplastic pollution is recognized as a global issue of increasing importance in environmental and ecological research [1, 9]. Soil microbiomes, encompassing bacteria, fungi, archaea, and viruses, play a crucial role in element cycling, nutrient transformation, and ecosystem services in terrestrial ecosystems. They are also widely used as biological indicators of soil quality and have significant functions in the remediation of soil pollutants [22, 23]. MPs in the soil can trigger substantial changes in the physicochemical and biochemical properties of the soil, and their accumulation in the soil profile has been proven to significantly impact soil structure, porosity, bulk density, water-holding capacity, pH value, cation exchange capacity, dissolved organic carbon (DOC) content, and nutrient availability [24]. Moreover, MPs provide an ideal niche for the colonization of soil microorganisms (such as plastic-degrading microorganisms, including Proteobacteria and Ascomycota) and the formation of biofilms [25, 26]. Therefore, it is imperative to systematically elucidate the response of soil microbiomes to MPs.

Although extensive research has been conducted on the impact of MPs on terrestrial ecosystems, most studies have primarily focused on understanding the occurrence, sources, and ecotoxicological effects of MPs in the soil, rather than exploring their in-depth impacts on microbiomes [27,28,29]. Soils contaminated with MPs possess unique microbial community structures and have become a current research focus; yet, there are limited reviews on the interaction between MPs and soil microbiomes [30]. Notably, previous research pays insufficient attention to the effects of soil microbiomes on the environmental behavior of MPs, such as degradation, adsorption, and additive leaching [31,32,33]. This review explores the impacts of MPs on soil microbiomes and their activity and ecosystem functions, such as carbon and nitrogen cycling and greenhouse gas emissions (Fig. 1b). Thereafter, it describes the impacts of soil microbiomes on the environmental behavior of MPs and summarizes the current progress in bioremediation strategies for soil MPs. In-depth research on the interactions between MPs and soil microbiomes not only aids in revealing the ecological risks of MPs in soil ecosystems but also benefits the scientific management and remediation of microplastic pollution.

Impacts of MPs on soil microbial community

Microplastics significantly impact soil microbial communities and soil ecosystem functioning through both direct and indirect pathways [34,35,36]. Direct effects include the biodegradation of MPs, the provision of metabolic substrates, leaching of additives, and potential cell damage [37, 38]. Indirect effects involve altering soil physicochemical properties, changing the bioavailability of co-existing pollutants, and affecting the performance of soil fauna and plants (Fig. 2) [38, 39]. For instance, the addition of micrometer-sized PE has been proven to increase the relative abundance of Proteobacteria and Firmicutes while decreasing that of Acidobacteria and Chloroflexi in the soil [40]. This, in turn, affects microbial processes related to the decomposition of soil organic matter and nutrient cycling.

Fig. 2

The potential mechanisms of MPs on soil microbial communities are multifaceted. MPs in soil form a unique ecological niche, the “plastisphere” which can influence microbial attachment, colonization, and survival. Additionally, plastic-degrading microorganisms within the plastisphere provide metabolic substrates through biodegradation, directly affecting microbial community structure. MPs can inhibit microbial metabolism by leaching toxic additives or penetrating microbial cells. They also alter soil properties such as aggregate structure, nutrient bioavailability, and dissolved organic carbon content, indirectly influencing microbial communities. The unique surface properties of MPs can change the bioavailability of coexisting pollutants through adsorption or desorption. Furthermore, the impacts of MPs on soil fauna and plants can indirectly alter gut, rhizosphere, and symbiotic microbial communities, further affecting the broader soil microbial community

The impacts of MPs on soil microbial richness and diversity are highly dependent on the properties of MPs (such as polymer type, aging degree, dosage, and morphological characteristics) and the physicochemical properties of the soil (soil porosity, bulk density, water-holding capacity, pH, cation exchange capacity, DOC content, and nutrient availability) [41,42,43]. The type of MPs represents a critical factor influencing soil microbial communities. For instance, biodegradable polylactic acid (PLA) with a particle size range of 50–300 μm exerts a milder impact on bacterial communities within microaggregates compared to PE; however, it still reduces the proportion of macroaggregates and impairs community stability [44]. Furthermore, MPs with varying particle sizes can affect the aggregation process of soil particles and the stability of aggregates, thereby influencing microbial colonization within aggregates [45]. The shape, type, and particle size of MPs can modulate their migration and distribution in soil, which in turn exerts cascading effects on the structure and function of microbial communities [46]. MPs exhibit strong affinity for heavy metals and organic pollutants in the soil, such as Cd, Pb, polycyclic aromatic hydrocarbons, and antibiotics [47, 48]. However, with continuous exposure to the soil environment or changes in external conditions such as temperature fluctuations, pH variations, and moisture content alterations, the adsorbed pollutants will gradually be released, further affecting microbial growth and metabolism [49]. Furthermore, the degradation of both conventional and biodegradable MPs in soil is frequently associated with the release of various additives, including plasticizers, light stabilizers, and flame retardants [50, 51]. These released organic compounds are recognized as significant factors influencing the structure of soil microbial communities.

MPs in soil can serve as a unique habitat for the colonization of soil microorganisms, forming a “plastisphere” [26], which has been confirmed in recent years to indirectly influence the structure of soil microbial communities [24, 52, 53]. The extracellular polymeric substances (EPS) produced by plastisphere microorganisms, composed of proteins, lipids, polysaccharides, and nucleic acids, can act as a barrier against ultraviolet radiation and toxic compounds, providing a suitable environment for microbial colonization and growth [54, 55]. Moreover, biofilms facilitate the exchange of metabolites, genetic material, and signaling molecules between different microbial cells, which in turn promotes the selective enrichment of specific microbial species, such as plastic-degrading microorganisms, antibiotic-resistant gene-carrying bacteria, and potential pathogens [56]. These characteristics enable the plastisphere microbial community to adapt to the unique environment of the plastic surface and may influence the structure and function of the bulk soil microbial community through mechanisms such as dispersal (Fig. 2).

The enrichment of plastic-degrading microbes is a major function of plastisphere microbiota [57]. Some bacteria and fungi within soil plastisphere have been reported to have potential to degrade plastic debris [58]. Deng et al. [59] found greater roles of bacteria in soil plastisphere microbial network formation and MPs degradation than fungi. Fungal genera like Aspergillus (such as Aspergillus tubingensis and Aspergillus japonicus) isolated from the plastisphere can effectively degrade plastic and play a significant role in plastisphere microbial communities [60, 61]. At present, there are relatively few studies on archaea in the plastisphere. However, archaea are generally involved in metabolic processes under extreme conditions in soil ecosystems, such as high salinity, high temperature, or low oxygen environments [62]. Therefore, it can be inferred that archaea in the plastisphere may be involved in the metabolism of toxic substances generated during the plastic degradation process. The plastisphere also enriches some potential pathogens, such as Staphylococcus aureus and Pseudomonas aeruginosa [63, 64]. These pathogens enhance their survival capabilities and antibiotic resistance and can spread through pathways such as the food chain, water sources, and air, thereby increasing the risk of humans contracting antibiotic-resistant infections and posing a potential threat to human health [63].

Potential effects of MPs on soil microbial activity

Soil enzymes are crucial bioindicators of ecosystem multifunctionality, participating in key processes such as nutrient cycling, organic matter decomposition, and carbon and climate regulation in the soil [36]. The impact of MPs on soil enzyme activities directly relates to whether the ecosystem can maintain its multifunctionality normally. MPs typically contain a high proportion of carbon, which can be utilized by certain soil microorganisms, such as bacteria, fungi, and algae, as a carbon source for biodegradation [65]. This utilization can, in turn, influence microbial growth and activity. Enzymes such as oxidases, laccases, hydrolases, and cutinase are capable of effectively participating in the degradation of polymers [66, 67]. For instance, biodegradable MPs like polyhydroxybutyrate-co-polyhydroxyvalerate (PHBV) can create microbial metabolic hotspots in the soil by providing metabolic substrates [67]. In these hotspots, the activities of β-glucosidase and leucine aminopeptidase are 60% and five times higher, respectively, than those in rhizosphere soil.

The release of DOC derived from MPs in the environment has been identified as a key factor influencing soil enzyme activity [68]. It is estimated that over 10,000 additives are used in plastic manufacturing, and the leaching of these additives in the soil can impact the metabolic processes of soil microorganisms [69]. Research has confirmed that additives in MPs, such as phthalates, bisphenol A, and decabromodiphenyl ether, can inhibit the activity of various functional enzymes in the soil, including polyphenol oxidase, catalase, and β-glucosidase, through direct or indirect pathways [70]. MPs can also act as potential carriers of pollutants in the environment, and changes in environmental conditions can cause these adsorbed pollutants to desorb from MPs and accumulate in the soil, thereby causing stress to the survival of soil biota. The impact of MPs on microbial activity often exhibits a particle size effect, with smaller-sized MPs more easily entering biological organisms. Nanoplastics (size < 1 μm), for example, can cause oxidative stress responses upon entering organisms, generating a large amount of reactive oxygen species [71]. These reactive oxygen species can attack the cellular structures of microorganisms, causing oxidative damage to cell membranes, proteins, and other components, thereby inhibiting microbial growth.

In addition to direct impacts, MPs can alter soil properties, thereby indirectly regulating soil microbial activity. Changes in the physical and chemical properties of soil, such as aeration, permeability, and water retention, caused by MPs, are major factors affecting soil enzyme activity [36, 41]. For example, the combination of plastics with soil aggregates can reduce soil bulk density by more than 15%, affecting the supply and distribution of oxygen and water in the soil and ultimately leading to changes in soil enzyme activity [72]. Soil enzymes typically have optimal activity within a specific pH range, and deviations in soil pH caused by MPs can inhibit or activate these enzymes, thereby affecting biochemical reactions in the soil. For instance, the activities of enzymes such as catalase and urease are particularly sensitive to soil pH [73]. Moreover, MPs, with their large specific surface area, can change the bioavailability of nutrients such as carbon, nitrogen, and phosphorus in the soil through adsorption, which may potentially affect the activity of functional enzymes involved in nutrient cycling in the soil, such as urease and phosphatase [74,75,76]. The impact of MPs on soil microbial activity is closely related to their type, concentration, and particle size [38, 76, 77]. Compared with traditional nondegradable MPs, biodegradable MPs are likely to accelerate the succession rate of soil bacterial communities and enhance the complexity and stability of their ecological networks [78]. MPs with smaller particle sizes tend to exert a more pronounced impact on the structure and function of soil microbial communities. It was demonstrated that compared with large-sized MPs (4500 μm), medium-sized (0.5–1500 μm) and small-sized (0.5 μm) MPs could induce more prominent changes in soil respiration, urease and invertase activities, and the diversity of bacterial and fungal communities [79]. However, due to the interwoven nature of these factors, assessing the relationships between them and microbial activity remains challenging.

The impact of MPs on the structure and activity of soil microbial communities may, in turn, exert cascading effects on the broader soil ecosystem, including the health and function of soil-dwelling animals and plants. The changes in soil physical properties, such as porosity and aeration, induced by MPs can influence the survival and activity of soil fauna [80, 81]. One study has demonstrated that earthworms (Lumbricus terrestris) are capable of surviving in soil contaminated with low concentrations of MPs [82]. However, a substantial increase in mortality rates was observed when the concentration of MPs in the soil was elevated to 28% [83]. The MPs ingested by soil animals can cause gut microbiota dysbiosis, which represents one of the mechanisms underlying MPs’ toxicity [29]. Additives released from MPs or pollutants adsorbed on their surfaces can exert toxic effects on soil animals, thereby affecting soil ecosystem stability and function [81]. Additionally, changes in soil microbial community structure caused by MPs can influence plant growth and health [84]. For example, MPs can modify plant symbiotic microorganisms, such as arbuscular mycorrhizal fungi (AMF) [85, 86] and rhizobia [43, 87], which further affect plant nutrition and performance in MPs-polluted soils.

Microplastics can disrupt soil ecosystems and have cascading effects on human health through the food chain [88]. For instance, MPs can accumulate in human tissues via the food chain, posing potential health risks [89]. Common polymer materials, such as PE, PP, PVC, and styrene-butadiene rubber, have been widely detected in human organs and may be associated with diseases like Alzheimer’s disease [90]. Furthermore, the biofilm on MPs can enrich soil pathogens and antibiotic resistance genes, which may be transferred to humans through the food chain, increasing disease risk [91]. Therefore, the impact of MPs on soil microbial activity extends beyond the microbial community itself, affecting the entire soil ecosystem through complex ecological interactions.

Influence of MPs on soil microbiome functions

Microplastics can disrupt the normal metabolic activities of soil microorganisms, thereby influencing critical ecological processes such as carbon and nitrogen cycling and greenhouse gas emissions (Fig. 3). MPs are carbon-rich polymers containing approximately 80% carbon, which may potentially contribute to soil organic carbon (SOC) content [92]. Research has shown that when low-density polyethylene (LDPE) MPs are present in soil at a concentration of 0.01% (w/w), SOC content can increase by more than 40% [93]. However, some researchers argue that organic carbon derived from MPs should be excluded from SOC storage because it differs in source and function from other SOC components, and the bio-refractory nature of most petrochemical-based plastics can affect microbial utilization [94,95,96]. DOC, the most labile fraction of soil organic carbon, also plays a key role in the biogeochemical cycling of carbon. Generally, DOC content in soil increases in a dose-dependent manner with the addition of MPs, as the addition of MPs enhances soil porosity and microbial activity, promoting the mineralization of SOC [96]. Compared to traditional petroleum-based MPs, the addition of biodegradable MPs, such as PLA, polybutylene adipate terephthalate (PBAT), and poly(butylene succinate) (PBS), can significantly increase the amount of microbially available carbon in the soil, with the degradation of MPs by microorganisms being considered the main reason [97,98,99,100]. Moreover, smaller-sized MPs can enhance the mineralization of SOC by increasing soil porosity and aeration, which in turn promotes the accumulation of DOC through enhanced microbial activity [74].

Fig. 3

The influence of MPs on the functions of soil microbiomes. MPs can influence soil functions related to carbon and nitrogen cycling and greenhouse gas emissions by affecting soil physicochemical properties, the bioavailability of carbon and nitrogen, microbial community structure, the abundance of functional genes, and the activity of functional enzymes

Nitrogen is a fundamental element for the functioning of organisms in terrestrial ecosystems. MPs in the soil can affect nitrogen transformation and its biogeochemical cycling [101, 102]. A meta-analysis based on 60 published studies confirmed that MPs can provide a suitable anoxic environment for denitrifying bacteria (such as Bacillus, Pseudomonas, Paracoccus, and Acinetobacter), facilitating their colonization and increasing their abundance [103]. This process significantly increases the copy numbers of denitrification-related genes (such as nap and nas) by 115.3% and 23.1%, respectively [103]. Adding 0.3–1% PVC to paddy soil decreases NH₄⁺-N and NO₃⁻-N levels by 16.1–55.9% and 23.9–28.9%, respectively, likely due to reduced ammonia-oxidizing archaea activity, lower amoA gene abundance, and plasticizer leaching [104]. MPs can enhance soil aeration, thereby promoting nitrification while also altering soil pH and affecting the activity of nitrogen-transforming microorganisms. It was reported that MPs can stimulate the activity of ammonia-oxidizing bacteria (e.g., Nitrosospira) and ammonia-oxidizing archaea (e.g., Nitrososphaera), facilitating the oxidation of NH₄⁺ catalyzed by ammonia monooxygenase (encoded by the amoA and amoB genes) [105]. Furthermore, the gradient change in dissolved oxygen concentration from the exterior to the interior of the biofilm can create hypoxic conditions, making the plastisphere more conducive to attracting denitrifying bacteria. This has been confirmed by the enrichment of Bacillus and Bradyrhizobiaceae within the plastisphere, along with an increase in the abundance of genes related to denitrification and nitrogen fixation [106]. Organic nitrogen is the main form of nitrogen in the soil, accounting for 92–98% of total soil nitrogen. PE MPs can effectively promote the mineralization of organic nitrogen by improving soil redox potential and altering functional genes [107]. Particularly, biodegradable MPs can act as a carbon source to select specific taxa involved in organic N mineralization to meet microbial N demand [108].

Soil MPs can influence the abundance and activity of soil functional microorganisms by altering soil carbon components (SOC, DOC, and microbial biomass carbon) and soil physical properties (soil pore structure, water content), ultimately increasing soil CO₂ and CH₄ emissions [109,110,111]. For example, after a 30-day incubation period, the introduction of PE MPs significantly increased soil CO₂ emissions by 24–28.7%, and there was a significant positive correlation between soil CO₂ emissions and the relative abundance of species such as Mycobacterium, Aeromicrobium, Amycolatopsis, Nocardioides, and Mortierella [112]. In addition, the small-molecule compounds released by MPs during degradation may be utilized by microorganisms as extra carbon or nitrogen sources, stimulating microbial growth and metabolism and thus increasing CO₂ emissions [113, 114]. A recent meta-analysis also reached similar conclusions, finding that the introduction of MPs significantly accelerated soil carbon cycling, increased CO₂ (54.3%) and CH₄ (9.7%) emissions, and promoted the increase in functional gene copy numbers involved in SOC decomposition (such as abfA, sga, and manB) [115]. The greenhouse effect of N₂O is approximately 273–300 times that of CO₂, and the addition of MPs can affect the rates and product distributions of nitrification and denitrification by altering soil redox conditions and microbial community structure. In addition, soil plastisphere can selectively enrich certain microbial groups related to the nitrogen cycle, such as Proteobacteria and Acidobacteria, which play important roles in nitrogen fixation, transformation, and cycling, thereby affecting soil N₂O emissions [52, 111]. The impact of MPs on greenhouse gas emission fluxes in soil systems is a current research hotspot, and future studies need to further investigate the mechanisms by which MPs affect soil greenhouse gas emissions to comprehensively assess the ecological risks of microplastic pollution.

Roles of soil microbiomes in regulating the environmental behavior of MPs

Extracellular polymeric substances are secreted to form biofilms, which significantly influence the environmental behavior of MPs. The biofilm increases the specific surface area and surface roughness of MPs, thereby providing additional adsorption sites for pollutants such as heavy metals (e.g., Pb and Cd) and organic contaminants (e.g., polycyclic aromatic hydrocarbons, antibiotics and pesticides) [11, 116]. The EPS and microbial metabolites within the biofilm can interact with pollutants through various mechanisms, including hydrogen bonding, electrostatic interactions, and π-π interactions (Fig. 4) [116, 117]. These interactions not only enhance the adsorption of pollutants but also alter their adsorption mechanisms. The formation of biofilms has been shown to significantly enhance the adsorption capacity of PLA for tetracycline by 88%, as EPS components (such as polysaccharides and proteins) within the biofilm can chelate with tetracycline [118]. 

Fig. 4

Mechanism of interaction between pollutants and MPs colonized by microorganisms. Biofilm-coated MPs can interact with pollutants through van der Waals forces and electrostatic attractions and repulsions. Moreover, functional groups within the EPS of the biofilm can bind to pollutants via hydrogen bonding, halogen bonding, and π-π conjugation, thereby enhancing the adsorption of pollutants

Soil plastisphere generally enriches plastic-degrading microorganisms [26, 58], which are believed to accelerate the aging, fragmentation, and biodegradation processes of MPs. Microorganisms within the plastisphere can also degrade and transform organic pollutants, converting complex organic pollutants into non-toxic or less toxic metabolites through biodegradation. This process reduces the concentration and toxicity of pollutants in the environment [119, 120]. In the absence of biofilms, pollutants are primarily released into the environment through physical desorption mechanisms (e.g., hydrodynamic effects). However, biofilm formation can increase the retention time of pollutants on the surface of MPs, making them more likely to be transformed or immobilized through biotransformation pathways [121]. This reduces the environmental risk associated with these pollutants.

The degradation of MPs in soil is a complex, time-consuming process primarily mediated by microorganisms. The degradation process can be divided into four stages.

• (1) Microorganisms secrete extracellular enzymes to form biofilms that adhere to the surface of MPs.

• (2) The biofilm and extracellular enzymes act on the polymer surface, creating cracks and altering the physicochemical properties of MPs.

• (3) Extracellular enzymes and free radicals produced by microorganisms further catalyze the polymer, cleaving it into smaller units such as oligomers. Intracellular transport molecules then integrate these smaller units into metabolic pathways, producing energy, biomass, and metabolites.

• (4) Finally, simple molecules such as CO₂, N₂, CH₄, H₂O, and oxidized salts are released into the environment through mineralization, completing the biodegradation and metabolic transformation of MPs [122, 123]. Studies have shown that soil bacteria (e.g., Bacillus and Pseudomonas), fungi (e.g., Penicillium), and indigenous soil organisms can all participate in the biodegradation process of MPs [124, 125].

Microorganisms can secrete a variety of enzymes, such as hydrolases and redox enzymes, which can directly act on plastic polymers, breaking them down into shorter chains or smaller molecules (e.g., oligomers, dimers, and monomers). For example, Pseudomonas nitroreducens S8 and Pseudomonas monteilii S17 can form stable biofilm layers on the surface of PET, using PET as a carbon source [126]. Simple short-chain polymers, such as PE and PP, are less biodegradable, while MPs with functional groups and flexible active sites typically have higher binding efficiencies with functional enzymes and degrade more rapidly. It has been reported that after cultivating soil with ¹³C-labeled biodegradable PBS for 425 days, the mineralization rate of PBS reached 65% [127]. Fungi play an irreplaceable role in the degradation of MPs. For example, Chaetomium globosum has been shown to oxidize the surface of PVC by secreting laccase and other extracellular enzymes, introducing oxygen functional groups and increasing surface hydrophilicity [128]. Currently, there are challenges in detecting and quantifying the biodegradation of MPs in soil, and the development of advanced in-situ detection technologies is a future research focus.

Microbial colonization can alter the release patterns of plastic additives, thereby affecting the health of soil environments and ecosystems [129]. During the production of MPs, various additives such as plasticizers, light stabilizers, flame retardants, antioxidants, and antimicrobial agents are incorporated to enhance their performance [130]. It is estimated that by 2030, the production of plastic additives in Europe is expected to exceed 2.3 million tons, including commonly used additives such as bisphenol A, phthalates, nonylphenol, and brominated flame retardants [131, 132]. Most additives are incorporated into plastics in a non-covalent manner, and these potentially hazardous additives continuously leach out of plastics during the degradation process [133]. They may even migrate into soil, groundwater, and the food chain.

Plastic additives can inhibit the growth of soil microorganisms and alter their community structure while causing adverse effects such as abnormal gene expression, oxidative stress, and intestinal toxicity in soil biota [134, 135]. The leaching of plastic additives may be the primary reason for the ecotoxicological effects of MPs on environmental organisms. Kim et al. [81] used ecotoxicological methods to assess the impact of 13 different MPs on nematodes and found that the acute toxicity of MPs is mainly attributed to extractable additives. When the additives are removed, the toxic effects of MPs disappear in soil toxicity tests. Plasticizer released from MPs was recognized as the main driver of changes in soil microbiota and their ecological functions [136]. Therefore, it is necessary to systematically explore the influence of soil microorganisms on the environmental behavior of MPs [134].

Bioremediation of MPs using soil microbiomes

Due to their small size and structural stability, MPs in soil are difficult to remove using traditional physical and chemical approaches. Irrespective of the difficult-to-degrade property of MPs, bioremediation such as biodegradation represents a promising way to mitigate soil MPs pollution [135, 137]. Soil microbiomes can serve as a reservoir of microorganisms capable of degrading various types of MPs [58, 135]. Many studies have reported various MPs-degrading microorganisms isolated from soils [135, 137, 138]. Five bacterial strains (Bacillus subtilis, Paracoccus aminophilus, Pseudomonas putida, P. aeruginosa, and Acinetobactor calcoaceticus) isolated from the soil samples of plastic waste disposal sites were found to effectively degrade LDPE MPs, with weight loss ranging from 11.73% to 18.21% after 4 months [139]. A soil isolate of Brevibacillus brevis was reported to degrade PE MPs by releasing laccase enzyme and organic acids, with a degradation rate of 19.8% after 35-day incubation [140]. Soil fungi with plastic-degrading capacity have also been reported, including the species from the genera Aspergillus, Penicillium, Fusarium, Trichoderma, Mucor, and Cladosporium [141] and the phyla Ascomycetes, Basidiomycetes, and Zygomycete [138]. Notably, plastisphere generally recruits more MPs-degrading microorganisms compared to bulk soil [142, 143]. Nocardia asteroides No.11 and Rhodococcus hoagii No.17 isolated from plastic residue samples were found to grow solely on PE and cause significant weight loss, and the genes encoding laccase, a multicopper oxidase able to catalyze the initial PE breakdown to alkanes, were identified in their genomes [144].

Plants and soil fauna generally benefit the remediation of MPs-polluted soil [135]. Rhizosphere microorganisms such as Serratia plymuthica and Laccaria laccata promoted the degradation of PLA and PET MPs and plant growth, indicating that they can be used to accelerate the remediation process of these MPs in cultivated soil [145]. Inoculation with plant-growth promoting rhizobacteria (PGPR) [146] and AMF [147] can reduce MPs-induced stress on plant-soil systems. In addition, soil faunal gut microbiota constitutes a key part of the soil microbiome. Studies have shown that earthworm gut bacteria have MPs-degrading capacity [148, 149]. Thus, introduction of soil fauna may facilitate the bioremediation of soil MPs.

Current studies have several limitations. The first one is that current MPs-biodegradation studies were conducted in synthetic media with strict incubation conditions (e.g., pH and temperature) [141]. However, since soil systems have complex and variable characteristics, whether the degrading microorganisms can grow normally and maintain their degradation activity in soils remains unknown. Another challenge is that microorganisms with plastic-degrading capability in soil ecosystems are often not dominant taxa and difficult to track [65, 150]. To address this limitation, high-throughput sequencing techniques (e.g., metagenomics) and DNA stable isotope probing can be employed to identify plastic-degrading microorganisms. These approaches effectively overcome the challenge of culturing most environmental microorganisms, thereby allowing the identification of microbial strains and communities with high-efficiency degradation capabilities. Finally, most studies selected only pure microbial strains [141], with little attention to microbial consortium [151]. Integrating synthetic biology and microbial community engineering can facilitate the design and construction of efficient microbial degradation systems. This not only enhances the bioremediation of soil MPs but also offers new insights for the sustainable management of plastic waste.

Conclusions and future perspectives

Microplastics can markedly alter the physicochemical properties of soil, including porosity, aeration, and pH, thereby influencing the structure of soil microbial communities. This influence manifests in changes to the relative abundance and diversity of bacteria, fungi, archaea, and viruses. MPs provide a unique habitat for microorganisms, facilitating biofilm formation and thereby affecting microbial metabolic activities. Moreover, MPs exert multifaceted impacts on soil ecosystem functions, such as disrupting the carbon and nitrogen cycles and altering greenhouse gas emissions. Soil microbiomes, in turn, play a significant role in the environmental behavior of MPs. For instance, biofilm formation enhances the adsorption capacity of MPs for pollutants, while microbial activity promotes the degradation of MPs. These findings not only help to reveal the ecological risks of MPs in soil ecosystems but also provide a theoretical basis for the scientific management and ecological risk assessment of MPs pollution.

Current research has paid insufficient attention to archaea and viruses, which may play unique and important roles in soil ecosystems. Future studies should focus on strengthening research on these microorganisms. Most current studies have been conducted through short-term cultivation experiments under controlled laboratory conditions, lacking support from long-term and field experiments. This may lead to discrepancies between research findings and the actual conditions in natural environments. Therefore, there is a need to enhance long-term experiments and field studies to better simulate the impacts of MPs in real environments.

Moreover, future research should integrate multi-scale approaches to comprehensively assess the impacts of MPs on soil microbiomes, spanning from the microscopic cellular and molecular levels to the macroscopic ecosystem level. This multi-scale perspective will provide a more holistic understanding of how MPs influence microbial community structure, function, and interactions within soil ecosystems. In the context of global climate change, the interactions between MPs and soil microbiomes are likely to be modulated by factors such as temperature, precipitation, and land-use changes. Therefore, it is essential to investigate the response mechanisms of soil microbiomes to MPs under changing environmental conditions.