Hidden Carbon: How Polymers Influence Soil Organic Matter and Carbon Cycling

Abstract

Anthropogenic polymers have become an increasingly important class of emerging contaminants in terrestrial ecosystems. While extensive research has focused on microplastics in aquatic environments, their interactions with soil systems and particularly with soil organic matter (SOM) remain insufficiently understood. Soil represents a major environmental sink for polymer residues originating from agricultural practices, urban activities, and atmospheric deposition. Accordingly, associations between polymers and SOM, including humic substances, may significantly influence the retention, mobility, and transformation of carbon in soil systems. This review synthesizes current knowledge on the influence of synthetic polymers on soil organic matter dynamics. A bibliometric and qualitative literature analysis based on publications indexed in Web of Science and Scopus from 1979 to 2025 was conducted to identify major research trends and knowledge gaps. The results indicate that polymer particles can alter soil structure, microbial activity, and sorption processes, thereby affecting the stability and cycling of soil organic carbon. Interactions between polymer surfaces and humic substances may modify aggregation processes and influence the persistence and mobility of both polymers and organic carbon compounds. Despite the rapid growth of research on microplastics, studies addressing polymer–SOM interactions remain limited and methodologically heterogeneous. Greater integration between polymer research, soil science, and land use studies is necessary to better understand the implications of polymer contamination for soil quality and carbon cycling. The findings highlight the need for standardized analytical approaches and interdisciplinary research frameworks to assess the long-term effects of polymers in soil ecosystems.

1. Introduction

The global production of synthetic polymers has increased rapidly over the past decades, exceeding 390 million tons annually, and continues to grow due to expanding industrial, agricultural, and consumer applications [1,2]. As a consequence, large quantities of plastic materials and polymer residues enter the environment, where they undergo fragmentation and transformation into smaller particles, including microplastics and nanoplastics [3,4]. Although the environmental impacts of plastic pollution have been widely investigated in marine and freshwater systems, terrestrial ecosystems, and particularly soils, have received comparatively less attention [5,6].

At the same time, soils represent one of the largest environmental reservoirs of anthropogenic polymers [7]. Polymer inputs to soil systems originate from multiple pathways, including agricultural practices such as plastic mulching, the application of sewage sludge and compost, atmospheric deposition, urban runoff, and the degradation of plastic materials in situ [8,9]. The magnitude and pathways of polymer accumulation often depend on land use systems, with agricultural soils frequently representing major entry points due to intensive management practices and the widespread use of plastic materials in crop production. In contrast, urban and peri-urban soils may receive polymer particles through atmospheric deposition, traffic-related emissions, and urban waste streams. As soils act as both sinks and transformation sites for polymer particles, their accumulation may influence key soil processes and ecosystem functions [9,10]. Carbon stabilization within soil organic matter (SOM) is strongly influenced by ecosystem structure, particularly in forest systems [11,12]. In these environments, factors such as canopy gaps, topography, and deadwood distribution can also regulate the input and distribution of polymer particles in the soil profile [13,14]. In this respect, preservation of ‘naturalness’ within these landscapes [15] can sustain soil as a carbon sink. However, these processes are sensitive to broader environmental stressors. For instance, shifts in seasonal surface runoff due to climate and land-use change simulated using hydrological modeling [16,17,18,19] can alter the transport and degradation rates of soil polymers, and managing these dynamics through torrent control structures and life cycle assessments [20,21,22] can improve long-term soil stability and carbon retention. Additionally, soil health and carbon sequestration potential is also influenced by the socio-ecological dimension and forest potential to provide essential ecosystem services [23,24].

Within soil systems, processes such as nutrient turnover, soil structure formation, and carbon storage are closely linked to soil organic matter (SOM), with soil organic carbon (SOC) representing the largest carbon pool in terrestrial ecosystems. This pool is a major component of the global carbon cycle and contributes to climate regulation. Interactions between anthropogenic polymers and SOM may therefore affect soil functioning and carbon stabilization through changes in soil aggregation, microbial communities, and sorption processes involving both organic and inorganic substances in soil systems [9,25].

In particular, humic substances—including humic acids, fulvic acids, and humin—represent highly reactive components of SOM that strongly influence the behavior of contaminants in soils [26]. Associations between polymer surfaces and humic substances may alter the physicochemical properties of both polymers and soil organic matter, potentially affecting the mobility, persistence, and environmental fate of these materials [27].

In soil environments, the behavior of polymer particles is controlled by a combination of physical, chemical, and biological processes. Soil texture, mineral composition, organic matter content, and microbial activity may significantly influence the transport, transformation, and persistence of polymer particles in soil systems [9,10]. Polymer particles may become incorporated into soil aggregates, interact with mineral surfaces, or be subjected to microbial colonization and biofilm formation, which can further modify their environmental behavior and degradation pathways [5]. As a result, the fate of anthropogenic polymers in soils is strongly linked to the structure and dynamics of the surrounding soil matrix.

Soil organic matter plays a particularly important role in regulating the mobility and environmental interactions of polymer particles. The complex structure of SOM, composed of diverse organic compounds and humified materials, provides numerous reactive sites that can participate in sorption and association processes with polymer surfaces [26,28]. Humic substances, due to their high molecular heterogeneity and functional group diversity, may influence the aggregation, surface charge, and sorption capacity of polymer particles [27]. These interactions may alter the stability and transport behavior of polymers in soils and may also affect the sorption of other contaminants associated with polymer particles (Boots et al. [25]).

At the same time, investigating polymer particles in soils remains methodologically challenging. The heterogeneous composition of soils, the small size of polymer fragments, and the presence of organic and mineral matrices complicate their detection and quantification [7]. Various analytical approaches, including spectroscopic and thermal techniques, have been applied to identify and characterize polymers in environmental samples [6]. However, differences in sampling procedures, extraction methods, and analytical protocols often limit the comparability of results across studies [7]. These methodological limitations highlight the need for systematic synthesis of available knowledge regarding the interactions between polymers and soil organic matter.

Despite increasing scientific interest in microplastics and other anthropogenic polymer particles, their effects on SOM dynamics and their role in soil carbon cycling remain insufficiently understood. Many existing studies focus primarily on the occurrence and distribution of microplastics, while fewer investigations address the mechanisms governing polymer–SOM interactions and their implications for soil ecosystem processes [7,9].

Although numerous reviews have addressed polymers in soils [29,30,31,32] and soil organic matter dynamics [33,34,35,36,37], a comprehensive synthesis explicitly linking polymer inputs to SOM transformation and carbon cycling mechanisms remains lacking. The aim of this review is to synthesize current knowledge on the effects of anthropogenic polymers on soil organic matter dynamics and associated soil organic carbon processes. Particular attention is given to polymer–SOM interactions, including those involving humic substances, and their implications for carbon stabilization, microbial activity, and soil structural dynamics. This review is based on peer-reviewed literature published primarily between 1979 and 2025, with an emphasis on studies from the last decade to capture recent advances in analytical techniques and conceptual understanding.

2. Materials and Methods

A systematic literature search was conducted using Scopus and Web of Science Core Collection, selected for their broad coverage of environmental and soil sciences

2.1. Bibliometric Assessment

Literature Search Strategy

A comprehensive and systematic literature search was conducted using two major multidisciplinary scientific databases: Scopus and the Science Citation Index Expanded (SCI-Expanded), accessed through the Web of Science (WoS) Core Collection. These databases were selected due to their extensive coverage of peer-reviewed research in environmental sciences, soil science, polymer science, ecology, agronomy, and related disciplines.

The search strategy targeted studies investigating the effects, interactions, transformations, and ecological implications of natural and synthetic polymers in soil systems, with particular emphasis on their influence on soil organic matter and soil organic carbon dynamics. Core concepts included: polymers (synthetic and natural), microplastics and nanoplastics, biodegradable polymers, soil organic matter, soil organic carbon, carbon sequestration, soil structure, aggregation, microbial processes, and biogeochemical cycling.

Search terms were constructed using controlled combinations of thematic and material-related keywords. Search strings combine keywords using Boolean operators and truncation to capture relevant variations. All retrieved records were screened in accordance with the PRISMA framework (Figure 1) to ensure transparency, reproducibility, and systematic reporting [38].

To ensure methodological reproducibility, database-specific search strings were developed to capture different logical constructions of polymer–soil interactions. Representative search expressions included

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(“influence of polymers on soil organic matter”) OR (“influence of polymers on soil organic carbon”)

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(polymer OR microplastic* OR nanoplastic* OR “plastic debris”) AND (“soil organic matter” OR “soil organic carbon” OR SOC OR SOM)

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(natural polymer* OR biodegradable polymer* OR polyester microfibers OR polyacrylate* OR nanopolymer* OR geopolymer* OR polypropylene OR polyethylene) AND (soil* AND (organic matter OR organic carbon))

Search strings were adapted to the indexing structure and field tags of each database.

No temporal restrictions were applied; all literature indexed up to the date of the search was considered. The inclusion criteria encompassed peer-reviewed research articles, review papers, book chapters, and conference proceedings published in English. Editorials, theses, non-peer-reviewed materials, and gray literature were excluded.

Duplicate records were removed through a two-stage procedure. First, automated matching based on Digital Object Identifiers (DOIs) and exact title correspondence was conducted using Microsoft Excel (2024). Second, manual verification was performed to resolve discrepancies arising from missing DOIs, variations in author names, typographical inconsistencies, or minor title differences.

Bibliographic metadata, including titles, abstracts, authors, affiliations, publication year, and journal sources were verified and standardized to harmonize author names and institutional affiliations prior to bibliometric analysis.

Study selection followed a two-stage screening protocol. Inclusion criteria included

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Peer-reviewed research article, review paper, book chapter, or conference proceeding;

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Explicit examination of polymers, microplastics, nanoplastics, or related materials in soil systems;

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Direct or indirect assessment of impacts on soil organic matter, soil organic carbon, soil aggregation, microbial activity, or carbon cycling;

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Complete and accessible bibliographic metadata.

Exclusion criteria:

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Non-peer-reviewed publications;

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Studies not involving soil systems;

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Studies where polymers were incidental contaminants without analysis of soil organic matter or carbon dynamics;

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Inaccessible full texts or incomplete abstracts;

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Insufficient methodological transparency.

Two independent reviewers screened titles and abstracts. Potentially relevant studies proceeded to full-text evaluation. Disagreements were resolved by consultation with a third reviewer. Exclusion reasons were categorized as: (A) out of scope; (B) non-peer-reviewed; (C) insufficient data; (D) inaccessible text; (E) inadequate methodology.

Following the screening process, the finalized corpus of publications was subjected to bibliometric analysis. Indicators were evaluated across nine dimensions: publication type, disciplinary focus, temporal trends, geographic distribution, authorship networks, institutional affiliations, journal sources, publishers, and keyword frequency.

Data processing and descriptive analytics were performed using Web of Science [39], Scopus analytical tools [40], Microsoft Excel (2024) [41], and Geochart [42]. Network visualizations of co-authorship, co-citation, and keyword co-occurrence were generated using VOSviewer (v.1.6.20) [43].

2.2. Qualitative Content Analysis

A qualitative content analysis was conducted on the full dataset to identify dominant mechanisms, conceptual frameworks, and methodological approaches related to polymer–soil organic matter interactions.

Publications were systematically coded and grouped into thematic clusters reflecting major research directions, including

• Effects of synthetic polymers and microplastics on soil organic carbon dynamics;
• Interactions between biodegradable polymers and soil microbial communities;
• Impacts on soil aggregation, structure, and carbon stabilization mechanisms;
• Polymer-induced changes in biogeochemical cycling processes;
• Methodological approaches for detecting and quantifying polymer–SOM interactions.

These thematic categories informed the structure of Section 3 and are presented schematically in Figure 2.

3. Results and Discussion

3.1. A Bibliometric Review

The dataset (328 publications) is dominated by research articles (86%), followed by reviews (10%), indicating a strong emphasis on primary research in this field (Figure 3a). The predominance of articles is typical for bibliometric studies in general [44,45,46,47]; however, in this case, a relatively large number of review articles can also be observed. This is likely due to the wide variety of polymer types and their numerous influences on soil organic carbon.

Our inventory revealed 29 research areas in which the published articles can be classified. Among these, the most important are Environmental Sciences & Ecology (95 articles), Agriculture (67 articles), and Engineering (39 articles) (Figure 3b).

During the period 1973–2025, the annual number of published articles increased continuously. While in the first 30 years only 1–4 articles per year were published, beginning around 2010, the number expanded markedly, reaching 56 articles in 2025 (Figure 3c). Similar trends have been reported in other cases [48,49,50,51]. This exponential increase reflects growing scientific attention to polymer contamination in soils, particularly following the expansion of microplastics research after.

Researchers from 47 countries across five continents have contributed to publications on this topic (Figure 4). The most represented countries are China (76 articles), the United States (40 articles), India (18 articles), and Germany (17 articles). As with other topics [52,53,54,55], the countries with the most publications are China and the United States. This can be explained by their large populations and, consequently, large research communities, the extensive land areas available in both countries, and the widespread use of polymers.

Countries can be grouped into seven clusters, of which 4 contain at least four countries. The first one includes: Australia, India, New Zealand and Taiwan; the second includes: Brazil, Finland, Mexico, Norway and Spain; the third includes: Denmark, England, Iran and Italy; the fourth includes the Czech Republic, Greece, Pakistan and Saudi Arabia (Figure 5). No clear pattern of clustering among these countries could be identified.

Articles on this topic have been published in 113 journals, with the most contributions coming from Journal of Hazardous Materials, Environmental Science & Technology (with 11 articles each), Soil Biology & Biochemistry (8 articles), Applied Soil Ecology, and Soil Science Society of America Journal (7 articles each) (

Table 1. The most representative journals publishing articles on the influence of polymers on soil organic matter.

Cur. No.	Journal	Documents	Citations	Total Link Strength
1	Journal of Hazardous Materials	11	1132	13
2	Environmental Science & Technology	11	1582	7
3	Environmental Research	3	44	4
4	Journal of Environmental Management	3	900	4
5	Sustainability	4	50	4
6	Applied Soil Ecology	7	128	3
7	Journal of Soils and Sediments	4	68	3
8	Geoderma	4	265	2
9	Environmental Pollution	6	351	1
10	Soil Biology & Biochemistry	8	320	1
11	Soil Science Society of America Journal	7	237	1
12	Water Research	4	190	1

and Figure 6).

Four major publishers dominate this field, consistently occupying the top positions in most bibliometric studies [56,57,58,59]. In this case, the ranking is as follows: Elsevier (85 articles), Wiley (19 articles), Springer Nature (16 articles), and MDPI (13 articles).

Apart from the terms used in our search query, the most frequently used keywords in the published articles are microplastics, nitrogen, carbon, and plastics. These keywords generally fall into two categories: (1) terms related to categories of polymers influencing soil organic matter, where plastics and particularly microplastics are the most represented; and (2) terms related to the processes resulting from these interactions, such as sorption, adsorption, pollution, and decomposition (

Table 2. The most frequently used keywords in articles on the influence of polymers on soil organic matter.

Crt. No.	Keyword	Occurrences	Total Link Strength
1	organic matter	38	104
2	microplastics	26	83
3	soil	27	66
4	nitrogen	21	61
5	carbon	20	55
6	plastics	14	47
7	sorption	16	46
8	pollution	13	45
9	microbial community	14	44
10	adsorbtion	17	43
11	water	14	41
12	diversity	12	40
13	sediments	12	39
14	bacteria	12	38
15	growth	14	38
16	decomposition	13	37

).

The keywords can be grouped into several clusters, three of which contain at least 10 terms. The first cluster includes terms related to the communities of organisms involved, such as bacteria, communities, community structure, enzymes, leaf litter, microbial community, and polymer. The second cluster mainly contains terms related to biological and physicochemical processes, such as adsorption, mechanisms, sorption, stability, pH, and impact. The third cluster predominantly includes terms related to identification and analytical approaches, such as extraction, growth, identification, substances, and sediments (Figure 7).

3.2. Literature Review

3.2.1. Global Research Trends on Polymer–Soil Organic Matter Interactions

Table 3. Geographic distribution and thematic focus of studies on polymer effects on soil organic matter.

Polymer Type	Soil System/ Environment	Main Effects on SOM & Carbon Cycling	Key Mechanisms	Representative Studies
Conventional microplastics (PE, PP, PET)	Agricultural soils, paddy soils, mangrove sediments	Altered CO2 emissions, disrupted SOM turnover, changes in aggregate-associated C	Physical soil structure modification; microbial habitat alteration	Zhang et al., 2019 [60]; Lin et al., 2024 [61]; Wang et al., 2024 [62]; Liu et al., 2026 [63]; Li et al., 2021 [64]; Du et al., 2025 [65]; Kumari and Chakraborty, 2024 [66]; Zhang and Zhang, 2020 [67]; Gao et al., 2021 [68]; Lu et al., 2025 [69]; Wang et al., 2024 [70]
Biodegradable plastics (PLA, PHB, PBAT)	Agricultural soils, controlled degradation studies	Enhanced or altered SOM mineralization; priming effects on native carbon	Microbial co-metabolism; labile carbon release	Huo et al., 2024 [71]; Shi et al., 2025 [72]; Fojt et al., 2022 [73]; Beltrán-Sanahuja et al., 2021 [74]; Sera et al., 2022 [75]; Fu and Zhou, 2021 [76]; Senko et al., 2024 [77]
Superabsorbent polymers/hydrogels	Arid and semi-arid soils	Increased water retention; changes in carbon dynamics and aggregation	Improved moisture availability; aggregation stabilization	Ali et al., 2021 [78]; Sroka et al., 2025 [79]; Guarda-Reyes et al., 2026 [80]; Gu and Doner, 1993 [81]; Ai et al., 2024 [82]
Polymer effects on microbial processes	Forest, paddy, and clay soils	Changes in microbial carbon pump, enzyme activity, SOM decomposition	Microbial community shifts; enzyme regulation	Xiao et al., 2021 [83]; Guo et al., 2021 [84]; Andreetta et al., 2013 [85]; Bottone et al., 2022 [86]
Polymer transport & soil structure interactions	Sandy, carbonate-rich, and porous soils	Modified transport/retention of organic carbon and contaminants	Sorption, aggregation, mineral interactions	Fan et al., 2025 [87]; Adrian et al., 2019 [88]; Kwarkye et al., 2025 [89]; Ai et al., 2024 [82]; Karnchanasest and Hawker, 2011 [90]; Bhuvaneswari et al., 2025 [91]; Breuckmann et al., 2022 [92]; Li et al., 2025 [93]

presents a structured overview of selected peer-reviewed studies examining the influence of various polymer types on soil organic matter, highlighting their geographic distribution, main research focus, and corresponding literature sources.

The dataset presented in Table 3 reveals several overarching patterns regarding research focus, geographical distribution, and thematic development in studies addressing polymers and soil organic matter (SOM).

The studies show a strong geographical concentration in Asia, particularly in China, which accounts for the majority of recent publications. Additional contributions originate from European countries (Czech Republic, Germany, Poland, Spain, Sweden, Italy, Russia), as well as Australia, Chile, India, Pakistan, Thailand, and the USA.

This distribution suggests:

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A particularly intensive research activity in East Asia.

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A balanced but smaller contribution from Europe.

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Limited representation from Africa, South America (except Chile), and high-mountain regions.

Based on article titles and stated research aims, the studies cluster into several dominant thematic groups:

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Microplastics in soil systems (transport, retention, vertical migration, aggregation, pollution characterization).

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Biodegradable polymers and bioplastics (degradation, carbon mineralization, soil interaction).

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Carbon cycling and greenhouse gas emissions (CO₂, CH₄, carbon dynamics, microbial carbon pump).

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Soil physical properties and aggregation (dispersion, aggregation, aggregate-associated carbon).

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Polymer–mineral interactions (clay minerals, carbonate media, humic substances).

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Methodological developments (analytical techniques, polymer quantification, removal of SOM for analysis).

The strongest thematic emphasis is clearly on microplastics and carbon cycling, particularly in publications from 2020 onward.

Temporal development. The publication years indicate a clear shift in research focus:

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Early studies (1990s–early 2010s) primarily investigated polymer effects on soil physical behavior and dispersion.

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Mid-period studies (2010–2018) increasingly examined degradation processes and environmental interactions.

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Recent studies (2020–2026) strongly emphasize microplastics, biodegradable plastics, greenhouse gas emissions, and detailed biogeochemical processes.

This progression reflects a transition from applied soil-conditioning research to environmental risk assessment and ecosystem-scale implications.

From the article topics, the dominant experimental approaches include

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Laboratory incubation experiments focused on CO₂ and CH₄ emissions.

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Column and leaching experiments examining transport and migration.

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Soil aggregation and physical stability analyses.

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Microbial and biochemical assessments.

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Analytical and spectroscopic characterization methods.

Field-scale and long-term ecosystem studies appear underrepresented compared to controlled laboratory experiments.

Most studies focus on: Agricultural soils; Urban soils; Forest soils; Paddy soils; Composting systems. There is little explicit attention to: Alpine or high-elevation soils; Periglacial environments; Steep slope systems

Across countries and years, three major conceptual research trends emerge:

• Polymers as contaminants (microplastics in soil ecosystems).
• Polymers as soil amendments (hydrogels, superabsorbents, biodegradable materials).
• Polymers as drivers of carbon cycle alteration (mineralization, priming, greenhouse gas fluxes).

These conceptual directions provide a structural basis for organizing the subsequent Section 3.2.2, Section 3.2.3, Section 3.2.4 and Section 3.2.5.

Overall, the literature shows a clear shift from early studies on soil physical properties toward recent research emphasizing microplastics, carbon cycling, and ecosystem-scale impacts.

3.2.2. Different Categories of Polymers and Their Influence on Soil Organic Matter

Microplastics (MPs)

Microplastics (MPs), defined as plastic particles < 5 mm [4], are increasingly detected in soils due to plastic mulching, sewage sludge application, compost amendment, flooding, landfill leakage, tire abrasion, and atmospheric deposition [94,95,96,97]. Terrestrial ecosystems are considered major sinks of MPs, with concentrations reported to be 4–23 times higher than in oceans [2]. Their persistence in soil for decades to centuries [98] allows gradual accumulation within soil aggregates.

Since plastics consist largely of carbon (~90% in polyethylene and polystyrene), MPs may contribute directly to measured SOC. Matthias C. Rillig et al. [10] highlighted that microplastic-derived carbon can be quantified as soil organic carbon using conventional methods, potentially leading to overestimation of actual biogenic carbon sequestration. In Swiss riparian soils, microplastic-derived carbon represented approximately 0.1–5% of total SOC [99,100], while Australian industrial soils contained up to 6.7% microplastic per soil weight. These findings raise the question of whether microplastic carbon should be considered part of “true” SOM.

Similarly, microplastics as carbon-rich synthetic materials may increase apparent soil carbon storage [10]. However, polyethylene releases only 0.11–0.48% of its carbon as dissolved organic carbon (DOC), of which only 22–46% is biolabile [101], suggesting limited real priming of native SOM. Apparent priming may instead occur via microbial biomass turnover [102].

MPs can also be incorporated into soil aggregates, altering the distribution and stabilization of organic carbon [9,10,99]. Residual plastic films reduce water permeability and modify soil organic matter content and available phosphorus [103,104]. A meta-regression analysis showed that increasing mulch residue decreased soil organic matter (−0.8%) and available phosphorus (−5%) [103].

Microplastic addition has been shown to activate organic C, N, and P pools and increase DOC [105]. In loess soil incubation experiments, high MP addition (28%) significantly increased DOC, DON, DOP, NH₄⁺, and NO₃⁻ concentrations and stimulated enzyme activities (FDAse and phenol oxidase), promoting the accumulation of high-molecular-weight humic-like substances [105]. MPs therefore influence not only total SOC but also its dissolved and bioavailable fractions.

MP exposure modifies microbial communities and functional genes related to carbon cycling. The presence of PE, PVC, PS, and PP promoted soil organic matter and enhanced the expression of N- and C-cycling genes at low concentrations (0.5–2%), increasing SOC storage by 6.7–93% [106]. In addition, plastisphere microbiomes showed higher organic carbon degradation capacity compared to bulk soils [107].

Conversely, field investigations in urban soils revealed a significant negative correlation between MP abundance and soil organic matter (Pearson r = −0.512, p = 0.013) [91], suggesting that long-term MP accumulation may disrupt SOM balance.

Beyond the previously discussed effects, microplastics significantly influence the SOM-mediated dynamics of heavy metals (HMs). A meta-analysis of 790 datasets showed that MPs increased the bioavailability of Cu, Pb, Cd, Fe, and Mn, with effects modulated by soil pH and organic matter [108]. Polystyrene MPs enhanced Pb bioavailability and uptake by ramie by altering soil pH, reducible Pb fractions, dehydrogenase activity, cation exchange capacity, and SOM [109].

Polyethylene MPs also accelerated microbial degradation of carbendazim by stimulating microbial activity and regulating soil organic carbon [110]. Taken together, available evidence suggests that MPs influence SOM both structurally and through interactions with contaminants.

Taken together, these studies indicate that microplastics do not simply increase soil carbon storage but rather redistribute carbon among functional pools while altering microbial processing and nutrient dynamics.

Biodegradable Polymers

Biodegradable polymers such as PBAT, PLA, PHB, and biodegradable mulch-derived MPs exhibit dynamic interactions with SOM. Mineralization of biodegradable mulch MPs was significantly influenced by soil type, temperature, and particle size [71]. Smaller particles enhanced mineralization, while larger particles induced stronger priming effects. At higher temperatures (40 °C), the results of the experiment suggested co-metabolism between biodegradable polymers and native SOM.

According to another study, PBAT microplastics significantly altered soil dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and nitrate levels, increased microbial biomass carbon and nitrogen, and shifted bacterial and fungal community composition [111]. These findings indicate that biodegradable MPs directly enter SOM cycling pathways.

Biodegradable polymer materials used as slow-release fertilizers increased soil pH, organic matter, and phosphorus availability while potentially reducing heavy metal activity [76].

In contrast to conventional plastics, biodegradable polymers actively participate in SOM cycling, often stimulating microbial activity and accelerating carbon turnover.

Polyester Microfibers (PMF)

Polyester microfibers represent a specific synthetic polymer category widely detected in soils [112]. In incubation experiments, PMF addition (0.1–0.3%) did not significantly change total SOC but redistributed carbon within aggregate fractions, decreasing organic carbon in large macro-aggregates while increasing it in smaller aggregates [99]. Less than 30% of the added PMF was incorporated into aggregates.

PMF increased cellulase and laccase activities and reduced natural organic carbon accumulation as PMF levels increased [84]. These results indicate that PMFs modify SOM distribution and microbial-mediated carbon cycling rather than increasing total carbon stocks.

Polypropylene (PP)

Polypropylene microplastics influence organic matter mineralization and greenhouse gas emissions. In mangrove sediments, low-dose PP (0.1%) increased CO₂ emissions, while higher doses (1–10%) decreased emissions, demonstrating dose-dependent SOM mineralization responses [61].

In composting systems, reusable polypropylene packing (RPP) suppressed organic matter degradation when used alone due to high moisture retention and reduced aeration. However, mixing RPP with sawdust improved DOC degradation and reduced nitrogen loss [64].

Polyethylene (PE)

Polyethylene interacts with SOM by influencing microbial biomass and humic substance formation. In long-term incubations, PE wax increased bacterial biomass and temporarily enhanced humic substances [75]. However, its mineralization remains minimal [101].

Polyacrylates (Superabsorbent Polymers)

Polyacrylates are crosslinked superabsorbent polymers used to improve soil moisture retention. Their addition increased soil respiration and CO₂ emissions in sandy soils, especially when combined with CaCO₃, due to alkalinization effects [79]. Thus, polyacrylates can indirectly influence SOM decomposition through pH modification.

Nanopolymers and Engineered Nanoparticles

Amphiphilic nanopolymers enhanced the removal of organochlorines from soils, with SOC and soil texture strongly influencing sorption processes [90].

Polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) exhibited strong sorption in soils rich in clay and SOM [88]. SOM influenced nanoparticle mobility through electrostatic interactions and steric repulsion [113,114]. These interactions demonstrate that SOM regulates the transport and retention of polymer nanoparticles.

Geopolymers

Geopolymers used for soil stabilization interact with humic and fulvic acids. Increasing organic matter content altered the mechanical properties and microstructure of geopolymer-stabilized soils [115], indicating that SOM composition influences geopolymer performance.

Natural Biopolymers

To contextualize the behavior of anthropogenic polymers in soil systems, it is essential to consider naturally occurring biopolymers that have long been integral components of soil organic matter. These natural polymers provide mechanistic analogs and contrasts for understanding how polymer chemistry, degradability, and interactions with minerals and microorganisms influence carbon cycling and stabilization processes. Therefore, this section examines key plant- and microbially derived biopolymers to highlight fundamental processes that are also relevant for interpreting the fate and function of synthetic polymer inputs in soils.

• Cutin and Suberin

Cutin and suberin are plant-derived aliphatic biopolyesters that constitute major structural components of leaf cuticles and root periderm, respectively. Due to their intrinsic chemical recalcitrance and hydrophobicity, they contribute significantly to relatively stable soil organic matter (SOM) pools. In Mediterranean forest soils, distinct stabilization patterns were observed between leaf- and root-derived biomarkers, depending strongly on soil physicochemical properties such as mineralogy and pH [85]. These findings underline that, unlike synthetic polymers introduced as external inputs, cutin and suberin are inherent components of plant residues that become selectively preserved during SOM formation, particularly within mineral-associated organic matter fractions.

Their persistence is attributed to long-chain aliphatic monomers (e.g., ω-hydroxy fatty acids, dicarboxylic acids) that resist rapid microbial mineralization and can associate with mineral surfaces or become occluded within aggregates. Consequently, cutin- and suberin-derived compounds are frequently considered biomarkers of plant inputs contributing to long-term carbon sequestration.

• Polysaccharides and soil aggregate dynamics: implications for natural polymer function

In contrast to the relatively recalcitrant aliphatic biopolymers (cutin, suberin), soil polysaccharides represent more dynamic, microbially mediated natural polymers that strongly influence soil aggregate (SA) formation and SOM turnover.

Experimental evidence demonstrates that the decomposability of added polysaccharides governs aggregate formation kinetics but not necessarily long-term stability. In a 99-day incubation study of a Japanese tropical soil Mizuta et al. [116], starch (readily decomposable) and cellulose (more resistant) both significantly increased macroaggregate (>1000 μm) formation compared to controls. However, starch induced a markedly faster increase in cumulative respiration and aggregate formation (maximum mean weight diameter, MWD, reached by Day 6), whereas cellulose led to a slower response. After initial formation, MWD declined similarly in both treatments, suggesting that substrate decomposability influences aggregate formation rate but not ultimate aggregate stability.

These findings are consistent with the macroaggregate–microaggregate conceptual model proposed in incubation studies using ^13C-labeled starch Guggenberger et al. [117]. Rapid fungal proliferation around substrate-rich microsites promoted macroaggregate formation through hyphal entanglement. Importantly, macroaggregate mass remained stable even after microbial biomass declined, implying that transient biological binding agents are replaced or complemented by more persistent stabilization mechanisms. Furthermore, starch-derived carbon became more effectively stabilized within microaggregates, where lower mineralization rate constants indicated enhanced physical protection.

The biochemical transformation pathways of added carbohydrates also support this dynamic interpretation. Radioisotope tracing studies Cheshire et al. [118] showed rapid redistribution of ^14C from glucose and starch into multiple sugar moieties, with substantial mineralization (60–80% of substrate C). Although total glucose declined rapidly, other sugars persisted, indicating microbial resynthesis and transformation into more complex carbohydrate fractions. This supports the view that soil polysaccharides are not merely residual plant polymers but include newly synthesized microbial products.

Long-term incubation data Novak [119] further demonstrated that starch initially decreased SOM stability by stimulating mineralization (priming effect), but newly formed organic compounds gradually regained stability. Nitrogen addition accelerated both decomposition and subsequent stabilization processes, highlighting nutrient-mediated regulation of polymer transformation.

• Structural role of soil polysaccharides in aggregation

Earlier investigations emphasized the binding role of soil polysaccharides in soil aggregation processes. It has long been recognized that the polysaccharide fraction of soil humus contributes significantly to structural stability. However, the structural nature of soil polysaccharides remains debated: whether they represent mixtures of partially decomposed plant and microbial polymers stabilized through interactions with minerals and humic substances, or newly synthesized, highly complex polymers inherently resistant to degradation.

Work by Martin [120] proposed that improved fractionation approaches—particularly those based on molecular weight distribution and uronic acid content—are necessary to better understand their binding interactions with clay minerals, metal ions, and humic substances. This perspective aligns with current understanding that organo-mineral interactions are fundamental to aggregate stabilization.

Experimental studies involving wheat straw amendments (Acton et al. [121]) demonstrated significant improvements in soil aggregation, with approximately 35% of aggregation variability explained by microbial polysaccharide (“gum”) content. Notably, peak aggregation did not coincide temporally with maximum microbial polysaccharide accumulation, suggesting that carbohydrate-derived carbon incorporated into humic acid–humin fractions also contributes to stabilization. Furthermore, nitrogen addition was found to reduce aggregate formation, indicating increased nutrients.

• Emerging context: bioplastics and polysaccharide-based materials

Recent research has extended these concepts to biodegradable polymer systems. A study investigating starch/PPst/GTR bioplastic films in riparian sediments Lakshmi [122] demonstrated progressive UV-induced degradation, release of humic-like dissolved organic matter, and increased aromaticity and humification indices during aging. Although conducted in sediment rather than soil, these findings suggest that starch-containing biopolymers can integrate into natural organic matter cycles with comparatively limited long-term persistence relative to conventional plastics.

• Integrated Interpretation

Collectively, the evidence indicates that natural biopolymers influence SOM through two contrasting but complementary pathways:

• Recalcitrant structural biopolymers (cutin, suberin)

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  Intrinsically resistant aliphatic matrices.

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  Contribute directly to stable SOM pools and mineral-associated fractions.

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  Serve as long-term carbon reservoirs.

• Labile polysaccharides (plant- or microbially derived)

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  Rapidly decomposed and microbially transformed.

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  Drive aggregate formation via transient biological binding agents.

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  Facilitate physical protection of SOM within microaggregates.

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  Undergo transformation into more stabilized humic-associated fractions.

Thus, while cutin and suberin primarily enhance SOM stability through chemical recalcitrance, polysaccharides promote SOM stabilization indirectly via aggregate dynamics, microbial synthesis, and organo-mineral interactions. Their functional role is therefore process-dependent: labile inputs stimulate structural reorganization of soil, whereas resistant aliphatic polymers contribute to long-term carbon persistence.

This duality underscores that natural polymers are not a homogeneous category in soil systems; rather, their ecological function depends on molecular structure, decomposability, microbial mediation, and physicochemical context.

Synthesis

Across polymer categories, influences on SOM include

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Apparent increases in measured SOC (microplastics);

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Redistribution of carbon within aggregate fractions (PMF, MPs);

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Stimulation or suppression of microbial activity and enzyme functions (MPs, PBAT, PE);

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Alteration of DOC, DON, and nutrient pools (biodegradable polymers);

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Modification of SOM–metal interactions and pollutant degradation;

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Indirect SOM changes through pH and moisture regulation (polyacrylates, hydrogels).

One the whole, polymer inputs do not simply increase soil carbon quantity but alter its composition, distribution, and biogeochemical cycling.

3.2.3. Mechanisms of Polymer Effects on Soil Organic Matter Transformation

Building on the identified polymer categories, this section examines the mechanisms through which polymers influence SOM transformation.

The main mechanisms by which polymers influence soil organic matter dynamics and carbon cycling are summarized in Figure 8.

Effects of Microplastics on Soil Physicochemical Properties and Aggregate Stability

The accumulation of microplastics (MPs) in soils induces measurable alterations in physicochemical properties that are directly linked to soil organic matter (SOM) dynamics. Changes in bulk density, porosity, water-holding capacity, pH, dissolved organic carbon (DOC), and aggregate stability have been widely reported [9,80,100,123]. Such modifications influence substrate diffusion, oxygen availability, and microbial habitat conditions, thereby indirectly affecting carbon turnover.

Experimental evidence indicates that MPs can modify aggregate formation depending on polymer characteristics. In purple soils, polyethylene (PE) MPs (0.40% w/w) promoted aggregation when applied as granules but inhibited it when applied as fibers or films, with effects mediated by changes in extracellular polymeric substances (EPS) and microbial community composition [63]. Structural equation modeling demonstrated both direct physical effects of MPs and indirect microbial-mediated pathways controlling aggregation. Since soil aggregates physically protect SOM from microbial decomposition, such structural alterations can substantially influence carbon stabilization mechanisms.

Beyond whole-soil structure, MPs also affect the supramolecular organization of SOM. Polyester-based plastics, including polyethylene terephthalate (PET) and poly-3-hydroxybutyrate (PHB), reduced SOM physical stability by weakening water molecule bridges and decreasing desorption enthalpy, thereby accelerating water release and destabilizing aliphatic crystallites [73]. These findings suggest that plastics influence not only bulk soil structure but also intrinsic SOM physicochemical properties.

Microplastics and Soil Organic Carbon (SOC) Storage and Mineralization

As carbon-rich materials (30–90% C in polymeric units), MPs represent an additional carbon input to soils [124]. Their persistence and relative recalcitrance suggest a potential contribution to stable SOC pools [10]. However, the net effect on SOC storage remains highly context-dependent.

Contrasting responses of SOC mineralization to MP exposure have been reported. Some studies observed enhanced SOC stability and reduced organic carbon mineralization at high MP concentrations (5–10% w/w), suggesting physical protection or dilution effects [125,126]. In contrast, other findings indicate that MPs disrupt soil aggregates, facilitating microbial access to previously protected SOM and thereby enhancing SOC decomposition [127,128]. These discrepancies likely reflect differences in polymer type, concentration, soil properties, and experimental duration.

Recent microcosm experiments further demonstrated that biodegradable MPs significantly increased soil CO₂ emissions, whereas conventional MPs exerted weaker or even inhibitory effects [72]. Notably, biodegradable MPs simultaneously enhanced microbial carbon use efficiency and promoted necromass accumulation in mineral-associated organic matter, highlighting their dual role in accelerating carbon release while contributing to stabilization pathways.

Similarly, incubation experiments with polyethylene (PE) and biodegradable polyhydroxybutyrate-valerate (PHBV) showed stronger positive priming effects under biodegradable plastic contamination. PHBV increased cumulative CO₂ emissions by 42–53% compared with PE, stimulated K-strategist dominance, and enhanced recalcitrant SOM decomposition, leading to negative net soil C balances [129]. These results indicate that biodegradable polymers may intensify SOM turnover through cooperative fungal–bacterial decomposition processes.

Dissolved Organic Matter (DOM) Dynamics and Mineral Associations

A key pathway linking polymers to SOM dynamics involves the production and transformation of dissolved organic matter (DOM). Aging and weathering of MPs promote the release of dissolved organic compounds, including additives and low-molecular-weight degradation products [78,125,130,131]. MP-derived DOM (MP-DOM) exhibits higher lability and lower humification compared with natural organic matter (NOM), resulting in increased microbial respiration and reduced mineral-associated organic carbon [132]. Specifically, MP-DOM induced 21–576% higher CO₂ emissions and 34–83% lower mineral-associated carbon relative to NOM, reflecting weaker sorption to mineral surfaces due to fewer polar functional groups and higher H/C ratios.

MPs also influence DOM formation and transformation indirectly by altering microbial biomass carbon (MBC) and enzyme activities [133]. Enhanced DOM availability may increase microbial processing and incorporation into SOC pools, but also stimulate mineralization depending on substrate quality and nutrient availability.

Interactions between synthetic polymers, microorganisms, and humic substances (HSs) further modulate degradation processes. HSs can either accelerate or inhibit polymer biodegradation through redox mediation, sorption interactions, and photo-oxidative processes, thereby influencing the fate of polymer-derived carbon in soils [77]. These interactions are critical for understanding long-term SOM transformation in MP-contaminated soils.

Greenhouse Gas Emissions and Biogeochemical Feedbacks

Microplastic contamination affects soil greenhouse gas (GHG) fluxes, including CO₂ and CH₄ emissions. Biodegradable MPs such as polylactic acid (PLA) significantly enhanced CO₂ emissions in reservoir water fluctuation belt soils, whereas conventional MPs (PE and PET) slightly inhibited emissions; aging modified these polymer-specific effects [93]. In agricultural soils, polymer-specific responses were also observed: PBAT and PLA markedly increased CO₂ and CH₄ emissions, whereas PBS reduced emissions [70]. These findings underscore the importance of polymer chemistry in determining soil carbon responses.

MP-induced shifts in microbial community structure, functional gene expression, and enzyme activities are key drivers of altered carbon cycling and GHG production [134,135,136,137]. Exposure to MPs can modify litter decomposition rates, root exudation patterns, and microbial carbon turnover, further influencing SOC formation and mineralization pathways [136].

Superabsorbent Polymers and Soil Carbon Persistence

Beyond microplastics, superabsorbent polymers (SAPs) also influence SOM dynamics by modifying soil water regimes and structure. Application of polyacrylate-based commercial SAPs and biodegradable nanocomposites reduced bulk density and increased porosity, particularly in Cambisols [80]. Water retention improved mainly at intermediate matric potentials. Importantly, the polyacrylate polymer enhanced mineralization of labile carbon, whereas the biodegradable nanocomposite favored less mineralized carbon fractions, suggesting potential for improved carbon persistence. These findings indicate that polymer-induced changes in soil hydrophysical properties can indirectly regulate microbial activity and SOM stabilization.

Synthesis and Implications for Soil Organic Matter Dynamics

Collectively, the evidence demonstrates that polymers influence SOM through multiple, interconnected pathways:

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Physical effects—modification of soil structure, aggregation, and water retention;

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Chemical effects—release of labile DOM and alteration of sorption processes;

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Biological effects—shifts in microbial community composition, enzyme activity, carbon use efficiency, and priming responses.

Conventional MPs may contribute relatively stable carbon but can destabilize protected SOM via structural disruption. In contrast, biodegradable polymers generally stimulate microbial activity, enhance priming effects, and increase short-term CO₂ emissions, although they may also promote microbial necromass formation and mineral-associated carbon under certain conditions.

The net impact of polymers on soil carbon sequestration is therefore polymer-specific and strongly modulated by soil type, nutrient status, polymer aging, morphology, and concentration. Current evidence indicates that while polymers can transiently increase SOC inputs, their overall effect often trends toward enhanced SOM turnover and greenhouse gas emissions, raising concerns regarding long-term soil carbon stability and climate feedbacks. Further integrative studies combining physicochemical and microbial approaches are required to disentangle stabilization versus destabilization mechanisms under realistic field conditions.

On the whole, polymer effects on SOM emerge from the interaction of physical, chemical, and biological processes, rather than from a single dominant mechanism.

3.2.4. Factors Controlling Polymer-Induced Changes in Soil Organic Carbon Dynamics

While the previous section focused on mechanisms, this section highlights the environmental factors controlling their magnitude and direction.

The interaction between synthetic and natural polymers and soil organic matter (SOM) is governed by complex physicochemical and biological mechanisms. These mechanisms include polymer–mineral–organic interactions, redox-mediated transformations, microbial responses, water regime, ionic strength, and pH. The reviewed studies demonstrate that polymer effects on SOM dynamics depend strongly on environmental context, polymer chemistry, and dosage.

Microplastics and Soil Organic Carbon Mineralization

The persistence of microplastics (MPs), particularly polyethylene (PE), in agricultural soils has raised concerns regarding their impact on soil organic carbon (SOC) turnover and carbon sequestration. In paddy soils, PE addition significantly altered SOC mineralization patterns under contrasting water regimes [69]. Over a 205-day incubation, PE reduced cumulative CO₂ emissions by 5.1–14.8% under both alternating wetting and drying (AWD) and continuous flooding (CF) conditions. Under CF conditions, SOC mineralization was suppressed through decreased cellobiohydrolase activity and increased microbial diversity, while under AWD, reductions in polyphenol oxidase activity were implicated.

Interestingly, PE addition increased dissolved organic carbon (DOC) concentrations, with the most pronounced effects at 1% (w/w) PE, where DOC increased by 37.2% under AWD and 18.5% under CF compared to the control. The positive correlation between DOC and mineral-associated organic carbon (MAOC) suggested that enhanced DOC adsorption onto mineral surfaces contributed to stabilized carbon pools. AWD management appeared to mitigate MP-induced alterations in SOC decomposition relative to CF [69].

Similarly, differential MP dosages influenced SOM decomposition and priming effects in paddy soils [83]. Low MP concentrations (0.01% w/w) reduced total SOM-derived CO₂ by 13.2% following straw addition and by 7.1% following glucose addition. Notably, glucose-induced priming effects were up to ten times stronger in low-MP soils than in high-MP soils (1% w/w), indicating a nonlinear response to polymer dosage. MPs had negligible effects on the mineralization of exogenous substrates but significantly altered native SOM turnover. These findings indicate that polymer concentration modulates microbial access to SOM and alters carbon cycling dynamics.

Collectively, these studies demonstrate that microplastic effects on SOM are dose-dependent and mediated by shifts in enzymatic activity, DOC dynamics, and microbial community structure. Water regime is a critical controlling factor in determining the magnitude and direction of these effects.

Polymer–Dissolved Organic Matter Interactions and Carbon Mobility

Polymer interactions with dissolved organic matter (DOM) further influence carbon transport and stabilization. Biosurfactants such as rhamnolipids and sophorolipids were shown to enhance polyethylene mobility in soil columns by adhering to PE surfaces and increasing particle stability [82]. However, the presence of DOM inhibited biosurfactant-mediated PE transport due to competition for adsorption sites on the polymer surface.

The hydrophobic fraction of DOM played a critical role in enhancing PE mobility, indicating that hydrophobic interactions significantly influence polymer–organic matter associations. Moreover, polymer transport increased with rising pH and decreasing ionic strength, highlighting the importance of electrostatic interactions. Sophorolipids produced greater PE mobility than rhamnolipids due to synergistic hydrophobic and electrostatic forces.

Although this study primarily focused on polymer transport, the findings have important implications for SOM dynamics. Enhanced polymer mobility may facilitate redistribution of associated organic compounds within soil profiles, potentially influencing vertical carbon transport and spatial heterogeneity of SOM pools.

Organic Polyanions, Clay Dispersion, and SOM Stabilization

The interaction between organic polymers and mineral surfaces strongly controls SOM stabilization mechanisms. Organic polyanions, including humic acid and anionic polysaccharides, were found to act as dispersing agents for Na-saturated clays rather than as flocculants [81]. Removal of native organic matter reduced clay dispersibility, whereas the addition of small amounts of polyanions increased dispersion.

In the absence of polyvalent cations, negatively charged humic substances did not promote stable aggregation. However, hydroxy-Al polycations effectively screened negative surface charges and acted as bridges between clay colloids and polyanions, preventing dispersion and increasing hydraulic conductivity by approximately two orders of magnitude during leaching.

These results emphasize that SOM stabilization via polymer interactions depends critically on cation bridging mechanisms. The coexistence of polyvalent cations and organic polyanions appears to enhance aggregate stability and reduce clay dispersion, thereby influencing the physical protection of organic matter within soil aggregates.

Synthetic and Biopolymer Amendments: Aggregate Stability and Transformation Products

The impact of added polymers on soil structure and SOM dynamics also depends on their biodegradability and transformation products. Amendments with polyvinyl alcohol (PVA), starch graft polymers (SGP), and microbial polysaccharide mixtures (LEV) significantly increased soil shear strength (1.5–5.5-fold) relative to untreated soil [82]. However, temporal patterns differed among treatments.

While PVA-treated soils followed similar temporal trends as controls, SGP and LEV amendments showed distinct response curves in soil detachment resistance. These changes were attributed to decomposition products formed during polymer degradation. Although shear strength measurements were insensitive to these transformations, soil detachment responses indicated that decomposition intermediates altered near-surface soil properties.

These findings highlight that polymer degradation products can exert secondary effects on SOM dynamics and soil physical behavior. Thus, polymer persistence and transformation pathways are essential determinants of long-term impacts on SOM stabilization.

Redox-Mediated Polymerization and Organo–Mineral Associations

Redox interactions between organic compounds and mineral phases further illustrate mechanisms of polymer formation and stabilization in soils. The reaction between catechol and Ce-bearing phosphate minerals demonstrated coupling between organic carbon oxidation and cerium redox transformations [138]. Under oxic conditions, catechol underwent oxidative polymerization, forming more complex organic molecules, including carboxyl- and carbonyl-rich polymers, while Ce³⁺ was oxidized to CeO₂ at the mineral surface.

Spectroscopic analyses confirmed the formation of higher molecular weight organic species associated with mineral grains. These results suggest that mineral-mediated oxidative polymerization can generate complex, potentially more stable organic polymers in soils. Such abiotic polymerization pathways may contribute to SOM complexity and long-term stabilization.

Synthesis of Controlling Factors

Across the reviewed studies, several key factors emerge as regulators of polymer influence on SOM:

• Polymer concentration—Nonlinear responses are common; low and high doses can produce contrasting effects on SOM mineralization.
• Water regime—Redox conditions and moisture dynamics modulate enzymatic activity and SOC turnover.
• Ionic strength and pH—Electrostatic interactions govern polymer adsorption, dispersion, and mobility.
• Presence of polyvalent cations—Essential for stabilizing organo–mineral complexes via cation bridging.
• Polymer degradability and transformation products—Biodegradation pathways determine secondary impacts on soil physical and biochemical properties.
• DOM composition—Hydrophobic fractions enhance polymer mobility, while competition for sorption sites modulates interactions.

Broadly, polymers influence SOM through intertwined physical, chemical, and biological mechanisms. Their effects range from enhanced carbon stabilization via mineral association and aggregation to altered carbon mineralization and priming effects. The direction and magnitude of these processes depend strongly on environmental conditions and polymer characteristics, underscoring the need for context-specific assessments when evaluating polymer impacts on soil organic matter.

3.2.5. Analytical Approaches for Determining the Influence of Polymers on Soil Organic Matter

A range of analytical techniques has been applied to characterize polymer–SOM interactions; however, methodological differences limit comparability across studies.

Elemental and Structural Characterization of Polymers and SOM

Reliable quantification of elemental composition is essential for assessing polymer contributions to soil carbon (C), nitrogen (N), hydrogen (H), and oxygen (O) pools. A novel approach combining wavelength-dispersive X-ray fluorescence (WDXRF) with partial least squares (PLS) regression enabled the determination of CHNO in polymers using Rayleigh and Compton scattering spectra of a Rh X-ray tube. This multivariate method successfully quantified matrix elements that are typically not directly measurable by XRF, and minimal spectral resolution provided the most accurate predictions [92]. Importantly, knowledge of matrix composition improved semi-quantitative fundamental parameter (FP)-based XRF analyses, particularly for plastics and soils with high organic content. Such developments are directly relevant for distinguishing polymer-derived carbon from native SOM in amended soils.

Spectroscopic techniques further provide insight into molecular-level interactions between polymers and SOM. Two-dimensional ^1H–^13C heteronuclear correlation NMR spectroscopy revealed domain-specific interactions between carbonyl-labeled benzophenone and various natural organic matter (NOM) types, including lignite, kerogen, peat, and humic acid [139]. Preferential π–π interactions between aromatic moieties of the sorbate and aromatic domains in SOM were identified, while alkyl-rich domains contributed variably depending on substrate composition. These findings highlight that SOM heterogeneity at the nanometer scale governs sorption selectivity, implying that polymer chemistry (e.g., aromaticity, polarity) will strongly influence its interaction strength and persistence in soils.

Complementary information on the SOM transformation state and radical content can be obtained using quantitative electron paramagnetic resonance (EPR). Stabilized semiquinone radicals in polyphenolic matrices of soils, peats, composts, and lignites were successfully characterized, and variations in g-values and spin concentrations were linked to the degree of organic matter transformation [140]. Because polymer inputs may alter redox-active functional groups within SOM, EPR offers a rapid monitoring tool for assessing oxidative or structural changes induced by polymer addition or degradation.

Polymer–SOM Interactions and Mobility in Solid Matrices

Beyond structural characterization, understanding polymer influence requires evaluating mobility and diffusion processes within SOM matrices. Magic angle spinning pulsed-field gradient (MAS PFG) NMR enabled direct measurement of contaminant diffusion in solid humic acid particles [141]. Diffusion coefficients of toluene were temperature-dependent and strongly influenced by matrix interactions, with a threefold decrease in activation energy above 50 °C attributed to structural reorganization of the humic matrix. These results demonstrate that SOM structural dynamics can regulate the mobility of organic molecules, suggesting that polymer presence may similarly modify matrix rigidity, pore accessibility, and transport behavior.

Surface-sensitive techniques also reveal how biogenic macromolecules mediate nanoparticle–soil interactions. Using quartz crystal microbalance with dissipation monitoring (QCM-D), the deposition kinetics of polyethylene glycol-coated TiO₂ nanoparticles were assessed in the presence of cellulase, either free in solution or adsorbed onto silica surfaces [142]. Cellulase layers significantly reduced nanoparticle deposition rates due to enhanced repulsive interactions, supported by optical NanoTweezer measurements. These findings indicate that extracellular enzymes—ubiquitous components of soil organic matter—can modulate polymer-coated nanoparticle mobility. Thus, biological constituents of SOM not only interact chemically with polymers but can also alter their environmental transport.

Extraction and Fractionation Approaches in Organic-Rich Soils

Reliable assessment of polymer occurrence and influence in soils requires effective separation from organic matrices. Oxidative digestion using Fenton’s reagent was identified as the most suitable protocol for removing organic matter while preserving the integrity of common polymer types during microplastic extraction from sludge and soil [143]. Alternative treatments (H₂O₂, NaOH, and KOH) either degraded particles or insufficiently reduced organic matter. The method’s compatibility with density separation enhances comparability across studies.

Similarly, sodium hypochlorite (NaOCl) was shown to efficiently oxidize SOM in organic-rich soils while largely preserving resistant polymers such as polypropylene, polylactic acid, low-density polyethylene, and polyethylene terephthalate [86]. Although minor surface chlorination occurred, NaOCl treatment significantly improved microplastic detectability in SOM-rich O horizons. However, recovery efficiency depended on particle size, polymer type, extraction technique, and soil characteristics, underscoring the complexity of developing standardized analytical methods.

Fractionation approaches also clarify how polymer-related processes influence SOM-bound contaminants and nutrients. A three-step fractionation using polyvinylpyrrolidone (PVP) isolated humic acids and fulvic acid subfractions in Sb-contaminated soils [144]. The PVP-non-adsorbed fulvic acid (FAA) fraction accounted for the majority of total nitrogen, total organic carbon, and antimony, demonstrating strong correlations between Sb and organic fractions. These findings emphasize the importance of operationally defined SOM fractions when evaluating polymer–metal interactions and nutrient cycling.

For hydrophobic organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), a carboxymethyl-β-cyclodextrin polymer (pCMCD) extraction method was developed to quantify bioaccessible fractions [145]. Bioaccessible PAH fractions were low (2.4–9.6%) in aged industrial soils and influenced by organic matter content, particle size, cation exchange capacity, and nitrogen content. Even the remaining bioaccessible fraction was only partially biodegradable in co-contaminated soils. These results illustrate how polymer-based extractants can provide environmentally relevant estimates of contaminant mobility in SOM-dominated matrices.

Polymer Degradation and Environmental Controls in Soil

Assessing the influence of polymers on SOM also requires evaluation of their degradation under terrestrial conditions. In situ experiments comparing conventional plastics (PE, PET, PA, PP/EVOH/PP) and polylactic acid (PLA)-based materials demonstrated minimal degradation (<2% weight loss) for conventional polymers after one year [74]. In contrast, PLA degradation was strongly enhanced at temperatures above 20 °C and high moisture availability, while soil texture exerted a minor influence. Early degradation signals were detectable via infrared spectroscopy and differential scanning calorimetry through changes in carbonyl and crystallinity indices. These findings confirm that polymer persistence in soil is highly dependent on environmental conditions and polymer chemistry, which in turn determines long-term contributions to SOM pools.

Mineral–Organic Interactions and Transformation Pathways

Interactions between organic matter and mineral phases further regulate polymer-associated processes in soil. Ferrihydrite–organic matter coprecipitates containing synthetic or microbiogenic extracellular polymeric substances (EPS) exhibited nonlinear effects on sulfidation pathways [137]. Under low sulfide loadings, transformation to goethite and lepidocrocite was favored, whereas high sulfide conditions promoted formation of Fe–S minerals such as mackinawite and pyrite. Increasing C/Fe ratios inhibited mineral transformation, particularly in the presence of microbiogenic EPS. These findings demonstrate that both quantity and chemical characteristics of associated organic matter strongly control mineralogical transformations, with implications for carbon stabilization and redox cycling in polymer-impacted soils.

Synthesis and Implications

Collectively, the reviewed methodologies demonstrate that evaluating polymer influence on soil organic matter requires an integrated analytical framework. Advanced spectroscopic (WDXRF, NMR, EPR), diffusometric (MAS PFG NMR), surface-sensitive (QCM-D), and selective extraction techniques provide complementary information on elemental composition, molecular interactions, mobility, degradation, and bioaccessibility.

The results consistently indicate that

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Polymer chemistry (aromaticity, polarity, functional groups) governs selective interactions with SOM domains.

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SOM heterogeneity and biological constituents (e.g., enzymes, EPS) modulate polymer mobility and transformation.

-

Environmental conditions (temperature, moisture, redox state) strongly influence degradation and mineral interactions.

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Operationally defined SOM fractions are critical for interpreting polymer-associated contaminant and nutrient dynamics.

Therefore, methodological refinement and standardization are essential for accurately quantifying polymer–SOM interactions and predicting their long-term ecological consequences.

3.2.6. Research Gaps, Management Actions and Future Directions

Despite the rapidly increasing number of studies addressing the influence of polymers on soil organic matter (SOM), significant knowledge gaps remain regarding their long-term environmental behavior, ecological impacts, and implications for soil carbon cycling. Addressing these gaps is essential for improving our understanding of polymer–soil interactions and for developing sustainable management strategies.

Research Gaps

One of the most important limitations in the current literature is the predominance of short-term laboratory experiments. Many studies rely on controlled incubation or microcosm experiments that evaluate polymer effects over relatively short time scales. While these experiments provide valuable mechanistic insights, they may not accurately represent long-term processes occurring in natural soil systems. Field-scale studies examining the persistence, transformation, and cumulative effects of polymers on SOM over multiple years remain scarce.

Another major research gap concerns the limited understanding of the long-term fate of polymer-derived carbon in soils. Although several studies indicate that synthetic polymers such as microplastics can contribute to measured soil organic carbon (SOC), it remains unclear whether this carbon should be considered part of functional SOM pools involved in nutrient cycling and ecosystem functioning. Distinguishing between polymer-derived carbon and biologically derived organic matter is therefore an important methodological and conceptual challenge.

Furthermore, interactions between polymers and soil microbial communities require deeper investigation. Existing research indicates that polymers can alter microbial diversity, enzyme activity, and carbon use efficiency, but the mechanisms underlying these responses are not fully understood. In particular, the role of microbial adaptation, plastisphere development, and microbial degradation pathways for different polymer types remains poorly characterized.

Another critical knowledge gap relates to the influence of environmental factors on polymer behavior in soils. Soil type, mineral composition, pH, moisture regime, and temperature all influence polymer degradation, sorption, transport, and interactions with SOM. However, comparative studies across contrasting soil environments remain limited. In addition, ecosystems such as alpine soils, permafrost regions, and tropical forest soils are still largely underrepresented in the current literature.

Methodological challenges also represent a major limitation. Analytical techniques for detecting and quantifying polymers in soils are still evolving, and differences in extraction protocols, identification methods, and reporting units make it difficult to compare results across studies. Standardized analytical frameworks are therefore needed to ensure reproducibility and comparability of research findings.

Management Actions

Given the growing evidence that polymers influence soil carbon dynamics and ecosystem functioning, several management strategies should be considered to mitigate potential negative impacts.

First, reducing the input of persistent plastic materials into soils is a critical step. In agricultural systems, this includes improving the management of plastic mulch films, irrigation pipes, and packaging materials to prevent fragmentation and accumulation of microplastics in soil environments. Enhanced recycling systems and the development of more efficient collection programs can significantly reduce plastic residues entering agricultural landscapes.

Second, the use of biodegradable polymers should be carefully evaluated before widespread adoption. Although biodegradable materials are often proposed as environmentally friendly alternatives, recent studies indicate that their degradation may stimulate microbial activity and increase carbon mineralization, potentially accelerating soil organic matter turnover. Therefore, the environmental performance of biodegradable polymers must be assessed under realistic field conditions and across different soil types.

Third, integrated soil management practices can help mitigate the impacts of polymer contamination. Practices that improve soil structure and organic matter content—such as organic amendments, reduced tillage, and diversified crop rotations—may enhance soil resilience to polymer-induced disturbances by promoting microbial diversity and stabilizing SOM within aggregates.

Finally, policies aimed at regulating plastic use and disposal in the agriculture and waste management sectors may contribute to limiting soil contamination. Environmental monitoring programs should include soil systems alongside aquatic ecosystems to better assess the distribution and impacts of polymer pollution.

Future Research Directions

Future research should prioritize long-term field experiments that assess polymer accumulation, degradation, and ecological impacts under realistic environmental conditions. Such studies are essential for understanding whether polymer inputs ultimately enhance or destabilize soil carbon storage.

Another promising research direction involves the integration of advanced analytical techniques to better characterize polymer–SOM interactions at the molecular level. Combining spectroscopic methods, isotopic tracing, and high-resolution imaging could help distinguish polymer-derived carbon from natural SOM and clarify its role in soil carbon cycling.

Interdisciplinary approaches linking soil science, polymer chemistry, microbiology, and environmental engineering will also be critical for advancing this field. These collaborations can improve our understanding of polymer degradation pathways, microbial adaptation mechanisms, and the environmental consequences of emerging polymer materials. Tests on assisted migration of tree species, including co-migration, could be conducted in different environmental conditions of actual and future genetic trials [146,147,148,149,150,151].

In addition, greater attention should be given to the development of sustainable polymer materials specifically designed to minimize environmental persistence while maintaining functional performance in agricultural and industrial applications. Designing polymers with controlled degradation rates compatible with soil ecological processes may represent an important step toward reducing long-term environmental impacts.

Broadly, future research should aim to move beyond isolated laboratory studies toward integrated ecosystem-scale assessments that consider physical, chemical, and biological processes simultaneously. Such approaches will be essential for accurately evaluating the role of polymers in soil systems and for developing effective strategies to protect soil organic matter and long-term soil health.

4. Conclusions

Anthropogenic polymers are emerging as active modifiers of soil organic matter dynamics, influencing aggregation, microbial processes, and carbon cycling.

Interactions between polymers and soil organic matter, particularly with humic substances, may affect the retention and mobility of both polymers and carbon compounds within soil systems. These interactions have the potential to influence soil organic carbon stability and carbon cycling processes, which are essential for maintaining soil fertility and regulating global carbon dynamics.

Despite increasing research interest in microplastics and polymer contamination, significant knowledge gaps remain. Many studies focus on the occurrence of polymer particles in soils, whereas fewer investigations address the mechanisms governing polymer–SOM interactions and their consequences for soil functioning. Furthermore, methodological inconsistencies and the lack of standardized analytical techniques complicate comparisons between studies and limit the interpretation of results.

Future research should prioritize the integration of soil science, environmental chemistry, and polymer research to better understand the long-term impacts of polymers in soil ecosystems. Particular attention should be given to the role of soil organic matter and humic substances in controlling polymer behavior, as well as to the influence of different land use systems on polymer accumulation and transformation in soils.

Improved analytical methodologies and interdisciplinary research frameworks will be essential for assessing the environmental risks associated with polymer contamination and for developing effective soil management strategies in a rapidly changing anthropogenic environment.

Anthropogenic polymers are increasingly recognized as emerging contaminants in terrestrial ecosystems. Available evidence indicates that polymer particles can influence soil organic matter (SOM) dynamics by altering soil aggregation, microbial activity, and sorption mechanisms. Interactions with SOM, particularly humic substances, may regulate the retention and mobility of polymers and associated carbon compounds, with potential implications for soil organic carbon stability and carbon cycling. Despite growing scientific interest, key knowledge gaps remain. Existing research largely focuses on the occurrence of polymer particles in soils, whereas fewer investigations address the mechanisms governing polymer–SOM interactions and their consequences for soil functioning. Methodological inconsistencies and the lack of standardized analytical approaches further limit cross-study comparisons. Addressing these gaps will require interdisciplinary efforts integrating soil science, environmental chemistry, and polymer science, with particular attention to SOM-mediated processes and land-use effects on polymer accumulation and transformation in soils.

In conclusion, current evidence suggests that polymers act less as passive carbon inputs and more as dynamic agents that can accelerate SOM turnover, with important implications for long-term soil carbon stability.