The study, published in the journal Microplastics and Nanoplastics, stands out for its methodological rigor. Rather than relying on bulk measurements that can be confounded by background carbon, the researchers used polyethylene enriched with carbon-13, a stable isotope that acts as a molecular fingerprint. By tracking the appearance of carbon-13 in carbon dioxide released from the incubated soil, they could measure microbial mineralization with extraordinary precision. Any carbon-13 dioxide detected had to come from the plastic, because natural soil carbon carries a far lower abundance of this heavy isotope. This allowed the team to separate the slow metabolism of plastic-eating microbes from the vast background noise of ordinary soil respiration.

The plastic itself was not simply dropped into the soil as pristine beads. The researchers first aged it with ultraviolet light, mimicking the weathering that plastic undergoes in the field before it is tilled into the ground. UV exposure breaks polymer chains and introduces oxygen-containing chemical groups at the surface, which is widely considered a prerequisite for microbial attack. Even under these favorable conditions, the soil microbial community managed to oxidize only a tiny fraction of the polymer over the 22-month incubation. The mineralization rate was highest early in the experiment and declined over time, suggesting that the most accessible, oxidized surface material was consumed first, leaving behind a polymer core that microbes could barely touch.

Extrapolated to real-world timescales, the numbers are sobering. If 0.12 percent mineralizes in less than two years, and the rate continues to fall as the remaining plastic becomes less accessible, complete degradation of polyethylene in soil would take centuries, if it happens at all under natural conditions. Agricultural soils are among the most plastic-contaminated environments on Earth, receiving fragments from mulch films, plastic-coated fertilizers, irrigation pipes, sewage sludge, and atmospheric deposition. The new findings imply that virtually every gram of polyethylene ever tilled into farmland is still there, either as visible fragments or as microscopic and submicroscopic particles dispersed through the soil matrix.

But persistence is only half of the story. The second major finding concerns where the plastic goes. Using nanoscale secondary ion mass spectrometry, or NanoSIMS, the team mapped the location of the labeled plastic inside individual soil aggregates — the small, crumb-like clusters of mineral particles and organic matter that give soil its structure. They found microplastics and even nanoplastics lodged inside pores within 1-to-2-millimeter aggregates, spaces that are typically sheltered from water flow and physical disturbance. This means plastic particles are not merely sitting on the soil surface; they are being transported into the interior architecture of aggregates, where they can reside for very long periods and become increasingly difficult to extract or study.

The physical integration of plastic into soil structure has consequences that go beyond simple contamination. Soil aggregates regulate water infiltration, aeration, root penetration, and the protection of organic carbon from decomposition. Introducing hydrophobic polymer surfaces into these delicate structures can alter how water and gases move through the soil, and may change how aggregates form and break apart. The study also found small but measurable amounts of polyethylene-derived carbon-13 incorporated into soil organic matter and into the microbial biomass itself. This indicates that some carbon from the plastic does enter the soil’s biological and chemical cycles — not through rapid mineralization, but through slow assimilation into the organic pool that sustains soil fertility.

That incorporation, however, was minor. The overwhelming majority of the labeled carbon remained as intact or partially oxidized polymer. For the researchers, this combination of extreme persistence and gradual integration is the key takeaway. Polyethylene does not vanish in soil; it becomes part of the soil. Over years and decades, fragments fragment further, migrate into smaller pores, associate with mineral surfaces and organic matter, and effectively become a permanent, synthetic component of the terrestrial environment. Unlike organic amendments that decompose into nutrients, this material accumulates, and its long-term effects on soil health remain largely unknown.

The work was carried out under the MINAGRIS project — MIcro- and Nanoplastics in AGRIcultural Soils — funded by the European Union’s Horizon 2020 research and innovation program. The project brings together institutions across Europe to assess how plastic debris affects soil biodiversity, productivity, and function. The new results provide a quantitative foundation for those assessments, offering hard numbers on mineralization rates that can feed into models of plastic accumulation in farmland. They also validate the use of isotope labeling combined with high-resolution imaging as a powerful toolkit for studying the fate of plastics in complex environmental matrices, where traditional extraction methods miss particles embedded deep within aggregates.

For farmers and policymakers, the message is clear: prevention matters far more than remediation. No known technology can remove microplastics from soil once they are incorporated, and the new data suggest there will be ample time for them to spread. Reducing plastic inputs to agricultural land — through biodegradable mulch alternatives, better recovery of plastic films, restrictions on sewage-sludge application, and improved waste management — is currently the only effective strategy for limiting the buildup. As the researchers demonstrate, every year of continued plastic input adds material that will remain in the ground long after current farming practices have changed.

The study also raises questions for future research. The incubation captured a single soil type under controlled laboratory conditions; field soils experience freeze-thaw cycles, wetting-drying pulses, root growth, and tillage, all of which can physically fragment plastic and redistribute it. Whether these processes accelerate mineralization or simply enhance the physical dispersion of particles into aggregates is an open question. What is already certain, however, is that polyethylene’s reputation as an inert, harmless filler material in soil is untenable. It persists, it infiltrates, and it slowly becomes one with the ground beneath our feet — a legacy that future generations of soil scientists, and farmers, will have to live with.

Subject of Research: Fate, mineralization, and physical integration of polyethylene microplastics in agricultural soil

Article Title: Polyethylene microplastics mineralize slowly in soil but integrate into soil structures and organic matter

Article References: Forsyth, H., Schweizer, S., Stricker, K., Höschen, C., Velescu, A., Wilcke, W., & Bigalke, M. (2026). Polyethylene microplastics mineralize slowly in soil but integrate into soil structures and organic matter. Microplastics and Nanoplastics, 6(1), Article 54. https://doi.org/10.1186/s43591-026-00223-3

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00223-3

Keywords: polyethylene, microplastics, nanoplastics, soil pollution, mineralization, carbon-13 labeling, soil aggregates, soil organic matter, biodegradation, agricultural soils, NanoSIMS, plastic degradation

The research, conducted by Leila Shafea and colleagues at the Soil Biophysics Group of Leibniz University Hannover in Germany, focused on two of the most common polymers in everyday waste: polyethylene terephthalate, or PET, the polar, relatively dense plastic of water bottles and textile fibres, and polystyrene, or PS, the non-polar, lightweight polymer of disposable cups and packaging. The team deliberately chose these two materials because they represent opposite ends of the density and surface-chemistry spectrum, and because earlier work by the same group had already examined their effects on soil physical properties. Polyethylene and polypropylene were excluded, the authors note, because their lower density and chemical stability make them less sensitive to ultraviolet ageing and less suitable for gravimetric recovery approaches.

To simulate environmental weathering in a controlled way, the researchers milled PET bottle fragments and PS plate material into three size classes ranging from roughly 400 to 1000 micrometres, then exposed half of the particles to intense ultraviolet-C radiation at 245 nanometres for 35 days in a custom-built irradiation chamber. While UVC light does not reach the Earth’s surface, the team used it deliberately to accelerate the ageing process, compressing the effects of long-term sunlight exposure into a laboratory timeframe. Earlier experiments had shown that this duration was sufficient to substantially alter the surface polarity of polystyrene particles without fragmenting them. Every four days the samples were stirred to ensure uniform exposure.

The surface transformation was dramatic and measurable by every analytical technique the team applied. Fourier-transform infrared spectroscopy revealed a marked increase in the carbonyl peak near 1730 wavenumbers and a higher carbonyl-to-methylene ratio, a standard indicator of photooxidation, in both polymers after ageing. Even polystyrene, a vinyl polymer with no inherent carbonyl groups in its backbone, acquired these oxygen-containing signatures, likely through a combination of photooxidation and thermal effects introduced during the milling process. The aged particles also developed an O-H absorption band around 3360 wavenumbers, pointing to the formation of hydroxyl groups alongside the carbonyl species.

The wettability of the particles shifted in parallel. Contact angle measurements, taken with a sessile drop method in which the behaviour of a water droplet on a bed of particles reveals how strongly the surface attracts or repels water, showed that UV-aged PET and PS were significantly more wettable than their pristine counterparts. X-ray photoelectron spectroscopy, which probes only the outermost ten nanometres of a surface, confirmed a higher oxygen-to-carbon ratio at the surface of aged particles and a decline in non-polar carbon species, with polar C-O and C=O species increasing in ways that differed between the two polymers, hinting at distinct photooxidation mechanisms for PET and PS. Nile red staining added a visual confirmation: the lipophilic dye fluoresces intensely on hydrophobic plastic surfaces, and the aged particles, having lost hydrophobic character to oxidation, stained noticeably darker and dimmer than pristine ones.

With the materials fully characterised, the team spiked samples of two contrasting topsoils, a sandy loam with 0.83 percent organic matter and a silt loam with 1.30 percent, both collected from an experimental site in Ruthe, Lower Saxony, with pristine and aged particles at a modest concentration of 0.5 percent by weight. Extraction then proceeded by density separation: the soil was first treated with an oversaturated sodium chloride solution at 1.2 grams per cubic centimetre, chosen as an environmentally benign bulk medium, followed by a smaller volume of the far denser sodium iodide solution at 1.8 grams per cubic centimetre to float out remaining particles. Residual soil organic matter was then destroyed with 33 percent hydrogen peroxide at 60 degrees Celsius for 24 hours, and the recovered particles were collected on 1-micrometre cellulose filters and weighed on a balance precise to a hundred-thousandth of a gram. Because co-recovered mineral grains inevitably inflate the gravimetric signal, the team ran blank soil controls for each texture and subtracted texture-specific correction factors.

The headline result was a robust average recovery of 81.0 percent across all 72 spiked samples, with individual rates ranging from 56.5 percent to 89.7 percent. Crucially, no statistically significant differences emerged between polymer types, between pristine and aged particles, among the three size classes, or between the two soil textures, although recovery trended slightly higher in the sandy loam, whose coarser structure and lower organic content presumably release particles more easily. The high-density sodium iodide step appears central to this robustness: the buoyant force it provides evidently overwhelmed any increase in particle-soil adhesion caused by the more hydrophilic surfaces of the aged plastics. This finding stands in sharp contrast to earlier reports, including one review citing recovery rates of only 13 to 39 percent for aged microplastics, and it suggests that the sequential chloride-iodide protocol is far less sensitive to weathering state than previous work implied.

Yet the study also uncovered a cautionary detail with implications well beyond method validation. When the recovered particles were re-examined, the aged ones had become measurably more hydrophobic than they were before extraction, while pristine particles were unchanged. A dedicated follow-up experiment, in which pristine and aged particles were incubated directly in the hydrogen peroxide treatment without any soil, pinpointed the cause: the oxidative cleaning step had partially stripped the oxidised surface layer from the aged particles, exposing fresh, unweathered polymer underneath. The fluorescence images showed bright spots on recovered aged particles, consistent with patches of newly exposed pristine surface. In other words, the very step that removes soil organic matter can also erase genuine environmental ageing signatures, meaning that post-extraction measurements of surface wettability may underestimate the true hydrophilicity, and by extension the mobility, of weathered plastics in soils.

This artefact matters because surface chemistry governs how microplastics move through soil, bind pollutants, and interact with organisms. Weathered particles with oxidised, polar surfaces are expected to be transported more readily by water through soil pores and to sorb hydrophilic contaminants differently than pristine ones. If standard extraction protocols inadvertently reverse the surface characteristics of aged particles, laboratory measurements could systematically misrepresent their environmental behaviour. The authors argue that contact angle and Nile red analyses performed after extraction should therefore be interpreted with care, and that the effect of oxidative cleaning must be considered whenever weathered plastics are processed.

The study is not without limitations, which the researchers themselves acknowledge. Only two polymer types and a relatively narrow size range of 400 to 1000 micrometres were tested, and larger particles are less dominated by surface forces than the smaller fractions, below roughly 300 micrometres, that dominate many environmental samples. The accelerated UVC ageing, while effective, cannot fully reproduce the combined photo-, thermal- and biodegradation that plastics experience over years in the field. Gravimetric quantification also remains sensitive to residual mineral particles despite the correction factors, and particle counting combined with automated imaging might yield more accurate results in future studies, albeit at the cost of a considerably more laborious workflow.

Still, the broader message is encouraging for the field of soil microplastic monitoring. Density separation with sodium chloride and sodium iodide, combined with moderate hydrogen peroxide oxidation, emerges as a protocol capable of reliably recovering both fresh and weathered PET and polystyrene from soils of differing texture and organic content, supporting the growing effort to standardise extraction methods and improve comparability across studies. As the authors point out, extending this approach to a wider range of polymers, shapes, particle sizes and multi-factor ageing regimes, including mechanical abrasion and biological degradation, will be the next step toward ageing-aware protocols that reflect real environmental conditions. In the meantime, the work provides soil scientists with both a validated tool and a warning: the plastics we pull from the ground may no longer be exactly the plastics that went in, and the difference lies in a layer only nanometres thick.

The research team developed a modified column experiment designed to reproduce that surface-release scenario. Rather than mixing microplastics into a water suspension and forcing the mixture horizontally or from the bottom of a packed column, the scientists placed buoyant low-density polyethylene particles on the soil surface and applied water from above. They tested the particles in different farmland soil types and used mathematical modeling to track how particles moved, became temporarily trapped, and were later released. The approach was intended to capture the behavior of particles under unsaturated conditions, when soil pores contain both air and water, as well as during wetting events that change the structure of the flow paths.

The experiments showed substantial penetration of buoyant microplastics into soil under both rainfall and irrigation conditions. That result is striking because buoyancy appears, at first glance, to work against downward movement. A particle less dense than water tends to rise or remain at the water surface, but the soil is not an open pool. As water infiltrates through connected pores, it can drag particles into narrow channels, particularly when the particles are small enough to enter the pore network. Surface water can also create thin films and preferential flow pathways around grains. The downward movement therefore depends not only on particle density, but on water flux, pore geometry, particle size, surface chemistry and the evolving balance between forces that retain particles and forces that mobilize them.

To describe that balance, the researchers applied a transport model incorporating mechanisms traditionally used for colloidal particles in porous media. One mechanism is attachment, in which a microplastic collides with a soil grain and remains held by surface forces. Detachment is the reverse process: a change in water chemistry or flow can dislodge a previously retained particle. Straining occurs when a particle becomes physically trapped because it is too large to pass through a pore throat. Blocking can develop when retained particles accumulate and alter the available pathways, potentially redirecting later particles or changing the local permeability. These processes do not simply remove microplastics from the moving water; they can produce pulses of retention and release, meaning particles may remain hidden in soil and then reappear during a later storm or irrigation cycle.

The soil itself strongly influenced the outcome. Buoyant microplastic transport was greater in silt than in silt loam, while silt loam retained more particles. Soil texture controls the size distribution and connectivity of pores: larger, better-connected pathways can permit particles to move farther, whereas finer or more complex structures increase the likelihood of trapping. Yet the relationship is not as simple as “coarser soil means more transport.” A particle must negotiate constrictions, grain surfaces and changes in water saturation, and even small differences in pore architecture can determine whether it travels downward or becomes lodged. The findings suggest that risk assessments based on a single standardized sand or artificially packed medium may fail to represent how microplastics behave in actual agricultural soils.

The study also examined natural organic matter, a chemically complex mixture derived from decomposed plants, microbes and other biological material. The presence of this material increased the transport of buoyant microplastics. Natural organic matter can coat both plastic surfaces and soil minerals, changing their surface charge, wettability and tendency to aggregate. It may act as a stabilizing layer that reduces particle clumping, keeping individual microplastics mobile in infiltrating water. It can also alter the interactions between a particle and a soil grain, weakening attachment or increasing electrostatic repulsion. In practical terms, the chemistry of a living, carbon-rich soil may allow more particles to remain suspended and move through pores than experiments using purified water would predict.

Weathering created another unexpected shift in particle behavior. The researchers found that ultraviolet-photodegraded microplastics traveled farther than pristine particles. Exposure to sunlight can oxidize the polymer surface, breaking or modifying chemical bonds and introducing new functional groups. These changes can increase the particle’s surface charge. When the particles and soil grains carry charges that repel one another, attachment becomes less favorable, allowing more microplastics to remain in the mobile water phase. Photodegradation can also roughen or fracture plastic surfaces, potentially changing their effective size and interaction with soil. The result is a paradox: environmental aging may make plastic fragments physically damaged, but chemically more mobile. A particle that has spent time exposed at the soil surface could therefore be more likely to enter the subsurface during a later wetting event than a freshly released fragment.

The work matters because agricultural soils are not isolated containers; they connect fields with drainage systems, streams, aquifers and crops. Microplastics that move below the surface may be difficult to recover, and retained particles can serve as a delayed source during future wetting and drying cycles. The study does not establish how much plastic reaches groundwater or enters plants under field conditions, nor does it measure ecological or human-health effects. Its importance is more immediate and foundational: it demonstrates that the starting conditions of an experiment can determine whether buoyant particles appear immobile or highly mobile. By combining a surface-release experiment with a model that accounts for attachment, detachment, blocking and straining, the researchers provide a framework for investigating real rainfall and irrigation events. As plastic use in agriculture continues and weather patterns become more intense or irregular, understanding these hidden transport pathways may be essential to predicting where microscopic fragments ultimately accumulate.