Plastic rain: Forests are trapping microplastics from the sky

When we talk about plastic pollution, oceans and rivers dominate the conversation. Beaches lined with plastic bottles have become the symbol of this crisis. Farms and city streets also show clear signs of contamination. But forests? Few would imagine plastic reaching there.

Researchers at TU Darmstadt have shown otherwise. Their study reveals, for the first time, that forest soils carry large amounts of microplastics.

The particles arrive mainly through the air, settle on leaves, and later end up in the ground. Forests, once thought of as clean and distant, are absorbing pollution every day.

Microplastics falling from the sky

Unlike farmland, forests do not receive fertilizers laced with plastic. They do not sit beside landfills or sewage plants. Yet they still collect microplastics. How? The atmosphere delivers them.

Leaves catch the particles first. Rain and falling foliage move them downward. Forests act like large nets that sweep pollution from the sky.

“The microplastics from the atmosphere initially settle on the leaves of the tree crowns, which scientists refer to as the ‘comb-out effect,'” explains lead author Dr. Collin J. Weber. “Then, in deciduous forests, the particles are transported to the forest soil by rain or the autumn leaf fall, for example.”

Soil as storage

Once in the soil, the particles do not stay at the surface. Leaf litter slowly breaks down, and with it, plastics sink deeper.

Insects, fungi, and other organisms speed up this process. The result is long-term storage of plastics in soil horizons that usually contain decomposed organic matter.

The researchers measured concentrations ranging from 120 to more than 13,000 particles per kilogram of soil. Stocks of up to nearly a million particles per square meter were recorded at some sites. Forest soils, it turns out, rival urban soils in plastic load.

Plastics in forest soil

The team analyzed the chemical makeup of the particles. Polypropylene, polyethylene, and polyamide dominated the samples. These are common in packaging and textiles.

The particles were mostly fragments and films, not fibers. Most measured less than 250 micrometers, far smaller than a grain of sand.

The similarity between what fell from the air and what lay in the soil points to the same source: atmospheric deposition. Other ground-based sources like littering or forestry practices may add some plastic, but their role is minor.

Forests collect plastics for decades

The story does not end with today’s measurements. The scientists modeled how long it would take to build up current soil levels.

The answer points back to the 1950s, when plastic production exploded worldwide. For 70 years, forests have been quietly gathering plastic particles from the air.

Even under conservative scenarios, the amounts found in soil today can be explained by steady deposition over decades. Forest soils, like silent records, tell the history of the plastic age.

Forests match city pollution

The surprise is not just the presence of plastics but the quantity. Compared to global data, these German forest soils contain as much plastic as urban soils and often more than agricultural, wetland, or coastal soils. The assumption that forests stand apart from pollution no longer holds.

Forests cover about a third of Earth’s land surface. If all are acting as sinks, the scale of storage is enormous. This changes how scientists think about the global flow of microplastics.

Forests as indicators

“Our results indicate that microplastics in forest soils originate primarily from atmospheric deposition and from leaves falling to the ground, known as litterfall. Other sources, on the other hand, have only a minor influence,” said Dr. Weber.

“We conclude that forests are good indicators of atmospheric microplastic pollution and that a high concentration of microplastics in forest soils indicates a high diffuse input – as opposed to direct input such as from fertilizers in agriculture – of particles from the air into these ecosystems.”

Forests, in other words, act like monitors. The amount of plastic in their soils reflects what is falling from the sky across regions and continents.

Risks for nature and people

What does this mean for ecosystems? Microplastics can change soil structure, affect microbes, and alter nutrient cycles. Over time, this could influence tree growth and forest health.

The combination of climate stress and plastic pollution could place new pressure on already fragile ecosystems.

“Forests are already threatened by climate change, and our findings suggest that microplastics could now pose an additional threat to forest ecosystems,” noted Dr. Weber.

There is also a human angle. The same particles found in forests travel through the air we breathe. What lands on a leaf today may be part of tomorrow’s inhaled dust.

Future of plastic in forest soils

This study is the first to show, with clear evidence, that forests are major recipients of microplastic pollution from the atmosphere.

The research raises urgent questions. How do different tree species capture particles? How do seasonal changes affect deposition? And what will decades more of plastic production mean for soils and air?

For now, one conclusion is unavoidable: forests, once considered safe from plastic waste, are firmly part of the story.

The study is published in the journal Communications Earth & Environment.

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