It is well known that microplastics and nanoplastics pollute oceans, rivers, and farmland. New research now reveals that they are also accumulating in forests. Geoscientists from the Technical University of Darmstadt in Germany have published a study highlighting a form of environmental pollution that has largely gone unnoticed.

The study reveals that forests are not only affected by local sources of pollution. On the contrary, most microplastic particles arrive via the air and gradually accumulate in forest soils.

“Microplastics from the atmosphere are initially deposited on the leaves in the tree canopies, which scientists call the ‘comb effect,’” explains lead author Dr. Collin J. Weber of the Institute of Applied Geosciences at the Technical University of Darmstadt. “Next, in deciduous forests, the particles are transported to the forest floor by rain or falling leaves in autumn, for example.”
Once on the forest floor, natural processes take over. The decomposition of fallen leaves plays a key role in capturing and storing microplastics in the soil. The researchers found the highest concentrations in the top layer of leaf litter, where decomposition has just begun. However, significant amounts were also detected at greater depths.
This movement to the lower soil layers is related not only to the decomposition of organic matter but also to biological activity, such as organisms that help break down leaves and redistribute the particles.
Measuring Microplastics in Soil, Leaves, and Air
To better understand how microplastics accumulate, the research team collected samples from four forest areas east of Darmstadt, Germany. They analyzed soil, fallen leaves, and atmospheric deposition (the transport of substances from Earth's atmosphere to the Earth's surface) using a recently developed method combined with spectroscopic techniques.

In addition, the scientists created a model to estimate the amount of microplastics that have entered forests from the atmosphere since the 1950s. This helped them assess what proportion of the total pollution stored in forest soils can be attributed to atmospheric sources.

"Our results indicate that microplastics in forest soils come primarily from atmospheric deposition and from leaves that fall to the ground, known as litter. Other sources, on the other hand, have only a minor influence," explains Weber. “We concluded that forests are good indicators of air pollution from microplastics and that a high concentration of microplastics in forest soils indicates a significant diffuse input—as opposed to direct input, such as from fertilizers in agriculture—of airborne particles into these ecosystems.”

A new environmental and health concern
This research, published in Nature Communications Earth & Environment, is the first to clearly show how forests become contaminated with microplastics and to directly link this contamination to airborne particles. Until now, this pathway had not been thoroughly studied.

The findings provide an important basis for assessing the environmental risks of microplastics in both the air and soil. “Forests are already threatened by climate change, and our findings suggest that microplastics could now pose an additional threat to forest ecosystems,” says Weber. The results may also have implications for human health, as they highlight how microplastics travel globally through the atmosphere and can be present in the air we breathe.

NATURE