Mulch films have changed the daily lives of many farmers by facilitating weed and water management. But when exposed to sunlight, moisture, and tillage, these plastics gradually break down into micro- and nanoplastics that accumulate in the soil. What happens to these invisible particles? Can biodegradable plastics really limit this pollution?

In market gardening and fruit growing, plastic mulch films have become indispensable in certain conditions of temperature, humidity, and soil aeration. Laid on the surface, they prevent weeds from growing, conserve moisture, and accelerate soil warming in the spring. The result? More productive crops and less dependence on herbicides, especially in temperate soils subject to seasonal water stress.

Today, the vast majority of mulch films—nearly 90%—are made from polyethylene, a petroleum-derived polymer. Strong, inexpensive, and waterproof, this material has become the norm in the fields. But it has one major drawback: it is not biodegradable and, once fragmented, can remain in the environment for centuries.

Once crops have been harvested, these films are not completely removed. The fragments that remain in the soil degrade slowly under the effect of sunlight, moisture, and mechanical tillage. They gradually fragment into microplastics (ranging in size from one micrometer to five millimeters), reaching several thousand particles per kilogram of soil on some farms. Over time, these fragments become even smaller and form what are known as nanoplastics (less than one micrometer in size).

Invisible and long understudied, these particles now raise many questions. How do they behave in agricultural soils? What are their potential effects on the environment? Above all, are so-called “biodegradable” alternatives really a solution to limiting this pollution?

European agriculture consumes 427,000 tons of plastic per year

Although nano- and microplastics in agricultural soils have multiple origins, not all sources contribute in equal proportions.

External inputs include sludge from wastewater treatment plants, which is sometimes used as an organic fertilizer. It can contain up to nearly 900 microplastic particles per kilogram of dry matter. On a European scale, spreading this sludge represents the introduction of 63,000 to 430,000 tons of microplastics into agricultural soils each year.

But the largest contribution often comes directly from the field. Plastic mulch films are now the main source of micro- and nanoplastics in agriculture. Every year, 427,000 tons of these products are used in Europe and 300,000 tons in North America. China alone accounts for nearly 30% of global consumption, with 2.6 million tons of agricultural films, including 1.3 million tons of mulching films, or about three-quarters of global usage.

Plastics, from field to plate

However, in agricultural soils, nano- and microplastics do not behave like simple inert waste. They can alter the physical, chemical, and biological properties of soils by affecting their structure, porosity, water retention capacity, and carbon and nitrogen cycles.

These particles can also attach themselves to roots, slowing down water absorption and plant growth. Some crops, such as lettuce and wheat, even absorb microplastics from the soil, which can then migrate to the edible parts we eat.

What's more, the smaller a piece of plastic is, the greater its surface area exposed to the surrounding environment in relation to its volume. However, interactions between plastic and soil occur mainly on its surface. Thus, for the same amount of plastic, microplastics and especially nanoplastics have a much larger contact surface with their environment than larger fragments.

This large surface area makes them particularly reactive: they interact more strongly with molecules present in the soil. Many chemicals can thus attach themselves to the surface of these particles, including organic pollutants and metals. In the presence of pesticides, plastics can then behave like real “chemical sponges,” capturing and concentrating undesirable and potentially toxic substances locally.

Biodegradable plastic? A promise that needs to be qualified

To address this issue, so-called “biodegradable” plastics have been developed. Presented as an alternative to conventional plastic films, they are based on an appealing principle: after use, they are transformed by microorganisms in the soil into natural compounds such as water and carbon dioxide.

But this promise is highly dependent on environmental conditions. Biodegradability is dependent on environmental conditions. Plastic can degrade quickly in an industrial composting facility, where it is subjected to high temperatures, but remain almost intact for a long time in cooler agricultural soil.

This is particularly the case for polymers commonly used in biodegradable mulch films, such as polybutylene adipate-co-terephthalate (PBAT) or polybutylene succinate (PBS). Under real-world conditions in agricultural fields, their degradation may remain incomplete.

This raises the question of their long-term fate and their actual ability to limit the accumulation of nano- and microplastics in soils.

Earthworms to the rescue?

In light of these failures, researchers are now working to understand the causes and identify the environmental and biological conditions that would improve the degradation of these plastics in agricultural soils. It is not just a question of designing new materials, but also of adapting agricultural practices to their presence in the soil, particularly in systems where mulch films are used intensively.

Scientists are therefore interested in the role of organic amendments, biostimulants, and microbial extracts, which can modify the biological activity of soils. Studies have shown that vermicompost—compost produced by earthworms—can significantly improve the performance of certain biodegradable plastics in agricultural conditions. This effect can be explained both by the addition of nutrients that promote soil microorganisms and by the presence of specific microbial flora from the digestive tract of earthworms.

To better understand these mechanisms, laboratory experiments are monitoring the degradation of plastics using respirometric systems that measure the conversion of carbon in plastics into carbon dioxide by microorganisms. These devices make it possible to quantitatively assess the conditions under which this degradation is accelerated.

Improving plastics and practices

While so-called biodegradable plastics represent an interesting avenue for combating the accumulation of nano- and microplastics in soil, their effectiveness depends heavily on actual conditions in the agricultural environment and they cannot be considered a systematic solution or taken for granted.

Research shows that improving their biodegradation requires a better understanding of the interaction between materials, the biological functioning of soils, and associated farming practices. Specific organic amendments, the addition of exogenous strains, and the stimulation of indigenous microbial activity are all potential levers.

Ultimately, reducing plastic pollution in agriculture will require a comprehensive approach, combining technological innovation, appropriate agronomic practices, and consistent regulatory frameworks in order to reconcile agricultural performance with sustainable soil protection.