Plastic is everywhere: Plastic residues have been detected in the most remote corners of the earth, even in Antarctica and the deep sea. Studies show that plants also absorb the particles. The consequences are still unclear – for example, whether the consumption of plastic-contaminated plants is problematic for people.
For years, scientists have been intensively researching plastic pollution in rivers, lakes and the seas. However, it is estimated that only around five percent of the plastic waste produced each year ends up in the ocean. The rest stays on land. Some of it ends up in the environment and breaks down into smaller and smaller particles over time.
However, the micro- and nanoparticles will never disappear completely – today they can be found in farmland, on mountain peaks and in remote forests, in glaciers and river meadows. And they become part of the natural cycles of matter, as recent experimental work suggests: Plants absorb the particles, trees in the forest just as well as crops in the field. The consequences of this are largely unclear.
The entry routes into the environment and subsequently into the soil are diverse. Films, which are often used to cover things such as when growing vegetables, are a particular problem in agriculture. Plastic also ends up in the fields with the compost if the organic waste was not properly separated when it was collected. Rivers deposit the plastic they take with them in flood plains, where it is then gradually transported into deeper soil layers.
Matthias Rillig, soil ecologist at the Freie Universität Berlin, also appreciates that there is probably nowhere more plastic-free soil. “The plastic particles rain down from the atmosphere, they are basically everywhere. When we conduct experiments on plastic soil pollution, we actually no longer find any uncontaminated control soils.” As part of the µPlastic (= microplastics) project funded by the Federal Ministry of Research, Rillig and his team are investigating how the tiny plastic particles influence the biological processes in the soil.
Specifically, the scientists are researching the effects of microplastics – particles under five millimeters in size – on the rhizosphere. This is the habitat in the immediate vicinity of the plant roots, with all the creatures contained therein such as worms, isopods, rotifers, bacteria and fungi. The composition and condition of the rhizosphere has a direct impact on plant health and growth.
Some studies from the past few years indicate that the presence of plastic can affect the growth of plants. In corresponding experiments, seeds of garden cress (Lepidium sativum) germinated more slowly in the presence of plastic particles, presumably because these clog the pores of the seeds. Wheat plants also grew worse when they were grown in the presence of plastic particles – in this experiment, for reasons that are still unclear, bio-plastic had a particularly unfavorable effect, while earthworms in the soil partially offset the negative effects.
“The whole thing is incredibly complex,” says Matthias Rillig. “The microplastic does not behave like a single chemical. There are countless different plastics, in different forms – as fibers, beads or foams. And they are mixed with different additives, which can ultimately be responsible for a possible toxic effect.”
In their own investigations, the researchers led by Rillig came to the surprising conclusion that plastic pollution does not necessarily have to be bad for soil and plants. “At first glance, the entry of plastic particles can actually have a positive effect, for example because the aeration of the soil or the transport of water is improved,” says Rillig. “It was difficult for us to digest at first.”
But that’s no reason for the biologist to give the all-clear. “We have to move away from the purely ecotoxicological point of view – is that harmful? – towards a global view: How is the Earth system changing overall. And: Is that desirable?” In view of the diverse uses of plastics and their extreme longevity, the flood of plastic will probably not be able to be stopped, at least for the foreseeable future. This means that long-term effects on ecosystems can also be expected.
Researchers from Switzerland recently provided evidence for the first time that forest trees absorb the plastic from the soil, i.e. store it in their tissue. They examined one to two-year-old seedlings of birch, spruce and sessile oak after the lowest roots had been in a solution containing tiny polystyrene nanoparticles for a few days. In fact, the scientists later found the plastic particles in the higher parts of the plant.
The researchers found significant amounts of plastic particles, especially in the trunk of the birch, which consumes a lot of water. “Normally everything that needs to go up in a tree is transported with the transpiration flow via the wood body, the xylem,” explains study leader Arthur Gessler from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). “We therefore assume that the nanoplastic is also transported in this way.”
It is not yet possible to answer whether the plastic pollution damages the trees. The amounts ingested were very small, explains Gessler. “But if trees are exposed to these concentrations for years, a significant transport to and consequently accumulation in the leaves, trunk and branches must be expected in any case.” The ecologist explains that the nanoparticles are small enough to interact with the membranes of the cells. “Once they are in the cells, it is conceivable that they interact with the organelles and in this way influence photosynthesis, for example.”
The consequences for the plants still have to be examined in more detail. It is conceivable that plastic pollution is an additional burden on forests, which are already under severe pressure from climate change.
Ultimately, the plastic could also end up in humans via contamination of plants. In experiments with rice plants, Chinese researchers demonstrated that the roots of the plantlets absorb the particles and transport them to the upper parts of the plant. Is the consumption of plastic-contaminated plants problematic for humans? There is no clear answer to this question.
Rillig points out that the particles also reach field plants via the air and can possibly get into the body this way alone. In addition, the plastics can contain various additives that are harmful to health or the environment, such as flame retardants and stabilizers. The problem might not then be the intake of the microplastic itself, but the toxic substances associated with it. Of particular concern in this context is microplastics from tire abrasion, which, according to the Federal Environment Agency (UBA), is the largest source of plastic emissions into the environment. This means that numerous toxic substances such as zinc, lead or cadmium are released into the environment.
There are still many unanswered questions about the extent and effects of plastic pollution in the soil. Most of the work to date on uptake in plants is laboratory testing. Even if the concentrations of plastic used correspond to those found in the field, the conditions are artificial. The plants are usually grown in a so-called hydroponic culture, in which their roots are immersed in an aqueous solution to which the plastic particles have been added. “These are pretty unrealistic scenarios,” admits Matthias Rillig. “The microparticles stick everywhere in the soil, the amounts absorbed are likely to be lower than in laboratory experiments.”
Direct measurements of microplastics in plants outdoors are currently difficult if not impossible – there are simply no reliable methods. After all, microplastics can be detected microscopically with great effort. There are currently no established measurement methods for the much smaller nanoplastic particles.
