Birch trees pull microplastics out of the ground

Birch trees could serve soil as garbage disposal. © JanBussan

A study shows that soil contaminated by tiny plastic particles could be rehabilitated with the help of trees: experiments show that birch trees absorb microplastics with their roots. The problematic particles could then be disposed of with the biomass. Further investigations should now show to what extent the process is actually suitable for combating the widespread environmental problem.

Shredded plastic bags and many other colorful rubbish objects – it is estimated that a third of all plastic waste ends up in the environment. The ugly remnants of our throwaway society are clearly visible in many places. But a large proportion can often only be seen with a magnifying glass or microscope: over time, many pieces of waste break down into particles smaller than five millimeters and then partly into nanoparticles with a size of less than 0.1 micrometers. The environment already abounds with these crumbs and they are even carried through the air. Studies show that this signature of civilization can already be detected in the most remote corners of our planet.

Can tree roots bind the particles?

The focus is often on the microplastic pollution of water bodies, but in some cases it is even more pronounced in the soil. An important factor in the spread on agricultural soils is the application of sewage sludge. As a result, fragments of synthetic clothing fibers in particular end up in the soil. Another major contribution is made by car traffic due to tire wear. Wherever it comes from – the artificial small things are considered problematic: microplastics can release pollutants that may be absorbed by crops. In addition, negative effects of microplastics on useful soil dwellers have been shown, and studies show that the particles can also offer certain pathogens good development opportunities.

One might think that once the soil is swarming with particles, there is no chance of getting them out again. But maybe there is a way to remediate, as the current study results of the researchers led by Kat Austen from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) give reason to hope. The key word here is phytoremediation – the extraction of pollutants by plants. This method is used to remove toxins from soil. For this purpose, certain types of plants are planted that absorb these substances particularly well with their roots. The biomass can then be disposed of together with the problematic substances.

Hope bearer birch

Studies on wheat and lettuce have already suggested that plants could also bind microplastics. In their study, however, Austen and her colleagues have now turned to a woody plant that has already been used successfully in phytoremediation: the silver birch (Betula pendula), which is widespread in Europe. In order to test their ability to absorb microplastics, they planted test plants in substrate that was mixed with microplastic beads (5 to 50 micrometers). These particles were tagged with a fluorescent dye for later easy detection in plant tissue. After five months, the scientists then examined root samples using fluorescence and confocal laser scanning microscopy.

As it turned out, the birch trees had actually absorbed particles deep into the root tissue: the researchers were able to detect fluorescent microplastics in various sections and layers of the root system. The percentage of root sections with enrichment was 5 to 17 percent. This shows that the extraction potential is relatively high, at least compared to the indications for wheat and lettuce, the scientists write. “This pilot study suggests that birch has real potential for long-term soil remediation solutions — including reducing the amount of microplastics in the soil,” says Austen. The birch also has another interesting aspect: its roots are particularly shallow below the soil surface, where microplastic pollution has been shown to be highest.

In order to confirm the potential of the birch trees for the remediation of terrestrial microplastic pollution, however, further investigations are now necessary, the researchers point out. They plan to examine the intake levels and other aspects more closely. It also needs to be clarified to what extent the trees can withstand their cleaning function at all: “The uptake rate of microplastics and the effects on the short- and long-term health of the trees still have to be investigated,” says Austen.

Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries, specialist article: Science of The Total Environment, doi: 10.1016/j.scitotenv.2021.152085

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