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Scientists identify plastic material that is biodegradable in soil

Massive amounts of polyethylene (PE) mulch films are used by farmers all over the world to combat weeds and to adjust the temperature and moisture of the soil to maximize crop yields. PE, however, is not biodegradable and accumulates in the soil over time, decreasing soil fertility and interfering with water transport.

A new study led by researchers at ETH Zurich have demonstrated that soil microbes can effectively break down mulch films composed of the alternative polymer poly(butylene adipate-co-terephthalate), or PBAT. The experts found that soil microorganisms metabolically utilized the carbon in PBAT for energy production and to build up microbial biomass.

“This research directly demonstrates, for the first time, that soil microorganisms mineralise PBAT films in soils and transfer carbon from the polymer into their biomass,” said study co-author Michael Sander.

PBAT is a petroleum-based polymer that can be used as a substitute for PE in producing mulch films. Since PBAT is known to be biodegradable in compost, the researchers set out to determine whether the polymer is also biodegradable in agricultural soils.

The study revealed that the soil microorganisms released carbon-13 from the polymer as the PBAT degraded. Using isotope-sensitive analytical equipment, the researchers found that the carbon-13 from PBAT was not only converted into carbon dioxide (CO2) as a result of microbial respiration, but was also incorporated into biomass at the surface of the polymer.

“The beauty of our study is that we used stable isotopes to precisely track PBAT-derived carbon along different biodegradation pathways of the polymer in the soil,” said study co-author Michael Zumstein.

The study authors emphasized that biodegradable plastics are distinctively different from plastics that just break up into tiny fragments and persist in the environment as microplastics.

“By definition biodegradation demands that microbes metabolically use all carbon in the polymer chains for energy production and biomass formation – as we now demonstrated for PBAT,” explained study co-author Hans-Peter Kohler.

The research team is the first to successfully demonstrate that a plastic material is effectively biodegrade in soils.

“We’ve developed analysis techniques that open the door for industry to test the environmental impact of their plastic products,” said co-author Kristopher McNeill. “Thanks to our method, they can switch to using biodegradable materials in the production of thin mulch films instead of non-degradable PE.”

The study is published in the journal Science Advances.

By Chrissy Sexton, Staff Writer

Image Credit: ETH Zurich / Environmental Chemistry Group

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