Thin
mulch films made of polyethylene are used in agriculture in numerous countries,
where they cause extensive soil contamination. Researchers have now identified
an alternative: films made of the polymer PBAT biodegrade in soils.
Our
world is drowning in a flood of plastic. Eight million tons of plastic end up
in the oceans every year. Agricultural soils are also threatened by plastic
pollution. Farmers around the world apply enormous amounts of polyethylene (PE)
mulch films onto soils to combat weeds, increase soil temperature and keep the
soil moist, thereby increasing overall crop yields.
After
harvest, it often is impossible for farmers to re-collect the entire films,
particularly when films are only a few micrometers thin. Film debris then makes
its way into the soil and accumulates in the soil over time, because PE does
not biodegrade. Film residues in soils decrease soil fertility, interfere with
water transport and diminish crop growth.
Researchers
at ETH Zurich and the Swiss Federal Institute of Aquatic Science and Technology
(Eawag) have now shown in an interdisciplinary study that there is reason to be
hopeful. In their recent study, they demonstrate that soil microbes degrade
films composed of the alternative polymer poly(butylene
adipate-co-terephthalate) (PBAT). Their work has just been published in the
journal Science Advances.
In
their experiments, the researchers used PBAT material that was
custom-synthesised from monomers to contain a defined amount of the stable
carbon-13 isotope. This isotope label enabled the scientists to track the
polymer-derived carbon along different biodegradation pathways in soil.
Upon
biodegrading PBAT, the soil microorganisms liberated carbon-13 from the
polymer.
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 also incorporated into the biomass of
microorganisms colonizing the polymer surface.
The
researchers are the first to successfully demonstrate -- with high scientific
rigor -- that a plastic material is effectively biodegraded in soils.
Because
not all materials that were labelled "biodegradable" in the past
really fulfilled the necessary criteria. "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," says Hans-Peter Kohler, environmental microbiologist at Eawag.
The
definition highlights that biodegradable plastics fundamentally differ from
those that merely disintegrate into tiny plastic particles, for instance after
exposure of the plastic to sunlight, but that do not mineralise.
In
their experiment, the researchers placed 60 grams of soil into glass bottles
each with a volume of 0.1 litre and subsequently inserted the PBAT films on a
solid support into the soil.
After
six weeks of incubation, the scientists assessed the extent to which soil
microorganisms had colonised the PBAT surfaces. They further quantified the
amount of CO2 that was formed in the incubation bottles and how much of the
carbon-13 isotope the CO2 contained. Finally, to directly demonstrate the
incorporation of carbon from the polymer in the biomass of microorganisms on
the polymer surfaces, they collaborated with researchers from the University of
Vienna.
At
this stage, the researchers cannot yet say with certainty over which timeframe
PBAT degrades in soils in the natural environment given that they conducted
their experiments in the lab, not in the field. Longer-term studies in
different soils and under various conditions in the field are now needed to
assess the biodegradation of PBAT films under real environmental conditions.
See:
Michael
Thomas Zumstein, Arno Schintlmeister, Taylor Frederick Nelson, Rebekka
Baumgartner, Dagmar Woebken, Michael Wagner, Hans-Peter E. Kohler, Kristopher
McNeill, Michael Sander. Biodegradation of synthetic polymers in soils:
Tracking carbon into CO2and microbial biomass. Science Advances, 2018; 4 (7):
eaas9024 DOI: 10.1126/sciadv.aas9024
Posted by Dr. Tim Sandle,
Pharmaceutical Microbiology
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Posted by Dr. Tim Sandle,
Pharmaceutical Microbiology
Posted by Dr. Tim Sandle,
Pharmaceutical Microbiology