Thin polyethylene (PE) mulch films are used in farming in many countries, where they cause massive soil contamination. Researchers at ETH Zurich and the Swiss Federal Institute of Aquatic Science and Technology (Eawag) have now shown that there are alternatives, i.e. films made of the plastic PBAT (polybutylene adipate-co-terephthalate) that are biodegradable in the soil.
Electron microscopy image: after a few weeks in soils, numerous soil microorganisms have colonised the surface of the PBAT film and begun to biodegrade the polymer.
ETH Zürich / Environmental Chemistry Group
Farmers around the world apply enormous amounts of PE mulch films onto soils to combat weeds, increase soil temperature and keep the soil moist, thereby increasing overall crop yields. After the harvest, it is often impossible for farmers to re-collect the films completely, particularly when films are only a few micrometres thick. As a consequence, PE film debris then makes its way into the soil and accumulates there. Film residues decrease soil fertility, disturb water supplies and diminish crop growth.
Researchers at ETH Zurich and Eawag have now shown in an interdisciplinary study that soil microbes can degrade alternative films of the plastic PBAT. Their work has been published in the journal "Science Advances". Like PE, PBAT is a petroleum-based polymer. Since PBAT is considered biodegradable in compost, the ETH and Eawag researchers also examined the biodegradability of this plastic in soils. By comparison, PE neither biodegrades in compost nor in soil.
Long-term studies required
In their experiments, the researchers used PBAT material that was custom-synthesised from monomers marked with a defined amount of the more stable, slightly heavier carbon-13 isotope. This isotope label enabled the scientists to track the polymer-derived carbon along pathways in the soil during biodegradation. If the soil microbes degrade PBAT, they also have to release carbon-13. Using suitable measuring instruments, the researchers have found that the carbon-13 from PBAT is not only converted into carbon dioxide as a result of microbial respiration but also incorporated into the cell structures that the microbes produce.
The researchers cannot yet predict how long PBAT will remain in agricultural soils. Long-term studies in different soils and under various conditions in the field are now needed to assess the biodegradation of PBAT films under environmental conditions. The results cannot, however, be directly transferred to other natural environments. For instance, biodegradation of polymers in seawater might be considerably slower because of its different environmental conditions and microbes.