Plastic is widely used in modern agriculture, particularly in the mulch film that farmers apply to cover field soils. This discourages weed growth and maintains soil moisture for crops.
However, farmers typically find that collecting and disposing of standard polyethylene (PE) film after usage takes a lot of effort and money. Additionally, because the thin PE films are so fragile, it is impossible to collect them all again. Because PE doesn’t disintegrate, it means that pieces of it remain on and in the soil and build up there.
A possible option is biodegradable mulch film because, unlike PE film, it ideally doesn’t leave any polymer residue in the soil environment. Its biodegradable polymers were specifically chosen so that microorganisms might utilize them to produce energy and create cell biomass. The backbone structure of the biodegradable polymers contains designated chemical “breaking sites.” Enzymes that attack and break down specific sites in the polymers can be released into the environment by naturally occurring microorganisms, such as those in soil. The microorganisms then absorb the minor breakdown products that are generated, and they eventually breathe out the final product, CO2.
Therefore, it is critical to demonstrate CO2 production from polymer carbon for biodegradation. Additionally, there are falsely called biodegradable plastics based on PE that contain particular additives in addition to truly biodegradable plastics. Only very tiny microplastics that are no longer visible to the human eye remain once these films have broken down. These also build up in the environment since microbes aren’t able to break them down.
New approach captures all aspects of biodegradation
It hasn’t been able to completely monitor polymer biodegradation up until now using current methods. However, the Environmental Chemistry Group at ETH Zurich has created a novel method to track and gauge if and how much a polymer degrades in a soil over the previous few years. Recent publication of their findings in Nature Communications.
These findings might alter future research on polymer biodegradation. Along with workers from the chemical business BASF, the initiative included researchers from the ETH Earth Sciences department, Eawag, and other organizations.
This innovative method relies on the use of polymers that have been labeled with stable carbon isotopes (13C). The researchers can clearly show that biodegradation is taking place thanks to this labeling, which enables them to trace the polymer’s 13C during disintegration in the soil. Up until this point, only non-isotopically tagged polymers have been used to examine the biodegradability of plastics. If the percentage of additional polymer carbon transformed into CO2 surpasses a set level over a specific incubation period, a polymer (or a plastic material made up of one or more polymers) is certified as biodegradable. For instance, the requirement for biodegradable mulch film calls for two-year soil incubations where at least 90% of the carbon in the mulch film is “mineralized” into CO2.
These tests have a solid track record of being effective tools for identifying polymer mineralization. They only monitor CO2 generation, therefore they don’t fully capture the scope of the biodegradation. As a result, scientists using current conventional techniques have been unable to determine how much polymer carbon is still present in the soil after an incubation time. Furthermore, it wasn’t obvious whether the remaining carbon was still present in the form of the additional polymer or had already been assimilated into the biomass by microbes.
Closed mass balances for carbon
These misunderstandings are resolved by the method created by the ETH researchers and their associates. They conducted their tests using PBS, a commercially significant biodegradable polyester that is also used in mulch films, which was 13C-labeled.
The authors showed complete mass balances for the PBS carbon by quantifying the residual amount of PBS-derived 13C that remained in the soil after incubation, in addition to determining mineralization to 13CO2. The researchers were now able to selectively track the 13C in the PBS during biodegradation.
“We were pleased to observe closed carbon mass balances during the 425 days of incubation in the soil. This demonstrated that over these extremely long incubation periods, we can properly predict where the polymer carbon ends up—roughly two-thirds in CO2 and one-third in soil “Taylor Nelson, the study’s lead author and a doctoral graduate of the Environmental Chemistry Group, adds.
The form in which the carbon added as PBS remained in the soil was another question the researchers wished to answer. How much PBS was still present as residue after being absorbed into the microbial biomass?
At the conclusion of the incubations, the authors removed and measured the residual PBS from the soil to provide an answer to this query. They were able to demonstrate that while the majority of the carbon was remained in the form of PBS, a sizeable portion — 7 percent — had been integrated into the microbial biomass.
Future research and the creation of novel biodegradable polymers depend on being able to precisely quantify the amount of polymer that is still present and the amount of polymer carbon that has been incorporated into biomass. According to Michael Sander, professor in the ETH Environmental Chemistry Group, “We can now systematically test for soil conditions and polymer properties that allow for complete biodegradation of the polymers to CO2 and to microbial biomass — and we can assess factors that may slow down polymer biodegradation over time.”
The group is currently looking into the biodegradation of additional polymers in diverse agricultural soils, including the field, utilizing the novel methodology. According to Kristopher McNeill, professor of environmental chemistry at ETH Zurich and leader of an eponymous research subgroup, “in this way, we want to ensure that biodegradable polymers live up to their name and don’t remain in the environment.”
Sander says that using biodegradable polymers instead of regular ones can help cut down on plastic pollution in situations where polymers are used directly in the environment and there is a good chance they will stay there after use.
