Improving our understanding of polymer biodegradation in soils using stable isotope labelling

Biodegradable polymers promise to be one important solution to plastic pollution, when used in certain applications. Tracking their carbon in the environment is important to understand and prove their biodegradation. New stable carbon isotope approaches allow us to do so selectively and sensitively.
Published in Earth & Environment
Improving our understanding of polymer biodegradation in soils using stable isotope labelling

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To avoid further accumulation of plastics in the environment, the current consensus is that we need to reduce our use of plastics, increase recycling and re-use of plastics and, in general, improve how we manage plastic waste. However, there is yet another option: replacing conventional, stable and persistent plastics with plastics made to biodegrade in the environment. The use of such biodegradable plastics offers unique advantages specifically in those applications in which plastics are directly being used in the environment and cannot be efficiently recovered from it after use. A picture-case example are thin agricultural mulch films that are placed onto agricultural soils to improve crop yields. It is difficult to completely remove these mulch films from the soils after use. For conventional mulch films composed of polyethylene, the remaining film fragments are stable in the soil and will accumulate over time when there are repeated inputs. By contrast, biodegradable mulch films composed of biodegradable polymers can be ploughed into soils after use where they subsequently undergo complete biodegradation by native soil microorganisms. This latter practice, however, demands having a high level of certainty that the mulch films will indeed biodegrade. The objective of our work was to advance an experimental approach that unequivocally demonstrates polymer biodegradation – and, at the same time, provides novel insight into the biodegradation process.

The experimental approach is based on incubating 13C-labelled polymers along with 13C-sensitive analytics, because this use of stable carbon isotopes allows delineating polymer-derived carbon from organic carbon that naturally occurs in the incubation medium (e.g., soil organic matter in soils) . We focused on poly(butylene succinate) (PBS) as a model polyester because it is of high commercial relevance and also used in mulching films. The ester bonds in PBS act as pre-determined breaking points which can be broken by extracellular esterases from microorganisms colonizing the PBS surface. The released low molecular weight hydrolysis products are then taken up and metabolically utilized by microorganisms under formation of CO2 and microbial biomass. Using 13C-labelled PBS in soil incubations, we could show extensive mineralization (i.e., approximately 65 %) of the PBS to 13CO2 over the course of long term (i.e., 425 days) of incubation. The stable carbon isotope labelling of PBS allowed us to also quantify the residual (non-mineralized) 13C that remained in the soil at the end of the incubation: mass balances on 13C were indeed closed after the long-term incubations (i.e., approximately 35% of the added 13C remained in the soil after 425 days). Finally, we extracted residual PBS from the soil to demonstrate that the majority of the residual 13C in soil was still present as PBS but that also a substantial part (i.e., 7% of the total added 13C) had been incorporated into microbial biomass. Modelling of the CO2 and residual PBS data showed that determining the latter is critical to accurately interpret the biodegradation dynamics: without the residual PBS data, the models would have predicted complete PBS biodegradation with more extensive incorporation of 13C into the microbial biomass.

Equations for carbon mass balances measured in our work. 13C from labelled PBS was tracked to 13CO2 (i.e., mineralized carbon) and non-mineralized PBS-13C in the soils was quantified. This non-mineralized carbon was further characterized by extracting and quantifying residual PBS in the soils, thereby revealing the extent of incorporation into microbial biomass.

This work is impactful for the field of polymer biodegradation from both methodological and process-understanding perspectives. Our approach overcomes limitations in previous biodegradation studies relying exclusively on incubating non-labelled polymers: without labelling, only polymer mineralization to CO2 can be followed - and with less certainty than for 13C-labelled polymers and without the possibility to close mass balances on added polymer carbon. Furthermore, residual polymers were not extracted and quantified in past studies. From a process-oriented perspective, our approach opens the door to systemically test for polymer and system factors (i.e., pH, temperature, microbial community) that control polymer biodegradation rates, including differences in microbial utilization patterns (i.e., extents of polymer mineralization vs. incorporation of polymer carbon into microbial biomass). This is possible because we look at CO2 and microbial biomass, the two endpoints of biodegradation, as well as at non-mineralized residual polymer. Many open questions remain about the factors controlling polymer biodegradation, and our ongoing and future work aims to answer these questions by applying the approach we developed here. Overall, an extensive understanding of polymer biodegradation processes enables not only the effective application of currently used biodegradable plastics –including the identification of situations where we must be cautious about the use of biodegradable plastics– but also the ability to better develop new biodegradable polymers and plastics thereof.


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