Tracing microplastics degradation in soil using stable isotope techniques

Sammanfattning

Microplastics (MPs) is an emerging concern in terrestrial ecosystems due to their persistence and potential effects on soil structure and microbial processes. Biodegradable plastics are proposed as a more sustainable alternative, but their behavior and degradation in soil remain poorly understood. In this study, stable carbon isotope (δ13C) analysis of respired carbon dioxide (CO2) emission was used to trace the mineralization of isotopically labeled bio-based MPs, made of sugarcane, added to a natural forest soil. Two types of bioplastics were tested at different concentrations of 1% and 5%, and CO2 emissions and isotopic signatures were monitored during four weeks using Cavity Ring-Down spectroscopy (CRDS). The plastics were a razor handle with a δ13C of -18.56‰ made of polylactic acid (PLA), polybutylene succinate (PBS) and polybutylene adipate terephthalate (PBAT) as well as a plant pot with a δ13C of -17.43‰ made of PLA, PBAT and calcium carbonate (CaCO3). The plastics were cryogenically ground using liquid nitrogen and a coffee grinder, then sieved to obtain an MP size of 0.5 mm and 1 mm, and mixed manually into 40 g dry weight equivalent forest soil. Elevated respiration and shifts in δ13C values were observed one hour after plastic addition, particularly in the higher concentration treatments. The respiration rate increased from 3 μg C g⁻¹ dm soil h⁻¹ in the blank to 16 μg C g⁻¹ dm soil h⁻¹ in the plant pot 5% treatment, representing a fivefold increase. At the same time the δ13C value shifted from -29‰ in the blank to -8‰ in plant pot 5%, indicating the emission of isotopically enriched carbon sources. However, rather than the degradation of the carbon backbone, the δ13C of CO2 beyond that of the δ13C of the original plastic suggests potential abiotic contributions and masking effects from plastic additives. The findings highlight both the potential and the limitations of this approach, emphasizing the need for further research into the degradation of plastic derived carbon and plastic additives.