Carbon Clues to Plastic Biodegradation: Capabilities, Limitations, and Interpretation of Stable Isotope Evidence

Stable isotope techniques provide a powerful framework for evaluating plastic biodegradation by tracing the fate of polymer-derived carbon. In particular, stable ¹³C- and radioactive ¹⁴C-labeled tracer experiments enable direct quantification of mineralization and carbon assimilation into biomass. In contrast, natural-abundance δ¹³C analysis offers a label-free approach but presents significant interpretative challenges, especially for solid polymers where degradation is typically surface-limited and isotopic shifts are small. In this review, we examine the theoretical basis of isotope fractionation and tracer methodologies and critically evaluate their application to plastic biodegradation systems. Synthesis of published data reveals systematic differences between degradation pathways: microbial systems typically exhibit small δ¹³C shifts (generally <1‰) that often approach analytical precision limits (0.1 to 0.3‰), whereas larger shifts are more frequently observed in insect-mediated systems. Based on these observations, we propose an empirical framework for interpreting Δδ¹³C values as qualitative indicators of the strength of biodegradation evidence, while emphasizing the limitations imposed by heterogeneous reactions and bulk polymer dilution effects. We conclude that stable isotope approaches are most powerful when integrated with complementary methods, including carbon mass balance, molecular characterization, and microbial analyses. This combined framework provides a more rigorous basis for verifying plastic biodegradation and understanding the environmental fate of polymer-derived carbon.