Microplastics-driven reconfiguration of organic carbon fractions in lake sediments: mineralization and stabilization...

While microplastics (MPs) are known to profoundly disrupt carbon cycling in lake sediments, the distinct mechanistic pathways by which biodegradable MPs reconfigure organic carbon fractions (OCFs) remain largely unknown. This study elucidates how poly(butylene adipate-co-terephthalate) (PBAT) and polyethylene (PE) MPs dynamically reconfigure OCFs and modulate carbon dioxide (CO₂) and methane (CH₄) emissions in anaerobic lake sediment microcosms. Incubation experiments showed that PBAT serves a dual role: it acts as a potent short-term carbon source that accelerates the release of bioavailable oligomers and dissolved organic carbon (exceeding PE), selectively enriching fermentative microbiota (e.g., p_Firmicutes) and enhancing carbon cycle-related functions, thereby increasing CO₂ and CH₄ emissions by up to 68.59 % and 191.07 % (compared to the control), respectively. Concurrently, PBAT-amended sediment accelerated the decomposition of potential mineralized carbon and readily oxidizable organic carbon, resulting in reduced absolute stability (heavy/light fraction organic carbon= 1.03-2.63) compared to natural sediment (4.45-6.64). Nonetheless, PBAT degradation appeared to inhibit late-stage methanogenesis while facilitating a net redistribution of carbon towards stabilized fractions. Crucially, PBAT enhanced carbon stabilization by promoting humification (increased SUVA₂₅₄ and humic-like fluorescence), mineral association, and humin accumulation. These findings reveal a significant environmental trade-off: biodegradable MPs, although less physically persistent, introduce complex biogeochemical disruptions by serving as substantial short-term carbon sources and modifying carbon sequestration processes. This challenges the assumptions of PBAT's environmental safety, emphasizing significant risks to carbon-sensitive lake ecosystems and necessitating the consideration of these cascading carbon-climate feedbacks in policy frameworks.