Biochar mitigates biodegradable microplastic-induced greenhouse gas emissions in lake sediments: Unraveling microbial mechanisms and particle-size effects

Accumulation of biodegradable microplastics (MPs) in freshwater sediments exacerbates greenhouse gas (GHG) emissions, though effective mitigation strategies are still poorly understood. This study investigated the potential of biochar (BC) for carbon sequestration and its role in suppressing MP-induced GHG emissions. Herein, we elucidated the effects of biodegradable MPs Poly (butylene-adipate-co-terephthalate) (PBAT) and different-sized (bulk- and nano-) BC on carbon dioxide (CO) and methane (CH) emissions. Results revealed that PBAT significantly reduced sediment pH and oxidation-reduction potential while increasing carbon content, leading to a 50.5 % rise in CO and 487.9 % in CH emissions. Nano-BC alone reduced CO emissions by 46.8 %, whereas no inhibitory effect was observed under MPs pollution. Critically, bulk- and nano-BC strongly inhibited cumulative CH emissions in MP-polluted sediment by 14.7 % and 50.6 %, respectively. BC countered PBAT-induced increases in sedimentary amino acids and humic substances by limiting key bacterial phyla involved in organic matter decomposition. Furthermore, BC reduced the abundance of MP-enriched microbes such as p_Firmicutes, f_Methanoregulaceae, and f_Methanotrichaceae, suppressed microbial metabolic functions and carbohydrate-active enzymes, and promoted CH oxidation and carbon fixation via genes including cutL, coxL, and coxA. Simultaneously, BC diminished methanogenic gene expression (mcrB, mcrA2). This work demonstrates the pronounced stimulatory effect of biodegradable MPs on sediment carbon mineralization and establishes the multi-level mechanism through which BC (particularly nano-BC) mitigates climate change effectively.