Changes in soil microbial functions involved in the carbon cycle response to conventional and biodegradable microplastics

Biodegradable mulch film has become an encouraging alternative to conventional petroleum–based plastic, but it is likely to generate more microplastics (MPs) than conventional films with lower resistance and thus profoundly affects the soil environment. However, the effects of biodegradable (BMPs) and conventional polyethylene MPs (PE–MPs) on soil microbial carbon (C) cycle remain unclear. Here, metagenomic sequencing was used to investigate microbial functional and relative taxonomic traits in response to PE–MP and BMP addition (5 % w/w) via a soil microcosm experiment. The results showed that the presence of BMPs, rather than PE–MPs, significantly changed soil microbial C cycling patterns at the functional and taxonomic levels. In soils, BMP addition significantly increased the abundances of genes regulating aerobic respiration, C decomposition (intracellular), C fixation, fermentation and CO oxidation pathways compared to those in the control and PE–MP added soils. Specifically, BMPs promoted soil starch and PHA (liable C) degradation, acetate and ethanol fermentation, Calvin–Benson–Bassham (CBB) cycle and 3–hydroxypropionate/4–hydroxybutyrate (3HP/4HB) associated with C fixation. Most of these C cycling pathways were further strengthened in the microplastisphere of BMPs. Furthermore, MPs selectively enriched specific taxa on their surface and subsequently increased their abundance in soils; Bradyrhizobium, Ramlibacter and Variovorax (enriched by BMPs) positively regulated labile C degradation, and Amycolatopsis and Nocardia (enriched by PE–MPs) promoted recalcitrant C degradation. Moreover, Pseudonocardia and Variovorax that were enriched by BMPs improved C dissimilation and assimilation by regulating fermentation and C fixation processes, respectively. Taken together, these results improve the understanding of the influence of conventional and biodegradable MPs on soil C cycling pathways and related microbial communities, highlighting that the presence of BMPs rather than PE–MPs accelerates soil C turnover.