Long-term aged fibrous polypropylene microplastics promotes nitrous oxide, carbon dioxide, and methane emissions from a coastal wetland soil

Microplastics (MPs) has been suggested that it can greatly affect soil greenhouse gases (GHGs) emissions via altering soil physical, chemical, and biological properties. However, the difference in GHGs emissions, especially for those from coastal wetland soils, between varied aged MPs was rarely explored and the underlying mechanisms of GHGs emissions affected by the aged MPs were poorly understood. Therefore, the implications of fibrous polypropylene MPs (FPP-MPs) exposure on NO, CO, and CH emissions were examined by a 60-day soil incubation experiment. Compared with the control, the additions of un-aged FPP-MPs with both two rates (0.2 and 2 %) and aged FPP-MPs with a low rate (0.2 %) showed an insignificant effect on NO emission, while the aged FPP-MPs added with a high rate (2 %) resulted in a remarkably increase in NO emission, especially for those of the 30-day-aged FPP-MPs. A significant increase in CO emission was only observed in the 30-day-aged FPP-MPs treatments, compared with the control, and a higher addition rate produced a higher increase of CO emission. Regarding CH emission, it was significantly increased by adding aged FPP-MPs, and a longer aging period or/and a higher addition rate generated a higher degree of promotion of CH emission. However, compared with the CO emission, the quantity of CH emission was extremely low. These increased GHGs emissions can be ascribed to the improvements in soil physical structure and other chemical properties (e.g., pH and contents of soil organic matter and dissolved organic carbon) and enhancements in the abundances of denitrification- and carbon mineralization-related microorganisms. Overall, our results highlight the risk of elevated GHGs emissions from the soil polluted with 30-day-aged FPP-MPs, which should not be ignored as long-term aged FPP-MPs continue to increase in coastal wetland soils.