Abstract: Microplastics (MPs), as a kind of emerging pollutant, have been widely found in soils. However, the effect of MPs on the mineralization of soil organic carbon (SOC), especially the difference between degradable and non-degradable MPs, is still unclear. The relevant mechanisms need to be further revealed. In this study, a soil incubation experiment (100-day) was conducted with adding degradable polylactic acid (PLA) and non-degradable polyethylene (PE) MPs to a coastal wetland soil, respectively. The results showed that, regardless MPs type, MPs addition significantly reduced the cumulative CO₂ emissions. Compared with the PLA-MPs, the PE-MPs had stronger inhibitory effect on the cumulative CO₂ emissions. Furthermore, the addition of both MPs types significantly increased SOC content and improved the stability of soil aggregates, while remarkably reduced the contents of dissolved organic carbon, NH₄⁺-N, and NO₃^--N. In terms of microbial property, the addition of MPs decreased the diversity of soil microbial community and the activity of carbon mineralization associated bacteria and enzymes. The reduction of available substrates for SOC mineralization, the enhancement of physical protection of soil aggregates, and the reduction of SOC mineralization-related bacterium and enzyme activities were identified as the main mechanisms for the lower SOC mineralization in MPs added soils. Overall, non-degradable PE-MPs had stronger effect on soil physical, chemical, and microbial indices than degradable PLA-MPs. Our results can provide important data support and theoretical basis for in-depth understanding of the environmental effects of MPs in coastal wetland soils.

Key words: microplastics, soil organic carbon mineralization, soil aggregate, bacterial community, enzyme activity