Effects of microplastics on carbon release and microbial community in mangrove soil systems

Mangrove ecosystems are major carbon sink biomes and also a sink of microplastics (MPs). The final enrichment of MPs in sediments may have a significant impact on the microbial community and carbon turnover in the soil. However, the effects of MP pollution on the mangrove soil microbial communities and carbon release remain unknown. Here, we conducted a manipulative incubation experiment by adding MPs to soil at different soil depths to examine the effect of enriched MPs on soil microorganisms and its function (i.e., decomposition of soil carbon). The results showed that the addition of MPs had no significant effect on the microbial diversity and CO₂ cumulative emission in the topsoil but significantly increased CO₂ release from the subsoil. The promoting effect of polylactide (PLA) on the release of CO₂ from the subsoil was stronger than that of polyethylene (PE) and aging PE. In the subsoil, the activity of soil extracellular enzymes related to N acquisition increased with the MP addition, indicating an increase in microbial N deficiency. The subsoil was more sensitive to MPs because of the exacerbated nitrogen limitation. MP addition reduced the microbial diversity of the subsoil and altered soil microbial interactions. The increasing abundance of some microbial taxa, especially bacteria related to the sulfur cycle, indicated more active electron transfer and organic carbon mineralization in the subsoil. Our findings suggest that MP contamination has potential effects on microbial communities, nutrient cycling, and carbon release in mangrove soils that vary depending on soil depth.