Reeds (Phragmites australis) modulate the impacts of microplastics on carbon and nitrogen metabolisms in wetland soil

The environmental prevalence of microplastics (MPs) has raised growing concerns about their potential to disrupt ecosystem processes and functions. Wetlands are an important hub for MPs diffusion, whereas the ecological impacts of MPs contamination in wetlands, particularly with regards to plant-microbe-MP interactions in element cycling pathways, remain underexplored. In this study, we investigated how polypropylene (PP) MPs at concentrations of 0.01 wt% and 0.1 wt% differentially affected soil carbon and nitrogen metabolic processes in the presence and absence of reed (Phragmites australis) plants. We found that PP MPs triggered contrasting alterations in carbon and nitrogen nutrients and enzyme activities in planted versus unplanted soils. Soil bacterial α-diversity was generally increased by MPs. The addition of PP MPs reshaped bacterial community composition by suppressing Acidobacteriota, Gemmatimonadota, and Nitrospirota, while enriching Thermomicrobiota and Bacillota. At the gene level, PP MPs had minimal effects on the abundance of carbon fixation genes in unplanted soils. By contrast, in planted soils, MPs addition enhanced the reductive citrate (rTCA) cycle by 14.9 % - 26.7 % and suppressed the reductive acetyl-CoA (rAc-CoA) pathway by 22.6 % - 24.0 %. For nitrogen metabolism, MPs-associated decrease (8.8 % - 11.8 %) in the abundance of denitrification genes was only observed in planted soils. These contrasts imply that PP MPs had differential impacts on the structure and functional stability of soil bacterial communities in planted and unplanted soils. Bacterial Simpson index and β-1,4-glucosidase were identified as the key factors associated with soil carbon and nitrogen cycling. This study highlights the significant impacts of MPs on soil carbon and nitrogen cycling in wetland ecosystems and the crucial role of plants in modulating the MPs-associated effects.