The Microplastic Menace: Polymer-Specific Impacts on Bacterial Communities and Carbon Cycling in Temperate Forest Soils

Microplastics (MPs) accumulate in forest soils and interfere with microbial processes that regulate carbon turnover. We hypothesized that polymer identity, MP content, and soil order jointly modulate microbial activity, enzyme allocation, and alpha diversity during the early exposure period. Ultisol and Andisol from temperate forests in southern Chile were incubated for 29 days with polyamide (PA), polyethylene (PE), and polyvinyl chloride (PVC) MPs at 0.5–5% (w/w) content. The incubation quantified CO production, fluorescein diacetate (FDA) hydrolysis, extracellular enzyme activities, and bacterial (16S rRNA) and fungal (ITS) metabarcoding. CO production converged toward the control by day 29, with a steeper empirical decline rate at 5% than at 0.5% across polymers. Bacterial Shannon diversity in Andisol declined from 6.4 in the control to 5.4 under PE and 5.8 under PVC, whereas in Ultisol, it ranged from 5.4 in the control to 6.1 under PVC and 5.6 under PA. Fungal Shannon diversity in Andisol ranged from 3.1 under PE to 4.2 under PA relative to 3.5 in the control, whereas Ultisol ranged from 3.7 under PVC to 4.2 in the control. Taxonomy-based functional assignments suggested polymer-and soil-dependent shifts in putative bacterial and fungal guild functions. Polymer-associated traits, MP content, and soil properties jointly shaped short-term functional and community responses, supporting the use of polymer identity, loading, and soil context in forest soil risk assessment.