Addition of polyester in soil affects litter decomposition rates but not microarthropod communities

Microplastics are defined as plastic particles that are <5mm. Manufactured in the production of many commercial products, microplastics have become an environmental threat for many organisms. Microplastics can be highly abundant in soil, and given their size, can interact with soil microarthropods. But how microplastics affect soil-dwelling organisms (mites and collembolans) and their role in ecosystem services such as decomposition is largely unknown. We studied the effects of polypropylene and polyester microfibers of two different lengths (2–3mm and 5–6mm) on microarthropod communities and decomposition rates in a sandy soil. Microplastic addition showed no effects on soil microarthropod communities for the groups Oribatida (abundance and species richness), Prostigmata, Astigmata and Mesostigmata, Collembola, nor other invertebrates present in the soil samples (abundance). The addition of microplastics in the soil did affect litter decomposition rates for litterbags on the soil surface; higher mass loss (i.e. decomposition) was found in polyester treatments compared to control and polypropylene treatments, regardless of the length of the fibers. However, no significant differences were found on feeding rates measured by bait-lamina sticks. Permanova results for microarthropod community structure among treatments were not significant, although non-metric multidimensional scaling analysis (NMDS) found that communities were less similar to one another in polypropylene addition treatments compared to polyester addition and to control treatments. This study is the first to test the effects of microplastics on soil microarthropod communities, and we find no direct negative effects of microplastic addition.