Supplementary material from "Microplastics negatively affect soil fauna but stimulate microbial activity: insights from a field-based microplastic addition experiment"

Microplastics are recognized as an emerging contaminant worldwide. Although microplastics have been shown to strongly affect organisms in aquatic environments, less is known about whether and how microplastics can affect different taxa within a soil community, and it is unclear whether these effects can cascade through soil food webs. By conducting a microplastic manipulation experiment, i.e. adding low-density polyethylene fragments in the field, we found that microplastic addition significantly affected the composition and abundance of microarthropod and nematode communities. Contrary to soil fauna, we found only small effects of microplastics on the biomass and structure of soil microbial communities. Nevertheless, structural equation modelling revealed that the effects of microplastics strongly cascade through the soil food webs, leading to the modification of microbial functioning with further potential consequences on soil carbon and nutrient cycling. Our results highlight that taking into account the effects of microplastics at different trophic levels is important to elucidate the mechanisms underlying the ecological impacts of microplastic pollution on soil functioning.