Multi-trophic interactions determine the impact of distinct microplastic types on soil bacteria

Microplastics are an emerging environmental problem with profound impacts on global biodiversity, as evidenced mostly in marine systems. However, the impacts of microplastics on the hotspot of biodiversity - soil - remain largely unexplored. Studies examining the influence of microplastics on a broader range of soil biodiversity, including trophic interactions between organisms, such as bacteria and nematodes, are rare. In a greenhouse experiment, we tested the impact of three different microplastics (MPs; low-density polyethylene (LDPE), polybutylene adipate terephthalate (PBAT), and a starch-based microplastic), both with and without nematode addition, on soil bacterial communities. The addition of MPs had limited effects on bacterial diversity. Bacterial alpha diversity was slightly increased by MPs (driven by LDPE and PBAT) and decreased by nematode addition. Nematode presence increased the effects of MP addition on bacterial beta diversity, with bacterial communities differing in all MP types from the control, whereas only PBAT affected the bacterial beta diversity without nematodes. We conclude that the effects of MPs on soil bacteria are MP-type-specific and elevated under more complex and thus realistic conditions found in soils, such as multitrophic conditions with nematodes. Therefore, we urge future studies on microplastics and other emerging pollutants to be conducted under conditions more representative of natural conditions, including the presence of microbiome predators.