The use of biodegradable plastics has been proposed as an alternative to mitigate the pollution problem caused by traditional non-biodegradable plastics. However, the relative impacts of both types of microplastics on plant community productivity and diversity are not known. Moreover, it is unclear whether soil biota can differentially mediate the impacts of biodegradable and non-biodegradable microplastics on plant communities. In this study, we investigated the effects of biodegradable and non-biodegradable microplastics on plant community biomass production and diversity, and whether soil biota mediate these effects. We employed a fully crossed factorial design, growing six plant communities in the presence or absence of 10 individual microplastics, and live soil versus sterilized soil. We hypothesized that: 1) Biodegradable microplastics have a less negative effect on plant community biomass production and diversity compared to non-biodegradable microplastics. 2) Soil biota differentially mediate the effects of biodegradable and non-biodegradable microplastics on plant community biomass production and diversity. Statistical analyses that included all 10 microplastics yielded two main findings. First, live soil ameliorated the negative effects of biodegradable microplastics on community shoot biomass. Second, the presence of microplastics, rather than their biodegradability, significantly reduced community diversity. Separate analyses of individual microplastics suggest that these patterns were driven by specific microplastics. The biodegradable microplastic Polybutylene succinate (PBS) was the main driver of the pattern observed in community shoot biomass. In contrast, the biodegradable microplastic Polycaprolactone (PCL) and non-biodegradable microplastics Ethylene-vinyl acetate (EVA) and Polyvinyl chloride (PVC) were the main drivers of the pattern observed in community diversity. Further analyses excluding PBS from the global models, but including the other nine microplastics, revealed no significant differences in community shoot biomass and diversity between biodegradable and non-biodegradable microplastics in live versus sterilized soil. Our findings suggest that biodegradable microplastics, often considered environmentally friendly, are not necessarily less harmful than non-biodegradable microplastics to the growth and diversity of plant communities. Some individual biodegradable microplastics, such as PBS, still pose significant ecological risks to plant community structure and productivity. However, the results also suggest that soil biota may mitigate the negative effects of some biodegradable microplastics.

We conducted a greenhouse experiment with six plant communities (each consisting of five plant species) and assigned them to the  2 soil treatments (live soil vs. sterilized soil) × 11 microplastic treatments (five biodegradable microplastics, five non-biodegradable microplastics, and one no-microplastic control) combinations. Four replicates per treatment. All plant data are collected from this experiment. Additionally, we collected soil from a parallel long-term experiment in a northeast China grassland (Hulunbuir; 119°38'E, 50°20'N) running since 2020, where we are examining the effects of microplastic pollution on plant community diversity and productivity in a grassland. We randomly collected 0.5 g of soil from each sample to extract. The 100 soil samples [10 microplastic (no-microplastic, 5 biodegradable microplastics, 4 non-biodegradable microplastics) ×10 replicates] were immediately stored at -80 ℃ until DNA extraction.