Soil biota modulate the effects of microplastics on biomass and diversity of plant communities

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.