The more microplastic types pollute the soil, the stronger the growth suppression of invasive alien and native plants

The ecological consequences of microplastic pollution for plants remain
largely unknown, and the few studies that tested the effects usually
focused on a single type of microplastic and a single plant species.
However, most plants will be exposed to multiple microplastic types
simultaneously, and the effects may vary among species. To test the
effects of microplastic diversity on plants, we grew single plants of
eight invasive and eight native species in pots with substrate polluted
with 0, 1, 3 and 6 types of microplastics. We found that the growth
suppression by microplastic pollution became stronger with the number of
microplastic types the plants were exposed to. This tended to be
particularly the case for invasive species, as their biomass advantage
over natives diminished with the number of microplastic types. The biomass
responses coincided with a positive effect of the number of microplastic
types on root allocation and thickness, which was also stronger for
invasive than for native species. In addition, the results of hierarchical
diversity-interaction models suggest that the negative impact of
microplastic diversity on the total biomass of invasive plant species was
influenced by both the identities of the microplastic and certain types of
microplastic with strong pairwise interactions. In contrast, the effect on
native species was determined solely by the microplastic identities.
Synthesis: Our multi-species study thus shows for the first time
that the negative effects of microplastic pollution on plant growth
increase with the number of microplastic types. We also found tentative
evidence that the negative impacts of microplastic diversity were more
pronounced for invasive plants compared to native plants, and that this
might be due to differences in the responses of root allocation and
thickness.