EPDM microplastic and earthworm effects on plants

Soil microplastic pollution can have negative effects on organisms,
including plants, but the underlying mechanisms are not fully understood.
We tested whether structural or chemical properties of a microplastic
cause its effects on plant above- and belowground growth and whether these
effects can be influenced by earthworms. We conducted a factorial
experiment in a greenhouse with seven common Central European grassland
species. Microplastic granules of the synthetic rubber ethylene propylene
diene monomer (EPDM), a frequently used infill material of artificial
turfs, and cork granules with a comparable size and shape to the EPDM
granules were used to test for structural effects of granules in general.
To test for chemical effects, EPDM-infused fertilizer was used, which
should have contained any leached water-soluble chemical components of
EPDM. Two Lumbricus terrestris individuals were added to half of the pots,
to test whether these earthworms modify effects of EPDM on plant growth.
As response parameters aboveground, belowground, and total biomass, as
well as several root traits were measured, and further parameters derived
from the measured values. EPDM granules had a clear negative effect on
plant growth, but since cork granules had a negative effect of similar
magnitude, with an average decrease in biomass of 37 % in presence of
granules, this is likely due to the structural properties of granules
(i.e., size and shape). For some belowground plant traits, EPDM had a
stronger effect than cork, which shows that there must be other factors
playing into the effects of EPDM on plant growth. The EPDM-infused
fertilizer did not have any significant effect on plant growth by itself,
but it had in interaction with other treatments. Earthworms had an overall
positive effect on plant growth and mitigated most of the negative effects
of EPDM. These results show that EPDM microplastic can have negative
effects on plant growth, and that these might be more related to its
structural than to its chemical properties.