Biodegradable plastics impair maize growth and reshape bacterial communities associated with roots and sand or soil

Biodegradable plastics entering terrestrial ecosystems raise environmental and food safety concerns, as they impact soil-plant systems either directly by releasing compounds during degradation, or indirectly, by altering soil biophysical properties and/or microbial communities. Here, we examined the impact of biodegradable micro- and macroplastics made from Mater-Bi, and starch (a simple carbon [C] source present in the plastic studied) on the growth of Zea mays seedlings and bacterial communities associated with roots or with the growth substrates, i.e., sand or silt loam soil. Increasing concentrations of biodegradable plastics and starch reduced plant biomass and chlorophyll content in a dose-dependent manner. Additionally, nitrogen (N) content was reduced, accompanied by lower transcript levels of genes commonly downregulated under N-limiting conditions. Biodegradable plastics and starch altered the bacterial community composition, resulting in decreased bacterial diversity in the growth substrates and increased diversity within maize roots. The effects on maize and bacteria were consistent but generally stronger in sand than in silt loam soil. A follow-up experiment confirmed that plastics- or starch-associated microbes did not affect maize growth when no plastics or starch was present. Our results suggest that biodegradable plastics significantly impair soil-plant systems through their C inputs.