Microplastics from biodegradable plastic bags alter soil properties and trigger stress response in sunflowers.

The widespread replacement of conventional plastics with biodegradable alternatives raises questions about their long-term effects in agricultural soils. We investigated the impact of microplastics (MPs) cryo-milled from home-compostable (PBAT_HOME) and industrial-compostable (PBAT_IND) polybutylene adipate terephthalate bags and compared it to the toxicity of MPs from conventional polyethylene (PE) bags. Sunflowers (Helianthus annuus L.) were grown in pots with soil spiked with 0 %, 0.05 %, 0.5 %, and 1.0 % (w/w) MPs for eight weeks under a 16/8 day-night photoperiod at room temperature (22/18 °C). Plant responses differed notably between biodegradable and conventional MPs as revealed by two-way hierarchical clustering analysis: both PBAT_HOME and PBAT_IND significantly reduced shoot and root biomass at 0.5 % and 1 %, while PE had no effect. At higher PBAT concentrations, proline levels increased, consistent with activation of drought-like stress response pathways that merit confirmation with additional molecular biomarkers. Soil water holding capacity increased at 1 % of all three MPs, indicating a lower water availability for plants in the MP-spiked soil. pH was slightly increased at 1 % PBAT_HOME and significantly at 1 % PBAT_IND soil. Despite differences in particle size, polymer composition, and starch content, no single factor explained the observed effects, suggesting a combined influence. These results highlight that biodegradable MPs can alter soil properties and trigger physiological responses in plants, emphasizing the need to assess their ecological impacts, especially in agricultural systems where soil health and resilience are critical for crop production.