Short-term exposure to biodegradable mulch-film microplastics at field-realistic levels shows no effect on lettuce performance and soil health

The increasing use of biodegradable plastic mulch films in agriculture is expected to reduce plastic pollution of soils. Yet, the effects of biodegradable microplastics on plant performance and soil health, under environmentally relevant microplastics concentrations and short-term exposure, are unclear. This study compares the impact of two biodegradable microplastics, polybutylene adipate-co-terephthalate (PBAT) and starch-based biodegradable (SBB), with conventional non-degradable polyethylene (PE) and two controls containing a natural carbon addition or just soil with no added carbon, on lettuce (Lactuca sativa) performance and soil nutrient availability. We conducted an outdoor pot experiment using a sandy loam soil. The soil was spiked to a concentration of 0.11% w/w with PBAT, SBB, and PE microplastics (~ 430 𝜇𝑚) or straw. Key plant performance metrics were monitored, and macronutrients were measured in the soil solution on a weekly basis. Furthermore, soil microbial activity, water-holding capacity, and chemical surface alterations of the polymers were measured after the experiment. We found no significant differences in most of the analyzed parameters. The absence of effects compared to previous studies could be due to comparatively low microplastics concentration and the short-term growth duration (8 weeks) leading to reduced aging of microplastic particles and thus reduced effect significance. Our findings suggest that lettuce performance and soil nutrient availability are not significantly impacted during a short-term experimental period and environmental-relevant concentrations of biodegradable microplastics.