Disentangling mechanisms by which microplastic films affect plant-soil systems: physical effects of particles can override toxic effects of additives

Abstract Background Microplastics, polymer-based particles < 5 mm, affect plant–soil systems positively or negatively, suggesting there are different modes of action. Microplastics, as particles, have physical effects but the leaching of additives likely contributes chemical mechanisms, both of which may be dependent on microplastic size. To disentangle such mechanisms, we established a controlled experiment involving polypropylene and polyethylene films of small, medium and large size, and we evaluated the individual and combined effect of plastic particles and additives (leachates from plastic particles) on soil properties and plant performance of the phytometer Daucus carota and on bare soils. Results We find that additives better explained variation in soil properties (e.g., 44.6% vs 1.3%). Soil respiration and aggregation were negatively affected for additives, likely due to the presence of toxic substances. Overall, such effects increased as plastic size decreased. By contrast, plastic particles better explained plant biomass responses. The positive effect of particles on aeration which may promote root penetration and nutrient uptake, and microplastics itself as a source of carbon potentially promoting soil microbial activity, help explain the positive effect of particles on plant biomass. Plants mitigated the negative effects of additives on bare soils while enhancing the positive effects of particles. This improvement was likely linked to an increase in root activity and rhizodeposition, as plastic particles improved soil aeration. The combined effect of additives and particles, which mimics the microplastic found in the soil, mitigated their individual negative effects on plant–soil systems. As the negative effect of additives could have been masked by the positive effects of particles, simply reporting net positive effects would capture only part of the response. Conclusions Additives and plastic particles differently affect soil properties and plant biomass. Additives primarily negatively affect soil properties due to toxic substances, while plastic particles enhance plant biomass likely by improving soil aeration. When examining microplastics effects on terrestrial systems (i.e., the combined effect of additives and particles), the negative effect of additives may be masked by the positive effects of plastic particles. Reporting only net positive effects risks overlooking these underlying negative effects. Plants can mitigate the negative impacts of additives and amplify the positive effects of plastic particles. Our study emphasizes the importance of investigating both the individual and combined effects of additives and particles to fully understand and address the impacts of microplastics on terrestrial ecosystems.