1. Plant invasions pose a major threat to terrestrial biodiversity, and microplastic pollution in soil could exacerbate this problem. Seed germination, a crucial stage for plants, can be affected by microplastics through both physical interference of plastic particles and chemical leaching from plastic additives. 2. We conducted a greenhouse experiment using native and invasive plant species of European grasslands, and evaluated individual and combined effects of plastic particles and additives on germination parameters. 3. We found that plastic particles primarily affected seed germination as physical agents, while additives from these particles exerted a comparatively lesser impact. 4. Particles negatively affected seed germination of all native species. Germination velocity, synchrony and total germination decreased by ~30%, ~11% and ~11%, respectively, in soils containing plastic particles compared to those without. Certain species were negatively affected by plastic additives. For Achillea millefolium and Dactylis glomerata, germination velocity and total germination decreased by ~26% and ~7%, respectively, while germination synchrony of Dactylis glomerata decreased by ~21% in soils with additives than without them. Plastic particles may have blocked seed pores and inhibited hypocotyl and radicle growth, while toxic compounds from additives may have disrupted key germination processes. 5. By contrast, particles and additives generally did not affect invasive species, suggesting that the negative effects of plastic experienced by natives, whether physical or chemical, were counteracted by effects resulting from the novel conditions created by microplastics, which may include the amelioration of soil physical properties such as increased soil porosity and aeration, and potential positive plant–soil feedbacks. Invasive species may profit from windows of variable resource availability, with positive effects on seed germination. 6. Synthesis. Plastic particles and additives may promote plant invasion by negatively affecting seed germination of native species while having neutral effects on invasive species. The delayed germination of natives due to microplastics poses a potential threat, leading to competitive disadvantages, reduced reproductive success and increased vulnerability to invasive species. Microplastic pollution appears to favour invasive species over natives during the early stages of seed germination, highlighting that the negative effect of microplastics on terrestrial ecosystems may be more severe than previously thought.
