Abstract: Microplastics in terrestrial ecosystems are relatively understudied compared to microplastics in well-studied marine and freshwater ecosystems. It is well-documented that soil microplastics can profoundly influence agricultural plant species; while terrestrial wild plants are primary producers and at the bottom of food chains, remarkably little is known about how microplastic abundances affect their growth and the related mechanisms. We selected 18 wild plant species, exposed them to an environmentally realistic microplastic gradient (ranging from 0 to 8000 items kg^–1 soil) for one growing season, and measured soil pH, nutrients and microbes, leaf fluorescence, and plant biomass. We also used structural equation modeling to link the associations between variables. 11% of the 18 wild plant species were inhibited by polypropylene (PP) microplastics, 39% were facilitated, and 50% were unaffected. Across all the 18 species, PP microplastics had no negative impacts on their whole-plant growth, as measured by the standardized effect size; microplastic abundance impacts on whole-plant growth exhibited hump-shaped reaction norms, and the abundance thresholds for positive impacts approximately ranged from 2000 to 6000 items kg^–1 soil. Soil pH played a key role in mediating microplastic impacts directly and indirectly by altering leaf chlorophyll and root nutrient uptake. These findings suggest that microplastic abundance thresholds could shape the whole-plant growth of terrestrial wild plants and microplastic abundance impacts might not rise consistently. Additionally, threshold effects highlight the importance of the full gamut of microplastic abundance gradients and provide insights into ecosystem management strategies.

Key words: chlorophyll fluorescence, microplastic abundance thresholds, plant growth, soil properties, standardized effect size