Microplastic abundance thresholds shape the growth of 18 wild plant species: the importance of soil pH

Abstract Microplastics in terrestrial ecosystems are relatively understudied while microplastic pollution in marine and freshwater ecosystems has been extensively studied. Although it is well-documented that soil microplastics can profoundly influence agricultural plant species, remarkably little is known about how microplastic abundances affect terrestrial wild plants, primary producers and foundational to food webs, particularly regarding their growth and the underlying mechanisms. We exposed 18 wild plant species to an environmentally realistic polypropylene (PP) 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 analyze variable relationships. Our results revealed that 11% of the 18 wild plant species were inhibited by 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; and growth responses followed hump-shaped curves along the microplastic gradient, with positive effects peaking between approximately 2000 and 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.