Polystyrene microplastic pollution induces species-specific shifts in root traits and rhizosphere conditions in a temperate forest

Microplastics (MPs) significantly alter soil physicochemical properties and adversely affect soil nutrient availability, potentially influencing plant root resource acquisition strategies. While extensive studies have examined the impact of MPs on agriculture ecosystems, knowledge about soil and fine root responses to MP pollution in forest ecosystems remains limited. This study assessed the impact of MP addition on fine-root traits and rhizosphere soil properties across four tree species (thick-root species: Pinus koraiensis and Phellodendron amurense; thin-root species: Tilia amurensis and Juglans mandshurica) in a mixed temperate forest. We found that the carbon/phosphorus ratio, the nitrogen/phosphorus ratio and/or the carbon/nitrogen ratio increased significantly, but the total and/or available phosphorus concentration decreased significantly in the rhizosphere soil in tree species characterized by thick and thin roots after the addition of MPs. Thin-root tree species significantly decreased root biomass and length proliferation, while their specific root area increased and root diameter decreased in response to MP addition for enhancing exploration and absorption ability within the root system. Conversely, thick-root tree species increased root biomass proliferation but reduced epidermal and cortical thickness following MP addition for decreasing construction and maintenance cost of the root system. Thus, root biomass, root length, epidermal and cortical thickness emerged as the most important root traits differentiating the responses of different tree species to MP addition. These results suggest that various root traits regulate root resource acquisition strategies across tree species in response to MP accumulation, offering novel insights into the ecological consequences of MP pollution in forest ecosystems.