Mycorrhizal-specific responses of rhizosphere soil properties and fine-root traits to polystyrene microplastic addition in a temperate mixed forest

While the impacts of microplastics on aquatic and agricultural ecosystems are well studied, the impacts on forest ecosystems involving soil and trees are scarcely investigated. Here, we assessed the impacts of microplastic addition on rhizosphere soil properties, and chemical, morphological and anatomical traits of fine roots for ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) tree species in a mixed temperate forest. We found that the concentration of available nitrogen in the soil rhizosphere increased, while the concentration of available phosphorus decreased in the ECM tree species after the addition of microplastics. The opposite pattern was true for AM tree species. Fine roots of ECM tree species exhibited shorter root length, smaller root diameter, lower root tissue density, lower branching ratio, lower branching intensity, and lower phosphorus concentration, but higher hyphal density, higher root carbon/nitrogen, and higher root carbon/phosphorus ratios with the addition of microplastics mediated by total phosphorus in the soil. Fine roots of AM tree species exhibited higher specific root length, tip density, epidermal thickness, vascular bundle diameter and root carbon/nitrogen ratio, but lower root diameter, branching intensity, cortical thickness, root tissue density and root phosphorus concentration after microplastic addition, which was mediated by soil water content, nitrate nitrogen and available phosphorus. These findings on mycorrhizal-specific responses to microplastic addition will deepen our understanding of carbon and nutrient cycling and species composition dynamics with increasing microplastic pollution in temperate mixed forest ecosystems.