Species-Specific Responses of Fine Root Morphology, Anatomy, and Mycorrhizal Traits to Nitrogen Addition in Temperate Forest Ecosystems

Excessive nitrogen (N) deposition alters plant-soil interactions and biogeochemical processes in forest ecosystems. Fine roots, which function as critical interfaces between plants and soil, exhibit morphological, anatomical, and mycorrhizal plasticity in response to nitrogen availability. This study examined species- and root-order-specific responses to nitrogen addition across a gradient and evaluated their ecological implications. The research, conducted in northeastern China's Fenglin National Nature Reserve, investigated three tree species across plots subjected to four N treatments. Root samples were analyzed for morphological traits, anatomical traits, and mycorrhizal characteristics. General linear models and three-way analysis of variance were employed to assess the effects of N treatments, tree species, and root orders. Nitrogen addition significantly influenced fine root morphology, with high-N (TH) increasing root diameter and medium-N (TM) enhancing specific root surface area. Anatomical traits exhibited dose-dependent changes, including increased cortex thickness and stele diameter under TH. Mycorrhizal traits showed divergent responses, with low-N promoting colonization and total hyphae length, while TM enhanced hyphae diameter. Species-specific and root-order-specific variations underscored the complexity of belowground functional responses. Fine root and mycorrhizal adaptations to nitrogen addition reflect trade-offs between structural reinforcement and absorptive efficiency, driven by species identity, root developmental stage, and soil nutrient status. These findings provide critical insights into nitrogen-induced root plasticity and its role in shaping forest ecosystem processes, offering valuable perspectives for sustainable forest management under global change scenarios.