Nitrogen Enrichment Modulates Fine-Root Morphology and Chemistry Through Mycorrhizal Type and Functional Differentiation in Temperate Forests

Anthropogenic nitrogen (N) deposition has intensified globally, yet its impact on root trait plasticity across mycorrhizal associations and root types remains poorly understood. To test how N addition differentially modulates fine-root traits via nutrient pathways and root functional differentiation, we conducted a field experiment. The study was conducted in a temperate mixed forest in northeastern China, where four nitrogen treatments (0, 25, 50, and 75 kg N ha⁻¹ yr⁻¹) were applied to long-term experimental plots containing both arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species. N addition significantly increased root N and phosphorus (P) concentrations, decreased root carbon (C)/N and C/P ratios, and enhanced specific root length and specific root surface area in absorptive and transport roots, particularly under high-N treatments. ECM roots exhibited greater plasticity and higher nutrient contents than AM roots. Soil dissolved organic N and soil total P were key drivers of AM root traits, whereas root C concentration, soil total N, and root P concentration were dominant predictors for ECM roots. Root responses were function-dependent, with contrasting regulation patterns between absorptive and transport roots. These findings demonstrate that ECM species achieve superior adaptability to N enrichment via enhanced morphological and chemical plasticity, and that root trait responses are strongly shaped by nutrient form and root functional role. This study provides a mechanistic basis for predicting forest belowground responses to N deposition under global change scenarios.