Mycorrhizal Type Steers Trait–Function Linkages in Root Nitrogen Uptake Across Subtropical Tree Species

Root nitrogen (N) uptake strategies are shaped by both functional traits and mycorrhizal type, yet how these factors interact remains unclear. We quantified inorganic N uptake rates using ¹⁵N tracers applied to intact, plant-attached absorptive roots across 63 co-occurring tree species in a subtropical forest. We found that while arbuscular mycorrhizal and ectomycorrhizal tree species exhibited convergent uptake rates, the underlying trait-uptake relationships diverged fundamentally. Leaf and root economic traits (e.g., N concentration) were the dominant predictors of N uptake rates in arbuscular mycorrhizal trees, whereas root morphology governed uptake rate variation in ectomycorrhizal trees. We developed a mycorrhizal-explicit whole-plant trait model that substantially increased prediction accuracy over conventional root-only models (R² = 0.37 vs. 0.19). These results reveal a hierarchical structure in which trait-function relationships are nested within mycorrhizal types, offering a mechanistic framework for understanding species coexistence and predicting forest N cycling.