Coordination between root exudation rates and root conservation traits under simultaneous warming and nitrogen addition

Fine-root traits and exudation shape plant-microbe interactions and nutrient cycling, yet it remains unclear whether exudation correlates with root traits or responds independently to soil conditions. In a 16-yr factorial warming × nitrogen addition experiment, we quantified fine-root morphology, chemistry, and exudation in two dominant temperate trees: Quercus rubra and Acer rubrum. Root exudation responded more strongly to experimental treatments than most root morphological traits. Nitrogen addition increased exudation in both A. rubrum and Q. rubra, whereas warming reduced exudation in Q. rubra. These shifts were not explained by root foraging traits, which were largely species-specific and stable. Instead, exudation was most closely associated with traits related to root construction and nutrient conservation, including root tissue density, root nitrogen concentration, and branching intensity. The contrasting effects of warming and nitrogen addition on exudation, despite stable root morphology, suggest that rhizosphere carbon inputs can shift without changes in root morphology. As such, future rhizosphere carbon fluxes may depend more on root tissue construction and belowground carbon allocation than on changes in root morphology. This suggests that physiological shifts in root carbon release may be a key pathway by which warming and nitrogen availability alter belowground carbon cycling.