Root resorption completes the tree nitrogen economy: Evidence from a 15-year nitrogen addition experiment across stand ages

Tree nitrogen (N) economy is typically framed as a balance between root acquisition and leaf resorption, leaving root resorption largely unresolved. We propose a triadic framework—integrating root N acquisition, leaf N resorption, and root N resorption—and evaluate it in a 15-year N addition experiment in Larix plantations, spanning young, intermediate, and mature ages. Roots resorbed N (mean 9.5%) but far less than leaves (72.8%). Chronic N addition reconfigured pathways with age: senesced root N concentration increased at all three ages; only young trees flipped from root N resorption to net accumulation, whereas mature trees showed no change. Ternary analyses revealed age-dependent rebalancing, with leaf and root N resorption covarying synergistically, while root N acquisition was decoupled from leaf N resorption and shifted from a trade-off with root N resorption in young trees to a synergy in mature trees. This ontogenetic switch stems from shifting N limitation and energy costs: young trees rely on cheap external uptake under N abundance, while mature trees conserve internally as demand grows. Our findings support a triadic N economy structured by age and highlight the need for flux-based approaches that dynamically quantify pathway contributions for more accurate predictions of nutrient cycling.