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Tree nitrogen (N) economy is typically framed as a balance between root acquisition from soils 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 <i>Larix</i> 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 intermediate and mature trees showed no significant 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 intermediate and mature trees. These results identify a triadic organization that structures the tree N economy and demonstrate how stand age reorganizes this balance under long-term N inputs. Our findings clarify when root resorption matters, why young plantations warrant particular attention under N enrichment, and how age-tailored management should account for shifting internal–external N supplies.