Root resorption completes the tree nitrogen economy: evidence from a 15-yr nitrogen addition experiment across stand...

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-yr N addition experiment in Larix plantations, spanning young, intermediate, and mature ages. Fine roots resorbed N (mean 9.5%) but far less than leaves (72.8%). Chronic N addition reconfigured pathways with age: fine-root N resorption efficiency declined only in young stands, whereas fine-root resorption proficiency decreased consistently across all ages. Ternary analyses revealed age-dependent rebalancing, with leaf and fine-root N resorption covarying synergistically, whereas fine-root N acquisition was decoupled from leaf N resorption and shifted from a trade-off with fine-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 a cheap self-reliant acquisition strategy under N abundance, whereas mature trees increasingly rely on mycorrhizal uptake while conserving 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.