Attribution-Underdetermined Signals in Long-Term delta-15N Chronologies: A Multi-Pathway Diagnostic Using TRIDENT

Long-term declines in tree-ring and foliar delta-15N in boreal forests have been attributed to progressive nitrogen limitation (PNL) under rising atmospheric CO2. However, delta-15N integrates multiple mechanistically distinct pathways, including (1) CO2-driven nitrogen constraint migration via progressive nitrogen limitation, (2) deposition isotope forcing resulting from changes in the isotopic composition of atmospheric nitrogen inputs, and (3) photosynthetic acclimation without true nitrogen scarcity. This technical note applies the TRIDENT diagnostic framework to formalise the attribution structure underlying long-term delta-15N chronologies. TRIDENT evaluates boundary-driven system claims through four gates: Bulk Lock, Boundary Flow, Constraint Field, and Coherence Dependency (G4). Existing evidence satisfies Gates 1–3. Gate 4 remains open because all three pathways generate identical directional delta-15N predictions under current observational constraints. We derive minimal formal expressions that render the pathways distinguishable and specify explicit falsifier conditions required for attribution closure. An empirical synthesis across multi-proxy studies, cross-biome isotope chronologies, deposition isotope records, FACE experiments, and Earth system model projections maps the current state of support and challenge. Contribution: methodological. A TRIDENT PASS without G4 closure constitutes a mechanism-class claim, not a causal claim. No new experimental data are presented. Part of the Sympathetic Harmonic Resonance Framework (SHRF) corpus. Keywords progressive nitrogen limitation nitrogen isotopes delta-15N boreal forest CO2 fertilisation carbon-nitrogen coupling TRIDENT diagnostic framework attribution methodology carbon sink Earth system models photosynthetic acclimation deposition isotope forcing proxy reliability Sympathetic Harmonic Resonance Framework SHRF