Nitrogen (N) accretion rates under N₂-fixing tree species can vary with site condition and possibly decline over time with down-regulation of N fixation. Tree-ring δ¹⁵N may depict these site-specific, long-term patterns in N dynamics, but field trials with N_2-fixing tree species are lacking. We examined whether tree-ring δ¹⁵N of N₂-fixing red alder (Alnus rubra) would mirror N accretion rates and δ¹⁵N of soils. We sampled a 27-year-old replacement series trial on southeastern Vancouver Island with red alder and coastal Douglas-fir (Pseudotsuga menziesii) in five proportions (0/100, 11/89, 25/75, 50/50, and 100/0, respectively). An escalation in forest floor N content was evident with an increasing proportion of red alder, equivalent to a difference of approximately 750 kg N ha^-1 between 100% Douglas-fir vs. 100% alder. The forest floor horizon was also enriched in δ¹⁵N under denser red alder treatments. Red alder had a consistent quadratic fit in tree-ring δ¹⁵N over time, with a net increase of 1.5‰, on average, before declining slightly. Douglas-fir tree-ring δ¹⁵N, in contrast, was largely unchanged over time (in 3 of 4 plots) but significantly enriched in the 50/50 mix. The minor differences in current litter N content and leaf δ15N between alder and Douglas-fir suggests the declining trend in alder tree-ring δ¹⁵N could coincide with lower N-fixation rates, either by down-regulation via nitrate availability or loss in alder vigour with shading and drought. We suggest tree-ring δ¹⁵N can provide insights into the abiotic constraints and facultative/obligate nature of N fixation for N₂-fixing trees.