Abstract: Population stability (PS) of functionally important species underpins ecosystem resilience, yet the mechanisms that maintain PS under heterogeneous nitrogen (N) conditions remain elusive, particularly for legumes in (sub)tropical forests. Legumes can stabilize their population performance through high abundance or symbiotic N₂ fixation, but how soil N “chooses” between these strategies is unknown. In a 20-ha subtropical forest plot, the sole dominant legume Ormosia glaberrima, with >2,500 individuals recorded over 15 years, provides an ideal system to test these pathways. We quantified PS from long-term demography as the inverse of inter- period variation in basal area growth, and assessed biological N fixation (BNF) ability using the N stable isotope natural abundance method. Structural equation modeling (SEM) was applied to disentangle the direct and indirect effects of soil N, abundance, and BNF ability on PS. Results showed no significant direct effect of soil N on the PS of O. glaberrima. Instead, it was positively correlated with abundance (R²_adj =0.22, P < 0.001) and negatively related to BNF ability (R²_adj=0.26, P < 0.001). SEM explained 34% of the variation in PS and revealed opposing mediating pathways. Elevated soil N indirectly enhanced PS by increasing abundance (β = 0.27) but reduced PS by suppressing BNF ability (β = -0.15). These findings reveal a context- dependent “abundance-fixation trade-off” as a novel mechanism regulating long-term legume population performance, with important implications for forest management aimed at maintaining ecosystem stability under global N deposition.

Key words: long-term demographic monitoring, soil nitrogen, abundance, biological nitrogen fixation, population stability, trade-off, subtropical lowland forest