Symbiotic dinitrogen (N2) fixation is often invoked to explain the N richness of tropical forests as ostensibly N2-fixing trees can be a major component of the community. Such arguments assume N2 fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N2 fixation in response to increased soil N availability. These contrasting views of N2 fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N2-fixing root nodules and nitrogenase activity of individual canopy-dominant legume trees (Inga sp.) across several lowland forest types. Fixation was substantial in disturbed forests and some gaps but near zero in the high N soils of mature forest. Our findings suggest that canopy legumes closely regulate N2 fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.