Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen(1-6), a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N-2)(7), but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use(1), or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N-2 fixation can overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronosequence in Panama, N-2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N-2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N-2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.