Dinitrogen (N2) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N2 fixation across major sectors of the land biosphere. A question remains as to why symbiotic N2-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here the authors present a unifying framework for terrestrial N2 fixation that can explain the geographic occurrence of N2 fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, the authors find a clear advantage to symbiotic N2 fixers in phosphorus-limited tropical savannas and lowland tropical forests.