Facultative versus obligate nitrogen fixation strategies and their ecosystem consequences

Symbiotic nitrogen (N) fixers are critical components of many terrestrial ecosystems. There is evidence that some N fixers fix N at the same rate regardless of environmental conditions (a strategy we call obligate), while others adjust N fixation to meet their needs (a strategy we call facultative). Although these strategies are likely to have qualitatively different impacts on their environment, the relative effectiveness and ecosystem-level impacts of each strategy have not been explored. Using a simple mathematical model, we determine the best facultative strategy and show that it excludes any obligate strategy (fixer or nonfixer) in our basic model. To provide an ex-planation for the existence of nonfixers and obligate fixers, we show that both costs of being facultative and time lags inherent in the process of N fixation can select against facultative N fixers and also produce the seemingly paradoxical patterns of sustained N limitation and N richness. Finally, we speculate on why the costs and lags may differ between temperate and tropical regions and thus whether they can explain patterns in both biomes simultaneously.