What determines the abundance of symbiotic N2 fixers within the tropical biome?

Biological nitrogen fixation is the largest nitrogen input to many natural terrestrial ecosystems, particularly tropical ecosystems, thereby influencing primary production, CO₂ uptake, and responses to climate change. However, our understanding of biological nitrogen fixation is still very limited, and the dominant plant family capable of fixing N₂ symbiotically, the Leguminasae, exhibits considerable geographic variation within the tropical biome. In particular, while such putative N fixers are abundant in lowland tropical forests and savannas, they are largely absent from more montane tropical environments. Basing on the principles of resource optimization, we developed a new model to help constrain our understanding of the geographic distribution of N fixers within the tropical biome. Our model treats N fixation according to the C cost of fixing N, coupled with the N cost associated with acquiring P from the soil for plant growth. We applied our model to study the competition between N-fixing and non-fixing plants in savanna, lowland and mountain rainforests in the tropics, and predict a shift in N fixers from high abundance in savannas to lowest abundance in montane tropical rainforests, consistent with empirical observations. Our model also predicts an increase in N fixation rate and greater competitive advantage of N fixers in the tropical environment under a double present CO₂ level. Our model offers promise for coupling the biogeochemical system of C, N, and P, and for understanding geographic variation of N fixation across a diversity of tropical ecosystems and their response to global change.