Symbiotic nitrogen (N) fixers are critical components of many terrestrial ecosystems, and play a crucial role in two mysteries in ecosystem ecology. The chronic N limitation that pervades mature temperate forests could easily be overcome by N fixers, who are conspicuous in these ecosystems only in their absence. Conversely, chronic N richness in many tropical forests might stem from N fixation by leguminous trees, which are ubiquitous in the tropics, but the potential reasons for fixing more than is necessary (“over-fixing”) are unclear.  There is evidence that N-fixing trees in temperate ecosystems are obligate - fixing N at the same rate regardless of their environment - while those in tropical forests are facultative - adjusting N fixation to meet their needs.  Can these disparate strategies explain the temperate and tropical paradoxes?  Why might these two different strategies persist?  In this project, we use a simple mathematical model to determine competitive outcomes and ecosystem-level impacts (i.e., N limitation versus N richness) of facultative and obligate N fixation strategies (including non-fixation).  Additionally, we examine how time lags associated with N fixation and costs of being facultative affect the competitive abilities and ecosystem-level impacts of different strategies.

Results/Conclusions

In the basic version of our model, a single facultative strategy excludes any obligate strategy (fixer or non-fixer), and never over-fixes. Therefore, it cannot account for the chronic N limitation of temperate forests, the chronic N richness of tropical forests, or the existence of non-fixers or obligate N fixers.  However, costs of being facultative and time lags inherent in the process of N fixation can.  If severe enough, costs of being facultative, such as a higher cost of N fixation per unit N fixed due to increased nodule construction costs, lead to selection against facultative fixers. The ensuing successional pattern is obligate N fixers being displaced by non-fixers and persistent N limitation, as is observed in temperate forests.  Even small time lags in N fixation produce substantial oscillations in N fixation, leading to periods of over-fixation and under-fixation.  Averaged over a landscape level, this is consistent with N richness, as is observed in tropical forests.  Much of our model remains untested due to the difficulty of studying N fixation in the field, but the degree of correspondence between our results and broad patterns in temperate and tropical forests is intriguing.