Functional ecology of free-living nitrogen fixation: A contemporary perspective

Nitrogen (N) availability is thought to frequently limit terrestrial ecosystem processes, and explicit consideration of N biogeochemistry, including biological N₂ fixation, is central to understanding ecosystem responses to environmental change. Yet, the importance of free-living N₂ fixation—a process that occurs on a wide variety of substrates, is nearly ubiquitous in terrestrial ecosystems, and may often represent the dominant pathway for acquiring newly available N—is often underappreciated. Here, we draw from studies that investigate free-living N₂ fixation from functional, physiological, genetic, and ecological perspectives. We show that recent research and analytical advances have generated a wealth of new information that provides novel insight into the ecology of N₂ fixation as well as raises new questions and priorities for future work. These priorities include a need to better integrate free-living N₂ fixation into conceptual and analytical evaluations of the N cycle's role in a variety of global change scenarios.