Biological nitrogen (N) fixation, a conversion of inert atmospheric N to a biologically available form, is a key driver of ecosystem function. N fixation is also a mediator of system responses to disturbance, ranging from land clearance to increasing atmospheric CO2, and the process may represent the only truly sustainable way to deliver new N to ecosystems. Despite its importance and the fact that N fixation has been identified as a vital area for research in ecosystem studies for several decades, N fixation remains poorly understood, and the understanding we do have often seems paradoxical. In addition, a scarcity of measurements, particularly outside of temperate systems, sometimes mires discussions in speculation. However, studies attempting to elucidate N fixation controls are now underway in many parts of the globe. This session will highlight new directions in N fixation research and will offer a venue for information integration. We hope not only to ask how N fixation inputs vary among ecosystems, but also to investigate what factors appear to drive variations both within and across systems. Talks will address symbiotic, associative, and free-living N fixation using a range of approaches (molecular, isotopic, computational) and will ask: What are the significant N fixation inputs in these systems? What are the key factors controlling fixation? How are N fixation rates expected to change as ecosystems respond to local and global environmental change? We hope that this session will offer insight into N fixation rates and controls for both scientists interested in N fixation and land managers considering N fixation for ecosystem maintenance and remediation.