Scaling from Molecules to Ecosystems: Exploring Controls Over Free-Living Nitrogen Fixation in Terrestrial Systems

All living organisms require nitrogen, and a single process ¿ biological nitrogen fixation ¿ accounts for more than 97% of natural nitrogen inputs into Earth¿s ecosystems. To date, most nitrogen fixation research has focused on the symbiotic nitrogen fixation occurring via the structured relationship of particular bacteria and plant roots. Yet free-living nitrogen fixation, the nitrogen fixation occurring outside of this symbiosis, may represent the dominant source of biologically-available nitrogen to a variety of ecosystems. Here we explored free-living nitrogen fixation controls, ranging from the effect of fertilization on nitrogen inputs to the role of microbial nitrogen-fixer community structure in regulating nitrogen fixation rates. We found that free-living nitrogen fixation was a dynamic process that supplied diverse ecosystems with nitrogen, and that we could elucidate multiple controls over this important process.