Global importance of nitrogen fixation across inland and coastal waters.

Biological nitrogen fixation is a key driver of global primary production and climate. Decades of effort have repeatedly updated nitrogen fixation estimates for terrestrial and open ocean systems, yet other aquatic systems in between have largely been ignored. Here we present an evaluation of nitrogen fixation for inland and coastal waters. We demonstrate that water column and sediment nitrogen fixation is ubiquitous across these diverse aquatic habitats, with rates ranging six orders of magnitude. We conservatively estimate that, despite accounting for less than 10% of the global surface area, inland and coastal aquatic systems fix 40 (30 to 54) teragrams of nitrogen per year, equivalent to 15% of the nitrogen fixed on land and in the open ocean. Inland systems contribute more than half of this biological nitrogen fixation. Editor's summary: Biological nitrogen fixation, which converts nitrogen gas into biologically available fixed nitrogen, has been studied extensively in terrestrial and open ocean systems, but less is known about this process in inland and coastal waters. Fulweiler et al. found that inland and coastal regions fix nitrogen at prodigious rates (see the Perspective by Brezonik). Although these habitats occupy less than 10% of Earth's surface, they are responsible for approximately 20% of the nitrogen fixation occurring on land and in the ocean. —Jesse Smith [ABSTRACT FROM AUTHOR]