N limitation to primary production and other ecosystem processes is widespread. To
understand the causes and distribution of N limitation, we must understand the controls of
biological N fixation. The physiology of this process is reasonably well characterized, but our
understanding of ecological controls is sparse, except in a few cultivated ecosystems. We
review information on the ecological controls of N fixation in free-living cyanobacteria,
vascular plant symbioses, and heterotrophic bacteria, with a view toward developing …

^ Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA;^
Department of Agronomy, University of Nebraska, Lincoln, NE 68583, USA;^ Department of
EPO Biology, University of Colorado, Boulder, CO 80523, USA;'^ Environmental Studies,
Prescott College, Prescott, AZ 86301, USA;^ Department of Plant Biology, Carnegie Institute
of Washington, Stanford, CA 94305, USA;^ Department of Biology, Arizona State University,
Tempe, AZ 85287, USA;'^ Ecology and Systematics, Cornell University, Ithaca, NY 14853 …