Diazotrophs are scattered across bacterial taxonomic groups (mostly in the Eubacteria but also a couple of Archaea). Even within a species that can fix nitrogen there may be strains that do not fix nitrogen (Postgate, 1998). Fixation is shut off when other sources of nitrogen are available*, and, for many species, when oxygen is at high partial pressure.** From Wikipedia
*So if fixation is shut of by the bacteria if N is present at some level, then at what level do you not need N fixing plants?If my soil test comes back with high or high enough N, are N fixing plants a waste? This would be good to know in a Polyculture Design.** N fixing is an anaerobic process. hmmmmm.