“…Another important control is phosphorus availability, which indirectly affects N 2 fixation by constraining organism growth. ,− ,− A high P requirement related to the large ATP demand of nitrogenase has been suggested to make N 2 -fixers more prone to P limitation than nonfixers . Inadequate moisture levels can also be problematic for terrestrial diazotrophs. ,, Finally, ecological interactions (e.g., grazing of diazotrophs, competition with nonfixers over nutrients and light, narrower range of growth conditions) are also important in explaining the distribution of diazotrophy in nature. ,,− Despite a variety of influencing factors, the dominant proximate control on diazotroph activity is fixed N availability (i.e., equivalent to supply minus demand). This dynamic is consistent with the genetic regulation, metabolic costs, and biological role of N 2 fixation.…”