title: Complex N acquisition by soil diazotrophs: how the ability to release exoenzymes affects N fixation by terrestrial free-living diazotrophs.
abstract: Terrestrial systems support a variety of free-living soil diazotrophs, which can fix nitrogen (N) outside of plant associations. However, owing to the metabolic costs associated with N fixation, free-living soil diazotrophs likely rely on soil N to satisfy the majority of cellular N demand and only fix atmospheric N under certain conditions. Culture-based studies and genomic data show that many free-living soil diazotrophs can access high-molecular weight organic soil N by releasing N-acquiring enzymes such as proteases and chitinases into the extracellular environment. Here, we formally propose a N acquisition strategy used by free-living diazotrophs that accounts for high-molecular weight N acquisition through exoenzyme release by these organisms. We call this the 'LAH N-acquisition strategy' for the preferred order of N pools used once inorganic soil N is limiting: (1) low-molecular weight organic N, (2) atmospheric N and (3) high-molecular weight organic N. In this framework, free-living diazotrophs primarily use biological N fixation (BNF) as a short-term N acquisition strategy to offset the cellular N lost in exoenzyme excretion as low-molecular weight N becomes limiting. By accounting for exoenzyme release by free-living diazotrophs within a cost-benefit framework, investigation of the LAH N acquisition strategy will contribute to a process-level understanding of BNF in soil environments.
authors.affiliation: Department of Plant Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Norman
authors.affiliation: Department of Plant Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Maren L
authors.initials: ML
authors.lastname: Friesen
mesh_terms: D001419:Bacteria / Q000201:enzymology*; D017753:Ecosystem; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D012988:Soil Microbiology
chemical_list: D009584:Nitrogen