Nitrogen Fixation Associated with Sand Grain Root Sheaths (Rhizosheaths) of Certain Xeric Grasses

Nitrogen fixation (acetylene reduction) was found to be associated with sand grain root sheaths (rhizoseaths) occurring on the following xeric grasses: Oryzopsis hymenoides (Roem. and Shult.) Ricker, Agropyron dasystachyum (Hook.) Scrib., Stipa comata Trin. and Rupr., and Aristida purpurea Nutt. Acetylene reduction rates associated with whole plant specimens of these species varied from 515 to 920 nmol C 2 H 4 /(g dry wt.) × (6 days). Nitrogenase activity was shown to be associated with the rhizosheaths. Bacillus polymyxa ‐like nitrogen fixers were isolated from the rhizosheaths of each grass. The isolates reduced acetylene and assimilated 15 N 2 .