Nitrogen fixation (acetylene reduction) was found to be associated with sand erain root sheaths (rhizoshcalhs) occurring on the following xeric grasses: Oryzopsis hymcnoiilcs (Rocm. and Shult.) Ricker, Agropyron dasyslachyum (Hook.) Scrib., Stipa comata Trin. and Rupr., and Aristidu purpurva Nutt. Acetylene reduction rates associated with whole plant specimens of these species varied from 515 to 920 nmol C^H^Ag dry wt.) x (6 days). Nitrogeniisc activity was shown to be associated with the rhizo sheaths. Bacillus potymyxa\\)/ic nitrogen fixers were isolated from the rhizosheaths of each grass. The isolates reduced acetylene and assimilated "N,.
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