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Free-living microaerophiles fixed 15 N 2 and reduced acetylene in fallen tree boles at two old-growth Pseudotsugamenziesii stands in western Oregon. Acetylene reduction was most rapid under an atmosphere of 2–10% O 2 , whereas under prolonged anaerobic conditions it was at or below detection limits. Acetylene reduction rates increased up to fourfold during long-term incubations in acetylene (> 12 h). Ratios of acetylene reduction to N 2 fixation frequently exceeded 6.0 during such long-term incubations but averaged 3.5 when samples were incubated < 7 h; consequently, long-term incubation of low-activity material in acetylene should be avoided. A preliminary survey indicated that N 2 fixation by free-living organisms in fallen boles was less than other potential N inputs to fallen boles and to the forest ecosystem.
Silvester et al. (1982) studied this question.