Dinitrogen fixation in a montane forest sere determined by 15 N 2 assimilation and in situ acetylene-reduction methods

Annual N input rates in a Rocky Mountain forest sere were determined by 15 N 2 assimilation in an aerobic atmosphere using Swaby-type electrolytic O 2 -producing incubation flasks, and by in situ enclosures employing C 2 H 2 reduction methods. It was determined that the input in a subalpine meadow was 0.33, in aspen 0.52 and in spruce sites 0.32 kg N ha −1 yr −1, respectively, by combined non-symbiotic and associative-symbiotic fixation. The in situ C 2 H 2 reduction values equated with the 15 N 2 fixation values if a ratio of 7.5 C 2 H 2 reduced to 1 N 2 fixed was used. Symbiotic fixation by a lupine in the aspen site added 0.63 kg N ha −1 yr −1. The associate-symbiotic fixation contributed 14% of the N input in the meadow, 30% in the aspen, and nil in the coniferous stands estimated by the C 2 H 2 reduction method in situ. The results supported the conclusions that the N 2 fixation, similar to other microbiological characteristics of the aspen and conifer sites, agrees with the accepted definition of a successional forest sere.