DINITROGEN FIXATION IN A MONTANE FOREST SERE DETERMINED BY N-14(2) ASSIMILATION AND INSITU ACETYLENE-REDUCTION METHODS

Annual N input rates in a Rocky Mountain forest sere were determined by 15N2 assimilation in an aerobic atmosphere using Swaby-type electrolytic O2-producing incubation flasks, and by in situ enclosures employing C2H2 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 C2H2 reduction values equated with the 15N2 fixation values if a ratio of 7.5 C2H2 reduced to 1 N2 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 in the coniferous stands estimated by the C2H2 reduction method in situ. The results supported the conclusions that the N2 fixation, similar to other microbiological characteristics of the aspen and conifer sites, agrees with the accepted definition of a successional forest sere.