Acetylene reduction in conifer logs during early stages of decomposition

Acetylene reduction was examined periodically for as long as 68 months in the outer and inner bark,sapwood, and heartwood of decaying logs of western hemlock [Tsuga heterophylla (Raf.) Sarg.] westernredcedar (Thuja plicata D. Don), Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], and Pacific silverfir (Abies amabilis Dougl. ex Forbes) in the western Oregon Cascade Mountains. Tissues from freshlycut logs from sound trees were unable to reduce acetylene. However, after 18 months of decomposi-tion, acetylene reduction was found in all log tissues except heartwood. Over the 68-month studyperiod, no significant relationship between reduction rate and tissue moisture was found. Acetylenereduction rates differed significantly among tissues, log species, and time of exposure to decomposers.Although acetylene reduction generally showed a steady increase with time, tissues of some speciesshowed a more complex, nonlinear pattern of change. Although the amount of nitrogen fixed is lowcompared to the total present in decaying logs, it might be an important source of readily availablenitrogen for the microbiota responsible for decomposition.