Nitrogen Fixation Associated with Increased Wood Decay in Douglas-fir Residue

Abstract

Abstract Nitrogen fixation rates, as estimated by the acetylene reduction technique, were determined for four decay stages of down Douglas-fir logs on two old-growth sites in northwestern Montana. Acetylene reduction rates increased as wood decay progressed, but were not affected by site location. Wood carbohydrate, soluble sugar, total and soluble nitrogen, and moisture content also varied among decay stages. Acetylene reduction rates were positively correlated with wood moisture content and nitrogen concentration, and negatively correlated with carbohydrate level. The annual nitrogen additions to the sites from nitrogen fixation in decaying Douglas-fir logs were small, 0.72 kg/ha/yr on the moister site and 0.26 kg/ha/yr on the drier site. These differences in nitrogen inputs were related to differences in residue loading between sites. Although small, such nitrogen gains may be significant over the rotation life of a stand. Forest Sci. 30:1038-1044.