Nitrogen fixation in woody residue of northern Rocky Mountain conifer forests

Abstract

N fixation rates, as estimated by the acetylene reduction technique, were determined for large woody residues on four old growth conifer sites in western Montana and northern Idaho. Residue loadings ranged from <50 Mg ha−1 on a warm, dry Douglas-fir site in Montana to >150 Mg ha−1 on a highly productive, wet, cedar–hemlock site in northern Idaho. Lignin and carbohydrate analyses indicated that wood on these sites was being decayed primarily by brown rot fungi. Ethylene production rates increased on all sites as wood decay progressed. Assuming that N-fixing bacteria were active for 180 days year−1, N fixed in woody residues ranged from a high of nearly 1.5 kg ha−1 year−1 on a cedar–hemlock site to a low of 0.16 kg ha−1 year−1 on a Douglas-fir site. The application of the N fixation results from this study to the northern Rocky Mountain region indicated that the majority of stands in the Douglas-fir, subalpine fir, and cedar–hemlock cover types would have N gains <0.5 kg ha−1 year−1. However, in some areas where residue loadings are heavy, such as overmature stands on cool, moist sites, or following harvesting, N gains could be much greater.