Nitrogen fixation in root-colonized large woody residue of Oregon coastal forests

Coarse woody residues are conspicuous features of the forest floor in coastal Oregon forests. They provide habitats for plants, animals, and a diversity of microorganisms. Live plants are partially or completely rooted in the woody residues. This study provides baseline information on nitrogenase activities (nitrogen fixation) and populations of nitrogen-fixating organisms in root-colonized and noncolonized woody residues on forest and clearcut sites. Coarse woody residue of decay classes IV-V were sampled at three sites of Douglas-fir stands having varying amounts of understory vegetation. Nitrogen-fixation activity in woody residues was detected on all three sites. The woody residues at lower elevation sites near the coast had the least nitrogen-fixing activity and nitrogen-fixing bacterial populations. Plant colonized and noncolonized woody residues had significantly higher nitrogenase activity than the adjacent soils, but the activity between the colonized and noncolonized woody residues did not differ significantly.