Asymbiotic nitrogen fixation in litter from Pacific Northwest forests

Asymbiotic nitrogen fixation in litter was assayed by acetylene reduction across a range of 25 forested sites in theWillamette Valley and Oregon Cascade and Coast ranges and periodically over a year at two Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) plantations in the Willamette Valley. Laboratory experiments showed that optimalconditions for N fixation by Douglas-fir litter were 200% moisture content and 22°C. Annual fixation was1.08 ± 0.13 kg/ha at one Willamette Valley plantation, 0.39 ± 0.06 kg/ha at the other. Fixation rates at the other23 sites, which were sampled less frequently, ranged from 0 to 5 g N ha I day-' and exceeded trace levels at onlysix sites, indicating annual totals much less than those at the Willamette Valley plantations. At four coastal and valleysites sampled by litter layer, older L layer Douglas-fir litter fixed the most N per gram dry weight. Percent N, percentC, and the C:N ratio of that litter layer did not differ significantly among sites or correlate with N-fixation rates. Forest-floor litter in most Northwest forests fixes no more than trace amounts of N, at most — 1 kg N ha-' year - 1. Theseamounts are smaller than N input from precipitation.