Graduate Thesis Or Dissertation
 

Levels of asymbiotic nitrogen fixation in leaf litter in Northwest forests

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  • Asymbiotic N fixation in leaf litter in the Northwest was assayed by acetylene reduction. Annual N input measured by periodic sampling in a young Willamette Valley Douglas-fir plantation at Adair, Oregon was 1.08 +_.13 kg/ha. Using different calculating methods, six other annual estimates at the Adair site ranged from 0 to 3.0 kg/ha. Field acetylene reduction rates at 25 other sites ranged from 0 to .005 kg N ha[superscript -1] day [superscript -1] (1.64 nmoles ethylene g[superscript -1] hr [superscript -1) with rates above trace levels at only six of the sites. None of these rates were at a level that indicated the annual total would approach that of the Adair site. The highest acetylene reduction rate in Douglas-fir litter was in the older L layer of litter. There were no significant differences between sites or between fixation levels for percent C, percent N, or C/N ratios. When acetylene reduction rates were modeled as a function of moisture and temperature, a significant amount of variation was explained both for a control led environment (R[superscript 2] = .74, p < .001) and in the field (R [superscript 2] = .69, p = .008). Caution is needed when using shortcut methods to calculate annual N input from asymbiotic fixation. Rates well in excess of 1 kg ha [superscript -1] yr [superscript -1] in leaf litter, which have been reported elsewhere, are unlikely to occur in the Northwest.
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