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15N Patterns of Douglas-Fir and Red Alder Riparian Forests in the Oregon Coast Range

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We used naturally occurring stable isotopes of N to compare N dynamics in near-stream and upslope environments along riparian catenas in N-fixing red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) forests in the Coast Range of western Oregon. Based on the existing literature, we expected soil 15N to be enriched closer to streams owing to inputs of isotopically heavy, marine-derived N by spawning salmon, higher rates of denitrification near the stream, or both. However, it has been unclear what effect red alder might have on soil 15N patterns near streams. We found a consistent −1‰ 15N signature in red alder foliage, and 15N of total N in soils under red alder averaged 2.2‰ along sampling transects extending 20 m upslope from the stream. Surprisingly, 15N of total N in soil under Douglas-fir was progressively depleted nearer to streams, opposite from the pattern expected from N losses by denitrification or N inputs from anadromous salmon. Instead, 15N of total N in soil under Douglas-fir converged toward soil 15N values typical of red alder sites. We consider that the historic presence of red alder may have contributed a legacy of lower soil 15N nearer to streams on sites that are currently dominated by young Douglas-fir forest.

Keywords: 15N natural abundance stable isotope; denitrification; marine-derived nutrients; nitrogen dynamics

Document Type: Research Article

Publication date: April 1, 2008

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