15N Patterns of Douglas-Fir and Red Alder Riparian Forests in the Oregon Coast Range
Abstract: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.
Document Type: Research Article
Publication date: April 1, 2008
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