δ 15 N 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 δ 15 N 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 δ 15 N patterns near streams. We found a consistent −1‰ δ 15 N signature in red alder foliage, and δ 15 N of total N in soils under red alder averaged 2.2‰ along sampling transects extending 20 m upslope from the stream. Surprisingly, δ 15 N 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, δ 15 N of total N in soil under Douglas-fir converged toward soil δ 15 N values typical of red alder sites. We consider that the historic presence of red alder may have contributed a legacy of lower soil δ 15 N nearer to streams on sites that are currently dominated by young Douglas-fir forest.