Organic and inorganic nitrogen nutrition of western red cedar, western hemlock and salal in mineral N-limited cedar–hemlock forests

Abstract

Western red cedar (Thuja plicata Donn.), western hemlock (Tsuga heterophylla Raf. Sarge) and salal (Gaultheria shallon Pursh) are the main species growing in cedar–hemlock forests on Vancouver Island, Canada. Based on the dominance of organic N in these systems, we tested the hypotheses that: (1) organic N can be utilized by the three plant species; and (2) salal, which is ericoid mycorrhizal and has high tannin concentration in its tissues, would absorb more N from the complex organic N compounds than the other two species. The abilities of cedar, hemlock and salal to take up 15N,13C-labelled glutamic acid were measured and the capacities of the three species to use nitrate (NO3–), ammonium (NH4+), glutamic acid, protein and protein–tannin N were compared over a 20-day period. Based on 13C enrichment, all three species absorbed at least a portion of glutamic acid intact. Cedar, hemlock and salal also showed similar patterns of N uptake from the NO3–, NH4+, glutamic acid, protein and protein–tannin treatments. The largest proportions of applied N were taken up from the NO3– and NH4+ treatments while smaller amounts of N were absorbed from the organic N compounds. Thus organic N was accessed to a modest degree by all three species, and salal did not have a greater capacity to utilize protein and protein–tannin–N.