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Natural Abundance of Nitrogen-15 as a Tool for Tracing Alder-Fixed Nitrogen¹

ABSTRACT

Ratios of ¹⁵N to ¹⁴N often differ between pools within ecosystems, and measurement of these natural-abundance ratios might allow transfers among pools to be traced. We tested this approach for its ability to trace biologically-fixed N in conifer plantations. Ratios of ¹⁵N to ¹⁴N were measured in soil total-N, ammonium, and nitrate, and in foliage of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and red alder (Alnus rubra Bong.) at four sites. Two ecosystems were sampled at each site, one a pure conifer stand and the other a mixed alder-conifer stand. Isotope ratios differed significantly among stands, but no pattern was consistent across all locations. Soil NH⁺₄ at all sites (and soil nitrate at one site) was significantly depleted in ¹⁵N relative to other N pools. Isotope discrimination clearly occurs during N transfers at these sites, but the ¹⁵N natural-abundance method does not provide a simple picture of N cycling at the ecosystem level.

¹ Paper 1852 of the Forest Research Laboratory, Oregon State Univ., Corvallis, OR 97331.

² Assistant Professor, School of Forestry and Environmental Studies, Duke University, Durham, NC 27706. Research Associate Professor, Dep. of Forest Science, College of Forestry, Oregon State University, Corvallis, OR 97331. Professor, Dep. of Soil Science, University of Alberta, Edmonton, Canada T6G 2E3.

Received for publication March 1, 1984. Accepted for publication October 10, 1984.