Natural Abundance of Nitrogen-15 as a Tool for Tracing Alder-Fixed Nitrogen1
- Dan Binkley,
- Phillip Sollins and
- William B. McGill2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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