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

Ratios of 15 N to 14 N 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 15 N to 14 N 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 15 N relative to other N pools. Isotope discrimination clearly occurs during N transfers at these sites, but the 15 N natural‐abundance method does not provide a simple picture of N cycling at the ecosystem level.