Natural abundance of nitrogen-15 as a tool for tracing alder-fixed nitrogen

Ratios of '5N to "N often differ between pools within ecosystems,and measurement of these natural-abundance ratios might allowtransfers among pools to be traced. We tested this approach for itsability to trace biologically-fixed N in conifer plantations. Ratios of'5N to "N were measured in soil total-N, ammonium, and nitrate,and in foliage of Douglas-fir 1Pseudotsuga menziesii (Mirb.) Franco!and red alder (Alnus rubra Bong.) at four sites. Two ecosystems weresampled at each site, one a pure conifer stand and the other a mixedalder-conifer stand. Isotope ratios differed significantly among stands,but no pattern was consistent across all locations. Soil NH; at allsites (and soil nitrate at one site) was significantly depleted in '5Nrelative to other N pools. Isotope discrimination clearly occurs dur-ing N transfers at these sites, but the '5N natural-abundance methoddoes not provide a simple picture of N cycling at the ecosystem level.
Additional Index Words: stable isotopes, isotope discrimination.N fixation, N cycling, N mineralization.