A Stable Isotope Tracer Study of Nitrogen Uptake and Transformation in an Old-Growth Forest Stream

The understanding of nitrogen dynamics in streams of temperate forest biomes historically has been constrained by a combination of anthropogenic disturbances and technical limitations. We report here on a study in an undisturbed stream in Oregon, USA, using a stable isotopic tracer to quantify uptake, transformation, and retention of nitrogen. We added 15NH4Cl for six weeks to Mack Creek, a third-order stream in a 500-year-old-growth coniferous forest and monitored 15N in dissolved, aquatic, and terrestrial riparian food web components. Data collected before, during, and for four weeks after the tracer addition allowed us to derive uptake rates of inorganic N and to trace its fates. Short uptake lengths (35–55 m) and residence times (8–12 min) of ammonium indicated strong demand. Despite nitrate concentrations of 55–68 μg/L, nitrification rates were also high, with 40– 50% of the 15NH4+ converted to nitrate over the 220-m study reach. Aquatic bryophytes and biofilm on large wood (“epixylon”) showed the highest biomass-adjusted uptake rates. All aquatic consumers sampled, both vertebrate and invertebrate, showed incorporation of tracer 15N by the end of the experiment; small invertebrate grazers were more strongly labeled than their food sources. Increased 15N label in 15 of the 17 riparian plant species sampled suggested transfer of aquatic N to the terrestrial ecosystem. At the end of the release, 81% of the added tracer was accounted for, with 49% exported (primarily as 15NO3−) and 32% retained within the stream and riparian biota (primarily by bryophytes, epixylon, and fine benthic organic material). Our results suggest that, in streams within undisturbed primary forests, uptake and retention of nitrogen may be highly efficient and that there may be strong connections between terrestrial and aquatic ecosystems.