Analysis of nitrogen cycling in a forest stream during autumn using a 15 N‐tracer addition

We added 15 NH 4 Cl over 6 weeks to Upper Ball Creek, a second‐order deciduous forest stream in the Appalachian Mountains, to follow the uptake, spiraling, and fate of nitrogen in a stream food web during autumn. A priori predictions of N flow and retention were made using a simple food web mass balance model. Values of δ 15 N were determined for stream water ammonium, nitrate, dissolved organic nitrogen, and various compartments of the food web over time and distance and then compared to model predictions. Ammonium uptake lengths were shortest at the beginning of the tracer addition (28 m) and increased through time (day 20 = 82 m, day 41 = 94 m), and ammonium residence time in stream water ranged from 4 min on day 0 to 15 min on day 41. Whole‐stream ammonium uptake rates, determined from the decline in 15 NH 4 in water over the stream reach, decreased from 191 mg N m −2 d −1 on day 0 to 83.2 mg N m −2 d −1 on day 41. Temporal trends in the NH 4 mass transfer coefficient ( ν f ) were similar to uptake rates; ν f was highest on day 0 (7.4 × 10 −4 m s −1 ) and lower on days 20 and 41 (2.7 and 2.8 × 10 −4 m s −1 , respectively). Rates of nitrification were estimated to be very low throughout the tracer addition and accounted for <3% of 15 NH 4 uptake on day 0. It appears that most of the N in epilithon was actively cycling based on comparisons of 15 N in stream water and biomass at the end of the experiment. In contrast, for allochthonous organic matter, we found that microbial 15 N represented 69% of the label in wood, 20% in leaves, and 31% in fine benthic organic matter (FBOM). Despite higher δ 15 N values in primary producers, 15 NH 4 uptake rates per unit stream bottom area were generally lower in epilithon compared to the detrital compartments, a result of the lower biomass of epilithon. Turnover times were similar for epilithon (47 d), leaves (38 d), and FBOM (53 d) based on the decline in 15 N tracer over the first 28 d after the addition stopped. Incorporation of 15 N varied among biomass compartments involved in ammonium uptake from water. Primary producers were more highly labeled than allochthonous organic matter. Epilithon δ 15 N values were higher than leaves or FBOM, appeared to reach isotopic equilibrium by day 42, and followed model‐predicted trends. The grazing mayfly Stenonema was more highly labeled than the epilithon, which suggests selective feeding or assimilation of the more highly labeled algal‐bacterial portion of the epilithon. Leaves had very low δ 15 N values, and δ 15 N values for the shredding stonefly Tallaperla were close to model predictions and followed labeling in leaves. Total retention of 15 N at the end of the experiment by the nine largest biomass compartments within the study reach accounted for only 12.3% of added 15 N, with leaves and FBOM representing the largest portions. Export of 15 N by suspended particulate and dissolved N accounted for an additional 11% and 30% of added 15 N, respectively. Results from the 15 N‐tracer addition in Upper Ball Creek demonstrate the high ammonium demand associated with microbes colonizing leaf detritus and the resultant linkage to invertebrate shredders. In Upper Ball Creek in autumn, spiraling of NH 4 is very tight, NH 4 residence time in water is short, and uptake rates are very high. Analyses of N spiraling in unimpacted streams provide an ecological foundation for assessment of spiraling in high‐N streams.