Testing a Nitrogen-Cycling Model of a Forest Stream by Using a Nitrogen-15 Tracer Addition

ABSTRACT Cycling of nitrogen (N) is commonly studied in aquatic ecosystems; however, most studies examine only parts of the N cycle, such as budgets, N uptake lengths, or oxidative transformations. To integrate conceptually and experimentally several aspects of the N cycle in a stream, we combined a N-cycling model and a tracer addition of nitrogen-15 (15N) to Hugh White Creek, a second-order forested mountain stream in North Carolina (USA). We calibrated a steady-state box model for N cycling in 5-m stream segments that included dissolved, detrital, and biotic compartments. This model was parameterized based on prior studies and used to predict the expected distribution of tracer 15N in all compartments through both time and distance downstream of the addition site. We tested the model results with a 23-day continuous addition of 15N-NH4+ to the stream. Deviations of field data from model predictions suggested areas in which we lacked understanding of the N cycle. Downstream distribution of 15N in epilithon and moss matched model predictions, indicating that our prior estimations of N uptake rates were correct. Leaves and fine detritus contained less label than predicted by the model, yet their consumers had both higher δ15N than predicted and higher δ15N than the detritus itself, suggesting selective assimilation of microbial N from ingested detritus. Splitting fine benthic organic N (FBON) into a microbial and recalcitrant pool gave better predictions of FBON and seston δ15N values relative to field data, yet overestimated invertebrate consumer δ15N possibly because our estimates of the fraction of invertebrate N derived from microbes were too high. We predicted that much of the labeled N would move downstream via FBON suspension and transport. We found that most of the 15N remained near the addition site 33 days after the addition was stopped, suggesting that the stream is highly retentive of particulate N.