Testing a nitrogen-cycling model of a forest stream by using a nitrogen-15 tracer addition

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.