A coupled field and modeling approach for the analysis of nitrogen cycling in streams

The stable isotope stream tracer model (SISTM) calculates the expected N-15 content in various stream ecosystem N compartments over distance and time during and after N-15 additions to streams. SISTM is a steady state compartment model that predicts delta(15)N values based on N stocks and fluxes and the experimental rate of N-15 addition. Predicted delta(15)N values are compared with observed delta(15)N values from a field tracer addition to evaluate our understanding of the N cycle. We demonstrated the use of this tool with information collected from field measurements and a 6-wk N-15-NH4+ addition to the Kuparuk River, Alaska, during the summer of 1991. SISTM was used to run a series of model calibrations that reflected increased information as the experiment progressed. Results of an a priori calibration (using pre-1991 data) yielded a predicted NH4+ uptake length (S-w) of 5.2 km compared with the observed S-w of 0.84 km, and underestimated the delta(15)N values of biota in all cases. When discharge and NH4+ concentrations measured during the 1991 experiment were added (model calibration Update 1), the predicted S-w, dropped to 0.44 km, indicating that the modeled fluxes overestimated the rate of NH4+ removal by the stream bottom. Adding N stocks and fluxes measured during the tracer addition (Update 2) did not improve predicted S-w, indicating faulty assumptions in our a priori calibration. The observed isotope data were used to estimate the form (NH4+ vs NO3-) of N taken up by primary producers and to improve our representation of the epilithon compartment (Update 3). Including this information brought the predicted S-w to 0.71 km compared with the observed 0.84 km, and resulted in a reasonable correspondence between predicted and observed delta(15)N values over the 6-wk addition. SISTM can be used as a framework to 1) summarize N-cycle information prior to a tracer addition, 2) generate testable predictions for field isotope studies, 3) improve our understanding of the N cycle using the field isotope data as constraints on nw: estimates, and 4) explore hypothetical N-cycle characteristics. The combined modeling and field tracer experiment approach efficiently provided a synoptic view of the N cycle in streams and rivers.