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

The stable isotope stream tracer model (SISTM) calculates the expected 15 N content in various stream ecosystem N compartments over distance and time during and after 15 N additions to streams. SISTM is a steady state compartment model that predicts δ 15 N values based on N stocks and fluxes and the experimental rate of 15 N addition. Predicted δ 15 N values are compared with observed δ 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 15 N-NH 4 + 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 NH 4 + uptake length (S w ) of 5.2 km compared wit...