Topographic regulation of nitrate-N export from catchments within an old-growth sugar maple forest in the Turkey Lakes Watershed, central Ontario, Canada

Nitrogen (N) saturation of Canada's forests is a potentially serious threat to the health and integrity of forested landscapes. To develop policies to protect our forests, policy-makers require from scientists a diagnosis of a forest's N saturation status for representative regions along N deposition gradients. A promising tool to assist in the diagnosis of N saturation lies in the catchment N export behavior. This tool assumes that the N deposition history is the causal factor for the N export behavior and that the behavior from a catchment can therefore be scaled to the region. The dissertation tests this assumption by establishing the natural variations in N export behavior among catchments within a region and by exploring the process controls that lead to these variations.; Within the test region, the Turkey Lakes Watershed located in central Ontario, the range in catchment-specific export was +/-25% the catchment-average export for dissolved organic N and +/-50% for dissolved inorganic N, indicating substantial natural variations in N export. The dissertation hypothesized that topography controlled the N export behavior. Empirical analyses of the concentrations of N in discharge waters suggested that for nitrate-N, the dominant N species, export was regulated by a topographically-regulated flushing mechanism and that the range in flushing activities among the catchments was a significant contributor to the variations in the N export behavior.; A hybrid modeling approach was adopted in which the processes identified in the empirical analysis as regulating N export behavior were simulated and correlated with observed N export behavior. Using the model, although two mechanisms for producing significant concentrations of nitrate-N in discharge waters became evident, a rapid flushing and a slow draining of N, flushing was clearly the dominant mechanism. The simulation analyses emphasized that for scientists to predict accurately the N export behavior, topographic influences on the interactive impacts of N formation and flushing activities must be considered.; The implications of these research findings are that N export behavior is tightly linked with topography and that within any given regional context, topographic complexity may take precedent over the landscape or regional controls on nitrate-N export behavior.