Heterogeneity in soil nitrogen within first-order forested catchments at the Turkey Lakes Watershed

Topographic positional regulation of nitrogen (N) dynamics in soil within Canadian Shield headwaters, located in calibrated catchments containing mature, tolerant hardwood forest, was examined to determine how N pools, mineralization, nitrification, and leaching in soil relate to N export in drainage waters. A uniformly high net N mineralization and nitrification potential for surficial soil layers rich in organic N was demonstrated for ridge, upper-middle slope, and lower slope - footslope topographic positions. Results from plot-scale studies revealed that NO3- concentrations in soil water from well-drained soils were very highly variable throughout the catchments, ranging from 25 to 175 (mu)mol.L^-1, with a median of 80 µmol.L^-1. The isotopic effects of denitrification were not detected in soil water collected from lower slope - footslopes, and in situ rates of N2O production from soils on lower slope - footslopes and valley bottoms were very low and highly variable. Higher N exports from soils than from streams were not explained by differences in soil water N with topographic position or denitrification in lower landscape topographic positions. Using an average soil water N value, based on the replicated plots within catchments, did not reduce the calculated substantial differences in N export between soils and streams.