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 µmol·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.

The role of forests in regulating water: The Turkey Lakes Watershed case study

Long-term experimental catchment studies, applied to relatively undisturbed ecosystems, provide reliable hydrologic data that are highly relevant to forest management decisions on water supply and quality. A number of large-scale, long term catchment studies have been conducted in North America to examine these linkages and processes in support of watershed management decisions. Among these the Turkey Lakes Watershed (TLW), a rare example of a long-term fully integrated examination of the biology and chemistry of the atmosphere, forests, soils, streams, and lakes, is presented as a case study. Multi-agency, interdisciplinary research at the TLW, which has strong links nationally and internationally, has included hydrological studies, examination of landscape influences on nutrient export to surface waters, and impacts of catchment disturbance on water yield, nutrient flux, carbon cycling, and sedimentation in streams. Application of partial cut harvest systems in the TLW tolerant hardwood forest resulted in reduced runoff and improved water quality (sediment, nitrate and calcium concentrations) relative to clearcut harvest. Twenty years after the initiation of reductions in atmospheric S emissions losses of SO42- from some headwater basins remain high and there is little evidence of acidification recovery in TLW surface waters. The TLW research approach can be used globally to scientifically assess how natural and human actions affect the important services provided by forested watersheds. For example, TLW results have contributed to international policy on acid rain reductions and air quality agreements. Key words: forest hydrology, water quality, hydrochemistry, forest disturbance, forest harvest, acid rain