Nitrogen chemistry of subsurface storm runoff on forested Canadian Shield hillslopes

The nitrogen dynamics of storm runoff was studied using throughfall trenche s on slopes with thin soils in a white pine forest catchment near Dorset, O ntario. Hydrologic data were combined with analysis of isotopic signatures and nitrogen chemistry in throughfall, soil water, and hillslope runoff. Tw o hypotheses were tested: (1) macropore preferential flow pathways are a so urce of nitrate flushing in storm runoff, and (2) the nitrogen chemistry of subsurface storm flow is controlled by the mixing of event water fluxes vi a macropores with preevent soil water. Most flow occurred at the soil-bedro ck interface on the slopes, and the use of O-18 indicated that a considerab le fraction of event water moved vertically to bedrock via preferential flo w paths. Despite high levels of inorganic N in throughfall, subsurface runo ff N losses during autumn storms were dominated by dissolved organic nitrog en, and little nitrate flushing occurred via. preferential flow paths. Comp arisons of observed NO3- and NH4+ concentrations versus concentrations pred icted from the mixture of event and preevent water in subsurface flow did n ot support hypothesis 2 and instead indicated depletion of inorganic N. Low rates of N mineralization and negligible nitrification in surface 0-0.1 m soil during June-October suggested high biological utilization of a limited soil N supply. Laboratory experiments in which soil cores were leached wit h solutions containing NO3- and bromide confirmed that the organic Ae horiz on was a sink for NO3-. These data suggest that the biogeochemistry of the organic horizon can regulate patterns of inorganic N loss in subsurface run off moving by preferential flow pathways in forest soils.