Nitrogen dynamics of storm runoff in the riparian zone of a forested watershed

The influence of storm runoff processes on stream nitrogen dynamics was investigated in a headwater riparian swamp on the Oak Ridges moraine in southern Ontario. Hydrologic data were combined with analysis of an isotopic tracer (¹⁸0) and nitrogen (NH ₄ ⁺ , NO ₃ ^– ) concentrations in saturation overland flow and stream discharge. Storm runoff was separated into its event and pre-event components using¹⁸O in order to examine the effect of water source on nitrogen chemistry. Laboratory experiments were also used to study nitrogen transformation associated with storm runoff-surface substrate interactions in the swamp. In most storms N0₃-N and NH₄-N concentrations in the initial 3–4 mm throughfall increment were 10–20x and 20–100x higher respectively than stream base flow concentrations. Maximum stream N0₃-N concentrations were < 2x to 6x higher than base flow concentrations and preceded or coincided with peak stream discharge. Storm-to-storm variations in stream N0₃-N behaviour also occurred during the hydrograph recession phase. NH₄-N concentrations attained an initial peak on the rising hydrograph limb, or at peak stream discharge. A second NH₄-N increase occurred during the late recession phase 3–5 h after maximum stream discharge. Inorganic-N concentrations in surface runoff were similar to peak streamflow.The close agreement between observed N0₃-N concentrations and values predicted from a chemical mixing model indicate that stream N0₃-N variations were controlled mainly by the mixture of throughfall and groundwater in surface stormflow from the swamp. Laboratory experiments also indicated that N0₃-N in surface runoff behaved conservatively when mixed with swamp substrates. With the exception of the late hydrograph recession phase, observed stream NH₄-N concentrations were much lower than concentrations predicted by the chemical mixing model. The rapid loss of NH₄-N from mixtures of surface stormflow and swamp substrates in laboratory experiments and the absence of uptake in sterilized substrates indicated that NH₄-N retention in surface storm runoff was due to biotic processes.