REGULATION OF NITRATE-N RELEASE FROM TEMPERATE FORESTS - A TEST OF THE N FLUSHING HYPOTHESIS

During the past decade, significant spatial and temporal variability i n the release of nitrate-nitrogen (N) from catchments in a sugar maple forest in central Ontario was observed. To explain this variability, we tested the flushing hypothesis [Hornberger ef al., 1994], where, wh en the soil saturation deficit is high, N accumulates in the upper lay ers of the soil and, as the soil saturation deficit decreases, the for mation of a saturated subsurface layer flushes N from the upper layers of the soil into the stream. We used the Regional Hydro-Ecological Si mulation System to simulate water, carbon, and N dynamics. A N flushin g index was modeled as S/S-30, the ratio of the current day saturation deficit to the previous 30-day average saturation deficit. A N source index was modeled as the ratio of N supply/demand. The relationship b etween the simulated N indices and the observed release of N indicated two mechanisms for the release of N from catchments: (1) a N flushing mechanism, where the N-enriched upper layer of the soil is flushed, a fter a period of low demand for N by the forest (e.g., during spring s nowmelt and autumn stormflow, the water table rising into previously u nsaturated parts df a N-enriched soil profile) or after a period of hi gh demand for N by the forest (e.g., during summer droughts, the water table rising into previously saturated parts of a N-impoverished soil profile following a period of enhanced rates of nitrification); and ( 2) a N draining mechanism, where spring snowmelt recharge of the groun dwater translocates N from the upper layer of the soil into deeper hyd rological flow pathways that are released slowly over the year.