EXPORT OF NITROGEN FROM CATCHMENTS WITHIN A TEMPERATE FOREST - EVIDENCE FOR A UNIFYING MECHANISM REGULATED BY VARIABLE SOURCE AREA DYNAMICS

Considerable variation in the export of dissolved inorganic nitrogen ( DIN) and dissolved organic nitrogen (DON) was observed among catchment s located within an old-growth sugar maple forest in central Ontario. Although discharge was a strong predictor of N-export, rates of export were variable for each catchment, ranging from -50% to +50% from the catchment-average response for DIN and -25% to +25% from the catchment -average response for DON. Among the catchments, a unifying flushing b ehavior was apparent for NO3--N, the dominant form of DIN in the disch arge waters, providing a basis for explaining the variation in the exp ort of DIN. Flushing occurs when a water table rises to the soil surfa ce with subsequent mobilization of nutrients stored near or at the soi l surface to surface waters. Catchment-specific flushing behaviors wer e captured in ''flushing'' characteristic time constants, defined as t he time interval required for a decline in N concentrations in dischar ge waters to e(-1) (37%) of their initial concentration. Variation in flushing behavior was linked to variation in N export; catchments with short flushing times (interpreted as catchments with source areas tha t are less variable) were observed to export less N than catchments wi th long flushing times (source areas that are more variable). A hypoth esis was formulated in which catchment topography and its influence on variable source area dynamics accounts for variation in flushing beha vior, hence variation in the export of NO3--N among the catchments. Th e implication of this hypothesis is that to predict accurately the exp ort of NO3--N from catchments within a landscape, we need first to con sider the influence of the topographic complexity of the catchments. O ur understanding of the mechanisms of processing and export of DON is not sufficient for accurate prediction at this point, highlighting the need for additional research on DON.