Two adjacent forested catchments: Dramatically different NO sub(3) super(-) export

Two adjacent catchments with similar temperate forest cover and podzolic soils have annual nitrate (NO sub(3) super(-)) export that differs by a factor of 10. Monthly rates of mineralization and nitrification measured by the buried bag technique, soil C/N ratios, and the contribution of microbial NO sub(3) super(-) to total NO sub(3) super(-) in the groundwater as determined by analysis of delta super(18)O in NO sub(3) super(-) are also similar. In both catchments, maximum NO sub(3) super(-) export occurs during spring melt, but in the catchment with higher export, NO sub(3) super(-) concentrations in the stream begin to increase in the fall period. Groundwater NO sub(3) super(-) concentrations measured in wells are very different in the two catchments with high groundwater NO sub(3) super(-) in the catchment exhibiting high NO sub(3) super(-) export. Following spring melt, steeper slopes in the high NO sub(3) super(-) catchment promote faster drainage, and the water table declines rapidly while high NO sub(3) super(-) concentrations are maintained in groundwaters. Deeper water tables will preserve high NO sub(3) super(-) in water infiltrating below the rooting zone and organic-rich upper soil horizons. In the low NO sub(3) super(-) catchment, slower drainage on shallower slopes lead to an increase in soil saturation, and the NO sub(3) super(-) disappears from the water before the water table declines. Analyses of delta super(15)N in NO sub(3) super(-) during NO sub(3) super(-) loss do not show evidence of denitrification, although denitrification proceeding to completion in isolated pockets followed by mixing with higher NO sub(3) super(-) groundwaters would yield the same result. Alternatively, active uptake of NO sub(3) super(-) by vegetation following spring melt will also deplete the groundwater NO sub(3) super(-) in the shallow soil depths without isotopic fractionation. The low NO sub(3) super(-) catchment also has lower NO sub(3) super(-) in shallow soil waters during spring melt. Shallower slopes promote near-surface flow paths in organic-rich soil horizons which may facilitate denitrification during spring melt. Although the catchment with low NO sub(3) super(-) export has a large wetland near the catchment outlet, the NO sub(3) super(-) attenuating capacity of this wetland is largely unused except in the late fall because growing season groundwater concentrations of NO sub(3) super(-) are undetectable and the wetland is frozen during snowmelt. I