Two adjacent forested catchments: Dramatically different NO₃⁻ export

Two adjacent catchments with similar temperate forest cover and podzolic soils have annual nitrate (NO 3 − ) 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 3 − to total NO 3 − in the groundwater as determined by analysis of δ 18 O in NO 3 − are also similar. In both catchments, maximum NO 3 − export occurs during spring melt, but in the catchment with higher export, NO 3 − concentrations in the stream begin to increase in the fall period. Groundwater NO 3 − concentrations measured in wells are very different in the two catchments with high groundwater NO 3 − in the catchment exhibiting high NO 3 − export. Following spring melt, steeper slopes in the high NO 3 − catchment promote faster drainage, and the water table declines rapidly while high NO 3 − concentrations are maintained in groundwaters. Deeper water tables will preserve high NO 3 − in water infiltrating below the rooting zone and organic‐rich upper soil horizons. In the low NO 3 − catchment, slower drainage on shallower slopes lead to an increase in soil saturation, and the NO 3 − disappears from the water before the water table declines. Analyses of δ 15 N in NO 3 − during NO 3 − loss do not show evidence of denitrification, although denitrification proceeding to completion in isolated pockets followed by mixing with higher NO 3 − groundwaters would yield the same result. Alternatively, active uptake of NO 3 − by vegetation following spring melt will also deplete the groundwater NO 3 − in the shallow soil depths without isotopic fractionation. The low NO 3 − catchment also has lower NO 3 − 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 3 − export has a large wetland near the catchment outlet, the NO 3 − attenuating capacity of this wetland is largely unused except in the late fall because growing season groundwater concentrations of NO 3 − are undetectable and the wetland is frozen during snowmelt. In the high NO 3 − catchment, organic‐rich soils and vegetation in the riparian zone cannot completely attenuate high NO 3 − in discharging groundwaters. In our study, factors controlling NO 3 − in groundwater such as slope, stratigraphy, and hydraulic conductivity can play a larger role than riparian zones in controlling differences in annual NO 3 − export observed between catchments.

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