| Abstrakt: |
Two adjacent catchments with similar temperate forest cover and podzolic soils have annual nitrate (NO3−) 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 NO3− to total NO3− in the groundwater as determined by analysis of δ18O in NO3− are also similar. In both catchments, maximum NO3− export occurs during spring melt, but in the catchment with higher export, NO3− concentrations in the stream begin to increase in the fall period. Groundwater NO3− concentrations measured in wells are very different in the two catchments with high groundwater NO3− in the catchment exhibiting high NO3− export. Following spring melt, steeper slopes in the high NO3− catchment promote faster drainage, and the water table declines rapidly while high NO3− concentrations are maintained in groundwaters. Deeper water tables will preserve high NO3− in water infiltrating below the rooting zone and organic-rich upper soil horizons. In the low NO3− catchment, slower drainage on shallower slopes lead to an increase in soil saturation, and the NO3− disappears from the water before the water table declines. Analyses of δ15N in NO3− during NO3− loss do not show evidence of denitrification, although denitrification proceeding to completion in isolated pockets followed by mixing with higher NO3− groundwaters would yield the same result. Alternatively, active uptake of NO3− by vegetation following spring melt will also deplete the groundwater NO3− in the shallow soil depths without isotopic fractionation. The low NO3− catchment also has lower NO3− 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 NO3− export has a large wetland near the catchment outlet, the NO3− attenuating capacity of this wetland is largely unused except in the late fall because growing season groundwater concentrations of NO3− are undetectable and the wetland is frozen during snowmelt. In the high NO3− catchment, organic-rich soils and vegetation in the riparian zone cannot completely attenuate high NO3− in discharging groundwaters. In our study, factors controlling NO3− in groundwater such as slope, stratigraphy, and hydraulic conductivity can play a larger role than riparian zones in controlling differences in annual NO3− export observed between catchments. [ABSTRACT FROM AUTHOR] |
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