Two adjacent forested catchments: Dramatically different NO 3 − export

Abstract

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.