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[1] 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 -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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On-site disposal of sewage in septic systems can lead to groundwater plumes with NO(3)(-)-N concentrations exceeding the common drinking water limit of 10 mg/L. Currently, denitrification is considered as the principal natural attenuation process. However, at a large seasonal-use septic system in Ontario (256 campsites), a suboxic zone exists where nitrogen removal of up to 80% occurs including removal of NH(4)(+)-N. This zone has both NO(3)(-)-N and NH(4)(+)-N at >5 mg/L each. In the distal NH(4)(+)-rich zone, NH(4)(+)-N concentrations (8.1 ± 8.0 mg/L) are lower than in the proximal zone (48 ± 36 mg/L) and NH(4)(+)-N is isotopically enriched (concentration-weighted mean δ(15)N of +15.7‰) compared to the proximal zone (+7.8‰). Furthermore, δ(15)N-NH(4)(+) isotopic enrichment increases with depth in the distal zone, which is opposite to what would result if nitrification along the water table zone was the mechanism causing NH(4)(+) depletion. Bacterial community composition was assessed with molecular (DNA-based) analysis and demonstrated that groundwater bacterial populations were predominantly composed of bacteria from two Candidatus genera of the Planctomycetales (Brocadia and Jettenia). Together, these data provide strong evidence that anaerobic ammonium oxidation (anammox) plays an important role in nitrogen attenuation at this site.

Diel (24-h) cycling of dissolved O2 (DO) in rivers is well documented, but evidence for coupled diel changes in DO and nitrogen cycling has only been demonstrated in hypereutrophic systems where DO approaches zero at night. Here, we show diel changes in N2O and DO concentration at several sites across a trophic gradient. Nitrous oxide concentration increased at night at all but one site in spring and summer, even when gas exchange was rapid and minimum water column DO was well above hypoxic conditions. Diel N2O curves were not mirror images of DO curves and were not symmetrical about the mean. Although inter- and intrasite variation was high, N2O peaked around the time of lowest DO at most of the sites. These results suggest that N2O must be measured several times per diel period to characterize curve shape and timing. Nitrous oxide concentration was not significantly correlated with NO3- concentration, contrary to studies in agricultural streams and to the current United Nations Intergovernmental Panel for Climate Change protocols for N2O emission estimation. The strong negative correlation between N2O concentration and daily minimum DO concentration suggested that N2O production was limited by DO. This is consistent with N2O produced by nitrite reduction. The ubiquity of diel N2O cycling suggests that most DO and N2O sampling strategies used in rivers are insufficient to capture natural variability. Ecosystem-level effects of microbial processes, such as denitrification, are sensitive to small changes in redox conditions in the water column even in low-nutrient oxic rivers, suggesting diel cycling of redox-sensitive compounds may exist in many aquatic systems.

ABSTRACT. The 14C content of dissolved organic carbon (DOC) in streams, soil water and groundwaters in the Harp Lake catchment in Ontario, Canada, reflect a mixture of DOC sources, including both contemporary plant material and 14C-depleted soil organic matter. The concentration and isotopic content of DOC in streams is highly variable, reflecting the complex flow path of the source water entering the streams. The characteristics of groundwater DOC are set in the soil column, either through DOC production in the deeper soil layers, or through preferential decomposition and/or sorption of 14C-enriched DOC components from percolating waters. We estimate the relative magnitudes of decomposition, transport and sorption as sinks for DOC produced in forested catchment soils.

ABSTRACT. Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We used carbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of the Precambrian Shield. Soil C02, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. S13C soil CO2 profiles (b13C values of -23%o in summer, slightly enriched during the fall and -25%o during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds.14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C