We report 15N/14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [NO3-] decreases (δ15N generally reaches 25–35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (εcell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of εcell, while uncertain, show a negative correlation with bottom water [O2]; we propose that this relates to the [NO3-] at the depth of denitrification. The N isotope effect at the scale of nitrate sediment–water exchange (εapp) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. εapp is much lower than εcell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O2] tends to yield more complete expression of εcell at the sediment–water scale, due to higher [NO3-] at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss (εsed, equivalent to εapp but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean εsed of ∼4‰ (∼2‰ if restricted to seafloor below 1km depth).

We report nitrate (NO3−) nitrogen isotope ratios for seawater samples collected in the Subantarctic Zone of the Southern Ocean during both winter and summer as part of the Australian Antarctic CRC Subantarctic Zone (SAZ) Project. The concentration and 15N/14N of the wintertime surface nitrate are very close to those of the subantarctic thermocline. The 15N/14N of nitrate in the surface increases sharply into the summer even though there is little seasonal change in nitrate concentration. There are two possible end‐member explanations for this observation. First, there may be significant equatorward nitrate transport during the summer, including a supply from the Antarctic surface. Second, the isotope effect of algal nitrate assimilation may be higher than has been estimated elsewhere, for example, for the seasonal sea ice zone of the Antarctic. We use a simple geochemical box model of the SAZ surface mixed layer as it evolves over the course of the summer to simulate salinity, nitrate concentration, and the 15N/14N of nitrate and sinking N. Our results suggest that a significant portion (∼30%) of the summertime SAZ nitrate is supplied from south of the Subantarctic Front and that N export is ≥3.5 mmol N m−2 d−1. Our approach also identifies the necessity of an isotope effect for nitrate assimilation in the SAZ of ≥7‰ and probably 8–9‰. Comparison to laboratory results suggests that this relatively high isotope effect may result from light limitation of algal growth in the SAZ.

Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3−) 15N/14N and 18O/16O associated with denitrification in the oxygen‐deficient thermocline waters of the eastern tropical North Pacific. The increase in δ18O is up to 3‰ greater than in δ15N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO3− δ18O and δ15N during NO3− consumption. Moreover, the maximum in NO3− δ18O is somewhat shallower than the maximum in NO3− δ15N. These two observations can be summarized as an “anomaly” from the 1:1 δ18O‐to‐δ15N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: (1) the addition of low‐δ15N NO3− to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO3− and NO2− (coupled NO3− reduction and NO2− oxidation) in the suboxic zone.

On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with pore water‐derived benthic nitrate fluxes in the deep BS basin, in order to gain new constraints on the mechanism of fixed nitrogen loss in the BS. The lack of any nitrate isotope enrichment into the deep part of the BS supports the benthic denitrification hypothesis. On the basis of the nitrate deficit in the water column with respect to the adjacent North Pacific and a radiocarbon‐derived ventilation age of ∼50 years, we calculate an average deep BS (>2000 m water depth) sedimentary denitrification rate of ∼230 μmol N m−2 d−1 (or 1.27 Tg N yr−1), more than 3 times higher than high‐end estimates of the average global sedimentary denitrification rate for the same depth interval. Pore water‐derived estimates of benthic denitrification were variable, and uncertainties in estimates were large. A very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep Bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope, owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down‐slope sediment focusing along the steep continental slopes that characterize the BS.

Nitrate and pesticide use in the 800 km(2) Raisin River agricultural watershed in eastern Ontario, Canada, threatens the quality of ground water in the highly exploited regional carbonate aquifer overlain by sandy till, To assess local recharge contributions through the cultivated fields, monthly monitoring of ground water levels, geochemistry, and environmental isotopes (delta(2)H(H2O), delta(13)C(DIC)) was carried out for 12 wells over a 14-month period,Seasonal water level variations suggest that recharge is constrained to spring and late fall when transpiration is minimized and the ground is not frozen, However, H-2 monitoring shows that early summer precipitation also contributes to recharge. Variations in delta(2)H values between monitoring sites suggest a local component to recharge,delta(13)C(DIC) was used to distinguish between dissolved inorganic carbon (DIC) originating from natural (C-3) vegetation and DIC from the cultivated corridors where corn is grown (C-4 vegetation), Seasonal variations in delta(13)C(DIC) are remarkably coherent for all wells, with uniform trends to positive values during periods of low water table elevation, During periods of high water table (spring and late fall), delta(13)C(DIC) values are between - 13 and - 16 parts per thousand VPDB (Vienna Peedee Belemnite), reflecting DIC originating from a dominantly natural (C-3) vegetation. When ground water levels are low (summer and mid-winter), delta(13)C(DIC) values shift to between - 11 and -7 parts per thousand, The seasonal enrichments in delta(13)C(DIC) are clear evidence for a local contribution to recharge by direct infiltration though the fields, This contribution is enhanced during periods of low water level, likely due to drainage from the phreatic aquifer, High DOC (dissolved organic carbon) concentrations (> 10 to 30 mg-Cn) correlate with periods of high water levels indicating infiltration of labile organics to the carbonate aquifer.The work carried out for this paper shows that the conjunctive use of environmental isotope geochemistry and physical parameters are fundamental to assessing the risk of ground water contamination in agricultural watersheds.