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In batch experiments, we studied the isotope fractionation in N and O of dissolved nitrate during dentrification. Denitrifying strains Thauera aromatica and "Aromatoleum aromaticum strain EbN1" were grown under strictly anaerobic conditions with acetate, benzoate, and toluene as carbon sources. (18)O-labeled water and (18)O-labeled nitrite were added to the microcosm experiments to study the effect of putative backward reactions of nitrite to nitrate on the stable isotope fractionation. We found no evidence for a reverse reaction. Significant variations of the stable isotope enrichment factor ε were observed depending on the type of carbon source used. For toluene (ε(15)N, -18.1 ± 0.6‰ to -7.3 ± 1.4‰; ε(18)O, -16.5 ± 0.6‰ to -16.1 ± 1.5‰) and benzoate (ε(15)N, -18.9 ± 1.3‰; ε(18)O, -15.9 ± 1.1‰) less negative isotope enrichment factors were calculated compared to those derived from acetate (ε(15)N, -23.5 ± 1.9‰ to -22.1 ± 0.8‰; ε(18)O, -23.7 ± 1.8‰ to -19.9 ± 0.8‰). The observed isotope effects did not depend on the growth kinetics which were similar for the three types of electron donors. We suggest that different carbon sources change the observed isotope enrichment factors by changing the relative kinetics of nitrate transport across the cell wall compared to the kinetics of the intracellular nitrate reduction step of microbial denitrification., (© 2012 American Chemical Society)

Concentrations and stable isotope compositions of nitrate from 11 karst springs in the Franconian Alb (southern Germany) were determined during low flow and high flow conditions to assess sources and processes affecting groundwater nitrate. During low flow, nitrate concentrations in groundwater were around 0.10 mM in springs draining forested catchments, whereas in agricultural areas nitrate concentrations were typically higher reaching up to 0.93 mM. The isotopic composition of groundwater nitrate during low flow (delta15N values of -3.1 to 6.7% per hundred, delta180 values of +2.1 to 4.0% per hundred) in concert with concentration data suggests that nitrate is formed by nitrification in forest and agricultural soils. In addition, synthetic fertilizer N that has undergone immobilization and subsequent remineralization likely constitutes an additional nitrate source in agriculturally used catchments. During recharge conditions, concentrations and delta15N values of groundwater nitrate changed little, but delta18O values were significantly elevated (up to 24.5%o per hundred suggesting that around 25% of the nitrate was directly derived from atmospheric deposition. Groundwater dating revealed that low nitrate concentrations in groundwater (_> or =0 years) are consistent with a mixture of old low nitrate-containing and young water, the latter being affected by anthropogenic N inputs predominantly in the agriculturally used catchment areas during the last few decades. Thermodynamic and hydrogeological evidence also suggests that denitrification may have occurred in the porous rock matrix of the karst aquifer. This study demonstrates that a combination of hydrodynamic, chemical, and isotopic approaches provides unique insights into the sources and the biogeochemical history of nitrate in karst aquifers, and therefore constitutes a valuable tool for assessing the vulnerability of karst aquifers to nitrate pollution in dependence on land use and assessing their self-purification capacity.