Experimental investigation of nitrogen and oxygen isotope fractionation in nitrate and nitrite during denitrification
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In batch experiments, we studied the isotope fractionation of nitrogen and oxygen during denitrification of two bacterial strains (Azoarcus sp. strain DSM 9056 and Pseudomonas pseudoalcaligenes strain F10). Denitrification experiments were conducted with succinate and toluene as electron donor in three waters with a distinct oxygen isotope composition. Nitrate consumption was observed in all batch experiments. Reaction rates for succinate experiments were more than six times higher than those for toluene experiments. Nitrogen and oxygen isotopes became progressively enriched in the remaining nitrate pool in the course of the experiments; the nitrogen and oxygen isotope fractionation varied between 8.6–16.2 and 4.0–7.3‰, respectively. Within this range, neither electron donors nor the oxygen isotope composition of the medium affected the isotope fractionation process. The experimental results provide evidence that the oxygen isotope fractionation during nitrate reduction is controlled by a kinetic isotope effect which can be quantified using the Rayleigh model. The isotopic examination of nitrite released upon denitrification revealed that nitrogen isotope fractionation largely follows the fractionation of the nitrate pool. However, the oxygen isotope values of nitrite are clearly influenced by a rapid isotope equilibration with the oxygen of the ambient water. Even though this equilibration may in part be due to storage, it shows that under certain natural conditions (re-oxidation of nitrite) the nitrate pool may also be indirectly affected by an isotope equilibration.
KeywordsNitrate Nitrite Denitrification Nitrogen isotopes Oxygen isotopes Kinetic isotope fractionation Isotope exchange
This work is integrated in the research and development program of the Helmholtz Centre for Environmental Research. We would like to thank Daniela Reichert and Wolfgang Städter of the Stable Isotope Laboratory Halle/Saale for conducting numerous nitrogen and oxygen isotope analyses. Special thanks are addressed to Ramona Hoffman who conducted the hydrochemical analyses of our experimental samples and to Stephanie Hinke for preparing the cultivation media. The study is part of the DFG research unit 580 “electron transfer processes in anoxic aquifers (etrap)” (FOR 580 grant Ri903/3-2) supporting Stefan Feisthauer.
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