Oxygen isotope systematics of nitrification

Abstract

During nitrification there is an exchange of oxygen atoms between water and nitrite, causing the [delta]¹⁸O of nitrate produced by nitrification to be closer to the [delta]¹⁸O of water than expected. A series of lab and field experiments were set up in order to quantify the exchange, and then calculate the [delta]¹⁸O of nitrate with these values. The lab experiments tested the exchange in ammonia oxidation, using ammonia oxidizing bacteria, Nitrosomonas sp. C113a and Nitrococcus oceani, and nitrite oxidation using cultures of the nitrite oxidizing bacterium, Nitrosococcus mobilis. The exchange value in the ammonia oxidation experiments could not be calculated because of unexpected complications in the analysis in the [delta]¹⁸O of nitrite. Although we weren't able to obtain a confident value for the exchange we were able to find a way to correct the [delta]¹⁸O of nitrite, for blank and exchange that affects the sample [delta]¹⁸O value for nitrite measured by the mass spectrometer. The exchange in the nitrite oxidation experiment could not be measured because there was full abiotic exchange in the bottle preventing us from calculating biotic exchange. A control experiment was successful in eliminating this exchange by adjusting the pH to a value higher than 8 prior to inoculation of the media during the experiment. In a future nitrite oxidation experiment this change in experimental design would make it possible to measure the exchange during nitrite oxidation. The experiments were a good step toward developing the best way to measure microbially-catalyzed exchange, and hopefully this value can be quantified in future analysis.