Denitrification, Dissimilatory Reduction of Nitrate to Ammonium, and Nitrification in a Bioturbated Estuarine Sediment as Measured with 15 N and Microsensor Techniques

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Abstract
Nitrogen and oxygen transformations were studied in a bioturbated (reworked by animals) estuarine sediment (Norsminde Fjord, Denmark) by using a combination of 15 N isotope (NO 3 - ), specific inhibitor (C 2 H 2 ), and microsensor (N 2 O and O 2 ) techniques in a continuous-flow core system. The estuarine water was NO 3 - rich (125 to 600 μM), and NO 3 - was consistently taken up by the sediment on the four occasions studied. Total NO 3 - uptake (3.6 to 34.0 mmol of N m -2 day -1 ) corresponded closely to N 2 production (denitrification) during the experimental steady state, which indicated that dissimilatory, as well as assimilatory, NO 3 - reduction to NH 4 + was insignificant. When C 2 H 2 was applied in the flow system, denitrification measured as N 2 O production was often less (58 to 100%) than the NO 3 - uptake because of incomplete inhibition of N 2 O reduction. The NO 3 - formed by nitrification and not immediately denitrified but released to the overlying water, uncoupled nitrification, was calculated both from 15 NO 3 - dilution and from changes in NO 3 - uptake before and after C 2 H 2 addition. These two approaches gave similar results, with rates ranging between 0 and 8.1 mmol of N m -2 day -1 on the four occasions. Attempts to measure total nitrification activity by the difference between NH 4 + fluxes before and after C 2 H 2 addition failed because of non-steady-state NH 4 + fluxes. The vertical distribution of denitrification and oxygen consumption was studied by use of N 2 O and O 2 microelectrodes. The N 2 O profiles measured during the experimental steady state were often irregularly shaped, and the buildup of N 2 O after C 2 H 2 was added was much too fast to be described by a simple diffusion model. Only bioturbation by a dense population of infauna could explain these observations. This was corroborated by the relationship between diffusive and total fluxes, which showed that only 19 to 36 and 29 to 62% of the total O 2 uptake and denitrification, respectively, were due to diffusion-reaction processes at the regular sediment surface, excluding animal burrows.