Estuarine sediments are critical for the remediation of large amounts of anthropogenic nitrogen (N) loading via production of N 2 from nitrate by denitrification. However, nitrate is also recycled within sediments by dissimilatory nitrate reduction to ammonium (DNRA). Understanding the factors that influence the balance between denitrification and DNRA is thus crucial to constraining coastal N budgets. A potentially important factor is the availability of different electron donors (organic carbon, reduced iron and sulfur). Both denitrification and DNRA may be linked to ferrous iron oxidation, however the contribution of Fe(II)-fueled nitrate reduction in natural environments is practically unknown. This study investigated how nitratedependent Fe 21 oxidation affects the partitioning between nitrate reduction pathways using 15 N-tracing methods in sediments along the salinity gradient of the periodically hypoxic Yarra River estuary, Australia. Increased dissolved Fe 21 availability resulted in significant enhancement of DNRA rates from around 10-20% total nitrate reduction in control incubations to over 40% in those with additional Fe 21 , at several sites.Increases in DNRA at some locations were accompanied by reductions in denitrification. Significant correlations were observed between Fe 21 oxidation and DNRA rates, with reaction ratios corresponding to the stoichiometry of Fe 21

The Yarra River estuary is a salt-wedge estuary prone to periods of stratification-induced anoxia and hypoxia (O 2 , 100 mmol L 21 ) during low-flow events. Nitrate reduction pathways were examined using the 15 N isotope pairing technique in intact sediment cores, emulating in situ conditions, to evaluate the fate of NO Whole-system estimates using deviations from conservative behavior and core incubations were in good agreement and showed that NH z 4 was regenerated more efficiently relative to DIC under hypoxic conditions. For the whole system, mean DDIC : DNH z 4 ratios under oxic (85 6 33) and hypoxic (20 6 3) conditions were significantly different. The more-efficient NH z 4 regeneration during hypoxia was attributed to rapid mineralization rates and cessation of nitrification; dissimilatory nitrate reduction to ammonium (DNRA) was not a significant contributor. Unexpectedly, the denitrification : DNRA ratio was significantly higher under hypoxic conditions, with denitrification contributing 99.1% 6 0.3% of total nitrate reduction. DNRA rates were significantly higher during oxic conditions (123.5 6 30.7 mmol m 22 h 21 ) when compared with rates during hypoxia (0.6 6 0.1 mmol m 22 h 21 ). The increase in DNRA in the presence of oxygen was attributed to the alleviation of NO { 3 limitation during these conditions.

The balance between estuarine denitrification and dissimilatory nitrate reduction to ammonium (DNRA) is critical for determining nitrogen loads received by oceans from inland waters. We aimed to determine the factors controlling the ratio between these processes and determining whether nitrogen was generally removed or recycled in estuaries. Rates of denitrification and DNRA with depth were measured in intact sediment cores in 11 estuaries along the coast of Victoria, Australia. The estuaries studied represent a range of biogeochemical conditions, land use, and catchment size. At a pore water profile scale, the ratio of denitrification to DNRA was well predicted by a multiple regression model with nitrate concentration in the overlying water, dissolved pore water iron, and ammonium as the predictor variables. Areal denitrification rates varied from 4 to 150 μmol · m−2 · hr−1, and DNRA rates varied from 2 to 30 μmol · m−2 · hr−1, with the ratio of denitrification to DNRA spanning a range from denitrification‐dominated (denitrification/DNRA = 8.4) to DNRA‐dominated (denitrification/DNRA = 0.3). DNRA dominated at sites with high iron pools and high organic carbon to nitrate ratios. We conclude that low nitrate and high Fe2+ availability generally enhances DNRA and drives an estuary toward being N recycling, rather than N removing.

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Cable bacteria represent a newly discovered group of filamentous microorganisms, which are capable of spatially separating the oxidative and reductive half-reactions of their sulfide-oxidizing metabolisms over centimeter distances. We investigated three ways that cable bacteria might interact with the nitrogen (N) cycle: (1) by reducing nitrate through denitrification or dissimilatory nitrate reduction to ammonium (DNRA) within their cathodic cells; (2) by nitrifying ammonium within their anodic cells; and (3) by indirectly affecting denitrification and/or DNRA by changing the Fe 2+ concentration in the surrounding sediment. We performed 15 N labeling laboratory experiments to measure these three processes using cable bacteria containing sediments from the Yarra River, Australia, and from Vilhelmsborg Sø, Denmark. Our results revealed that in the targeted systems, cable bacteria themselves did not perform significant rates of denitrification, DNRA, or nitrification. However, cable bacteria exhibited an important indirect effect, whereby they increased the Fe 2+ pool through iron sulfide dissolution. This elevated availability of Fe 2+ significantly increased DNRA and in some cases decreased denitrification. Thus, cable bacteria presence may affect the relative importance of DNRA in sediments and thus the extent by which bioavailable nitrogen is lost from the system.