Use of N-Isotope Distribution in Nitrogen Gas in the Study of Denitrification

The oxygen minimum zone (OMZ) of the Arabian Sea is a significant source of nitrous oxide (N 2 O), yet the metabolism responsible for N 2 O production is unclear. High-resolution profiles identified peaks and troughs of N 2 O and NO 2 2 in the top 500 m of the water column. The first peak in N 2 O was not in the oxycline, but deeper at the oxic-suboxic interface. Peaks and troughs were targeted with a suite of 15 was not binomially distributed and therefore was not entirely derived from the same source as N 2 O. Although indicative of an alternative N 2 source to denitrification, the lack of significant production of labeled N 2 with 15 NH þ 4 discounts anaerobic ammonium oxidation (anammox), as we understand it. Dissolved organic nitrogen or nitrate/nitrite reduction to ammonium are suggested as the additional sources of N in N 2 production.Large areas of the surface waters of the global ocean are at equilibrium with respect to N 2 O and the atmosphere, but regions that are oxygen deplete are significant sources of N 2 O (Elkins et al. 1978). The Arabian Sea is one such important source, which is estimated to contribute to between 5% and 18% of the total oceanic N 2 O emission (Law and Owens 1990; Naqvi and Noronha 1991). Nitrous oxide is an important atmospheric trace gas, contributing to global warming, ozone depletion, and acid rain formation (Stein and Yung 2003). Although the constraints on the global N 2 O budget have improved in recent years, the mechanisms of the major sinks and sources have not yet been fully resolved (Stein and Yung 2003).In the classic nitrogen (N) cycle, N 2 O is formed as an intermediate of denitrification, i.e., facultative anaerobic respiration with NO 2 3 or NODenitrification is dependant on low oxygen concentration (hypoxia), or complete anoxia, and is coupled to the oxidation of organic carbon (Tiedje 1988). In contrast, N 2 O is formed as a byproduct of autotrophic nitrification under low oxygen, i.e., NH along the oxycline, and nitrification is considered by some to be the dominant source (Elkins et al. 1978;Codispoti et al. 1992 and references therein; Dore et al. 1998). While this offers one explanation, the observed d 15 N of N 2 O (,+7-8%) from these waters is much higher than that expected for N 2 O formed during nitrification, i.e., ,260% relative to NH þ 4 , and the issue has been disputed (Yoshida 1988;Kim and Craig 1990; Dore et al. 1998). Naqvi et al. (1998) argue several possible modes of N 2 O production in the ocean, involving different isotopic fractionations. Alternatively, a great deal of discussion has focused on denitrification in the ocean, and, indeed, the Arabian Sea is considered to account for one-third of the global water column denitrification (Naqvi 1987), despite it never being measured directly, and actual measurements of denitrification in other open waters being comparatively scarce (Elkins et al. 1978;Brettar and Rheinheimer 1992; Castro-González and Farías 2004). Denitrification activity in an OMZ has largely been inferred from eith...

Section: Discussionmentioning

confidence: 74%

High‐resolution profiles and nitrogen isotope tracing reveal a dominant source of nitrous oxide and multiple pathways of nitrogen gas formation in the central Arabian Sea

Nicholls

Davies

Trimmer

2007

“…Strictly this definition should only be used to describe the proportion of 15 N in the produced gases if they were generated from random isotope pairing (Hauck et al 1958). …”

Section: Methodsmentioning

confidence: 99%

Production of nitrogen gas via anammox and denitrification in intact sediment cores along a continental shelf to slope transect in the North Atlantic

Trimmer

Nicholls

2009

132

111

We measured the production of N 2 gas from anammox and denitrification simultaneously in intact sediment cores at six sites along a transect of the continental shelf (50 m) and deeper slope (2000 m) in the North Atlantic. Maximum rates of total N 2 production were measured on the shelf and were largely due to denitrification, with anammox contributing, on average, 33% of this production. On the continental slope, the production of N 2 gas decreased but the proportion due to anammox reached a maximum of 65%. This change in both amount and dominant pathway of N 2 production could be explained largely by the concentration of organic carbon at each site. With increasing carbon the total production of N 2 increased rapidly while the response of anammox was not significant. On the continental slope, total N 2 production fell below 2 mmol N m 22 h 21 and anammox was strongly related (r 5 0.95) to denitrification but the relative magnitude of anammox to denitrification (1.65 : 1) suggested that anammox could not be fuelled by NO { 2 from denitrification alone. On the shelf, however, where total N 2 production was predominantly greater than 2 mmol N m 22 h 21 , no relationship between anammox and denitrification was found and anammox remained constant at 1.4 mmol N m 22 h 21 . Despite the constancy and greater availability of NO { 3 and lower temperatures on the continental slope, the significance of anammox to the total production of N 2 appears primarily controlled by the overall rate of N 2 production.

“…comm.). Atom-% 15N was estimated by the method of Hauck et al (1958), assuming isotopic equilibrium among the three N2 species.…”

Section: Methodsmentioning

confidence: 99%

The coupling of nitrification and denitrification in two estuarine sediments1,2

Jenkins

Kemp

1984

385

213

Nitrification and denitrification in estuarine sediments were measured by 15N isotopic tracer techniques at two stations in the Patuxent River estuary for April and August. A close coupling of the two processes was demonstrated directly from the evolution of labeled N, following 15NH,+ amendments during spring experiments: > 99% of the added 15NH,+ which was oxidized to r5N0,-was subsequently reduced to 15N-labeled N, during 48-h incubations, suggesting that the two reactions were occurring in close proximity to one another. In contrast, this coupled nitrificationdenitrification was decreased by two orders of magnitude in the summer, even though the capacity for denitrification (NZ production from N03-enrichment) remained similar to spring levels. This pattern of sharply seasonal nitrification was corroborated with measurements of bacterial relative abundance. Indirect evidence suggests low redox conditions (and reduced 0, concentrations) as the possible cause of decreased summertime nitrification. Ambient rates of nitrification-denitrification were inferred from 15N experiments in these sediments. Estimated springtime rates of about 77-89 pmol N*m-2*h-' are similar to previously reported values for denitrification supported by NO,-diffusion from overlying water to coastal sediments.