We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the ...Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting δ15N distributions. Pacific surface waters are marked by strongly positive values for δ15N-NO3–) reflecting fractionation associated with subsurface N-loss and partial NO3– utilization. This contrasts with negative values in NO3– depleted surface waters of the Atlantic which are lower than can be explained by N supply via N2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (ϵd=11.4 ‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations (O2 < 5 μmol l−1) in both regions reflect a nitrification source. Sharp decrease in N2O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 < 5 μmol l−1.
We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the ...reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2‰ for nitrogen and 0.5‰ for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 μM (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.
Most global biogeochemical processes are known to respond to climate change, some of which have the capacity to produce feedbacks through the regulation of atmospheric greenhouse gases. Marine ...denitrification-the reduction of nitrate to gaseous nitrogen-is an important process in this regard, affecting greenhouse gas concentrations directly through the incidental production of nitrous oxide, and indirectly through modification of the marine nitrogen inventory and hence the biological pump for CO2. Although denitrification has been shown to vary with glacial-interglacial cycles, its response to more rapid climate change has not yet been well characterized. Here we present nitrogen isotope ratio, nitrogen content and chlorin abundance data from sediment cores with high accumulation rates on the Oman continental margin that reveal substantial millennial-scale variability in Arabian Sea denitrification and productivity during the last glacial period. The detailed correspondence of these changes with Dansgaard-Oeschger events recorded in Greenland ice cores indicates rapid, century-scale reorganization of the Arabian Sea ecosystem in response to climate excursions, mediated through the intensity of summer monsoonal upwelling. Considering the several-thousand-year residence time of fixed nitrogen in the ocean, the response of global marine productivity to changes in denitrification would have occurred at lower frequency and appears to be related to climatic and atmospheric CO2 oscillations observed in Antarctic ice cores between 20 and 60 kyr ago.
The oceanic nitrogen cycle is critically important for the partitioning of greenhouse gases between ocean and atmosphere. Baffin Bay connects ocean regions that are major sources (North Atlantic) and ...sinks (North Pacific and western Arctic) of biologically available nitrogen and further harbors supersaturation of nitrous oxide and a coincident deficit in nitrate in the deep basin. Isotopic tracer profiles of both nitrogen species presented here provide novel insights into the origin and cycling of reactive nitrogen in Baffin Bay, highlighting the connectivity between different Arctic systems and horizontal components of basin‐scale nutrient transport. Baffin Bay bottom water properties are derived from export production in northern Baffin Bay, which is largely fueled by Pacific‐derived nutrients. In situ remineralization at depth gives rise to benthic denitrification, evidenced by a pronounced accumulation of nitrous oxide with a distinctively high site preference (<44‰) in the deep basin. Nutrients supplied to Baffin Bay are hence stripped from surface waters and trapped at depth over long timescales, where sedimentary denitrification further adds to the N removal capacity of the Arctic Ocean.
Key Points
Nitrate isotope ratios indicate substantial remineralization of organic matter in deep Baffin Bay, fueled by Pacific‐derived nutrients
Nitrous oxide isotopomers support a predominantly sedimentary source of N2O propagating well into the oxygenated water column
Sedimentary denitrification is a significant sink of dissolved inorganic nitrogen in deep Baffin Bay
Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO₃⁺) and nitrite (NO₂⁺), to the gases nitric oxide (NO), nitrous oxide (N₂O), and dinitrogen (N₂), is important to primary ...production, water quality, and the chemistry and physics of the atmosphere at ecosystem, landscape, regional, and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses, and future prospects for the different methods. Methodological approaches covered include (1) acetylene-based methods, (2) ₁₅N tracers, (3) direct N₂ quantification, (4) N₂:Ar ratio quantification, (5) mass balance approaches, (6) stoichiometric approaches, (7) methods based on stable isotopes, (8) in situ gradients with atmospheric environmental tracers, and (9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well-defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.
Francois et al suggest that reduction of the CO2 "leak" from ocean to atmosphere by increased surface-water stratification south of the Polar Front made a greater contribution to the lowering of ...atmospheric CO2 concentrations during the Last Glacial Maximum than did the increased export of organic carbon from surfaces to deep waters occuring further north.
Abstract Nitrous oxide (N 2 O) is a climate relevant trace gas, and its production in the ocean generally increases under suboxic conditions. The Atlantic Ocean is well ventilated, and unlike the ...major oxygen minimum zones (OMZ) of the Pacific and Indian Oceans, dissolved oxygen and N 2 O concentrations in the Atlantic OMZ are relatively high and low, respectively. This study, however, demonstrates that recently discovered low oxygen eddies in the eastern tropical North Atlantic (ETNA) can produce N 2 O concentrations much higher (up to 115 nmol L −1 ) than those previously reported for the Atlantic Ocean, and which are within the range of the highest concentrations found in the open-ocean OMZs of the Pacific and Indian Oceans. N 2 O isotope and isotopomer signatures, as well as molecular genetic results, also point towards a major shift in the N 2 O cycling pathway in the core of the low oxygen eddy discussed here, and we report the first evidence for potential N 2 O cycling via the denitrification pathway in the open Atlantic Ocean. Finally, we consider the implications of low oxygen eddies for bulk, upper water column N 2 O at the regional scale, and point out the possible need for a reevaluation of how we view N 2 O cycling in the ETNA.
Microbial nitrogen processing in direct association with marine animals and
seaweeds is poorly understood. Microbes can both attach to the surfaces of
macrobiota and make use of their excreted ...nitrogen and dissolved organic
carbon (DOC). We tested the role of an intertidal mussel (Mytilus californianus) and red alga (Prionitis sternbergii), as well as
inert substrates for microbial activity using enclosed chambers with seawater
labeled with 15N-enriched ammonium and nitrate. Chambers with only
seawater from the same environment served as a control. We found that 3.21 nmol
of ammonium per gram of dry mass of mussel, on average, was oxidized per
hour, while 1.60 nmol of nitrate was reduced per hour. Prionitis
was associated with the oxidation of 1.50 nmol of ammonium per gram of wet mass per
hour, while 1.56 nmol of nitrate was reduced per hour. Inert substrates
produced relatively little change compared to seawater alone. Extrapolating
to a square meter of shoreline, microbial activity associated with mussels
could oxidize 2.5 mmol of ammonium and reduce per 1.2 mmol of nitrate per
day. A square meter of seaweed could oxidize 0.13 mmol ammonium per day and
reduce the same amount of nitrate. Seawater collected proximal to the shore
versus 2–5 km offshore showed no difference in ammonium oxidation or nitrate
reduction. Microbial nitrogen metabolism associated with mussels was not
influenced by the time of day. When we experimentally added DOC (glucose) as
a carbon source to chambers with the red alga and inert substrates, no change
in nitrification rates was observed. Added DOC did increase dissolved inorganic
nitrogen (DIN) and phosphorus uptake, indicating that DOC addition stimulated heterotrophic
microbial activity, and suggests potential competition for DIN between
heterotrophic and chemolithotrophic microbes and their seaweed hosts. Our
results demonstrate that microbes in direct association with coastal animals
and seaweeds greatly enhance nitrogen processing and likely provide a
template for a diversity of ecological interactions.
During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (d15N) and their control by relative NO3− drawdown were examined in detail. Near‐surface d15N data clearly conform to ...Rayleigh isotopic fractionation during phytoplankton utilization of NO , with a fractionation factor of about 5‰. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large‐scale characteristic of the system. Decreasing near‐surface NO3− with distance from the equator strongly correlates with increasing d15N in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Niño) and 2 (non‐El Niño) cruises. Despite a doubling of NO3− accompanying relaxation of El Niño conditions, d15N values in near‐surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since d15N varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled NO3− was consumed in the immediate vicinity of the equator, as estimated from the d15N data. The moored sediment trap time series provided a more highly resolved temporal view of the 15N dynamics of this system. At the equator and to the south, there is little temporal variation in d15N and, hence, in relative NO3− utilization over the 1‐yr duration of the program. At 28 and 58°N, there is a large 5‰ decrease in d15N during the transition to non‐El Niño conditions in response to the northward movement of NO3− ‐rich waters, which probably reflects the intensification of the north equatorial current. Overall, % NO3− utilization at the equator remained between 40 and 60% over the observation period, despite large hydrographic and dynamical changes. This observation implies a tight control of relative NO3− utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative NO3− utilization is likely determined by the product of the ratio of Fe to NO3− in upwelled waters and the NO3− : Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Niño, will affect surface NO3− new and export production, but it will not affect relative NO3− utilization. In this light, downcore d15N records in the equatorial Pacific should reflect past changes in the limitation of NO3− drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.
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