scholarly journals Vertical segregation among pathways mediating nitrogen loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem

2017 ◽  
Vol 14 (20) ◽  
pp. 4795-4813 ◽  
Author(s):  
Alexander Galán ◽  
Bo Thamdrup ◽  
Gonzalo S. Saldías ◽  
Laura Farías
Keyword(s):  
Coastal Upwelling ◽  
Nitrite Reduction ◽  
15N Tracer ◽  
Ammonium Oxidation ◽  
N Loss ◽  
N2o Production ◽  
Coastal System ◽  
Central Chile ◽  
N Removal ◽  
Bottom Waters

Abstract. The upwelling system off central Chile (36.5° S) is seasonally subjected to oxygen (O2)-deficient waters, with a strong vertical gradient in O2 (from oxic to anoxic conditions) that spans a few metres (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g. anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N loss and its speciation, in the form of N2 and/or N2O, were studied using 15N-tracer incubations, inhibitor assays, and the natural abundance of nitrate isotopes along with hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth) over the continental shelf off Concepción, Chile. Results of 15N-labelled incubations revealed higher N removal activity during the austral summer, with denitrification as the dominant N2-producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline, where a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 µmol L−1) relative to the hypoxic bottom waters ( <  20 µmol O2 L−1). Different pathways were responsible for N2O produced in the oxycline and bottom waters, with ammonium oxidation and dissimilatory nitrite reduction, respectively, as the main source processes. Ammonium produced by dissimilatory nitrite reduction to ammonium (DNiRA) could sustain both anammox and nitrification rates, including the ammonium utilized for N2O production. The temporal and vertical variability of δ15N-NO3− confirms that multiple N-cycling processes are modulating the isotopic nitrate composition over the shelf off central Chile during spring and summer. N removal processes in this coastal system appear to be related to the availability and distribution of oxygen and particles, which are a source of organic matter and the fuel for the production of other electron donors (i.e. ammonium) and acceptors (i.e. nitrate and nitrite) after its remineralization. These results highlight the links between several pathways involved in N loss. They also establish that different mechanisms supported by alternative N substrates are responsible for substantial accumulation of N2O, which are frequently observed as hotspots in the oxycline and bottom waters. Considering the extreme variation in oxygen observed in several coastal upwelling systems, these findings could help to understand the ecological and biogeochemical implications due to global warming where intensification and/or expansion of the oceanic OMZs is projected.

scholarly journals Vertical segregation among pathways mediating nitrogen-loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem

2017 ◽  
Author(s):  
Alexander Galán ◽  
Bo Thamdrup ◽  
Gonzalo S. Saldías ◽  
Laura Farías
Keyword(s):  
Coastal Upwelling ◽  
Nitrogen Loss ◽  
Austral Summer ◽  
Vertical Gradient ◽  
15N Tracer ◽  
Austral Spring ◽  
N Removal ◽  
Bottom Waters ◽  
Vertical Segregation ◽  
Summer Maximum

Abstract. The upwelling system off central Chile (36.5&amp;degree: S) is seasonally subjected to oxygen (O2) deficient waters, with a strong vertical gradient in O2 varying from oxic to anoxic conditions on a scale of a few meters (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g., anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N-loss and its speciation, in the form of N2 and N2O, were studied using 15N-tracer incubations, inhibitor assays, the natural abundance of nitrate isotopes, and hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth), over the continental shelf off Concepción, Chile. Results of 15N-labeled incubations revealed a higher N removal activity during the austral summer, with denitrification as the dominant N2 producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline. Notwithstanding, a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 μmol L−1), relative to the hypoxic bottom waters (

scholarly journals Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

2016 ◽  
Vol 13 (4) ◽  
pp. 1105-1118 ◽  
Author(s):  
Damian L. Arévalo-Martínez ◽  
Annette Kock ◽  
Carolin R. Löscher ◽  
Ruth A. Schmitz ◽  
Lothar Stramma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Coastal Upwelling ◽  
Mesoscale Eddies ◽  
South Pacific ◽  
N Cycling ◽  
Gene Marker ◽  
N Loss ◽  
Mode Water ◽  
N2o Production

Abstract. Recent observations in the eastern tropical South Pacific (ETSP) have shown the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production–consumption processes in the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source–sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked, and sampled during two surveys carried out in the ETSP in November–December 2012. A two-peak structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification, ammonium monooxygenase (amoA), and the gene marker for N2O production during denitrification, nitrite reductase (nirS). Conversely, we found strong N2O depletion in the core of the OMZ (O2 < 5 µmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the centre of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, “ageing” of mesoscale eddies tends to decrease N2O concentrations through the water column in response to the reduced supply of material to fuel N loss, although hydrographic variability might also significantly impact the pace of the production–consumption pathways for N2O. Our results evidence the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

scholarly journals High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea

2018 ◽  
Vol 15 (23) ◽  
pp. 7333-7346 ◽  
Author(s):  
Neus Garcias-Bonet ◽  
Marco Fusi ◽  
Muhammad Ali ◽  
Dario R. Shaw ◽  
Pascal E. Saikaly ◽  
...  
Keyword(s):  
Red Sea ◽  
Coastal Lagoon ◽  
N2 Fixation ◽  
Nitrogen Loss ◽  
Anaerobic Ammonium Oxidation ◽  
Ammonium Oxidation ◽  
N Loss ◽  
Seagrass Meadows ◽  
Bare Sediment ◽  
N Removal

Abstract. Nitrogen loads in coastal areas have increased dramatically, with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (N2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N2 fixation in a tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. We detected high annual mean rates of denitrification (34.9±10.3 and 31.6±8.9 mg N m−2 d−1) and anammox (12.4±3.4 and 19.8±4.4 mg N m−2 d−1) in vegetated and bare sediments. The annual mean N loss was higher (between 8 and 63-fold) than the N2 fixed (annual mean = 5.9±0.2 and 0.8±0.3 mg N m−2 d−1) in the meadow and bare sediment, leading to a net flux of N2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, N2 fixation can contribute substantially to seagrass growth since N2 fixation rates found here could contribute up to 36 % of plant N requirements. In vegetated sediments, anammox rates decreased with increasing organic matter (OM) content, while N2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while N2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the N2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.

scholarly journals Coastal hypoxia/anoxia as a source of CH<sub>4</sub> and N<sub>2</sub>O

2009 ◽  
Vol 6 (5) ◽  
pp. 9455-9523 ◽  
Author(s):  
S. W. A. Naqvi ◽  
H. W. Bange ◽  
L. Farías ◽  
P. M. S. Monteiro ◽  
M. I. Scranton ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Black Sea ◽  
Coastal Upwelling ◽  
Dead Zone ◽  
Nitrous Oxide Emissions ◽  
Quantitative Prediction ◽  
The Black Sea ◽  
N2o Production ◽  
Bottom Waters

Abstract. We review here available information on distributions of methane (CH4) and nitrous oxide (N2O) from major, mostly coastal, oxygen (O2)-deficient zones produced due to both natural processes and human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O2 levels. In the case of CH4, bottom-water O2 content probably affects emission from sediments, believed to be the main source of water-column CH4, as well as its oxidative loss in water itself. Highest CH4 accumulation (several μM) occurs in silled basins having anoxic deep waters such as the Black Sea and the Cariaco Basin. One to two orders of magnitude smaller, but still significant, accumulation also occurs in bottom waters of open margins experiencing anoxia and in silled basins containing suboxic/severely hypoxic waters. In highly eutrophic waters over open continental shelves (such as the upwelling zone off Namibia and the "dead zone" in the Gulf of Mexico) high CH4 concentrations (several hundred nM) may occur in non-sulphidic waters as well, but in these regions it is difficult to differentiate the hypoxia-induced enhancement from in situ production of CH4 in the water column and, sometimes, large inputs of CH4 associated with freshwater runoff or seepage from sediments. Despite the observed CH4 build-up in low-O2 bottom waters, methanotrophic activity severely restricts its emission from the ocean. As a result, an intensification or expansion of coastal hypoxic zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH4 budget. The situation is different for N2O, the production of which is greatly enhanced in severely hypoxic waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of semi-enclosed/land-locked anoxic basins such as the Black Sea. Most O2-deficient systems serve as strong net sources of N2O to the atmosphere. This is especially true for regions of coastal upwelling with shallow oxygen minimum zones where a dramatic increase in N2O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N2O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N2O as well as insufficient data from some key coastal regions.

scholarly journals High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea

2018 ◽  
Author(s):  
Neus Garcias-Bonet ◽  
Marco Fusi ◽  
Muhammad Ali ◽  
Dario R. Shaw ◽  
Pascal E. Saikaly ◽  
...  
Keyword(s):  
Red Sea ◽  
Coastal Lagoon ◽  
N2 Fixation ◽  
Nitrogen Loss ◽  
Anaerobic Ammonium Oxidation ◽  
Ammonium Oxidation ◽  
N Loss ◽  
Seagrass Meadows ◽  
Bare Sediment ◽  
N Removal

Abstract. Nitrogen loads in coastal areas have increased dramatically with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (N2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N2 fixation in tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. We detected high annual mean rates of denitrification (34.9 ± 10.3 and 31.6 ± 8.9 mg N m−2 d−1) and anammox (12.4 ± 3.4 and 19.8 ± 4.4 mg N m−2 d−1) in vegetated and bare sediments. The annual mean N loss was higher (8 and 63-fold higher) than the N2 fixed (annual mean = 5.9 ± 0.2 and 0.8  ± 0.3 mg N m−2 d−1) in the meadow and bare sediment, leading to a net flux of N2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, N2 fixation can contribute substantially to seagrass growth since N2 fixation rates found here could contribute up to 36 % of plant N requirements. In vegetated sediments, anammox rates decreased with increasing organic matter (OM) content, while N2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while N2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the N2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.

scholarly journals Regulation of nitrous oxide production in low oxygen waters off the coast of Peru

2019 ◽  
Author(s):  
Claudia Frey ◽  
Hermann W. Bange ◽  
Eric P. Achterberg ◽  
Amal Jayakumar ◽  
Carolin R. Löscher ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Organic Matter ◽  
Particulate Organic Matter ◽  
15N Tracer ◽  
Marker Genes ◽  
Ammonium Oxidation ◽  
Low Oxygen ◽  
N2o Production ◽  
Carbon Supply ◽  
Nitrification And Denitrification

Abstract. Oxygen deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N-tracer experiments in combination with qPCR and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118 ± 27.8 nmol L−1 d−1 just below the oxic-anoxic interface. Highest N2O production from ammonium oxidation (AO) of 0.16 ± 0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with highest archaeal amoA transcripts/genes. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

scholarly journals Isotopic evidence for alteration of nitrous oxide emissions and producing pathways contribution under nitrifying conditions

2019 ◽  
Author(s):  
Guillaume Humbert ◽  
Mathieu Sébilo ◽  
Justine Fiat ◽  
Longqi Lang ◽  
Ahlem Filali ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Biofilm Reactor ◽  
Nitrite Reduction ◽  
Nitrous Oxide Emissions ◽  
Site Preference ◽  
N2o Emissions ◽  
Ammonium Oxidation ◽  
N2o Production ◽  
Oxidation Rates ◽  
Nitrifier Denitrification

Abstract. Nitrous oxide (N2O) emissions by a nitrifying biofilm reactor were investigated with N2O isotopocules. The site preference of N2O (15N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 °C), and ammonium concentrations (from 6.2 to 62.1 mg N L−1). Nitrite reduction, either nitrifier-denitrification or heterotrophic denitrification, was the main N2O producing pathway under the tested conditions. Nitrite oxidation rates decreased as compared to ammonium oxidation rates at temperatures above 20 °C and sub-optimal oxygen levels, increasing N2O production by the nitrite reduction pathway. Below 20 °C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in the N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium.

scholarly journals Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru

2020 ◽  
Vol 17 (8) ◽  
pp. 2263-2287 ◽  
Author(s):  
Claudia Frey ◽  
Hermann W. Bange ◽  
Eric P. Achterberg ◽  
Amal Jayakumar ◽  
Carolin R. Löscher ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Organic Matter ◽  
Particulate Organic Matter ◽  
15N Tracer ◽  
Marker Genes ◽  
Ammonium Oxidation ◽  
Low Oxygen ◽  
N2o Formation ◽  
N2o Production ◽  
Nitrification And Denitrification

Abstract. Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH4+ and the other from other substrates such as NO2-) was the dominant species, comprising 70 %–85 % of total produced N2O from NH4+, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

scholarly journals Isotopic evidence for alteration of nitrous oxide emissions and producing pathways' contribution under nitrifying conditions

2020 ◽  
Vol 17 (4) ◽  
pp. 979-993
Author(s):  
Guillaume Humbert ◽  
Mathieu Sébilo ◽  
Justine Fiat ◽  
Longqi Lang ◽  
Ahlem Filali ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Biofilm Reactor ◽  
Nitrite Reduction ◽  
Nitrous Oxide Emissions ◽  
Site Preference ◽  
N2o Emissions ◽  
Ammonium Oxidation ◽  
N2o Production ◽  
Oxidation Rates ◽  
Nitrifier Denitrification

Abstract. Nitrous oxide (N2O) emissions from a nitrifying biofilm reactor were investigated with N2O isotopocules. The nitrogen isotopomer site preference of N2O (15N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 % to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 ∘C) and ammonium concentrations (from 6.2 to 62.1 mg N L−1). Nitrite reduction, either nitrifier denitrification or heterotrophic denitrification, was the main N2O-producing pathway under the tested conditions. Difference between oxidative and reductive rates of nitrite consumption was discussed in relation to NO2- concentrations and N2O emissions. Hence, nitrite oxidation rates seem to decrease as compared to ammonium oxidation rates at temperatures above 20 ∘C and under oxygen-depleted atmosphere, increasing N2O production by the nitrite reduction pathway. Below 20 ∘C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium. The approach and results obtained here, for a nitrifying biofilm reactor under variable environmental conditions, should allow for application and extrapolation of N2O emissions from other systems such as lakes, soils and sediments.

scholarly journals Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

2015 ◽  
Vol 12 (12) ◽  
pp. 9243-9273 ◽  
Author(s):  
D. L. Arévalo-Martínez ◽  
A. Kock ◽  
C. R. Löscher ◽  
R. A. Schmitz ◽  
L. Stramma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Coastal Upwelling ◽  
Mesoscale Eddies ◽  
South Pacific ◽  
N Cycling ◽  
Gene Marker ◽  
N Loss ◽  
Mode Water ◽  
N2o Production

Abstract. Recent observations in the eastern tropical South Pacific (ETSP) demonstrated the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production/consumption processes on the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source-sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked and sampled during two surveys carried out in the ETSP in November-December 2012. A "two peak" structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification amoA and the gene marker for N2O production during denitrification, nirS. Conversely, we found strong N2O depletion in the core of the OMZ (O2 < 5 μmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the center of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, "aging" of mesoscale eddies tends to decrease N2O concentrations through the water column in response to reduced supply of material to fuel N-loss, although hydrographic variability might also significantly impact the pace of the production/consumption pathways for N2O. Our results demonstrate the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

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