Nitrous oxide production rates and stable isotopes in the Benguela Upwelling System during METEOR cruise M157

doi:doi:10.1594/PANGAEA.966690

Nitrous oxide production rates and stable isotopes in the Benguela Upwelling System during METEOR cruise M157

Upwelling systems are significant sources of atmospheric nitrous oxide (N₂O). The Benguela Upwelling System is one of the most productive regions worldwide and a temporally variable source of N₂O. Strong O₂ depletions above the shelf are favoring periodically OMZ formations. We aimed to assess underlying N₂O production and consumption processes on different temporal and spatial scales during austral winter in the Benguela Upwelling System, when O₂-deficiency in the water column is relatively low. The fieldwork took place during the cruise M157 (August 4th – September 16th 2019) onboard the R/V METEOR. This expedition included four close-coastal regions around Walvis Bay at 23°S, which presented the lowest O₂ concentrations near the seafloor and thus may provide hotspots of N₂O production. Seawater was collected in 10 L free-flow bottles by using a rosette system equipped with conductivity-temperature-depth (CTD) sensors (SBE 911plus, Seabird-electronics, USA). Incubation experiments were performed using stable isotope ¹⁵N-tracers. Seawater samples for ¹⁵N-tracer incubations and natural abundance N₂O analysis were collected from 10 L free-flow bottles and filled bubble-free into 125 mL serum bottles. The samples for natural abundance N₂O analysis were immediately fixed with saturated HgCl₂ and stored in the dark. To perform the incubation, we added ¹⁵N-labeled NO₂-, NO₃⁻ and NH₄⁺ to estimate the in-situ N₂O production rates and associated reactions. To determine a single rate, the bottles were sacrificed after tracer addition, and within the time interval of 12 h, 24 h and 48 h by adding HgCl₂. Rates were calculated based on a linear regression over time. Total N₂O and natural abundance isotopologues of N₂O were analyzed by using an isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Scientific). NO₂- production was additionally analyzed by transforming ¹⁵NO₂- to ¹⁵N₂O following the azide method after McIlvin & Altabet (2005) and the nitrogen isotope ratio of N₂O was measured by an IRMS. N₂ production was determined via an IRMS (Flash-EA-ConfloIV-DELTA V Advanced, Thermo Scientific) by injecting headspace from exetainers. The N₂O yield per nitrite produced and the N₂O yield during denitrification was calculated. Samples for natural abundance N₂O was sampled and measured in triplicates and is shown as an average with standard deviation (SD). In order to estimate the contribution of different N₂O producing pathways by major biological processes and the extent of N₂O reduction to N₂, the dual-isotope mapping approach was applied to natural abundance isotopologues of N₂O, which uses the relative position of background-subtracted N₂O samples in a δ¹⁵Nˢᴾ-N₂O vs. δ¹⁸O-N₂O diagram (Yu et al., 2020; Lewicka-Szczebak et al., 2020).

BibTex:

@dataset{Dangl_Gabriela_and_Frey_Claudia_and_Hassenrück_Christiane_and_Sabbaghzadeh_Bita_and_Wäge-Recchioni_Janine_and_Lehmann_Moritz_F_and_Ley_Martin_and_Rehder_Gregor_and_Jürgens_Klaus_2024,
    abstract = {Upwelling systems are significant sources of atmospheric nitrous oxide (N₂O). The Benguela Upwelling System is one of the most productive regions worldwide and a temporally variable source of N₂O. Strong O₂ depletions above the shelf are favoring periodically OMZ formations. We aimed to assess underlying N₂O production and consumption processes on different temporal and spatial scales during austral winter in the Benguela Upwelling System, when O₂-deficiency in the water column is relatively low. The fieldwork took place during the cruise M157 (August 4th – September 16th 2019) onboard the R/V METEOR. This expedition included four close-coastal regions around Walvis Bay at 23°S, which presented the lowest O₂ concentrations near the seafloor and thus may provide hotspots of N₂O production. Seawater was collected in 10 L free-flow bottles by using a rosette system equipped with conductivity-temperature-depth (CTD) sensors (SBE 911plus, Seabird-electronics, USA).
Incubation experiments were performed using stable isotope ¹⁵N-tracers. Seawater samples for ¹⁵N-tracer incubations and natural abundance N₂O analysis were collected from 10 L free-flow bottles and filled bubble-free into 125 mL serum bottles. The samples for natural abundance N₂O analysis were immediately fixed with saturated HgCl₂ and stored in the dark. To perform the incubation, we added ¹⁵N-labeled NO₂-, NO₃⁻ and NH₄⁺ to estimate the in-situ N₂O production rates and associated reactions. To determine a single rate, the bottles were sacrificed after tracer addition, and within the time interval of 12 h, 24 h and 48 h by adding HgCl₂. Rates were calculated based on a linear regression over time. Total N₂O and natural abundance isotopologues of N₂O were analyzed by using an isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Scientific). NO₂- production was additionally analyzed by transforming ¹⁵NO₂- to ¹⁵N₂O following the azide method after McIlvin & Altabet (2005) and the nitrogen isotope ratio of N₂O was measured by an IRMS. N₂ production was determined via an IRMS (Flash-EA-ConfloIV-DELTA V Advanced, Thermo Scientific) by injecting headspace from exetainers. The N₂O yield per nitrite produced and the N₂O yield during denitrification was calculated. Samples for natural abundance N₂O was sampled and measured in triplicates and is shown as an average with standard deviation (SD). In order to estimate the contribution of different N₂O producing pathways by major biological processes and the extent of N₂O reduction to N₂, the dual-isotope mapping approach was applied to natural abundance isotopologues of N₂O, which uses the relative position of background-subtracted N₂O samples in a δ¹⁵Nˢᴾ-N₂O vs. δ¹⁸O-N₂O diagram (Yu et al., 2020; Lewicka-Szczebak et al., 2020).},
    author = {Dangl, Gabriela and Frey, Claudia and Hassenrück, Christiane and Sabbaghzadeh, Bita and Wäge-Recchioni, Janine and Lehmann, Moritz F and Ley, Martin and Rehder, Gregor and Jürgens, Klaus},
    doi = {10.1594/PANGAEA.966690},
    institution = {PANGAEA (Chemistry)},
    keyword = {'15N', 'Benguela Upwelling System', 'Denitrification', 'Field observation', 'N2O production rates', 'Namibia', 'Stable isotope', 'ammonium oxidation', 'nitrate reduction', 'nitrification', 'oxygen minimum zone', 'tracer'},
    title = {Nitrous oxide production rates and stable isotopes in the Benguela Upwelling System during METEOR cruise M157},
    url = {https://service.tib.eu/ldmservice/vdataset/png-doi-10-1594-pangaea-966690},
    year = {2024}
}