Publication Date:
2024-06-05
Description:
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).
Keywords:
15N-tracer; Ammonium, oxidation rate; Ammonium, oxidation rate, limit of detection; Ammonium, oxidation rate, standard error; ammonium oxidation; Anammox rate; Anammox rate, standard error; Benguela Upwelling System; BUSUC 1; Calculated; CTD/Rosette; CTD-RO; DATE/TIME; Denitrification; Denitrification rate, standard error; DEPTH, water; Event label; Field observation; Gas Chromatograph (GC), Manufacturer unknown, custom built; coupled with Isotope Ratio Mass Spectrometer (IRMS), Thermo Scientific, Delta V Plus; Isotope Ratio Mass Spectrometer (IRMS), Thermo Scientific, Delta V Advantage [Conflo IV interface]; LATITUDE; LONGITUDE; M157; M157_14-14; M157_16-25; M157_17-16; M157_2-9; Meteor (1986); N2O production rates; Namibia; Nitrate, reduction rate; Nitrate, reduction rate, limit of detection; nitrate reduction; nitrification; Nitrous oxide, limit of detection; Nitrous oxide, yield; Nitrous oxide production; Nitrous oxide production, standard error; oxygen minimum zone; Sample code/label; Site preference, N2O; Site preference, N2O, standard deviation; Stable isotope; Station label; δ15N, nitrous oxide; δ15N, nitrous oxide, standard deviation; δ15N-alpha, nitrous oxide; δ15N-alpha, nitrous oxide, standard deviation; δ15Nbeta, nitrous oxide; δ15Nbeta, nitrous oxide, standard deviation; δ18O, nitrous oxide; δ18O, nitrous oxide, standard deviation
Type:
Dataset
Format:
text/tab-separated-values, 801 data points
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