Year-round N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O production by benthic NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; reduction in a monomictic south-alpine lake

Freymond, Chantal V.; Wenk, Christine B.; Frame, Caitlin H.; Lehmann, Moritz F.

doi:10.5194/bg-10-8373-2013

Cited by 18 publications

(16 citation statements)

References 45 publications

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“…With the onset of seasonal anoxia in the south basin of Lake Lugano (Lehmann et al, 2004; Blees et al, 2014), the sediments (95 m) and deep redox transition zone (70–90 m) become important in the production and consumption of deep N 2 O in this system (Freymond et al, 2013; Wenk et al, 2016). Here we have restricted our discussion to the top 50 m of the water column.…”

Section: Resultsmentioning

confidence: 99%

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

et al. 2017

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Ammonia-oxidizing microorganisms are an important source of the greenhouse gas nitrous oxide (N2O) in aquatic environments. Identifying the impact of pH on N2O production by ammonia oxidizers is key to understanding how aquatic greenhouse gas fluxes will respond to naturally occurring pH changes, as well as acidification driven by anthropogenic CO2. We assessed N2O production rates and formation mechanisms by communities of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in a lake and a marine environment, using incubation-based nitrogen (N) stable isotope tracer methods with 15N-labeled ammonium (15NH4+) and nitrite (15NO2−), and also measurements of the natural abundance N and O isotopic composition of dissolved N2O. N2O production during incubations of water from the shallow hypolimnion of Lake Lugano (Switzerland) was significantly higher when the pH was reduced from 7.54 (untreated pH) to 7.20 (reduced pH), while ammonia oxidation rates were similar between treatments. In all incubations, added NH4+ was the source of most of the N incorporated into N2O, suggesting that the main N2O production pathway involved hydroxylamine (NH2OH) and/or NO2− produced by ammonia oxidation during the incubation period. A small but significant amount of N derived from exogenous/added 15NO2− was also incorporated into N2O, but only during the reduced-pH incubations. Mass spectra of this N2O revealed that NH4+ and 15NO2− each contributed N equally to N2O by a “hybrid-N2O” mechanism consistent with a reaction between NH2OH and NO2−, or compounds derived from these two molecules. Nitrifier denitrification was not an important source of N2O. Isotopomeric N2O analyses in Lake Lugano were consistent with incubation results, as 15N enrichment of the internal N vs. external N atoms produced site preferences (25.0–34.4‰) consistent with NH2OH-dependent hybrid-N2O production. Hybrid-N2O formation was also observed during incubations of seawater from coastal Namibia with 15NH4+ and NO2−. However, the site preference of dissolved N2O here was low (4.9‰), indicating that another mechanism, not captured during the incubations, was important. Multiplex sequencing of 16S rRNA revealed distinct ammonia oxidizer communities: AOB dominated numerically in Lake Lugano, and AOA dominated in the seawater. Potential for hybrid N2O formation exists among both communities, and at least in AOB-dominated environments, acidification may accelerate this mechanism.

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Section: Resultsmentioning

confidence: 99%

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

et al. 2017

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“…In both years, the anoxic sediments were a significant N 2 O source, as revealed in independent sediment-core flow-through incubations (Freymond et al 2013). In 2010, however, the high sedimentary N 2 O production rates observed by Freymond et al (2013) in combination with the low N 2 O accumulation in the anoxic bottom water suggested efficient N 2 O consumption in the RTZ from the beginning of the stratification period. Moreover, in 2010, the highest N 2 O concentrations were detected at the oxic-anoxic interface, implying that N 2 O production by nitrification or nitrifier denitrification is an important N 2 O source.…”

Section: Large Amounts Of N 2 O Accumulate In the Lake Lugano South Bmentioning

confidence: 90%

Differential N₂ O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions

et al. 2016

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Lakes are a nitrous oxide (N2O) source to the atmosphere, but the biogeochemical controls and microbial pathways of N2O production are not well understood. To trace microbial N2O production (denitrification, nitrifier denitrification, and nitrification) and consumption (denitrification) in two basins of Lake Lugano, we measured the concentrations and N and O isotope compositions of N2O, as well as the intramolecular 15N distribution, i.e., site preference (SP). Our results revealed differential N2O dynamics in the two lake basins, with N2O concentrations between 12 nmol L−1 and > 900 nmol L−1 in the holomictic South Basin, and significantly lower concentrations in the meromictic North Basin (<13 nmol L−1). In the South Basin, the isotope signatures reflected a complex combination of N2O production by nitrifying bacteria through hydroxylamine (NH2OH) oxidation, N2O production through incomplete denitrification, and N2O reduction to N2, all occurring in close vicinity within the redox transition zone (RTZ). In the North Basin, in contrast, the N2O isotopomer signatures suggested that nitrifier denitrification was the main N2O source. The pronounced decrease in N2O concentrations to undetectable levels within the RTZ, in tandem with an increase in δ15N‐N2O, δ18O‐N2O, and SP indicated quantitative N2O consumption by microbial denitrification. In the northern basin this was primarily sulfide‐dependent. The apparent N and O isotope enrichment factors associated with net N2O consumption were 15ε ≈ 3.2‰ and 18ε ≈ 8.6‰, respectively. The according 18O to 15N enrichment ratio (18ε: 15ε ≈ 2.5) is consistent with previous reports for microbial N2O reduction, underscoring its robust nature across environments.

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“…A possible explanation for this phenomenon is that benthic NO { 3 reduction during these months was incomplete, and that significant portions of the reduced N accumulated as N 2 O. However, benthic N 2 O fluxes ranging between 0.4 and 1.2 mmol N m 22 h 21 (Freymond et al 2013) did not account for more than 15% of total benthic NO { 3 reduction, even during periods of maximum N 2 O production (5 mmol N m 22 h 21 in October 2010). Another potential pathway for benthic NO { 3 reduction, which could explain the imbalance between observed NO { 3 fluxes and N 2 production through denitrification, is its reduction to NO { 2 and subsequent consumption by anammox.…”

Section: Discussionmentioning

confidence: 99%

“…Continuous-flow sediment core incubations-Within 6 h after sampling, the continuous-flow sediment core incubations were set up in a cold room at near in situ temperature (6.5uC), as described previously (McCarthy and Gardner 2003;Freymond et al 2013). Briefly, gas-tight plungers containing two holes were positioned , 10 cm above the sediment surface to yield , 255 mL of overlying water volume.…”

Section: Methodsmentioning

confidence: 99%

Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

Wenk

Zopfi

Gardner

et al. 2014

Limnology & Oceanography

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We evaluated the seasonal variation of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the sediments and the integrative N (and O) isotopic signatures of dissolved inorganic nitrogen (DIN) compounds in the overlying water column of the monomictic Lake Lugano south basin. Denitrification was the dominant NO { 3 reduction pathway, whereas the contribution of anammox and DNRA to total benthic NO { 3 reduction was , 6% and , 12%, respectively. Sedimentary denitrification rates were highest (up to 57.2 6 16.8 mmol N m 22 h 21 ) during fully oxic bottom water conditions. With the formation of seasonal bottom water anoxia, NO { 3 reduction was partitioned between water column and sedimentary processes. Total benthic NO { 3 reduction rates determined in 15 N-label experiments and sediment-water interface N 2 fluxes as calculated from water column N 2 : Ar gradients revealed that sedimentary denitrification still accounted for , 40% of total N 2 production during bottom water anoxia. The partitioning between water column and sedimentary denitrification was further evaluated by the natural abundance stable N isotope composition of dissolved NO from approximately 7% to 20% and from 2% to 14%, respectively. Using a closed-system (Rayleigh) model, the N and O isotope effects associated with community NO { 3 consumption were 15 e < 13.7% and 18 e < 11.3%, respectively. With the assumptions of a relatively low net N isotope effect associated with sedimentary denitrification (i.e., 15 e sed 5 1.5-3%) vs. a fully expressed biological N isotope fractionation during water column denitrification (i.e., 15 e water 5 20-25%), our results confirm that 36-51% of NO { 3 reduction occurred within the sediment. The general agreement between the indirect (isotopic) approach and the flux and rate measurements suggests that water column nitrate isotope measurements can be used to distinguish between benthic and pelagic denitrification quantitatively.

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Year-round N<sub>2</sub>O production by benthic NO<sub>x</sub> reduction in a monomictic south-alpine lake

Cited by 18 publications

References 45 publications

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

Differential N₂ O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions

Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

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Year-round N&lt;sub&gt;2&lt;/sub&gt;O production by benthic NO&lt;sub&gt;x&lt;/sub&gt; reduction in a monomictic south-alpine lake

Cited by 18 publications

References 45 publications

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

Differential N2 O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions

Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

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Year-round N<sub>2</sub>O production by benthic NO<sub>x</sub> reduction in a monomictic south-alpine lake

Differential N₂ O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions