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  • 1
    Language: English
    In: Frontiers in Environmental Science, 01 April 2018, Vol.6
    Description: Soil-borne nitrous oxide (N2O) emissions have a high spatial and temporal variability which is commonly attributed to the occurrence of hotspots and hot moments for microbial activity in aggregated soil. Yet there is only limited information about the biophysical processes that regulate the production and consumption of N2O on microscopic scales in undisturbed soil. In this study, we introduce an experimental framework relying on simplified porous media that circumvents some of the complexities occuring in natural soils while fully accounting for physical constraints believed to control microbial activity in general and denitrification in particular. We used this framework to explore the impact of aggregate size and external oxygen concentration on the kinetics of O2 consumption, as well as CO2 and N2O production. Model aggregates of different sizes (3.5 vs. 7 mm diameter) composed of porous, sintered glass were saturated with a defined growth medium containing roughly 109 cells ml−1 of the facultative anaerobic, nosZ-deficient denitrifier Agrobacterium tumefaciens with N2O as final denitrification product and incubated at five different oxygen levels (0–13 vol-%). We demonstrate that the onset of denitrification depends on the amount of external oxygen and the size of aggregates. Smaller aggregates were better supplied with oxygen due to a larger surface-to-volume ratio, which resulted in faster growth and an earlier onset of denitrification. In larger aggregates, the onset of denitrification was more gradual, but with comparably higher N2O production rates once the anoxic aggregate centers were fully developed. The normalized electron flow from the reduced carbon substrate to N-oxyanions (edenit-/etotal- ratio) could be solely described as a function of initial oxygen concentration in the headspace with a simple, hyperbolic model, for which the two empirical parameters changed with aggregate size in a consistent way. These findings confirm the important role of soil structure on N2O emissions from denitrification by shaping the spatial patterns of microbial activity and anoxia in aggregated soil. Our dataset may serve as a benchmark for constraining or validating spatially explicit, biophysical models of denitrification in aggregated soil.
    Keywords: Greenhouse Gas Emissions ; Denitrification Kinetics ; Microbial Hotspots ; Microsites ; Anoxic Aggregate Centers ; Agrobacterium Tumefaciens ; Environmental Sciences
    E-ISSN: 2296-665X
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  • 2
  • 3
    Language: English
    In: Biogeosciences Discussions, 03/08/2019, pp.1-31
    ISSN: Biogeosciences Discussions
    E-ISSN: 1810-6285
    Source: CrossRef
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  • 4
    Language: English
    In: Biogeosciences, Sept 27, 2019, Vol.16(18), p.3665
    Description: pSoil denitrification is the most important terrestrial process returning reactive nitrogen to the atmosphere, but remains poorly understood. In upland soils, denitrification occurs in hotspots of enhanced microbial activity, even under well-aerated conditions, and causes harmful emissions of nitric (NO) and nitrous oxide (N.sub.2 O). The timing and magnitude of such emissions are difficult to predict due to the delicate balance of oxygen (O.sub.2) consumption and diffusion in soil. To study how spatial distribution of hotspots affects O.sub.2 exchange and denitrification, we embedded microbial hotspots composed of porous glass beads saturated with growing cultures of either Agrobacterium tumefaciens (a denitrifier lacking N.sub.2 O reductase) or Paracoccus denitrificans (a "complete" denitrifier) in different architectures (random vs. layered) in sterile sand that was adjusted to different water saturations (30thinsp;%, 60thinsp;%, 90thinsp;%). Gas kinetics (O.sub.2, CO.sub.2, NO, N.sub.2 O and N.sub.2) were measured at high temporal resolution in batch mode. Air connectivity, air distance and air tortuosity were determined by X-ray tomography after the experiment. The hotspot architecture exerted strong control on microbial growth and timing of denitrification at low and intermediate saturations, because the separation distance between the microbial hotspots governed local oxygen supply. Electron flow diverted to denitrification in anoxic hotspot centers was low (2thinsp;%-7thinsp;%) but increased markedly (17thinsp;%-27thinsp;%) at high water saturation. X-ray analysis revealed that the air phase around most of the hotspots remained connected to the headspace even at 90thinsp;% saturation, suggesting that the threshold response of denitrification to soil moisture could be ascribed to increasing tortuosity of air-filled pores and the distance from the saturated hotspots to these air-filled pores. Our findings suggest that denitrification and its gaseous product stoichiometry depend not only on the amount of microbial hotspots in aerated soil, but also on their spatial distribution. We demonstrate that combining measurements of microbial activity with quantitative analysis of diffusion lengths using X-ray tomography provides unprecedented insights into physical constraints regulating soil microbial respiration in general and denitrification in particular. This paves the way to using observable soil structural attributes to predict denitrification and to parameterize models. Further experiments with natural soil structure, carbon substrates and microbial communities are required to devise and parametrize denitrification models explicit for microbial hotspots.
    Keywords: Nitrogen Oxides – Analysis ; Soil Microbiology – Analysis ; Denitrification – Analysis ; Soil Carbon – Analysis
    ISSN: 1726-4170
    E-ISSN: 17264189
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  • 5
    Language: English
    Description: Soil-borne nitrous oxide (N2O) emissions have a high spatial and temporal variability which is commonly attributed to the occurrence of hotspots and hot moments for microbial activity in aggregated soil. Yet there is only limited information about the biophysical processes that regulate the production and consumption of N2O on microscopic scales in undisturbed soil. In this study, we introduce an experimental framework relying on simplified porous media that circumvents some of the complexities occuring in natural soils while fully accounting for physical constraints believed to control microbial activity in general and denitrification in particular. We used this framework to explore the impact of aggregate size and external oxygen concentration on the kinetics of O2 consumption, as well as CO2 and N2O production. Model aggregates of different sizes (3.5 vs. 7 mm diameter) composed of porous, sintered glass were saturated with a defined growth medium containing roughly 109 cells ml-1 of the facultative anaerobic, nosZ-deficient denitrifier Agrobacterium tumefaciens with N2O as final denitrification product and incubated at five different oxygen levels (0-13 vol-%). We demonstrate that the onset of denitrification depends on the amount of external oxygen and the size of aggregates. Smaller aggregates were better supplied with oxygen due to...
    Keywords: Agrobacterium Tumefaciens ; Anoxic Aggregate Centers ; Denitrification Kinetics ; Greenhouse Gas Emissions ; Microbial Hotspots ; Microsites ; Physically-Based Modeling ; Dewey Decimal Classification::300 | Sozialwissenschaften, Soziologie, Anthropologie::330 | Wirtschaft::333 | Boden- Und Energiewirtschaft::333,7 | Natürliche Ressourcen, Energie Und Umwelt
    Source: DataCite
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