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| Titel |
Measurement of dinitrogen fixation by Biological soil crust (BSC) from the Sahelian zone: an isotopic method. |
| VerfasserIn |
F. Ehrhardt, G. Alavoine, I. Bertrand |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067549
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| Zusammenfassung |
| Amongst the described ecological roles of Biological Soil Crust, N fixation is of importance
for soil fertility, especially in arid and semi-arid ecosystems with low inputs. In BSC, the
quantification of N fixation fluxes using an indirect method is widespread, usually with the
Acetylene Reduction Assay (ARA) which consists in measuring the nitrogenase activity
through the process of acetylene reduction into ethylene. A converting factor, still discussed
in the literature and greatly depending of the constitutive organisms of the BSC,
is the tool used to convert the amount of reduced ethylene into quantitative fixed
Nitrogen. The aim of this poster is to describe an isotopic direct method to quantify the
atmospheric dinitrogen fixation fluxes in BSC, while minimizing the variability due to
manipulations.
Nine different BSC from the Sahelian zone were selected and placed in an incubation
room at 28Ë C in dark and light conditions during three days, while moisture equivalent to
pF=2 was regularly adjusted using the gravimetric method with needles and deionized water,
in order to activate and reach a dynamic stability of their metabolisms. Subsequently, each
crust was placed into a gas-tight glass vial for incubation with a reconstituted 15N2 enriched
atmosphere (31.61 % atom 15N, while the proportion of each main gas present in the air was
conserved, i.e. 78% N2, 21% O2 and 0.04% CO2). Principal difficulties are to guarantee the
airtighness of the system, to avoid crust desiccation and to keep the crust metabolically active
under stable conditions for six hours. Several tests were performed to determine the optimum
time for 15N2 incubation. Three replicated control samples per crust were also stabilized for
three days and then dried at 105Ë C, without any incubation with 15N2 enriched
atmosphere.
Total N and 15N were then measured in the grounded (80μm) and dried (105Ë C) crust,
using a Flash EA elemental analyzer (Eurovector, Milan, Italy) coupled to a DeltaPlus
Advantage mass spectrometer (Finnigan Thermo Fisher Scientific, Bremen, Germany).
N2fixation fluxes were calculated from the difference between the amount of 15N in
incubated and in control samples. Mean values ranged from 1.32.10-3 ± 1.02.10-4 to
8.47.10-2 ± 2.63.10-3 mgN.m-2.h-1. Concerning the variability, differences observed
between crusts and between replicates are probably related to the characteristic of each crust
as well as to field sampling which integrates the important heterogeneity and sensitivity of the
material. |
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