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| Titel |
Non-homogeneity of isotopic labelling in 15N gas flux studies: theory, some observations and possible lessons |
| VerfasserIn |
Reinhard Well, Caroline Buchen, Marianna Deppe, Wolfram Eschenbach, Andreas Gattinger, Anette Giesemann, Hans-Martin Krause, Dominika Lewicka-Szczebak |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111507
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| Publikation (Nr.) |
 EGU/EGU2015-11636.pdf |
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| Zusammenfassung |
| Quantifying dinitrogen (N2) and nitrous oxide (N2O) fluxes from different soil N
pools and processes can be accomplished using the 15N tracer technique but this
is subject to four different sources of bias (i. – iv.). This approach includes 15N
labelling of selected N pools in soil and subsequent isotope analysis of all relevant N
pools as well as of gas samples from enclosures, i.e. mixtures of soil-derived and
atmospheric N2 and N2O. Depending on the processes of interest, there may be 15N
labelling of one or several N pools, were several labelling treatment are needed in the
latter case (e.g. Müller et al., 2004). Measuring pool-derived N2 or N2O has been
shown to include two calculation problems, (i.) arising from multiple pools (e.g.
Arah, 1992) and (ii.) dealing with the non-random distribution of N2 and N2O mole
masses (Hauck et al., 1958). Non-randomness can be solved if m/z 28, 29 and 30 are
correctly analysed and the 15N enrichment of one (to distinguish two pools, i.e. soil
and atmosphere) or two pools (in case of three pools) is known (Spott & Stange,
2008).
Moreover (iii.), NO3- pools generating N2 and N2O via denitrification can be identical or
different, e.g. if N2O evolved from higher enriched NO3- in deeper soil was more reduced to
N2 compared to N2O evolved from N2O from shallow soil with lower enrichment, or vice
versa.
Apportioning N2O fluxes to NH4+ (nitrification and/or nitrifier denitrification) and NO3-
(denitrification) is often conducted by NO3-labeling, measuring δ15N of emitted N2O and
applying mixing equations were the measured 15N enrichment of NH4+and NO3-pool is
used. However, this assumes that the average 15N enrichment of NH4+and NO3-in the soil is
identical to the enrichment in the active soil domain producing N2 and/or N2O. Violation of
this precondition must lead to bias in source apportionment (iv.), but to our knowledge this
has not been investigated until now.
Here we present conceptual models and model calculations addressing cases iii. and iv..
Furthermore we present some experimental data illustrating this. These include two data sets
from denitrification experiments exhibiting substantial deviations in 15N enrichment between
the N pools producing N2 and N2O. Moreover, results from a lab incubation study to quantify
NH4+-derived N2O with increasing NH4+ amendment under conditions favouring
nitrification are shown, were non-labelled NH4+ was added together with 15N labelled
NO3-. Here we found large deviations between the 15N enrichment of NO3- in
extracted soil water and the 15N enrichment of the labelled N pool as calculated from
N2O isotopologues (Bergsma et al., 2001). We think that this reflects type iv. bias,
probably because enrichment of NO3- in anoxic micro-sites was less diluted by
non-labelled NO3- from nitrification compared to NO3- in oxic zones. Our data
analysis provides a means to overcome bias iv. and thus to obtain correct source
apportionment.
References:
Arah, J.R.M. (1992): Soil Sci. Soc. Am. J. 56, 795 – 800, 1992.
Bergsma, T. et al. (2001): Env. Sci. & Technol. 35(21): 4307-4312.
Hauck, R.D., et al.(1958): Soil Science 86, 287 – 291, 1958.
Lewicka-Szczebak, D. et al.(2013): Rapid Comm. Mass Spectrom., 27 1548-1558.
Müller, C. et al. (2004): Soil Biol. Biochem. 36(4): 619-632.
Mulvaney, R.L.(1984):. Soil Sci. Soc. Am. J. 48:690 - 692.
Spott, O, et al.. (2006): Rapid Comm. Mass Spectrom., 20: 3267-3274.
Spott, O. and C. F. Stange (2007): Rapid Comm. Mass Spectrom., 21: 2398-2406. |
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