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| Titel |
Does nitrogen deposition affect the turnover of plant- and microbially-derived organic matter compounds differently? |
| VerfasserIn |
Marco Griepentrog, Alexander Heim, Frank Hagedorn, Rienk H. Smittenberg, Michael W. I. Schmidt |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250045563
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| Zusammenfassung |
| The IPCC report 2007 assesses soil nitrogen availability as a key factor in predicting future
carbon sequestration by terrestrial ecosystems. However, various studies reported
contrasting N effects on SOM dynamics, but the reasons for this are not well understood.
One potential explanation is that decomposition processes of individual organic
molecules respond differently to N, depending on their internal N content. According to
this hypothesis, decomposition of compounds with high N content, like microbial
amino sugars, would not respond to N additions, in contrast to compounds with low
N content like plant-derived lignin. A significant shift in the ratio between these
biomarkers would be indicative of major changes in the C fluxes within the soil system.
Localization of isotopically labeled biomarkers in soil density fractions will identify
the soil compartments where important changes occur. This information may also
be used to test current hypotheses on organic matter stabilization mechanisms in
soil.
We use soil samples from a four year CO2 enrichment and N deposition experiment with
model forest ecosystems (open-top chambers with beech / spruce) on two soil types (acidic
loam / calcareous sand). The ecosystems have been fumigated with two levels of 13C-labeled
CO2 (370 / 570 μmol mol-1) and treated with two levels of 15N-labeled fertilizer (0.7
g / 7 g NH4NO3-N m-2 yr-1). We will apply compound-specific stable isotope
analysis to trace added CO2 into plant- and microbially-derived molecular markers
(lignin / amino sugars) within soil density fractions (free / occluded light and heavy
fraction).
In a methodological evaluation we first tested which density cut-off and ultrasonic
dispersion energy yielded the maximum organic matter in the light fraction. Results indicate
that a density cut-off at 1.6 g cm-3 optimally separates light from heavy fractions. At higher
density C concentrations decrease while mass increases, suggesting incorporation
of mineral material into the light fractions. An ultrasonic energy input of 250 J
ml-1 seems to be adequate to release light fractions occluded within soil aggregates
since no changes in C concentration and mass recovery occur at higher dispersion
energy.
Additionally we did a literature survey of 28 studies using combinations of
density fractionation and ultrasonic dispersion to separate occluded fractions. A
wide range of dispersion energies (22 – 1700 J ml-1) with a median of 240 J ml-1
and a mean of 299 ± 40 J ml-1 were used. Interestingly only 15 studies reported
calibration of the ultrasonic equipment used and almost none of them discussed
why they used specific densities or ultrasonic energy for separation of occluded
material.
Besides density fractionation results, we will present first data on molecular markers
together with initial interpretations. |
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