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| Titel |
Increased nitrogen deposition did not affect the composition and turnover of plant and microbial biomarkers in forest soil density fractions |
| VerfasserIn |
Marco Griepentrog, Samuel Bodé, Pascal Boeckx, Frank Hagedorn, Guido L. B. Wiesenberg, Michael W. I. Schmidt |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072829
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| Zusammenfassung |
| Increased atmospheric nitrogen (N) deposition and elevated CO2 concentrations affect many
forests and their ecosystem functions, including organic matter cycling in soils, the largest
carbon pool of terrestrial ecosystems. However, it is still not clear how, and what
the underlying mechanisms are. Specific molecules of plant and microbial origin
(biomarkers) might respond differently to N deposition, depending on their internal N
content. Microbial cell-wall-constituents with high-N content like amino sugars are
reliable biomarkers to distinguish between fungal- and bacterial-derived organic
residues. Individual lipids are plant-specific biomarkers that lack N in their molecular
structure. Here, we tested the effects of elevated CO2 and increased N deposition on the
dynamics of plant and microbial biomarkers by studying their composition and turnover
in forest soil density fractions. Furthermore, we tested the hypothesis that these
biomarkers respond differently to increased N deposition, depending on their internal N
content.
We used soil samples from a 4-year elevated CO2 and N deposition experiment in model
forest ecosystems (open-top chambers), that were fumigated with ambient and
13C-depleted CO2 and treated with two levels of 15N-labeled fertilizer. Bulk soil
was separated into free light fraction, occluded light fraction and heavy fraction
by density fractionation and ultrasonic dispersion. The heavy fraction was further
particle-size fractionated with 20 μm as a cut-off. We determined carbon and N
concentrations and their isotopic compositions (δ13C, δ15N) within bulk soil and
density fractions. Therein, we extracted and quantified individual amino sugars and
lipids and conducted compound-specific stable-isotope-analysis using GC- and
LC-IRMS.
Results show that amino sugars were mainly stabilized in association with soil minerals.
Especially bacterial amino sugars were preferentially associated with soil minerals,
exemplified by a consistent decrease from fungal- to bacterial-derived amino sugars from
light (plant-like) to heavy (mineral) soil fractions. Other than expected, elevated
CO2 and increased nitrogen deposition did not affect the distribution of amino
sugars within and between soil fractions. One explanation could be that the four
years of the experiment were too short to reach a new equilibrium of fungi and
bacteria.
For the first time we were able to determine isotope ratios of individual amino
sugars in soil density fractions from a natural abundance field experiment. Our
results show that, in the presence of soil minerals amino sugars turn over slower
than in light, physically unprotected fractions. Surprisingly, fungal amino sugars
turn over at the same rate than total organic carbon, while bacterial amino sugars
turn over slower. Furthermore, nitrogen deposition did not affect the turnover of
individual amino sugars in soil density fractions, indicating that microbial community
distribution was not affected after four years of increased nitrogen deposition. This is
in contradiction to the often observed reduction of fungal biomass after nitrogen
additions.
Data from the lipid analysis (plant biomarkers) are still under investigation and will be
presented in conjunction with the results for amino sugars (microbial biomarkers). |
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