25th International Meeting on Organic Geochemistry IMOG 2011

P-243
Organic matter composition and degradation in Siberian
permafrost soils
Silke Höfle 1 , Pascal Boeckx 2 , Dries Roobroeck 2 , Gesine Mollenhauer 3 , Janet
Rethemeyer 1
1 Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany, 2 Faculty of Bioscience
Engineering, Ghent University, Ghent, Belgium, 3 Alfred-Wegner Institute for Polar and Marine Research,
Bremerhaven, Germany (corresponding author:silke.hoefle@uni-koeln.de)
Enhanced microbial activity in thawing permafrost soil
is assumed to increase decomposition and carbon
emissions to the atmosphere [1]. Microorganisms are
the main drivers of the decomposition of soil organic
matter (SOM). It remains difficult to estimate changes
in organic carbon stocks for future warming scenarios
because of the heterogeneous mixture of organic
components and spatial variability in permafrost
landscapes. Thus, it is essential to better characterize
SOM composition and identify temperature sensitive
carbon pools which are preferentially degraded.
We analyzed depth intervals in the active layer
thawing during summer of two characteristic
cryogenic structures (rim and centre) of the polygonal
tundra in the Lena Delta, Siberian Arctic. SOM
composition was characterized by elemental and lipid
analysis including n-alkanes and n-fatty acid.
Microbial communities were studied by phospholipid
fatty acid (PLFA) analysis indicative of viable
microorganisms and the degradation of labile/'young'
and stable/'old' SOM will be identified by 14 C analysis
of individual PLFAs.
The very different SOM quality in the polygon rim and
centre profiles is reflected by bulk soil parameters and
lipid distributions. Total organic carbon (TOC)
contents decrease from 2.8 to 0.8 wt-% in the centre,
while an increase with depth (1.6 to 3.3 wt-%TOC) is
observed in the rim. The polygon centre consists of
peaty, mostly undecomposed plant material
dominated by terrestrial plant lipids with C22 and C24
fatty acid and C27 and C29 n-alkanes being the most
dominant compounds. High C/N ratios (35-51) and
CPI values for both lipid fractions are further
indicators for slow decomposition and low microbial
activities, respectively in the water saturated active
layer during the summer season. Likewise PLFA
concentration decline strongly with depth (Fig.1)
indicating that (aerobic) bacterial activities are mainly
taking place in the uppermost layer. Declining 14 C
contents of bulk SOM (113 to 95 pMC) are assumed
to reflect mainly the 14 C signature of plant and root
biomass.
The polygon rim contains organic rich mineral soil
with fewer recognizable plant remains. The stronger
decomposition of SOM is displayed by lower C/N
ratios (16-20) and CPI values for n-fatty acids and nalkanes
throughout the profile. Similar long-chain
plant lipids are observed as in the polygon centre.
The abundances of the n-alkanes decrease with
depth. However, long-chain fatty acid concentrations
show a considerable increase with depth and yield
higher concentrations near the permafrost table and
even in the uppermost frozen soil layer than in the
surface soil suggesting better preservation or in-situ
production of these compounds. Regarding the strong
decrease in 14 C concentration (90 to 68 pMC) parallel
to increasing TOC contents in the rim profile, we
assume that both factors may be important here. High
abundances of gram- and gram+ bacterial PLFAs
within the active layer give indication for a large
microbial community. 14 C data of individual PLFAs –
still in process – will indicate if production or
preservation is more important and if the 'old' SOM in
the permafrost is microbial metabolized.
Fig. 1: PLFA concentrations for gram+ (i14:0, i/a15:0, a16:0,
i/a17:0), gram- (16:1‘9c, 16:1‘11c, 18:1‘11c), fungal (18:1‘9c)
biomarkers and 14 C values of the polygon rim and centre
References:
[1] Khvorostanov D V et al., 2008, Tellus, 60B, 265-275
379