Research Highlights
Earth and
Environmental Science
02
PRINCIPAL CONTACT:
Adam W. Gillespie
Research Associate – Soil Chemistry
Canadian Light Source Inc.
adam.gillespie@lightsource.ca
360-657-3651
Journal/Principal
publication:
Soil Biology and Biochemistry,
2014. 68:106-116
DOI: 10.1016/j.soilbio.2013.09.021
Perennially and annually frozen soil
carbon differ in their susceptibility
to decomposition: analysis of
Subarctic earth hummocks by
bioassay, XANES and pyrolysis
Introduction
Authors:
Gillespie, A.W.
Sanei, H.2
Diochon, A.3
Ellert, B.H.4
Regier, T.Z.5
Chevrier, D.5
Dynes, J.J.5
Tarnocai, C.1
Gregorich, E.G.1
1,5
1. Agriculture and Agri-Food Canada,
Science and Technology Branch,
Ottawa, ON
2. Geological Survey of Canada,
Calgary, AB
3. Department of Geology, Lakehead
University,Thunder Bay, ON
4. Agriculture and Agri-Food Canada,
Science and Technology Branch,
Lethbridge, AB
5. Canadian Light Source Inc.,
Saskatoon, SK
Compared to global averages, Polar Regions are
expected to experience disproportionately higher
and earlier surface temperature changes under
current global change predictions. These regions
also have been identified as especially vulnerable to
disturbance, largely because temperature constraints
are the drivers for high carbon (C) accumulation
[1]. It is estimated that circumpolar regions contain
over 50% of the total terrestrial C stocks, yet
comprise only ca. 15% of global terrestrial surface
area [2]. Recent evidence suggests that the C pools
in permafrost soils are particularly biodegradable
and therefore have the potential to release significant
quantities of CO2 to the atmosphere with increases
in temperature [3]. This combination of large
C stocks, high substrate biodegradability and a
disproportionate temperature increase suggests a
scenario where polar regions, which currently are
considered a sink for atmospheric C, may become
a C-source by contributing to a positive feedback
cycle of enhanced respiration and accelerated CO2
production [1].
Our objectives in this study were to characterize and
develop relationships between the chemistry and
bioavailability of SOC in the horizons of Subarctic
earth hummocks. Biodegradability was assessed in
a controlled mineralization study in the laboratory.
The chemical composition of soil organic matter
(SOM) was characterized by X-ray absorption nearedge structure (XANES) spectroscopy at the carbon
(C) K-edge and its thermal stability was determined
by Rock-Eval pyrolysis.
Science
The study site was located in a high Subarctic
forest near Inuvik, Northwest Territories, Canada,
at 68.17° N, 133.31° W. Soils are classified as
Turbic Cryosols, which are permafrost soils
marked by evidence of cryoturbation, a process
of soil movement by freeze-thaw action [4].
Earth hummocks were excavated to the depth of
the permafrost using a shovel so that the vertical
face exposed one complete hummock (example
hummock Figure 1). Soils below the permafrost
layer were obtained using a gas-powered corer. The
soil profiles were sampled in bulk from genetic soil
horizons.
The biologically available C was assessed by
measuring the quantity of CO2 released by
incubation at 25°C for 98 d. Analytical pyrolysis was
conducted using the Rock-Eval® 6 system, where soil
samples were subjected to a temperature heating
program under an N2 atmosphere.
For XANES analysis, subsamples of the fresh soil
were slurried in water, deposited onto Au-coated
Si wafers and air-dried at room temperature.
The wafers were then affixed to sample holders
using double-sided carbon tape for insertion into
the X-ray absorption vacuum chamber. XANES
spectra were collected using the Spherical Grating
Monochromator (SGM) beamline 11ID-1 at the
Canadian Light Source. Spectra were acquired using
the slew scanning mode of the SGM beamline
which continuously scans the energy of the
monochromator while acquiring data in order to
minimize X-ray exposure. Each slew scan took 20
seconds and multiple unique spots were scanned on
each sample. The beamline exit slit was set at 25 µm
and fluorescence yield data was collected using a two
stage microchannel plate detector.
Data were averaged from a minimum of 10 scans
from each sample, and background corrected
by a linear regression fit through the pre-edge
region followed by normalization to an edge step
of one. X-ray absorption data were compared by
deconvoluting the normalized spectra and using
identified resonances corresponding to C types [5,
6]. Ratios of peak areas were calculated to assess the
relative abundances between different compound
classes. In this study, ratios between phenol:ketone,
carboxylic acid:ketone, and carbohydrate:ketone
areas were retained, where increases in these
2013 research Report
[77]
ratios ratios were interpreted as evidence
for increases in the extent of microbial
decomposition [7].
Discussion
The mineralization bioassay showed
that buried organic horizons were
less susceptible than surface SOM to
biodegradation, and not significantly more
susceptible than the adjacent mineral soil
(Table 1). Analysis by XANES showed the
accumulation of ketones in buried organic
horizons, and the loss of carbohydrate,
phenolic and carboxylic compounds (Figure
2). This suggests that ketones can be used
as biomarkers for microbially transformed
SOM. This is consistent with other research
suggesting that SOC in the permafrost was
likely exposed to decay in the active layer
prior to incorporation into permanently
frozen soil [3]. Phenolic signals likely
originate from lignin and therefore
represent plant-derived organic matter.
Carbohydrates and carboxylic acids are
also found in plant and microbial biomass,
but are lost as mineralization proceeds [8].
Ketones are metabolic products of microbial
metabolism of fatty acids and aromatic
compounds and have been detected in
XANES studies of bacterial biomass [9].
Ketones also have been identified as
products of microbially transformed
SOM [7]. The inverse relationship observed
between mineralization data and ketones
suggests that these compounds represent
SOM which has already been stripped of its
most bioavailable components.
In contrast, SOM in perennially frozen
mineral soils (i.e., below the permafrost
table), was more susceptible to
biodegradation than that in buried mineral
and organic soils in the annually frozen
active layer. The SOM in these horizons did
not show ketone signals but instead showed
strong phenolic content. Although phenols
are typically thought of as recalcitrant
compounds, there is growing consensus
that any C source can be decomposed and
mineralized if the ecological conditions
permit [10]. Analysis by pyrolysis indicated
that the thermolabile fraction was related
to the bioavailability of C, and that in
perennially frozen soils, this fraction
contained proportionately higher oxygencontaining functional groups. These results
point to a pool of labile SOC, relatively
rich in phenolic compounds, in perennially
frozen soils which may be susceptible to
decomposition in a warming climate. Future
warming-induced C losses, therefore, may
mostly occur not from annually-frozen
[78]
Canadian Light Source
Table 1. ANOVA of grouped soil horizons across four Subarctic pedons. Values followed by
the same letter are not significantly different at P≤0.05. OC, soil organic C; TN, total N; Cmin,
mineralizable C over 98 days. Total organic C, total N, C:N ratio and potential mineralization data
all were log-normally distributed (Anderson-Darling test), and were log-transformed prior to
analysis of variance. Table shows non-transformed values.
Figure 1. Illustrations and taxonomic descriptions of horizons in the experimental
cryoturbated pedons located near Inuvik, NWT, Canada. Horizon classification is in
accordance with the Canadian System of Soil Classification. Horizons with ‘y’ suffix are
affected by cryoturbation and those with ‘z’ suffix are permanently frozen.
Research Highlights
SOM buried by cryoturbation, but from
perennially-frozen C made accessible by the
falling permafrost table.
Conclusion
Overall, the chemical composition of
SOM in these earth hummocks shows
distinct patterns and characteristics with
location in the pedon. The results did not
support our initial hypothesis that buried
organic matter would be more susceptible
to biodegradation. The organic C-rich
horizons that have been subducted and
have accumulated along the permafrost
table through cryoturbation show the
same C mineralization potentials as
mineral soils within the same pedon.
Our hypothesis that the thermolabile
fraction would relate to the mineralization
potential was supported by the results. We
observed a strong relationship between
the chemical composition of SOM and its
C mineralization potential, whereby the
ratios of carboxylic acids:-, carbohydrates:or phenols:ketones were correlated with
decreasing C mineralization potential. This
finding suggests that ketones are biomarkers
of SOM which has been previously
undergone some biodegradation and has
been stripped of its most bioavailable
components. We originally hypothesized
that carbohydrates would increase and
lignins would decrease with increased
C mineralization potentials. Instead,
evidence from XANES coupled with the
pyrolysis data suggests that in perennially
frozen soils a labile pool of SOM exists
and comprises phenolic compounds, but
not carbohydrates. Together, these results
show that annually frozen, buried C-rich
organic horizons are not as biologically
labile as surface organic matter. Conversely,
perennially frozen C below the permafrost
table was more labile than C in the
active layer and may be susceptible to
decomposition if the permafrost table falls.
References
[1] Davidson, E.A., et al., Nature, 2006. 440:165-173.
[2] Tarnocai, C., et al., Global Biogeochem. Cycles, 2009. 23.
[3] Kuhry, P., et al., Permafr. Periglac. Process., 2010. 21:208214.
[4] Tarnocai, C., et al., Can. J. Soil Sci., 2011. 91:749-762.
[5] Cooney, R.R., et al., J. Phys. Chem. B, 2004. 108:1818518191.
[6] Urquhart, S.G., et al., J. Phys. Chem. B, 2002. 106:85318538.
[7] Gillespie, A.W., et al., Biogeochem., 2013:1-14.
[8] Sollins, P., et al., Geoderma, 1996. 74:65-105.
[9] Hitchcock, A.P., et al., Geobiology, 2009. 7:432-453.
[10] Kleber, M., et al., Glob. Change Biol., 2011. 17:10971107.
Figure 2. Representative deconvolutions of C K-edge XANES spectra from selected horizons, highlighting the
ketone (Ket, 286.5 eV), phenol (Phe, 287.3 eV), carboxylic acid (Cbx, 288.5 eV) and carbohydrate (Cbh, 289.5 eV)
features.
Acknowledgements
Research described in this paper was performed at
the Canadian Light Source, which is supported by the
Natural Sciences and Engineering Research Council
of Canada, the National Research Council Canada, the
Canadian Institutes of Health Research, the Province
of Saskatchewan, Western Economic Diversification
Canada, and the University of Saskatchewan. AWG, AD,
BHE, CT and EGG acknowledge financial support for
this research through Agriculture & Agri-Food Canada’s Science
and Technology Branch and the SAGES program. We thank S. Wu
and A. Spasojevic for their skilled help in the laboratory analyses
and to R. Stewart and R. Robinson of the Geological Survey of
Canada for Rock-Eval analysis.
Beamline information
SGM, XANES, 270-320 eV.
2013 research Report
[79]