SUMMARY: ATLAS Winter C-Flux Soils Findings
Investigators C.L Ping1, G.J. Michaelson1, X.Y. Dai1 and J.M. Kimble2
1
2
University of Alaska Fairbanks, School of Natural Resources and Agricultural Sciences
USDA-NRCS National Soil Survey Laboratory, Lincoln, NE
Introduction
The soils research activities of the NSF-ARCSS
ATLAS Winter-Flux Study have provided a significantly better
understanding of arctic soils and their organic carbon
composition. Field study sites were along the Barrow-Council
(western) and Dalton Highway (eastern) transects (Figure 1).
Soils studies were undertaken in order to advance our
understanding of arctic soils with regard to the quantities of
soil organic carbon (SOC) they contain, and the relationship of
SOC to CO2 respiration from the soil to the atmosphere. The
following is a summary of some of the study findings.
Soil OC Stocks
The Arctic encompasses a large land area that holds a significant portion (up to 26%) of
terrestrial carbon in its soils. Many factors contribute to the large stores and distribution
patterns of SOC in arctic soils. Factors such as cool-moist conditions and landscape processes
contribute significantly to increase SOC stocks. Cryoturbation for example, results in mixing
of soil materials including SOC within the soil profile. Thaw lake cycles and gelifluction due
to slope movement, result in mixing and burial of surface SOC. Findings about soil organic
matter from both of the ATLAS and the Flux project have provided a better assessment of
quantities of SOC present in arctic soils, and an understanding of the distribution patterns with
depth, both issues key to determining the impact of changing climate conditions on the arctic
system.
Major Findings:
Arctic soils contain more SOC than
previously thought: Analysis of ATLAS
soils data have supported the Flux study
findings that SOC stocks in arctic soils are
on average about twice as high as
literature values previously reported. SOC
stocks in soils of the Arctic Coastal Plain
were found to average 62 kg SOC m-2
while stocks of the Arctic Foothills
average about 44 kg SOC m-2 each to a 1m
depth (Figure 2).
Surface layer SOC is inadequate to
explain SOC stocks: Analysis of ATLAS
soils data also shows that the SOC of
surface organic layers is correlated to
pedon SOC stocks (r=0.70) however this
only explained 50% of the variation found
in stocks overall (Figure 3).
Subsurface soils including upper
permafrost contain significant SOC
stocks:
A detailed study of SOC distribution to 1 m depth revealed that on the average soil stocks are
evenly split between the active-layer and the upper permafrost and that active-layer stocks
are evenly divided between the surface organic and subsurface mineral soil horizons (Figure
4).
 Contribution of SOC stocks from different soil horizons: The Oa-horizon, a component
of the surface organic-layer, is the major contributor to SOC stocks for most arctic soils.
However, the upper permafrost Cf horizons also contain large portions of SOC stocks for both
soils of the coastal plain and foothills regions. Surface organic horizons are cryoturbated into
the underlying mineral horizons and these mixed horizons hold large portions of SOC stocks.
This is especially true for the Arctic Foothills where cyoturbation is strong and the active layer
is deep (Figure 4).
SOC and Respiration of CO2
Little is known about the quality of SOC in arctic soils or how its quality relates to
decomposition under changing or static conditions. The SOC in soil organic matter was
characterized for soils at each Flux study site. Selected soils were studied further to evaluate
analytical methods in characterizing bioactivity of SOC and to determine the relative
bioactivity of SOC in various soils. ATLAS study efforts were focused on SOC and
wintertime bioactivity or CO2 production. Water-soluble portions of SOC are presumed to be
more available under winter conditions. Bioactivities or production of CO2 at low temperatures
were compared with soil stocks of this wintertime reactive water-soluble SOC (wsSOC).
Major Findings:
SOC of arctic soils can be effectively assessed with various techniques: Instrumental
methods (CPMAS 13C NMR, and Pyrolysis GC-MS) show some promise for accessing
quality of SOM from arctic soils. The CPMAS 13C NMR used in conjunction with chemical
fractionation techniques (Ping et al., 2001 and Dai et al., 2002) was able to elucidate
chemical functionality differences for SOC fractions with some fractions correlating to
bioactivity (CO2 evolution, Figure 5, left) in laboratory incubations (Dai et al., 2001, 2001a).
Bioactive fractions of SOC were isolated: The most highly bioactive SOC fractions were
the lower molecular-weight hydrophilic acids and neutrals fractions in extractable SOM (Dai
et al., 2000) and in soil waters (Michaelson et al., 1998). These highly bioactive SOM
fractions may account for up to 70% of CO2 production in soils at warm-season temperatures
and respiration of CO2 from soil water correlates strongly to the amount of these fractions
present (Figure 5, right).
Cryoturbated SOC is of increased importance to bioactivity as temperature drops: As
temperature drops below freezing in the soil, respiration of CO2 from surface organic
horizons slows more than that of the subsurface mineral horizons. Cryoturbated SOC where
present in arctic soils could serve as a primary substrate for winter respiration of CO2
(Figure 6).
Water-Soluble SOC is important to
winter soil respiration: Stocks of
water-soluble SOC (wsSOC) in soils
rather than total SOC are well correlated
to respiration of CO2 at sub-zero
temperature (Figure 7). Total SOC
stocks do not correlate to wsSOC stocks
across the wide range of soils and thus
total SOC may not be useful in
predicting cold-season processes in the
same manner it is used for the warmseason. Stocks of wsSOC may be
useful to predict or model cold-season soil
respiratory CO2 flux (Michaelson and
Ping, 2003).
Conclusions
Soil morphological, physical and chemical data are now for the first time, available for sites
across arctic Alaska. These data are being used with integrated C-Flux and ATLAS research
projects examining arctic terrestrial systems. This soils research while designed to provide
essential support and data to the C-flux and ATLAS integrated study groups, has also been key
in the field-testing of the newly instituted Gelisol order in Soil Taxonomy, this new order of
soils in the US soil classification system provides recognition for permafrost-affected soils. It
has also provided data for field validations and reference, and supported the development of
the N. American Soil Carbon Map. The soils data also contributed to the site characterization
for other research projects, including the Ameriflux project, the NSF-CALM, the USDA
Global Change Initiative and soil parameters for climate change models.
The detailed and specific studies of SOC stocks under the ARCSS-LAII program have
indicated that arctic soils likely contain twice as much of the terrestrial C pool as previously
reported. This newly accounted for SOC is of significance not only in magnitude, but also in
its quality as it relates to the Arctic and Global C cycles under changing climate. Organic
matter characterization study indicates that soil active-layers contain relatively large amount of
their C in fractions that are in intermediate states of decomposition and are susceptible to
further decomposition under warmer temperatures and changing moisture levels. Large
amounts of SOC stocks are found in both the active-layer and upper permafrost due to
cryoturbation. This portion of SOC is not highly decomposed, and thus is susceptible to
increased decomposition with warming winter and shoulder-season conditions such as those
that are now being observed in arctic Alaska.
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