SUMMARY: ATLAS Winter C-Flux Soils Findings
Investigators C.L Ping1, G.J. Michaelson1, X.Y. Dai1 and J.M. Kimble2
1University of Alaska Fairbanks, School of Natural Resources and Agricultural Sciences
2USDA-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 warm-season. 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.
Literature Cited
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Dai, X.Y., C.L. Ping, R. Candler, L. Haumaier, and W. Zech. 2001a. Characterization of soil organic matter fractions of tundra soil in Arctic Alaska by carbon-13 nuclear magnetic resonance spectroscopy. Soil Sci. Soc. of Am. J. 65:87-93.
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Michaelson, G.J., C.L. Ping and J.M. Kimble. 2001. Effects of soil morphological and physical properties on estimation of carbon storage. Chap. 23. p.339-347. In R. Lal, J.M. Kimble, R.F. Follett, and B.A. Stewart (eds.) Assessment Methods for Soil Carbon. Lewis Publishers, Boca Raton, FL.
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Ping, C.L., G.J. Michaelson, X.Y. Dai, and R.J. Candler. 2001. Characterization of Soil Organic Matter. p.273-283. In R. Lal, J.M. Kimble, R.F. Follett and B. Stewart (eds.) Assessment Methods for Soil Carbon. Lewis Publishers, Boca Raton, FL.
Ping C.L., G.J. Michaelson, X.Y. Dai, L. Everett, J.M. Kimble and D.A. Walker. 2002. Characterization of Soils Associated with ATLAS Sites in Western Alaska, Manuscript in contribution to Arct. Antarct. Res.