High stocks of soil organic carbon in North American ...

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John M. Kimble. 3. , Howard. Epstein ... Fireweed, Palmer Alaska 99645, USA,. 2 ... m. 2. ) and 3 of those five points are in the High Arctic at one location. Other.
High stocks of soil organic carbon in North American Arctic region

Chien-Lu Ping*1, Gary J. Michaelson1, Mark T. Jorgenson2, John M. Kimble3, Howard Epstein4, Vladimir E. Romanovsky5, & Donald A. Walker6 1

Agriculture and Forestry Experiment Station, University of Alaska Fairbanks, 533 E.

Fireweed, Palmer Alaska 99645, USA, 2Alaska Biological Research, Box 80410 Fairbanks, Alaska 99708, USA, 3151 East Hill Church Road, Addison, NY 14801, USA, 4

Department of Environmental Sciences, University of Virginia, P.O. Box 400123

Charlottesville, Virginia 22904, 5Goephysical Institute, University of Alaska Fairbanks, P.O. Box 755780 Fairbanks, Alaska 99775, 6Institute of Arctic Biology, University of Alaska Fairbanks, P.O. Box 757000 Fairbanks, Alaska 99775. *Correspondence and requests for materials should be addressed to C.L. Ping Agriculture and Forestry Experiment Station, University of Alaska Fairbanks, 533 E. Fireweed, Palmer Alaska 99645, USA. Email: [email protected].

ABSTRACT The Arctic soil organic carbon (SOC) pool is important but poorly defined. The most cited pool size estimates are based on one study that uses 48 North American soils of which only 5 are actually in the Arctic1. Three of these 5 are in the colder high arctic not representative of soils across the bioclimatic gradient of the region. Other soil map-based pool estimates2,3 use largely or solely subarctic soils to project

the Arctic. In all of these studies soil depth and direct measurements are lacking. We studied 70 soils across the whole bioclimatic gradient of the North American Arctic to a 1 meter depth and found SOC pools in different bioclimatic subzones (subzones A to E, north to south) to average 11, 17, 55, 53, and 43 kgSOCm-2 respectively. When projecting our data to the circumarctic based on the subzones as is done for the life zone and soil map-based previous studies we found a higher SOC stores of 41.0kgSOCm-2 compared to 21.8kgSOCm-2. We also detail the SOC depth distribution not previously available but important for climate warming studies.

The Arctic occupies about 13% of the land area and is estimated to hold about 14% of the soil organic carbon pool (SOC)1. This estimate is from the 1982 study of SOC in the world’s life zones1 and is for the entire tundra life zone but is widely cited for the estimation of Arctic SOC pools. The region was represented by an average value of 21.8 kg m-2 SOC for tundra areas (8.8x1012m2). A comparison of the 48 tundra data points used in the Post et al. study1 with the current Circumpolar Arctic Vegetation Map (CAVM) reveals that only 5 of the 48 points are in the Arctic as defined by the CAVM (5.05x1012m2) and 3 of those five points are in the High Arctic at one location. Other studies of tundra soils find values ranging from 20 kg m-2 to 29 kg m-2 SOC2,4,5,6, that includes only the upper 20 to 40cm in arctic Alaska and values estimated to100 cm in Russia and Canada. These studies have offered the only data easily available and are cited repeatedly in the literature even though soils of the Arctic were known to contain considerable carbon at depths up to a meter or more due to cryoturbation7,8,9. The Post study1, used mostly subarctic soils many with data to less than 1 meter depth and bulk

density (BD) estimated, the Chapin et al.4 and Oechel studies5 use only one Alaska site with limited depth, the Stolbavoi study6 of Russian Arctic SOC used estimated BD, depth and SOC contents, and the Tarnocai study2 extrapolated to a meter depth based on cores from one area. Arctic SOC data directly measured and to depth is very difficult to find.

In the circum-arctic patterned ground features, caused by frost cracking and heave, are prominent on the landscape10,11. These features include sorted and nonsorted circles, stripes, desiccation polygons and ice wedge polygons10. Associated with these features, the arctic tundra soils show strong evidence of cryoturbation that is characterized by warped, broken and distorted soil horizons and surface organic matter being churned down to the lower active layers and upper permafrost8,9,12. However, recent global change studies such as SOC temperature sensitivity13, global SOC distrubution14 and carbon emission stabilization assessments15 in both regional and world-wide assessments are all using the average value of 21.8 kg m-2 that largely ignores a significant part of the cryoturbated SOC in deeper part of the soils. Recent synthesis of Arctic ecosystem study results have pointed to the importance of the deeper carbon in the soil profile for effecting and being affected by climate change16,17. These C-stores are affected by warming temperatures and may affect up to 80% of the seasonal C-flux from tundra soils.

Our study has taken a systematic approach to examine and assess the SOC pools of arctic soils in order to minimize the uncertainties associated with previously SOC data. First we approach this by selecting study sites along the north-south bioclimate gradient in Arctic North America according to the most current Circumpolar Arctic Vegetation

Map18(Figure 1). Secondly we examined soils representing major landform units (toposequence). Thirdly, we examined soils across the complete cycle of pattern ground expression to a 1 meter or more depth at each site. Fourth, we used the state of the art techniques to describe and sample the cryoturbated soil profiles including direct measurement of soil bulk density, moisture/ice content, and thicknesses of warped (cryoturbated) horizon profiles19,20.

RESULTS AND DISCUSSION

Profile SOC distribution The presence, form and abundance of patterned ground plays a key role in determining patterns of tundra vegetation21,22, the morphology and properties of cryogenic soils12,23,24 and carbon dynamics and sequestration12,25,26. Movement of SOC from the surface to depth is accomplished through cryoturbation. This movement is caused by cracking due to soil freeze-thaw cycles and by soil hydrothermal gradients that produce differential frost heave27. Nonsorted circles are a common form of patterned ground in the Mid and Low Arctic (Bioclimatic Subzones B-E) while polygonal cracking and striping patterns are common in the High Arctic21,23,24 (Subzone A). Although based on surface observations, the influence of nonsorted circles appears to decrease from the north to south we found nonsorted circles were also common in the Low Arctic (Subzones D and E), only their surface appearance is masked by vegetation. The SOC in a typical soil profile under a nonsorted circle is unevenly mixed and distributed in recognizable masses within the active layer and upper permafrost12,20 (see supplementary material). Chunks of

surface organic matter, even whole-plant tussocks, were found subducted along the bowlshaped depression found in the permafrost table under nonsorted circles. Our research indicates that most cryoturbated SOC occurs within the depths of 80–120 cm on upland tundra. In subzones A and B, nearly all cryoturbated carbon is within 80 to 100 cm depth. The reason seems to be that the subduction of surface generated SOC goes to the top of the contemporary permafrost table and over the century time-scale the top of the permafrost can fluctuate on the order of tens of centimeters. This permafrost SOC is distict from that in Yedoma23,28 which is meters deeper under older landscapes. We estimate that cryoturbation has affected about 70% or more of the SOC and has affected stocks found in the active-layer and upper permafrost (Table 1). The SOC stocks varied widely across Arctic subzones, variation associated with pattern ground type and landscape position. Mean SOC for the subzones generally increased from High to Low Arctic reaching a maximum in Subzone C. In contrast, the highest stocks found for the subzones tended to increase to the southernmost Subzone E (Table 1). In subzone A the active layer contained an average of 76% of SOC stocks while for Subzones B thru E, the active-layer averaged 55 to 61% of the SOC stocks to 1 meter depth. The larger portion of SOC in the upper profile for the High Arctic is consistent with less surface SOC being produced through reduced biomass production21 and less cryoturbation. In the high arctic surface cracking dominates with stripe patterning on slopes. In the Mid and Low Arctic the more intensive nonsorted circle patterning dominates. We observed more nonsorted circle patterns from north to south along with increase cryoturbation and subduction of SOC. Overall the Arctic subzones, only 21% of the profile SOC to 1 meter was in the

surface horizons of the active layer, while the subsurface active layer contained about 41% with the remaining 38% of SOC resides in the permafrost.

Arctic SOC pools As discussed above the widely cited study of Post et al.1 is commonly used as the pool estimate of Arctic SOC but is actually based on 48 north American soils only 5 of which are arctic soils (3 of these are from the high arctic) as defined in the newest CAVM18. We used data or averaged data for soils found under zonal vegetation in each of the CAVM bioclimatic subzones and applied it to the map area given in the bioclimatic subzone map (Table 2). This was done in a similar manner used by Post et al.1 with the life zone approach, the difference is that we used only the arctic area and only directly measured data from across the Arctic. In doing so we provide a systematic approach to look at Arctic SOC pools and distribution within specific subzones and soil profiles with depth. Our data represents latitudinal and depth distribution for Arctic SOC stocks. Distributions are especially relevant and useful to climate warming assessments. In lieu of any additional data, our estimates are based on what we feel are the best available to date. The range and standard deviation of SOC data for each subzone given in Table 1 can provide an idea of the variablility found for the landscapes and deviations are similar to those listed by Post et al. 1. Our calculations (Table 2) are done using the SOC found in soils under zonal vegetation. Our estimate of the Arctic18 SOC pool for glacial-free areas is 207 Gt OC or an overall average of 41 kg m-2 OC. This estimate of overall average SOC stocks is nearly double that of the 21.8 kg m-2 OC previously estimated for tundra lifezones1 commonly used to represent the Arctic but is similar to 42 kg m-2 estimated

recently for Siberia and Alaska Steppe Tundra soils28. The profile depth distribution of SOC across the subzones was on average about 25% in the surface organic enriched horizons, 40% in the subsurface active layer and 35% in the permafrost (to 1m). In light of recent studies emphasizing the importance of deeper carbon and the whole season, we provide the first data based estimate for deeper profile SOC stocks across the Arctic. Although this estimate is based on the SOC stocks found in the North American transect sites it could be the best until more measured data to include the Eurasian Arctic becomes available. With strong similarities in circum-arctic vegetation, landscapes11,18 and soil forming factors29 it is reasonable to assume that with systematic investigation of circumarctic soils, SOC stores will be found to be similar to those we found and higher than those of the earlier estimates that rely largely on Subarctic and projected or estimated soils data. Such a wide difference in range of estimates combined with the recently recognized importance of deeper SOC in C-flux from tundra under changing climate should be incentive to better delineate the SOC stocks of the Arctic region.

METHODS In this study a total of 70 soils pits were excavated across the bio-climate zones from the foothill of the Brooks Range of northern Alaska to the Archipelago of the Canadian High Arctic between 1993 and 2006. The carbon stocks were calculated based on soils profile excavated across the whole cycle of predominate surface patterns where present (such as from the center of a nonsorted circle to the area in between the circles, or intercircle). Excavations were to a meter depth and OC expressed as kgOCm-2. For soils with

highly cryoturbated, broken and warped horizons8, thicknesses were calculated by the relative proportion of each horizon in an exposed meter square profile and for all soil horizons soil bulk density (block, core or clods samples) and organic carbon content were measured19. Sampling sites were selected along a toposequence in each study site, namely from the well drained ridge tops to the poorly drained lowlands. The ice and rock fragment contents measured were used to correct the measurements. The boundary of the upper permafrost was identified by cryogenic and ice structures30. Soil organic carbon was measured with samples treated with HCl to remove inorganic carbon then determined on LECO CHN analyzer at 800C with continuous flow of O2.

REFERENCES 1. Post, M.W. Emanuel, W.R. Zinke, P.J. & Stangenberger, G. Soil carbon pools and world life zones. Nature 298, 156–159 (1982). 2. Tarnocai, C. Carbon pools in soils of the Arctic, Subarctic, and Boreal regions of Canada. p91-103. In Global Climate Change and Cold Regions Ecosystems, Adv. In Soil Sci. (Lal, R., Kimble, J.M., and Stewart, B.A. eds.) (Lewis Publishers, 2002) 3. Esarwan, H, Van Den Berg, E. &Reich, P. Organic carbon in soils of the world. Soil Sci. Soc. Am. J. 57,192-194 (1993). 4. Chapin III, F.S. Miller, P.C. Billings, W.D.& Coyne, P.I. Carbon and nutrient budgets and their control in coastal tundra. p458-486. In Brown, J., Miller, P. C., Tieszen, L. L. and Bunnnell, F.L. (eds.), An Arctic ecosystem: The coastal Tundra at Barrow, Alaska. (Stroudsburg, Penn.: Dowden, Hutchison and Ross, 1980). 5. Oechel, W.C. & Billings, W.D. Effects of global change on the carbon balance of

arctic plants and ecosystems. p.139-168. In Chapin F. S., III, Jefferies, R.L., Reynold, J. F., Shaver, G. R. and Svoboda J. (eds.), Arctic Ecosystems in a Changing Climate. (Academic Press, San Diego, 1992). 6. Stolbovoi, V. Carbon in Russian soils. Climatic Change, 55:131-156 (2002). 7. Douglas, L.A. & Tedrow, J.C.F. Tundra soils of arctic Alaska. Transactions of the 7th International Congress on Soil Science 4, 291-304 (1960). 8. Tarnocai, C. & Smith, C.A.S. The formation and properties of soils in the permafrost regions of Canada. p. 21-42. In: Proc. Of the 1st International Conference on Cryopedology ( Russian Academic of Sciences, Pushchino, Russia, 1992). 9. Bockhiem, J.G. &Tarnocai, C. Recognition of cryoturbation for classifying permafrost affected-soils. Geoderma 81,281-293 (1998). 10. French, H.M. The periglacial environment 2nd ed., (Longman, White Plains, N. Y., 1996). 11. Chernov, Y.I, & Matveyeva, N.V. Arctic ecosystems of Russia. p361-506. In: Ecosystems of the World, Polar and Alpine Tundra (Wielgolaski, F.E. ed.)(Elsevier, 1997). 12. Ping, C.L. Bockheim, J.B. Kimble, J.M. Michaelson, G.J. & Walker, D.A. Characteristics of cryogenic soils along a latitudinal transect in arctic Alaska, J. of Geophysical Res. 103(D22), 28,917-28,928 (1998). 13. Davidson, E.A. & Janssens, I.A. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change, Nature. 440,165-173 /9 March (2006). 14. Jobbagy, E.G. & Jackson, R.B. the vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecol. Appl. 10, 423-436 (2000).

15. Jones, C. et al. Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil. Glob. Change Biol. 11, 154166 (2005). 16. Sturm, M. Schimel, J. Michaelson, G. Welker, J. Oberbauer, S.F. Liston, G. Fahnestock, J. & Romanovsky, V.E. The Role of Winter Biological Processes in Converting Arctic Tundra to Shrubland, Bioscience 55(1), 17-26 (2005). 17. Hobbie, S.E. et al. Controls over carbon storage and turnover in high-latitude soils. Global Change Biol. 6(1), 196-210 (2000). 18. Walker, D.A. Raynolds, M. Daniels, D.J.A. et al. The circumpolar vegetation map. Journal of Vegetation Science 16,267-282 (2005). 19. Kimble, J.M., Tarnocai, C., Ping, C.L., Ahrens, R., Smith C.A.S., Moore, J. & Lynn, W. Determination of the amount of carbon in highly cryoturbated soils. p277-291, In: Proceedings of the Joint Russian-American Seminar on Cryopedology and Global Change. (Gilichinsky, D.A. ed.) (Russian Acad. Of Sci., Inst. of Soil Sci. and Photosynth., 1993). 20. Michaelson, G.J., Ping, C.L., Kimble, J.M. Effects of soil morphology and physical properties on estimation of carbon storage in arctic soils. In: Assessment Methods For Soil Carbon, Advances in Soil Science. (Lal, R., Kimble, J.M., Follet, R.F., Stewart, B.A. eds.) (Lewis Publishers, 2001). 21. Walker, D.A. Epstein, H.E. Gould, W.A. Kelley, A.M. Kade, A.N. Knudson, J.A. Krantz, W.B. Michaelson, G.J. Peterson, R.A. Ping, C.L. Raynolds, M.K. Romanovsky, V.E. & Shur, Y. Frost-boil ecosystems: complex interactions between landforms, soils, vegetation, and climate. Perma. Perigl. Proc. 15, 171-188 (2004).

22. Bliss, L.C. & Svoboda, J. Plant communities and plant production in the western Queen Elizabeth Islands. Holarctic Ecology 7, 325-344 (1984). 23. Tedrow, J.C.F. Soil of the Polar Regions (Rutgers Univ. Press, New Brunswick, N. J., 1974). 24. Washburn, A.L. Geocryology: A Survey of Periglacial Processes and Environments (John Wiley and Sons, New York, 1980). 25. Michaelson, G.J. & Ping, C.L. Soil organic carbon and CO2 respiration at subzero temperature in soils of arctic Alaska, J. of Geophy. Res. 108(D2), 8164, doi:10.1029/2001JD000920 (2003). 26. Bockheim, J.G. Walker, D.A. Everett, L.R. Nelson, F.E. & Shikolmanov N.I. Soils and cryoturbation in moist nonacidic and acidic tundra in the Kuparuk river basin arctic Alaska, USA. Arct. Alp. Res. 30, 166-174 (1998). 27. Peterson, R.A. Walker, D.A. Romanovsky, V.E. Knudson, J.A. Raynolds, M.K. & Krantz, W.B. A differential frost heave model: cryoturbation-vegetation interactions. p885–890. In Permafrost: Proceedings of the Eighth International Conference on Permafrost, Vol. 2, Phillips M, Springman SM, Arenson LU (eds). (A. A. Balkema Publishers, Lisse, 2003). 28. Zimov, A.Z. Schuur, E.A.G. & Chapin III, F.S. Permafrost and the global carbon budget. Science 312, 1612-1613 (2006). 29. Goryachkin, S.V., Blume, H.P., Beyer, L., Campbell, I., Claridge, Bockheim, B.J., Karavaeva, N.A., Targulian, V., and Tarnocai, C. Chapter 1. Similarities and differences in Arctic and Antarctic soil zones. p49-70. In Kimble, J. (ed.), Cryosols: PermafrostAffected Soils. (Springer, 2004).

30. Hoefle, C.M., C.L. Ping, and J.M. Kimble. Properties of permafrost soils on the Northern Seward Peninsula, Northwest Alaska. Soil Sci. Soc. Am. J. 62,1629-1639 (1998).

AKNOWLEDGEMENTS The following National Science Foundation projects contributed to this work: Arctic Transitions in the Land-Atmosphere System, the Flux Study, Biocomplexity of the Arctic System, and Arctic Coastal Transfer. Also significant contributions from the U.S Dept. of Agric. Hatch Project and Global Change Initiative.

Figure 1 Site locations. Colors are bioclimatic subzones with points for soil study sites.

Study Sites Isachsen Mould Bay Green Cabin N

A B C D E

67 o

Bioclimate Subzones

Alaska Sites

ALASKA

CANADA

Table 1. Summary of SOC storage data (data set given in supplementary information) by Bioclimatic subzone. Active Layer Permafrost Total SOC Organic Enriched Subsurface Upper Permafrost Bioclimatic Surface Horizons Horizons To 1 Meter Depth To 1 Meter Depth Subzone -2 ------------------------------------- kg SOC m -------------------------------A ave. all (n=9) 2.1 6.3 2.6 11.0 Zonal veg. Soil 2.2 12.0 1.0 15.2 (n=1) Range Std. Dev.

B ave. all (n=6) Zonal veg. soil (n=1) Range Std. Dev.

C

ave. all (n=8) Zonal veg. soil (n=3) Range Std. Dev.

D ave. all (n=24) Zonal veg. soil (n=12) Range Std. Dev.

E ave. all. (n=23) Zonal veg. soil (n=6) Range Std. Dev.

(0.1-6.8) 2.0

(0.1-12.0) 3.8

(0.1-6.6) 2.5

(0.3-19.8) 6.7

2.4 1.9

6.9 18.2

7.4 1.0

16.8 21.1

(0.9-4.9) 1.5

(3.1-18.2) 5.7

(1.0-12.1) 4.4

(7.2-21.1) 5.2

10.8 11.8

22.5 16.4

21.6 14.9

54.9 43.1

(2.9-20.2) 5.8

(9.7-57.6) 15.8

(3.4-68.1) 20.4

(24.0-84.5) 21.3

11.2 7.1

20.7 23.2

21.6 12.7

53.1 42.2

(2.1-43.7 11.3

(0.1-46.2) 14.5

(1.4-63.5) 17.2

(20.0-93.5) 18.3

13.2 14.2

13.1 10.6

16.4 19.5

42.8 44.3

(0.1-63.5) 14.8

(0.1-56.1) 12.2

(0.1-53.0) 15.6

(2.7-104.3) 26.8

Table 2. Estimate of Arctic SOC pool and portioning based on the Bioclimatic subzones18 and soils sampled under zonal vegetation for each subzone (averages in Table 2). Active Layer Permafrost Subzone Area17 Total SOC OrganicSubsurface To 1 meter % of total (% Area of (unglaciated) (to 1 enriched horizons SOC pool Arctic) meter) surface hor. km2 x 106 ----------------------------Gt OC--------------------------% A - (2.0) 0.101 1.5 0.2 1.2 0.1 0.7 B - (8.9) 0.449 9.5 0.9 8.2 0.4 4.6 C - (23.1) 1.166 50.3 13.8 19.1 17.4 24.3 D - (29.6) 1.494 64.2 10.6 34.7 19.0 31.1 E - (36.4) 1.837 81.4 26.1 19.5 35.8 39.3 Total (100) 5.047 206.9 51.5 82.6 72.7 100.0 average = 41.0 kgSOC m-2