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DIVISION S-5-PEDOLOGY

Soil Organic Carbon Storage and Distribution in Arctic Tundra, Barrow, Alaska

^a Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299 USA
^b Ohio State Univ., Byrd Polar Research Ctr., 1090 Carmack Rd., Columbus, OH 43210-1002 USA
^c Dep. of Geography, Univ. of Cincinnati, Cincinnati, OH 45221 USA
^d Dep. of Geography, Univ. of Delaware, Newark, DE 19716 USA
^e International Permafrost Assoc., P.O. Box 7, Woods Hole, MA 02543 USA

bockheim{at}facstaff.wisc.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil organic C (SOC) levels were determined to a depth of 100 cm for the nine units designated on a 1957 1:20000 soil map of Barrow, AK prepared by J.V. Drew. The legend was updated by converting Drew's map units into the recently adopted Gelisol order in U.S. soil taxonomy and field verified. The SOC varied from 2.5 kg m^-3 in modern beach sediments to >73 kg m^-3 in Typic Sapristels in high-centered, ice-wedge polygons developed in reworked organic-rich lake sediments. The SOC averaged 50 kg m^-3 for the entire 64-km² area (excluding open water). Considerable variability in SOC exists within individual soil map units. For example, SOC levels in a Typic Aquiturbel (formerly classified as a Meadow Tundra, Normal Phase soil) ranged from 24 to 109 kg m^-3 . Substantial variation in SOC occurs within individual patterned-ground units. For a high-centered, ice-wedge polygon with a diameter of 15 m, SOC levels are 24, 32, and 64 kg m^-3 for the wedge trough, rim, and center, respectively. In a low-centered, ice-wedge polygon, SOC levels are 28 and 83 kg m^-3 for the trough and center. The variation in SOC within soil map units and individual patterned-ground units is due primarily to differences in the amount of ground ice. Active-layer thickness varies within and between soil map units, ranging from 31 cm in Typic Sapristels to >100 cm in modern beach sediments. About 47% of the SOC in the upper meter of soil was in the active layer at the time of sampling; the remainder occurring in frozen ground, much of it meeting the definition of permafrost. Some of the SOC originates from past reworking of organic-rich lake sediments. Carbon stocks in near-surface permafrost may be of global significance and should be inventoried in other tundra regions.

Abbreviations: ARCSS, Arctic Systems Science • CRREL, U.S. Army Cold Regions Research and Engineering Laboratory • IBP, International Biological Program • SOC, soil organic C

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
THE SOILS of Barrow, AK have been studied for more than 40 yr in the context of tundra ecosystem processes (Douglas and Tedrow, 1959; Brown and Johnson, 1965; Waelbroeck et al., 1997), polar soil classification (Drew, 1957; Drew and Tedrow, 1957; Douglas and Tedrow, 1960; Everett et al., 1981), soil–plant relations (Gersper et al., 1980), soil disturbance effects (Gersper and Challinor, 1975; Challinor and Gersper, 1975), and pedon-scale soil variation within areas of patterned ground (Brown, 1969; Everett, 1974; Everett and Brown, 1982; Hinkel et al., 1996). Many of these studies were undertaken in conjunction with activities of the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) (Brown and Johnson, 1965) and the U.S. Tundra Biome component of the International Biological Programme (Brown et al., 1980; Everett et al., 1981). Although abundant literature exists for the Barrow region and elsewhere in arctic Alaska (e.g., Weller et al., 1995), much of it is focused on the active layer, the shallow zone above permafrost that experiences seasonal freezing and thawing (Nelson et al., 1998a, 1998b). Few data have been reported for near-surface permafrost, which exists from the base of the active layer to a depth of 100 cm and where as much as 60% of the SOC occurs (Michaelson et al., 1996; Bockheim et al., 1998).

Predictions from general circulation models, summarized in a recent report by the Intergovernmental Panel on Climate Change (Kattenberg et al., 1996), indicate that climate warming in the high northern latitudes will exceed the global average. The mean air temperature during July and August at Barrow has increased 1.5°C since 1970 (Oechel et al., 1995). Permafrost temperatures in arctic Alaska may have increased 2 to 4°C during the last century (Lachenbruch and Marshall, 1986; Osterkamp and Romanovsky, 1996). Soils of the tundra biome contain 14% of the world soil C pool (Post et al., 1982; Gilmanov and Oechel, 1995). During the last several decades, the Alaskan arctic tundra may have changed from a net CO₂ sink to a source (Oechel et al., 1995; Waelbroeck et al., 1997). These observations demonstrate the importance of determining the amounts and spatial distribution of SOC in arctic tundra ecosystems.

Accordingly, the objectives of this study were (i) to determine the amount of SOC in each soil map unit from a reconnaissance map of the Barrow area prepared in 1957 by J.V. Drew, (ii) to determine the statistical variability in SOC within a single, dominant soil map unit, (iii) to evaluate pedon-scale (<7 m) variability in SOC within individual ice-wedge polygons, and (iv) to determine the amount and proportion of SOC in the active layer and upper permafrost to a depth of 100 cm.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Site Description
At 71°N latitude, Barrow is the northernmost point in the United States (Fig. 1). Located at the tip of the Arctic Coastal Plain physiographic province (Wahrhaftig, 1965), the Barrow area is underlain by continuous permafrost to depths of >400 m. The active layer ranges from 30 to 90 cm in thickness. The Barrow area has low relief, with elevations ranging from 0 to 20 m above sea level, and its landforms are dominated by drained lake basins and oriented thaw lakes (Sellmann et al., 1975). About 65% of the surface is covered by polygonal ground (Brown, 1967). Cores taken from around Barrow indicate that the volume of segregration ice in pores and lenses averages 50 to 75% in the upper 2 m (Sellmann et al., 1975). Ice wedges may contain an additional 10 to 20% of the volume of soil in the upper 2 m. The soil parent materials are reworked glaciomarine sediments of Pleistocene age, including the clay- and silt-rich Gubik Formation (Black, 1964).

Barrow has a cold maritime climate. Winters are long, dry, and cold, and summers are short, moist, and cool. The mean annual air temperature is -12.6°C; on average, July is the warmest month at 4.1°C, and February is the coldest at -27.7°C (National Oceanic and Atmospheric Administration, 1996). Mean annual precipitation is 124 mm, 37% of which falls as rain during July and August. The winter snowpack averages 20 to 40 cm, but snow accumulation on the landscape is highly variable because of variations in terrain roughness and drifting from strong easterly winds.

Eight major vegetation associations have been recognized and mapped in the Barrow region (Brown et al., 1980). Sedge meadow covers about 75% of the area and contains predominantly sedge (Carex aquatilis Wahlenb.), tall cottongrass (Eriophorum angustifolium Honckeny), white cottongrass (E. Scheuchzeri Hoppe), and Fisher's tundragrass (Dupontia fisheri R. Br.) (Webber, 1978).

Data Collection and Analysis
Despite abundant published material on the Barrow area, particularly from the CRREL and Tundra Biome studies, available data were inadequate for determining profile quantities of SOC. Except for a recent study by Michaelson et al. (1996) that included two profiles from the Barrow area, none of the published studies for Barrow provided SOC and bulk density values by horizon for the soil profile to a depth of 100 cm.

We used SOC data (K.R. Everett, 1994, unpublished) from soil cores collected along the CRREL transect at Barrow (Brown and Johnson, 1965; Hinkel et al., 1996) and collected new data from eight soil map units for which such information was lacking.

Established in 1962, the CRREL transect extends 2.1 km southeastward from Elson Lagoon to Central Marsh (Brown and Johnson, 1965). In its eastern half, the transect runs parallel with Wohlschlag Slough (Fig. 1). Twenty-seven cores were collected from plots along the transect in November 1993 and 21 additional cores were taken from the same plots in May 1994 (Hinkel et al., 1996). Obtained with a modified CRREL core barrel auger, the cores were taken to maximum depths of . The cores were cut into 5- or 10-cm segments, placed in stainless steel moisture containers, and shipped to Ohio State University, where they were oven dried at 105°C for 24 h. The volumetric field moisture content and bulk density were determined (Hinkel et al., 1996), along with organic C (K.R. Everett, 1994, unpublished data). Eighteen of the 20 cores collected in 1994 for which complete data were available are from a single soil map unit, the Meadow Tundra, Normal soil (Typic Aquiturbel); these data were used to determine the statistical variability in SOC within a single soil map unit.

Because all of the soil cores except two were collected from Meadow Tundra, Normal Phase soils, we subsequently collected samples from the remaining eight soil map units. We described and sampled 24 pedons during August of 1995, 1996, and 1997. The pedons were dug by hand to the top of the frozen layer, encountered at an average depth of 38 cm. The sampling pits were excavated or cored further to a depth of 100 cm using a gasoline-powered Pico shovel or a CRREL core barrel. Pedon-scale (<7 m) variability in SOC was determined for two types of patterned ground: a high-centered, ice-wedge polygon within the Arctic Systems Science (ARCSS) grid and a low-centered, ice-wedge polygon on plot 203 at the former International Biological Program (IBP) site (Fig. 1). Minimally disturbed cores were taken from each horizon for bulk density determination. For horizons in which cores could not be collected, bulk density was estimated from the equation:

(1)

where y = bulk density (g cm^-3) and x = organic C (%) (Bockheim et al., 1998).

Soils were analyzed for total C using a Leco 1000 CHN analyzer (Leco Corp., St. Joseph, MI) for core samples collected in 1993 and 1994 (K.R. Everett, unpublished) and bulk samples collected by horizon in 1995 and a Dohrmann C analyzer (Dorhmann, Santa Clara, CA) for bulk samples collected in 1996 and 1997.¹ Carbon determinations on samples treated with dilute HCl suggested that carbonates were not present; therefore, the total C values represent organic C. Cross-checking revealed no statistical differences in total C determinations between the two instruments. Each pedon was classified by great group according to Tedrow's (1974) classification scheme and by subgroup in U.S. soil taxonomy (Soil Survey Staff, 1998). Soil organic C pools were calculated by taking the product of horizon thickness or core depth interval, bulk density, and percentage of SOC, and summing the values for the upper 100 cm (Michaelson et al., 1996). In cases where cores or pit excavations did not reach 100 cm, percentage of SOC and bulk density for the last horizon were projected to 100 cm. No corrections were necessary for coarse fragments except for the Typic Haplorthel (formerly Arctic Brown) on a gravelly, raised beach ridge.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Distribution of Soil Taxa
The 1:20000 map of the soils around Barrow was prepared by J.V. Drew in 1957 when the zonal soil classification system was still in use in the USA (Tedrow et al., 1958). Arctic soils were differentiated into four great groups based on drainage characteristics ranging from somewhat excessively drained to very poorly drained: Arctic Brown, Upland Tundra, Meadow Tundra, and Half Bog (Drew, 1957). The correlation between these soil taxa and those recognized in the new Gelisol order (Soil Survey Staff, 1998) is shown in Table 1 . Gelisols include three suborders: Histels (organic soils with permafrost within 100 cm of the surface), Turbels (mineral soils that are cryoturbated and have permafrost within 200 cm of the surface), and Orthels (other mineral soils that are not cryoturbated and have permafrost within 100 cm of the surface).

Modern beach sediments with little evidence of soil development and deep (>100 cm) permafrost account for 4.4% of the area, and open water makes up the remaining 9.0% (Table 2).

Soil Organic Carbon in Relation to Soil Taxon
Soil organic C ranges from 2.5 kg m^-3 for modern beach sediments to 73 kg m^-3 for Typic Sapristels (Table 2). Because Typic Aquiturbels and Typic Histoturbels (formerly Meadow Tundra soils) comprise nearly 75% of the area, the areally weighted mean SOC for the map area (excluding open water) is 50 kg m^-3. This is less than the 62 and 65 kg m^-3 values for the Prudhoe Bay coastal plain reported by Michaelson et al. (1996) and Bockheim et al. (1998), respectively, possibly because well-developed ice wedges are more extensive and higher ice contents occur in permafrost of the Barrow region. The depth distribution of organic C in representative pedons is shown in Table 3 , and the cumulative amounts of organic C stored in the active layer and the upper permafrost are shown in Table 4 .

Variability of Soil Organic Carbon in Ice-Wedge Polygons
There is considerable pedon-scale (<7 m) variability in SOC in arctic soils because of cryoturbation and the formation of patterned ground, especially ice-wedge polygons. For example, in a high-centered, ice-wedge polygon within the ARCSS grid measuring 15 m across, the SOC was 24 kg m^-3 in the polygon trough and 32 kg m^-3 on the rim (Glacic Aquiturbel) and 64 kg m^-3 in a Ruptic-Histic Aquiturbel occupying the polygon center (Fig. 3A). In a low-centered, ice-wedge polygon at the former IBP site (Plot 203; Fig. 3B), SOC was 28 kg m^-3 in the polygon trough (Glacic Aquiturbel) and 83 kg m^-3 in the center (Terric Hemistel). Cryoturbation, which refers to all soil movements due to frost action, plays an important role in mixing surface organic matter into the subsoil (Tarnocai and Zoltai, 1978; Kimble et al., 1993; Bockheim and Tarnocai, 1998). In highly cryoturbated soils, the organic matter accumulates as tongues beneath the ice-wedge troughs in areas of active ice-wedge growth (Brown, 1969). Some of the organic matter moves from the tongues as pockets and streaks under the polygon centers (involutions), but much of it spreads across the permafrost table as a result of cryoturbation. Soil organic matter at the base of the active layer and in the upper permafrost at Barrow yields radiocarbon ages of <2500 to 10500 yr before present (Brown, 1965, 1969).

Depth-Distribution of Soil Organic Carbon
The proportion of SOC in the upper permafrost at Barrow is directly related to the influence of soil moisture on active-layer thickness. All of the SOC in the upper 100 cm of Typic Haplorthels and modern beach sediments was present in the active layer. Those soils lack permafrost in the upper 100 cm and are coarse-textured and somewhat-excessively to excessively drained (Table 2).

From 51 to 65% of the SOC was present in the active layer of the well-drained to somewhat poorly drained Typic Haploturbels, Typic Umbriturbels, and Typic Aquiturbels (Table 2). The active layer is 40 to 53 cm deep in these soils. However, in the poorly and very poorly drained Typic Histoturbels, Typic Aquorthels, and Typic Sapristels, which have active-layer depths of 31 to 37 cm, from 28 to 35% of the SOC was in the active layer. On average, 47% of the SOC (to a 100-cm depth) was in the active layer, with the remainder in near-surface permafrost. These findings are consistent with those of Michaelson et al. (1996) and Bockheim et al. (1998) for the coastal plain in the Prudhoe Bay region and indicate that SOC estimates for much of the Arctic may require upward adjustment.

Global Change and Soil Organic Carbon Dynamics in Arctic Alaska
Recent experiments with general circulation models (e.g., Manabe et al., 1991) predict significant increases in mean annual air temperature for many parts of the Arctic by 2050. Warming of the magnitude forecast by transient general circulation models could reduce the total area of the Northern Hemisphere underlain by equilibrium permafrost by 12 to 22% (Anisimov and Nelson, 1997) and induce increases of active-layer thickness of 20 to 30% across much of the permafrost region (Anisimov et al., 1997). The influence of climate warming on the C cycle in permafrost regions is uncertain, but could accelerate the rate of organic matter decomposition. Conversely, warming could increase the productivity of arctic vegetation, with the possible result that little net change in future C emissions would occur. Modeling experiments by Waelbroeck et al. (1997) indicate that a temporary increase of CO₂ emissions would accompany a substantial increase in active-layer thickness, followed by a long-term increase in C accumulation. These issues require further investigation.

The upper 30 to 50 cm of permafrost in the Barrow area contain an average of 0.41 kg C m^-2 per centimeter of material compared with 0.63 kg C m^-2 per centimeter of material in the active layer (derived from data in Table 2). Therefore, lowering the permafrost table 40 cm, that is, doubling the average active-layer depth, would add an average of 16.4 kg C m^-3 to the active layer. This would increase SOC levels by 33%, since the current average SOC content of Barrow soils is 50 kg C m^-3.

Not all of this SOC can be expected to undergo decomposition. Douglas and Tedrow (1959) measured in situ organic matter decomposition rates in several soils (0–8 cm layer) of the Barrow region, reporting values of 340 to 1900 kg ha^-1 yr^-1. Some models (Konyushkov, 1998) predict that global warming will enhance cryoturbation in the Arctic, which could operate to bring C sequestered in the uppermost permafrost to the surface and increase the release of CO₂ to the atmosphere.

In addition to increasing release of CO₂, warming in the Arctic could cause extensive thermal erosion, in that ice-wedge polygons underlie 65% of the Barrow region (Brown, 1967). Thermal erosion is evident today around Barrow, particularly along naturally eroding sea cliffs and in areas where organic layers have been disturbed by humans (Ferrians et al., 1969; Brown and Péwé, 1973).

Climate warming could initiate an intensification of processes associated with the thaw–lake cycle in the arctic coastal plain. Thaw lakes presently comprise 9% of the map area (Fig. 1), but drained basins occupy much of the remaining terrain. Drainage of thaw lakes can be induced by local disruption or thawing of ice-wedge networks and through headward or lateral erosion by streams (Britton, 1957; Billings and Peterson, 1980). Lake drainage exposes the former bottomland to subaerial processes and aerobic decomposition.

The high spatial variability at soil map unit (polypedon) and pedon scales makes any prediction about the effect of climate warming on SOC dynamics in arctic Alaska difficult. However, it is clear that Arctic soils, including those of the Barrow region, contain abundant SOC and that less than half of the SOC in the upper 100 cm exists in the active layer.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
More than 75% of the soils in the Barrow area are cryoturbated (Turbels) and have moderately high SOC values. The highest SOC values were found in Histels and in bogs containing organic-rich lake sediments. There is considerable variability in SOC in relation to soil taxon and within patterned-ground features such as high-centered and flat-centered polygons. About 53% of the SOC in the upper 100 cm occurs in the near-surface permafrost. With climate warming, the active layer could thicken and SOC in the upper permafrost could be released to the atmosphere or hydrosphere as CO₂ (Christensen and Cox, 1995; Waelbroeck and Louis, 1995; Waelbroeck et al., 1997). Although this process could be countered by increased cryoturbation under conditions of aggrading permafrost (Konyushkov, 1998) induced by increased vegetation productivity, the discovery of substantial C stocks in the uppermost layers of permafrost warrants revision of SOC estimates for the tundra biome and its fate under the influence of climatic warming.

	ACKNOWLEDGMENTS

 
We appreciate the cooperation of the Ukpeagvik Inupiat Corp. for administrative assistance and the Barrow Arctic Science Consortium (BASC) for facilitating research in the Barrow Environmental Observatory. This research was supported by NSF awards OPP-9318528 to K.R. Everett and F.E. Nelson, OPP-9612647 to F.E. Nelson, and OPP-9529783 to K.M. Hinkel.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Contribution number 1126 from the Byrd Polar Research Center.

¹ Mention of company or product name does not constitute endorsement by the Univ. of Wisconsin.

Received for publication April 30, 1998.

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