(NDVI) classes from the Circumpolar Arctic Vegetation Map. On the basis of ..... pixels in each class was counted in Photoshop (Adobe Systems. Incorporated ...
Click Here
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 24, GB3012, doi:10.1029/2009GB003660, 2010
for
Full Article
Spatial distribution of soil organic carbon in northwest Greenland and underestimates of high Arctic carbon stores Jennifer Horwath Burnham1,2 and Ronald S. Sletten1 Received 25 August 2009; revised 10 January 2010; accepted 4 March 2010; published 29 July 2010.
[1] The amount of soil organic carbon (SOC) in the high Arctic is generally poorly constrained. Because of periglacial processes such as frost churning and sequestration in frozen soils, a substantial amount of SOC is typically not inventoried. This study provides a detailed study of SOC content by depth in 55 soil pits in a high Arctic ecosystem of northwest Greenland. Sampling sites spanned ecosystems from mires to polar deserts, from sea level to the margins of the Greenland Ice Sheet, and across various periglacial features. The amount of SOC in the various ecosystems was mapped using a correlation of SOC with high‐resolution ASTER satellite imagery and the normalized difference vegetation index (NDVI) classes from the Circumpolar Arctic Vegetation Map. On the basis of this correlation, the total carbon was extrapolated to greater areas of the high Arctic. Our study found the amount of SOC in the high Arctic has typically been grossly underestimated, remarkably by the greatest amount in the most barren environments of the polar desert. We estimate that the high Arctic contains about 12 Pg SOC, a factor of over 5 times greater the most cited values previously reported. Since our estimate was only assessed in seasonally frozen ground, additional carbon frozen in the permafrost is likely present and potentially available in the event of permafrost thawing due to warming of the Arctic. Citation: Horwath Burnham, J., and R. S. Sletten (2010), Spatial distribution of soil organic carbon in northwest Greenland and underestimates of high Arctic carbon stores, Global Biogeochem. Cycles, 24, GB3012, doi:10.1029/2009GB003660.
1. Introduction [2] One direct feedback to rising greenhouse gases in an anthropogenically warmed environment is the release or storage of organic carbon in vegetation and soils [Smith and Shugart, 1993; Oechel and Vourlitis, 1994; Christensen et al., 1999; Kirschbaum, 2000; Joos et al., 2001; Melillo et al., 2002; Luo, 2007]. To assess this potential release of carbon and evaluate its effect on climate change, accurate estimates of the quantity and distribution of soil organic carbon (SOC) are required. Current estimates assert that global soil ecosystems contain 2–3 times as much carbon (1395–2400 Pg) [Bohn, 1982; Post et al., 1982; Schlesinger, 1984; Batjes, 1996] as is in the atmosphere (750 Pg) [Intergovernmental Panel on Climate Change (IPCC), 2001], elucidating the importance of this pool and its potential to both affect and be affected by future global climate change [Jenkinson et al., 1991; Smith and Shugart, 1993; Oechel and Vourlitis, 1994; Christensen et al., 1999; Cox et al., 2000; Kirschbaum, 2000; Joos et al., 2001; Melillo et al., 2002; Jones et al., 2005; Knorr et al., 2005]. 1 Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA. 2 Now at Department of Geography, Augustana College, Rock Island, Illinois, USA.
Copyright 2010 by the American Geophysical Union. 0886‐6236/10/2009GB003660
[3] While it has been recognized that the Arctic (particularly the low Arctic) contains substantial stores of SOC [Post et al., 1982; Miller et al., 1983; Gorham, 1991; Bliss and Matveyeva, 1992; Kimble et al., 1993; Gilmanov and Oechel, 1995; Michaelson et al., 1996; Tarnocai, 2000; Ping et al., 2008], few comprehensive, quantitative studies of SOC have been conducted in the high Arctic (a region defined as north of the 4°–6°C July mean average temperature [Bliss, 1979]). One widely cited reference of high Arctic SOC content sampled only carbon in the upper 25 cm of soil [Bliss and Matveyeva, 1992]. As soil profiles in the high Arctic generally extend beyond 25 cm depth, and cryoturbation transports carbon to depth, this estimate does not quantify fully the amount of high Arctic SOC. Horwath et al. [2008] reported that 62% of SOC associated with pervasive periglacial nonsorted stripes in fine‐textured soils of the high Arctic was located below 25 cm. [4] Permafrost conditions drive many physical processes that shape the landscape and mediate soil carbon dynamics in the Arctic. Water migration toward freezing fronts results in ice lens formation in soils and resultant frost heave. Repeated heave cycles lead to soil mixing (cryoturbation), often expressed as broken or irregular horizons and convoluted deposits of buried organic carbon [Tedrow, 1962, 1965; Vandenberghe, 1988; Bockheim and Tarnocai, 1998]. Soil carbon also accumulates along the permafrost contact or is frozen within the permafrost as it aggrades [Tedrow, 1965; Kimble et al., 1993; Bockheim and Tarnocai, 1998]. SOC in
GB3012
1 of 14
GB3012
HORWATH BURNHAM AND SLETTEN: SOIL CARBON IN NW GREENLAND
the low Arctic has been underestimated substantially by not accounting for SOC at depth [Kimble et al., 1993; Michaelson et al., 1996; Bockheim et al., 1999], hence it is probable that SOC in the high Arctic has also been underestimated since most estimates are based on the top 25 cm of soil. [5] Because of its short growing season and near‐surface permafrost, Arctic ecosystems are expected to elicit profound responses to global climate change (summaries given by IPCC [2001]). As air temperatures continue to rise and permafrost thaws, previously inactive stores of Arctic SOC may enter the active soil carbon cycle and be released as CO2 or CH4 [Gorham, 1991; Smith and Shugart, 1993; Christensen et al., 1999; Tarnocai, 1999; Hobbie et al., 2000]. The high Arctic region encompasses approximately 2 million km2 of ice‐free land [Bliss and Matveyeva, 1992] and therefore represents a large region of terrestrial carbon storage with the potential to release or store more carbon. With the Arctic region already exhibiting warming [e.g., Lachenbruch and Marshall, 1986; Oechel et al., 1995; Osterkamp and Romanovsky, 1996; Serreze et al., 2000; Comiso and Parkinson, 2004; Steffen et al., 2004; Overpeck et al., 2005; Ekstrom et al., 2006; Jorgenson et al., 2006], it is increasingly relevant that accurate quantifications of high Arctic SOC are completed. [6] The objectives of this research were to (1) provide a detailed quantitative estimate of active layer SOC in northwest Greenland and determine if previous estimates of SOC in the high Arctic have been underestimated, (2) examine the spatial distribution of SOC in northwest Greenland by investigating correlations of SOC with surface variables (e.g., slope, aspect, NDVI, aboveground biomass), and (3) further determine the utility for using such correlations to estimate SOC in the circumpolar high Arctic.
2. Methods 2.1. Site Description [7] Field research was conducted out of Thule Air Base on the northwest coast of Greenland (76°N, 68°W). The air base is located on an 800 km2 sparsely vegetated, ice‐free peninsula that is bounded to the east by an outlet glacier of the Greenland Ice Sheet (Store Landgletscher), and by Baffin Bay to the south and west (Figure S1).1 The region near Thule Air Base is known by its Greenlandic name, Pitugfik, but henceforth in this paper, the study area will be simply referred to as the Thule region or the Thule peninsula. Place names will be introduced by Greenlandic name, followed by the English name in parentheses. [8] Portions of the region have been covered by three to four ice sheet advances during the Quaternary [Funder and Houmark‐Nielsen, 1990; Kelly et al., 1999], which deposited a relatively thin veneer of glacial drift across the majority of the peninsula. The timing and extent of these glaciations is somewhat uncertain, with periods of ice‐free coverage ranging from 9,000 to 30,000 years across the peninsula [Davies et al., 1963; Funder and Houmark‐Nielsen, 1990; Kelly et al., 1999]. The vegetation is generally prostrate and is 1 Auxiliary materials are available with the main article. doi:10.1029/ 2009GB003660.
GB3012
dominated by Salix, Dryas, Cassiope, Saxifraga, and Carex species. Using the Circumpolar Arctic Vegetation Map (CAVM) classification system, the vegetation communities of the study area are classified as (P1) prostrate dwarf shrub, herb tundra; (P2) prostrate/hemiprostrate dwarf shrub tundra; (W1) sedge/grass, moss wetland; (B1) cryptogam, herb barren; (B3c) noncarbonate, mountain complex; (G1) rush/grass, forb, cryptogam tundra; and small regions of (B4c) carbonate mountain complex [CAVM Team, 2003] (Table 1). Vegetation cover ranges from 0% to 100%, consisting of vascular plants, lichens, and cryptogamic crusts within nonvegetated bare soil and rock (Table 1). Limited wetland (W1) communities occur in small areas of low‐lying valleys in the north, extreme south (Manîserqat (Green Valley)), and in isolated pockets across the peninsula (Figure S1). The B3c community is confined to the crystalline rock region of Pingorssuit (P Mountain) in the south central region of the Thule peninsula (Figure S1). The sparsely vegetated B1 and P1 communities are generally confined to regions of higher elevation or in close proximity to the Store Landgletscher, while the remaining plant communities (P2 and G1) are widely distributed across the peninsula and occur normally at lower elevations. [9] The generalized bedrock geology of the Thule region consists of Proterozoic‐age carbonate‐rich sedimentary rocks in the northern portion, Archaean‐age crystalline metamorphic basement rocks the southern region, and interspersed diabase sills and dikes [Davies et al., 1963; Escher and Pulvertaft, 1995]. Soils meet the criterion for Cryosols and Histosols [Food and Agriculture Organization (FAO), 2006] and are generally developed in glacial drift of mixed lithology or in bedrock outcrops as listed above. [10] On the basis of weather data from Thule Air Base (1978–2003), the mean annual air temperature of the study area is −11.9°C. The average maximum temperature over this record is 8.0°C, and the average minimum is −37°C (Thule Air Base, unpublished data, 2004). Growing season (June, July, and August) temperatures from 1978 to 2002 average 3.8°C. Mean annual precipitation is approximately 108 mm, with the growing season averaging 14.3 mm (Thule Air Base, unpublished data, 2004). Soil thaw usually begins in the first week of June, with freeze back starting in early to mid‐ September. 2.2. Soil Pit Locations [11] Seventy‐five soil pits were excavated and sampled during midsummer (July to August) in 2003, 2004, and 2005. Data from 55 of these pits are presented in this paper, as some pits were outside the bounds of the satellite imagery or were not described in sufficient detail for this study. Soil pit widths were typically 1 m wide with depths dictated by the base of the annual thaw layer, or in some cases the depth of high water tables or extremely gravelly soil conditions. The soil pits depths ranged from 20 cm in organic‐rich soils (peat over permafrost) to 96 cm in mineral soils. These active layer depths are similar to those observed in soil studies of the Canadian high Arctic by Muc et al. [1994]. Soil horizons were field mapped to scale, and digital photos were taken. Soil samples (∼500 g) were collected from diagnostic soil horizons (i.e., O, A, C, etc.), and soils were classified according
2 of 14
3 of 14
Endoskeleti‐Turbic Cryosol Turbic Cryosol Turbic Cryosol Gleyi‐Histic Cryosol Skeleti‐Leptic Cryosol Turbic Cryosol Turbic Cryosol Endoleptic Cryosol Turbic Cryosol Oxyaquic Cryosol Gleyi‐Turbic Cryosol Turbic Cryosol Lepti‐Turbic Cryosol Turbic Cryosol Turbic Cryosol Turbic Cryosol Chromi‐Calcic Cryosol (Skeletic) Endoleptic Cryosol Lepti‐Turbic Cryosol Gleyi‐Turbic Cryosol Parayermi‐Turbic Cryosol Turbic Cryosol
Plot
06‐2004 07‐2004 08‐2004 11‐2004 12‐2004 14‐2004 15‐2004 19‐2004 22‐2004 30‐2004 33‐2004 39‐2004 46‐2004 49‐2004 50‐2004 52‐2004 59‐2004 61‐2004 64‐2004 71‐2004 74‐2004 75‐2004
PSD PSD PSD Mire PD PSD PSD PSD PSD Mire PSD PSD PSD PSD Mire PSD PD PSD PD PD PD PSD
Vegetation Communitya P1 P1 P1 W1 B1 P1 P2 P1 P1 W1 P1 P1 P2 P2 W1 P1 B1 P1 B3c B1 B1 P1
CAVM Class 2 2 n/a 4 1 2 4 2 2 4 2 n/a 3 4 5 3 2 n/a 1 2 1 3
NDVI Class 0.12 0.08 n/a 0.32 0.01 0.09 0.28 0.08 0.09 0.32 0.08 n/a 0.18 0.29 0.39 0.17 0.04 n/a 0.00 0.05 0.00 0.16
NDVI Value 35 27 58 100 1 37 91 23 41 95 n/a 26 69 95 97 68 1 n/a 46 51 5 67
Percent Vegetation Cover b 23 15 32 74 1 23 53 15 22 41 n/a 22 60 54 53 51 1 n/a 12 11 4 39
Percent Vascular Cover Dryas moss crypto sedge n/a Dryas Cassiope Dryas Dryas moss n/a Salix Cassiope Cassiope moss Dryas n/a n/a crypto crypto Salix crypto
Dominant Species 1.9 20.0 n/a 5.8 0.0 2.3 4.3 17.1 n/a 6.2 n/a n/a 24.6 3.0 10.0 8.0 0.0 n/a 8.3 1.3 0.0 n/a
Aboveground Biomass (g/100 cm2) 82 47 71 34 80 86 60 54 97 50 70 80 24 70 44 54 80 40 59 80 60 52
Deepest Depth (cm) 160 267 343 107 381 236 236 259 267 221 15 343 213 168 183 152 152 213 533 472 328 262
Elevation (m)
0–1 5 1–2 0–1 0 6 5–6 0–1 1–2 0–1 1–2 3 17 1 0–1 2–4 6 16 8 1–2 2 0–1
Percent Slope
202 180 0 225 n/a 135 0 247 135 180 0 292 180 0 0 225 180 270 90 225 315 0
Aspect
7.92 7.88 6.11 15.16 4.83 7.38 11.19 15.15 1.49 8.00 4.84 14.15 9.44 7.52 22.87 6.65 0.46 2.54 1.33 2.95 3.09 3.07
Total Organic Carbon (kg/m2)
b
Bliss and Matveyeva [1992]. Includes nonvascular species; PSD, polar semidesert; PD, polar desert; P1, prostrate dwarf shrub, herb tundra; P2, prostrate/hemiprostrate dwarf shrub tundra; W1, sedge/grass, moss wetland; B1, cryptogam, herb barren; B3c, noncarbonate mountain complex; and G1, rush/grass, forb, cryptogam tundra.
a
FAO Soil Classification
Table 1. Summarized SOC Data and Site Characteristics for 55 Soil Pits in Northwest Greenland
GB3012 HORWATH BURNHAM AND SLETTEN: SOIL CARBON IN NW GREENLAND GB3012
GB3012
HORWATH BURNHAM AND SLETTEN: SOIL CARBON IN NW GREENLAND
GB3012
Figure 1. Color‐classified NDVI map from a 26 July 2004 ASTER image. The range of NDVI values in each of the eight classes is identical to those used in the CAVM map [CAVM Team, 2003]. Small NDVI values represent sparse vegetation cover, while high values represent dense or greener vegetation. to the World Reference Base for Soil Resources classification system [FAO, 1998, 2006] (Table 1). Additional parameters recorded for each site include slope, aspect, elevation, aboveground biomass, vascular vegetation cover, total vegetation cover (including lichens and cryptogamic crusts), degree of surface disturbance (i.e., patterned ground formation), and the dominant geological setting. Vegetation coverage was obtained by making point descriptions every 25 cm along a 25 m transect in four cardinal directions from the center of the pit. Aboveground biomass was obtained from a 10 × 10 cm sod sample collected at each site. Living biomass and litter were plucked from the sod, dried for 24 h at 60°C, and then weighed. [12] The selection of soil pit locations was based primarily on normalized difference vegetation index (NDVI) classes derived from an Advanced Spaceborne Thermal Emission and Reflection (ASTER) satellite image (15 × 15 m pixels in the visible near infrared) (refer to Horwath [2007] for details). Eight NDVI classes, representing an index of relative vegetation greenness, were selected on the basis of classes used by the Circumpolar Arctic Vegetation Map (Figure 1). Low NDVI values represent sparse vegetation cover, while high values indicate bright green vegetation and/or densely vegetated areas. Translating these NDVI classes to CAVM vegetation classes, B1 and B3c correspond to NDVI classes 1 and 2; P1, P2, and G1 correspond to NDVI classes 2–4; and W1 corresponds to NDVI classes 3–8. As the surface of the study area is extremely heterogeneous, NDVI provided an objective criterion for selection of sampling sites. Secondary land
surface characteristics such as bedrock geology, surficial geology (Quaternary sediments), and elevation were also considered in site selection in order to broadly distribute sites across the region and to capture the variability in geology and topography. Using GIS maps of the above characteristics, sampling sites were chosen by randomly selecting locations in the center of contiguous patches with similar NDVI values (to avoid possible edge effects) prior to beginning fieldwork. In the field, a GPS and laptop computer were used to navigate to the preselected locations. This was done to avoid a sample bias in the field. The number of pits in each NDVI class is loosely weighted by the areal coverage of each class across the study area to account for the amount of subsurface carbon variability. 2.3. Remote Sensing [13] A ratio of the red (R) and near‐infrared bands (NIR) (0.63–0.69 mm and 0.76–0.86 mm, respectively) (equation (1)) from a level 2 (reflectance) 25 July 2004 ASTER image was used to produce eight classes of NDVI (Figure 1). Of the cloud‐free, summer images available, this date was selected because of its proximity to peak biomass. Approximately 15% of the study area was covered by clouds and cloud shadows. These pixels were eliminated prior to the areal estimation of each NDVI class. To estimate areal coverage, the number of pixels in each class was counted in Photoshop (Adobe Systems Incorporated, San Jose, California, United States, version 6.0) on a density‐sliced color image generated using ENVI soft-
4 of 14
GB3012
GB3012
HORWATH BURNHAM AND SLETTEN: SOIL CARBON IN NW GREENLAND
Table 2. Summary of SOC and pH Data Based on Eight NDVI Classes in the Thule Regiona NDVI Class
2
SOC average (kg/m ) SOC maximum SOC minimum Standard deviation Average soil pH Number of pits Number of pixels Percent cover Area of class (km2) SOC per class (1012g)
1
2
3
4
5
6
7
8
Total
3.2 5.3 1.3 1.8 5.98 5 408,984 11.5 0.0 0.30
6.2 19.5 0.5 5.4 6.16 15 1,714,500 48.3 0.0 2.38
8.5 15.8 2.0 4.8 5.10 16 537,689 15.1 0.0 1.08
12.7 26.4 3.6 8.2 4.74 10 174,716 4.9 0.0 0.50
22.9 – – – 4.92 1 10,237 0.29 0.0 0.05
14.2 15.8 12.6 2.3 4.79 2 738 0.02 0.17 0.002
16.9 – – – 4.79 1 398 0.01 0.00 0.0015
25.9 – – – 4.35 1 124 0.003 0.000 0.0017
– – – – – 51b n/a 80.2c 640.7c 4.3
a
These data show how SOC varies across NDVI classes, how SOC is unevenly distributed across the study area, and the total estimated SOC storage to the top of the permafrost table for the Thule peninsula. b Additional four pits not included in these calculations, as they were outside of the ASTER image boundaries. c Remaining 19.8% (158.5 km2) found in snow, ice, lakes, and cloud cover.
ware (Research Systems, Incorporated, Boulder, Colorado, United States, version 3.5). The number of pixels was multiplied by the 15 m ASTER pixel size to estimate the areal coverage of each NDVI class (Table 2). NDVI ¼
NIR R NIR þ R
ð1Þ
Numerous “external” variables (e.g., shade, rock, bare soil, litter, etc.) contribute to the spectral signals used to calculate NDVI [Hope et al., 1993]. The combination of these variables produces a “mixed” pixel which integrates the various signals. Each variable contributes a unique spectral signature that affects the NDVI value above the soil pit (1 m2) and, in turn, is affected by the satellite averaging spectral reflectance signals over a 15 × 15 m area (one pixel size). While this may become an important issue later, no adjusted indexes (such as soil adjusted vegetation index (SAVI)) were used to remove external factors from the spectral signals. One goal of this research was to find correlations between SOC storage and surface parameters such as remotely sensed NDVI, so that this technique might be extended to other areas of the high Arctic to better estimate SOC storage. To that end, NDVI, one of the most commonly used indexes of surface vegetation assessment, was used for comparison to other Arctic carbon studies for potential extrapolation to larger areas. 2.4. Soil Particle Size Analysis [14] Soil samples were oven‐dried at 60°C for 24 h and sieved with a 2 mm mesh sieve on an electric shaker (Model 150, Derrick Manufacturing, Buffalo, New York, United States). To determine the percent gravel content of each sample, the
