Distribution of Organic Matter

Compared to other ecosystem types, arctic and alpine vegetation have low plant biomass and stocks of carbon, mainly because of the lack of a tree stratum and the often spotty vegetation. For

GLOBAL BIOGEOCHEMICAL CYCLES i.V THE CLIMATE SYSTEM

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TABLE 1 Comparisons of Carbon Pools in Arctic-Alpine Tundra, in the Neighboring Boreal Zone, and the World's Total.

Total carbon (1012 kg)

Area (106 km) Soil(gm 2) Vegetation (g m-2) Soil/Vegetation Soil Vegetation Soil + Vegetation

Arctic and Alpine tundra 10.5 9200 550 17 96 5.7 102

Boreal woodlands 6.5 11,750 4150 2.8 76 27 103

Boreal forest 12.5 11,000 9450 1.2 138 118 256

Terrestrial total 130.3 5900 7150 0.8 772 930 1702

['he soil pools do not include the most recalcitrant fractions. (After McGuire et al., 1997).

instance, in the Terrestrial Ecosystem Model (TEM), which has been used to simulate regional and global C fluxes, the stock of fixed C per unit area in the arctic and alpine vegetation is assumed to be about 550 g/nr. This corresponds to 6-7% of the amount per unit area in the neighboring boreal forest and 8% of the global average (McGuire et al, 1997; Table 1). In contrast, excluding the most recalcitrant fractions with turnover times of a millennium or more, the amounts of "reactive" soil C per unit area approach those of the boreal forest and of the transition zone of open woodlands between the forest and the tundra and are about 50% higher than the terrestrial average. These differences in distribution give a soil to plant C ratio of approximately 17 in the arctic/alpine regions, compared with 1.2 in the neighboring boreal forest and a terrestrial average of 0.8 (Tablel).

The C content in the soils of the Arctic is usually estimated at about 14% of the total global soil carbon (Post et al, 1982). However, estimates of both soil and vegetation carbon in the northern ecosystems vary considerably, both in estimates across the entire region and in estimates of the content in major vegetation types (Bliss and Matveyeva, 1992; Oechel and Billings, 1992; Gilmanov and Oechel, 1995; Shaver and Jonasson, in press). In spite of the variability, all estimates agree that there is a general trend from the southern to the northern Arctic of decreasing amounts of organic matter incorporated into both soils and plant biomass (Table 2). It also appears that the soil organic matter (SOM) pool decreases from oceanic toward continental regions. For instance, Chris-

tensen et al (1995) found much lower C pools both in wet coastal polygonal tundra and in mesic sedge-grass tundra in the region east of the Yamal Peninsula than in western Siberia. The much lower C pool in east than in west Siberia coincides with lower summer temperature and precipitation, reflecting the higher con-tinentality of the climate. Also, the l4C gradients in soil profiles were steeper in the east, indicating lower C accumulation rates there (Christensen et al, 1999).

Despite these patterns, there is generally more variation among ecosystem types within a given region than within a single ecosystem type across the entire latitudinal range of the Arctic (Shaver and Jonasson, in press; Table 2). For instance, organic matter content and primary production in both vegetation and soil can vary by more than three orders of magnitude across ecosystem types within the same region while the between-region variation in single ecosystem types is less than 10-fold (Shaver and Jonasson, in press). Similarly, the estimated ratio of organic matter content in soil and vegetation between similar (and dominating) ecosystems across the regions varies 2- to 5-fold, while the within-region variation among ecosystem types is at least 200-fold (from less than unity to 40), as in the low Arctic. It is reasonable to assume that the ratio of soil to plant carbon reflects the "end result" of various processes that control atmospheric C sequestration through photosynthesis and net primary production (NPP) on one hand, and organic matter turnover and respiratory C losses in soils and plants on the other. This can be

TABLE 2 Estimates (g/m2) of Orgai

nie Matter Mass in

Soil, Plant Biomass, and Net Primary Production (NPP)

in Main Arctic Ecosystem Types.

Soil

Vegetation

NPP

Soil/Vegetation

Soil/NPP

Vegetation/NPP

Area (% of total)

Low arctic

Tali shrub

400

2600

1000

0.15

0.4

2.6

3

Low shrub

3800

770

375

4.9

10

2.1

23

Tussock/sedge

dwarf shrub

29000

3330

225

8.7

129

15.8

17

Wet sedge/mire

38750

959

220

40

176

4.3

16

Semidesert

9200

290

45

32

204

6.4

6

High arctic

Wet sedge/mire

21000

750

140

28

150

5.4

2

Semidesert

1030

250

35

4.1

29

7.1

18

Polar desert

20

2

1

10

20

2.0

15

After Shaver and Jonasson (in press) based on data from Bliss and Matveyeva (1992) and Oechel and Billings (1992).

After Shaver and Jonasson (in press) based on data from Bliss and Matveyeva (1992) and Oechel and Billings (1992).

exemplified by almost identical C fixation and respiratory C loss measured in a high arctic wet sedge ecosystem in northeast Greenland (Christensen et al, 2000) and in a similar ecosystem type in southern low arctic Alaska (Shaver et al., 1998). The relatively small latitudinal differences relative to the local variability suggest that landscape-scale variations in environment and disturbance are stronger than the control associated with large-scale variations in climate.

Part of the large variation in estimates of biomass and NPP is, however, methodological. There is a 5-fold variation among studies and years in estimates of aboveground biomass and NPP at a single site (Toolik Lake tussock tundra)(Epstein et al., 2000). The largest variation reflects whether nonvascular plants were included or excluded and the definition of "live moss." Moss biomass varies 10-fold among studies at that site. However, even aboveground vascular biomass is more variable than can be readily explained by spatial or annual variability and probably depends on where the ground surface is defined by different investigators—a detail seldom described in published methods. Belowground biomass is even more variable among studies, and estimates are highly sensitive to methods and to the care with which roots are separated from organic matter.

There appears to be a bimodal pattern of soil-to-plant - C ratios across the entire Arctic, with large areas of both low and high ratios. About 75% of the Arctic consists of ecosystems with a maximum ratio of 10 and the remaining 25% has a ratio of over 20 (Table 2). Hence, the overall "Arctic mean" of about 13 (Table 2) or cold regions mean of 17 willi alpine regions included (Table 1) has limited biological relevance, assuming that the ratios actually reflect biological processes. The differences become even more evident in the ratios of accumulated soil organic matter and net primary production, ranging from less than unity to about 200 (Table 2). Basically, this indicates a pronounced difference in organic matter incorporation and turnover among the different arctic ecosystem types. The differences also indicate that cold regions have a large potential capacity to both sequester and lose C if the climatic conditions change and that these changes affect the controls of C accumulation and loss.

The variations in the ratio of soil organic matter and NPP are closely coupled to variations in snow accumulation patterns and hydrologic regime, which often depends strongly on topography. Wet ecosystem types always have high content of soil organic matter and high SOM/NPP ratio, regardless of latitudinal position. In contrast, wind-exposed and dry ecosystems have lower amounts of soil organic matter and low SOM-to-NPP ratios, except for low arctic semideserts with a very high ratio (Table 2). This suggests that hydrological conditions play an overriding role for the ecosystem processes and that the hydrological regime is the main determinant shaping the ecosystems. The topographic variation in SOM/NPP ratio also indicates that water exerts its effects on carbon storage primarily by restricting decomposition in wet sites rather than by restricting production in dry sites. This coupling between topography and hydrological features creates a mosaic of ecosystem types with distinctly different structure and function within short distances in the arctic landscape (Shaver and Jonas-son, in press).

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