| Abstract: Soil formation in the far north takes place under drastic ecological conditions, including freezing temperatures most of the year, short periods of thaw, permafrost and minimum biological activity within the soil. Unfavorable soil conditions limit the development of bacteria and fungal organisms, and the acid reaction of humus horizons and the lack of nutritional material preclude the vital functions of nitrification microorganisms. All these factors taken together preclude the full development of many soil horizons. In tundra soils, the soil microfabric is concentrated in the form of coatings and cryogenic-coagulated aggregates which are common to the soil affected by permafrost. The main pedogenetic processes in tundra soils are cryogenetic aggregation, cryoturbation and the weakening of migrational processes. The process of global warming may have a profound effect on the development of soils of the far north.
Key words: tundra; pedogenesis; Gelisols; permafrost
Introduction
The tundra is the coldest of all biomes. The term tundra comes from the Finnish word meaning treeless plain. Tundra is noted for its frost-molded landscapes, extremely low temperatures, little precipitation, poor nutrients, low biotic diversity, simple vegetation structure, limited drainage and short growing seasons.
Tundra is located in the northern hemisphere that encircles the north pole and extends to south of the coniferous forests of the tiaga. The arctic is known for its cold, desert-like conditions. Where the growing season ranges from fifty to sixty days. The average winter temperature is -340 C, but the average summer temperature is 30 to 120 C, which allows the biome to sustain life. Yearly precipitation, including melting snow, averages 15 to 25 cm. Soil is formed very slowly in the tundra environment (Campbell and Clairidge 1987). Underneath the thin layer of soil is a layer of permafrost, consisting of mostly gravel and finer material. The permafrost prevents the water from percolating downward, so the water gathers at the top forming bogs and ponds. As a result fo the permafrost, there are no deep-root systems in the vegetation of the tundra.
Arctic Soils
Arctic soils fall under the major soil category known as Gelisols, which are the permafrost-affected soils (Soil Survey Staff 1999). These soils comprise 18 million km2 or about 13% of the Earth’s land surface and occur in the Arctic, Antarctic, Subarctic, Boreal and some alpine regions. The Gelisol soils are of global concern because they contain many protected areas, support numerous indigenous populations, and may be subject to considerable impacts from human development and global warming.
Gelisols are defined as soils having permafrost within 100 cm of the soil surface, or gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface. Gelic materials are seasonally or perennially frozen mineral or organic materials that have evidence of cryoturbation (frost churning), ice segregation or cracking from cryodessication (Bockheim, et al 1997). Gelic materials contrast with other kinds of materials in being entirely defined on the basis of phyical and thermal characteristics, rather than chemical properties.
Permafrost and the Occurance of Gelisols
Permafrost is defined as a thermal condition as "earth material that remains continuously at or below 00 C for at least two consecutive years" (van Everdingen, 1998). At the surface is the "active layer" which thaws in summer and refreezes in winter (Bockheim and Hinkel). Permafrost may be ice-cemented, or, in the case of insufficient interstitial water, dry. In the frozen layer, a variety of ice lenses, vein ice, segregated ice crystals and ice wedges are evident.
An important consideration is that the permafrost is in a dynamic equilibrium with the environment (Bockheim, 1999).
Classification of Gelisols
There are three suborders within the Gelisol order: Histels, Turbels and Orthels. These suborders are differentiated on the basis of organic matter content and for mineral soils whether or not there is cryoturbation.
Histels: Histels have more than 80% (by volume) of organic materials from the surface to a depth of 50 cm. Histels are commonly associated with palsas (a permafrost mound containing a core of alternating layers of segregated ice or peat or mineral soil), peat hummocks and centered lowland polygons.
Turbels: Turbels are mineral soils that occur on areas with patterned ground. Patterned ground is a general term for any ground surface with a discernibly ordered, more-or-less symmetrical morphological pattern of ground. This suborder has marked influence of cryoturbation.
Orthels: Orthels are mineral soils containing permafrost within the upper 100 cm, but lack cryoturbation. These soils occur in areas with dry permafrost such as floodplains and dry valleys.
How Tundra Soil Forms
Tundra soils are in an unfrozen state for some two or three months of the year during which time the active layer is in a relatively dry condition. The rate and depth of seasonal thaw varies with texture. Because of the electrolyte content of the soil solution, tundra soils do not freeze until the temperature drops considerably below the freezing point of water (Tedrow 1955). These soils are characterized by a cold, wet active layer, which is that layer of ground above the permafrost which thaws in summer and freezes again in winter. This active layer is comprised of various mineral and organic matter. Because vascular plants are nearly absent in this environment, the organic content of the soil is fairly low and is mainly provided by lichens, algae and diatoms (Birkland). Though the general range of thickness of the permafrost is approximately 200 to 350 meters in northern Alaska (Bryan), at numerous sites, such as escarpment areas and narrow ridges, the active layer is unusually thick...up to three to five feet. At certain well-drained locations, a distinctive layer of brown soil forms, on top of which forms a cover of dwarf heaths, herbs, sedges, grasses, lichens and mosses. Since the permanently frozen layer is impervious to water that accumulates, drainage water moves sluggishly downslope through the surface layer above the frozen soil and eventually finds its way to a stream. In essentially flat areas drainage is extrememly slow.
Gelisols tend to form on sites where there is considerable depth to the permafrost and where the surface layers remain stable and are not subject to flooding or solufluction, the slow downslope movement of waterlogged soil. Frost action and related phenomena are widespread throughout the arctic regions, and evidence of them is visible in the soil profiles.
Cryopedogenic processes that lead to gelic materials are driven by the physical volume changes from water to ice, moisture migration, or thermal contractions of the frozen material by continued rapid cooling. These processes include freezing and thawing, cryoturbation, frost heaving, thermal cracking and ice segregation (Tedrow 1977).
It should be emphasized that cryopedogenic processes are soil-forming processes characteristic of soils with permafrost and should not be viewed as operating against other soil-forming processes in lower-latitude soils; rather, they are distinctive processes producing horizons and properties that are uncommon to other soil orders. Processes that are common to other soil orders operate in Gelisols but at a lesser magnitude because of the dominance of cryopedogenic processes (Bockheim & Tarnocai 1998).
Special Problems in Managing Gelisols
Gelisol present special problems in terms of management, not only because of frost churning, heaving sorting and cracking, but also because of melting of segregated ice following a disturbance to the thermal regime leading to subsidence, or thermokarst. To preserve the integrity of structures such as buildings, roads and pipelines in permafrost soil, it is important to maintain the negative thermal balance of the soil. This is achieved by using special construction methods. For agricultural development, it is important to determine the ice content of the soil; otherwise, after clearing the land or within a few years after cultivation begins, severe subsidence and thermokarst can develop (Pewe).
References
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