The tundra biome is a treeless landscape found in high latitudes and mountain elevations, characterized by extremely cold temperatures and short growing seasons. The soil in these regions is particularly unique, playing a defining role in shaping the landscape and its ecosystems. Its properties are distinct from soils found in warmer climates, influencing all aspects of the tundra environment and supporting specialized plant and microbial communities.
Key Characteristics of Tundra Soil
A defining feature of tundra soil is permafrost, which refers to ground that remains at or below 0°C for at least two consecutive years. This permanently frozen layer can extend to significant depths, sometimes exceeding 1,500 meters (4,900 feet) in continuous permafrost zones. Permafrost underlies approximately 15% of the Northern Hemisphere’s land area, including large portions of Alaska, Canada, and Siberia. It does not necessarily contain ice, but its temperature must remain below freezing.
Above the permafrost lies the active layer, which is the uppermost soil layer that thaws during the brief summer and refreezes in winter. This layer can vary in thickness, typically ranging from about 12 inches (30 centimeters) to 10 feet (3 meters), depending on local climate, vegetation, and soil type. Plant roots can only establish themselves in this thawed active layer. The active layer also harbors microbial communities that process organic matter during the thawed period.
The presence of the impermeable permafrost layer directly beneath the active layer leads to poor drainage in tundra soils. Water from melting snow and summer precipitation cannot percolate deeply, causing the active layer to become saturated and waterlogged. This creates anaerobic, or oxygen-deprived, conditions within the soil, which influences chemical processes and microbial activity. The saturated conditions contribute to the unique wetland characteristics observed across much of the tundra.
Cold temperatures in tundra environments severely limit the rate at which organic matter decomposes. This slow decomposition leads to the accumulation of dead plant material, forming thick layers of peat, especially in poorly drained areas. Consequently, tundra soils often have high organic content, storing substantial amounts of carbon. This stored carbon is a significant component of the global carbon cycle.
Tundra soils also tend to be acidic, with pH values often ranging from 5.3 to 7.0, and sometimes as low as 3.5 in some areas. This acidity is linked to the slow decomposition of organic matter and the waterlogged conditions. The low temperatures and saturation inhibit microbial activity, preventing the breakdown of organic acids that would otherwise be neutralized. Soil pH influences nutrient availability and the composition of plant and microbial communities.
How Tundra Soil Forms and Is Classified
Soil development in tundra environments proceeds slowly due to consistently low temperatures and limited biological activity. The formation of these soils is heavily influenced by repeated freeze-thaw cycles. These cycles contribute to a process known as cryoturbation, or frost churning, which physically mixes soil layers.
Cryoturbation involves the expansion and contraction of water as it freezes and thaws, which can displace and deform soil particles. This churning action can move material from the surface downward and from the permafrost table upward, leading to irregular or disrupted soil horizons. Over time, this process results in a lack of distinct layering typically seen in soils from warmer climates.
The specific soil order that characterizes tundra regions is called Gelisols, a term derived from the Latin word “gelare,” meaning “to freeze.” Gelisols are defined by the presence of permafrost within 100 centimeters (about 3.3 feet) of the soil surface and show evidence of cryoturbation.
Gelisols are further categorized into suborders based on their characteristics. For example, Turbels show significant evidence of cryoturbation, often with irregular soil horizons. Histels contain substantial amounts of organic carbon, typically found in wet areas. Orthels encompass other Gelisols with less evidence of cryoturbation or high organic matter content.
Visible Patterns on the Tundra Surface
The unique soil dynamics of the tundra give rise to distinct, symmetrical patterns on the ground, known as patterned ground. These geometric shapes are a direct manifestation of freezing and thawing processes, with cryoturbation being the primary mechanism.
One common type of patterned ground is polygons, which appear as hexagonal or polygonal shapes on the surface. These often form around ice wedges, which are large, vertical ice masses that grow in cracks in the ground. As the ground contracts in winter, cracks form, and water seeps in and freezes, expanding the ice wedges and pushing up the surrounding soil to create raised borders.
Another observable feature includes circles, which can be sorted or non-sorted. Sorted circles typically have coarser materials like stones arranged at their edges, while finer soil is concentrated in the center. Stripes, on the other hand, are elongated patterns that form on slopes, resulting from the downslope movement of soil and stones influenced by repeated freeze-thaw cycles and gravity.