The development of Arctic soils is dominated by cryogenic processes, which are driven by the formation of ice in the soils. A number of models have been developed to explain the mechanisms involved in cryoturbation, which is one of the most common cryogenic processes in these soils. The most recent model involves the process of differential frost heave (heave-subsidence), which produces downward and lateral movement of materials (Walker et al. 2002; Peterson and Krantz 2003). Other processes, such as brunification and, especially, podzolization, are not common, probably because of the lack of leaching resulting from the shallowness of the active layer. Gleyic processes are common, and can occur in soils developed on various parent materials.

Soil properties such as soil texture, pH, salinity and the presence of carbonates depend on the parent materials. The nitrogen content of Arctic soils is generally very low, and has been regarded as a more limiting factor for plant growth than phosphorus and potassium contents (Broll et al. 1999). Other limiting factors for plant growth are low soil temperatures, high stone content and, in some cases, high carbonate content and the occurrence of salts (Bolter et al. 2006).

The high amounts of organic carbon stored in Arctic soils, and the relatively rapid warming of this region as a result of climate change, are probably the main reasons so much attention has been focused on these soils in recent times. These soils (both mineral and organic) have operated as carbon sinks for thousands of years. In general, small amounts of organic matter are produced annually by the vegetation. This organic matter is then deposited as litter on the soil surface, with some decomposing as a result of biological activity. A large portion of this litter, however, builds up on the soil surface, forming an organic soil horizon. Cryoturbation causes some of this organic material to move down into the deeper soil layers (Bockheim and Tarnocai 1998). In addition, roots contribute organic carbon that is also translocated by cryoturbation. Soluble organic materials move downward because of the effect of gravity and the movement of water along the thermal gradient toward the freezing front (Kokelj and Burn 2005). Once the organic material has moved down to the cold (0 to -15°C), deeper soil layers, where very little or no biological decomposition takes place, it may be preserved for many thousands of years. As a result, the average carbon content of cryoturbated, permafrost-affected mineral soils is approximately 49-61 kg m-2, while that of organic (or peatland) soils is 43-144 kg m-2 (Tarnocai et al. 2007).

Little is known about soils in much of the Arctic, because the harsh climatic conditions and the relative inaccessibility of most of this vast region have made such studies very difficult. We know even less about how the climate-warming that is already affecting this region will transform these northern soils and their properties.

Acknowledgments Thanks are due to Dr. Chien-Lu Ping of the University of Alaska, Fairbanks, for providing his unpublished pedon data.

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