Hot desert soils—Global distribution and unique characteristics
Introduction
Hot desert soils are areas on Earth's land surface with sparse vegetation and animal life caused by extreme heat and low amounts of precipitation (Fig. 1). As a consequence of residing in a hot dry climate, these soils have unique profiles, long recognized as “non-flushing” types of soil (Rode, 1955). Because of their large global extent, hot desert soils are important sinks and sources of inorganic carbon, as well as sources of aerosol nutrients for oceans and continents downwind (Schlesinger, 1982; Monger et al., 2015a; Yu et al., 2015). Despite their dryness and heat, when irrigated hot desert soils can be very agriculturally productive, provided sufficient irrigation water is available to remove salts from the soil profile (Dregne, 1983).
As defined here hot deserts include the areas of Earth with aridic soil moisture regimes and thermic-hyperthermic soil temperature regimes (Soil Survey Staff, 1999). Hot desert soils, therefore, fall into the arid (< 250 mm) and hyperarid (< 50 mm) classes based on mean annual precipitation classes alone (Fig. 2a). When taking into account the effects of mean annual precipitation plus temperatures and seasonality of precipitation, hot desert soils also fall into some areas classified as steppes (Fig. 2b).
Hot deserts occur on all continents except Antarctica (Fig. 3). Total arid soils, when including all temperature regimes, comprise some 26,324,771 km2 (Table 1). This area not only includes all Aridisols, but also the dry Vertisols, Oxisols, Andisols, and Entisols that occur within the aridic soil moisture regime. The area of all arid soils, including those with mesic, frigid, and cryic soil temperature regimes, comprises 20% of the total ice-free land. Of this amount, hot desert soils are the dominant component given their vast sizes in Africa, the Middle East, North and South America, and Australia (Fig. 3). These hot arid regions include the Sahara, Kalahari-Namib, Somali-Chalbi, Arabian, Iranian, Thar, southern half of the Turkestan, Mojave, Sonoran, Chihuahuan, Peruvian, Atacama, Monte, Gran Chaco, and Australian Deserts (McGinnies et al., 1968).
An overview of the characteristics and distinguishing features of arid-region soils are well discussed by Dregne (1976). Arid-region soils possess many unique characteristics that distinguish them from their more well-known counterparts in the humid regions. They commonly have a low level of organic matter, slightly acid to alkaline reaction (pH) in the surface, calcium carbonate accumulation somewhere in the upper 125 cm of soil, weak to moderate profile development, coarse to medium texture, and low biological activity. Frequently, in both the cold and the hot arid zones, soils will be covered by a thin layer of stones and gravels that constitutes a desert pavement. Soluble salts may be present in quantities sufficient to influence plant growth, particularly in poorly drained depressions, in irrigated areas, and in soils containing appreciable amounts of gypsum. The detailed discussions of desert soils are also presented by Dunkerley (2011). The importance of these soils extends beyond the margins of the global drylands, and extends their significance even to global environments.
Increased comprehension of hot desert soils can be obtained by placing them in the framework of factors ➔ processes ➔ features (Targulian and Goryachkin, 2004). Most of the soils developed in the arid regions of Iran and New Mexico, USA are classified in Aridisols and Entisols (Roozitalab et al., 2018; Monger et al., 2011). Soil forming processes and the formation of diagnostic horizons in different regions of Iran and New Mexico, are mainly influenced by the overall semi-arid and arid climate and high calcareous conditions of the soils. Calcium carbonate redistribution, gypsum accumulation, soil salinization and alkalization are therefore the major processes in these climatic and soil conditions. In New Mexico USA, most Pleistocene Aridisols that formed in igneous parent material on stable geomorphic surfaces have argillic horizons (Gile, 1975b).
This paper discusses the climatically-controlled interdependent factors, how those factors provide the framework for pedogenic processes, and how those pedogenic processes produce morphological features unique to hot desert soils.
Section snippets
Factors operating on hot desert soils
Soil has long been recognized as being the product of five soil-forming factors (Dokuchaev, 1883). For hot desert soils, four of the factors are very interdependent as shown in Fig. 4. Time, the 5th factor, can be conceived as Fig. 4 coming toward the reader along the Z-axis, being steered through time in an X-Y direction by any combination of the other factors, but especially climate.
Locations of hot desert soils is primarily controlled by atmospheric high pressures at the latitudes of 20 to
Pedogenic processes operating in hot desert soils
The same pedogenic processes of additions, transformations, transfers, and removals that occur in humid-temperate soils also occur in hot desert soils, but at much different rates (Simonson, 1959). As the result of little water and much heat, the pedogenic processes listed below are affected in the following ways as compared to humid-temperate soils.
Surface horizons
Surface horizons in hot desert soils include (1) the low-organic matter ochric epipedon, (2) the silty vesicular yermic horizon often occurring beneath desert pavements, (3) the barren takyric horizon of dried lakebeds, and (4) small areas of the anthropic epipedon formed by human activity. Desert pavement, desert varnish, and biological soil crusts are also uniquely common surface features of hot desert soils as described below.
- (1)
Desert pavement is a surficial layer of rock fragments usually one
Summary
Hot desert soils are a member of soil types formed in an extreme environment on Earth. Low moisture and high heat differentiate these soils from normal soils that have ample moisture and moderate temperatures necessary to produce ecosystems of vascular plants and support agriculture. These moisture/temperature extremes also differentiate hot desert soils from other extreme soils, such as cold polar soils, high-precipitation/high-heat tropical soils, and chemically toxic soils. Hot desert soils
Declaration of Competing Interest
None.
Acknowledgements
The senior author would like to thank Dr. Hossein Tazikeh and Mohsen Soleimanzadeh (Gorgan University, Iran) for their help in preparation of the thin sections.
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