deserts soil


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Climate, deserts and desertification Deserts are dry areas characterized by low precipitation (<25 cm/yr), high evaporation potential and a lack of sufficient vegetation to support abundant animal life. Desertification is the conversion of non-desert areas into desert primarily due to human activity. Annually, about 70 000 sq. km of land are converted to desert (about the size of West Virginia).

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Why deserts? Global weather patterns Surface air at the Equator is warmer than surface air at high latitudes (near the poles). Convective flow of air occurs as warm surface air at the Equator rises buoyantly and migrates to higher latitudes, whereas cold surface air at high latitudes stays near the surface because it is dense, and migrates to the Equator. This pattern is complicated by the effect of the rotation of the Earth (Coriolis effect), resulting in a number of convective cells from Equator to pole.

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Warm surface air at the Equator rises producing a decrease in air pressure near the surface (equatorial low). As this warm air rises, it cools and its ability to hold water drops, so clouds form and precipitation occurs. The resulting dryer air continues to cool (and become denser) and moves to higher latitudes. Eventually this air sinks, producing a zone of high pressure (subtropical high). A similar process occurs at higher latitudes where cold polar air moves towards lower latitudes, warms and picks up moisture, and eventually rises to produce a low pressure zone (polar front). At the pole, cold, dry air sinks resulting in high pressure (polar high). The ability of air to hold water decreases with temperature

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Many deserts on Earth are located in subtropical or polar areas, where dry air associated with zones of high atmospheric pressure dominates. Note that much of the polar areas are deserts because they are characterized by low annual precipitation. Dry highs and wet lows

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World distribution of dry regions Although most deserts are either subtropical or polar, and related to global weather patterns, there are exceptions.

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Orographic effects Deserts also occur in the rain shadow of mountain belts. Moisture-rich air rises as it goes over the mountains, and loses moisture (precipitation). By the time the air makes it across the mountains, it is dry. Death Valley and the Great Basin are in the rain shadow of the Sierra Nevada mountains Death Valley

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Desertification Much (>70%) of the World’s non-desert dry areas (i.e., still capable of sustaining agriculture prior to degradation) have been degraded to some extent as a result of human activity. Dry land degradation worldwide Populations in dry areas subsist by exploiting a relatively fragile and unproductive land. Improper exploitation methods can lead to extensive deterioration of the soil, to where it can no longer sustain any cultivation.

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Causes of desertification Desertification results from changes in land use which lead to increased soil erosion. Increased soil erosion is promoted by processes which expose soil to erosive agents (surface runoff and wind). They include removal of native vegetation (including trees), overgrazing, leaving plowed soil exposed between growing seasons, etc.

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Soil as a resource Regolith is the layer of unconsolidated material at the Earth's surface. Soil is the layer of unconsolidated material at the Earth's surface which can support plant growth. Because of its ability to sustain plant growth, soil constitutes an extremely valuable resource. The nature of the soil influences the type of agriculture, and the agricultural productivity of a region. Loss of soil represents potential loss of food productivity.

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Weathering Soil forms by weathering of pre-existing rock or unconsolidated deposits (e.g., sediments, glacial till, etc.), and from the breakdown of organic material at or near the surface. Mechanical weathering is the physical breaking-up of bedrock or coarse unconsolidated material into smaller-sized fragments. This size reduction facilitates chemical weathering which is the transformation of original minerals in the bedrock or unconsolidated deposit into other minerals, through chemical reactions, and changes in composition due to removal or addition of certain chemical components.

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Mechanical weathering Frost wedging results from expansion of freezing water in fractures in the rock. It is a very effective mode of mechanical weathering in temperate to cold climate. Plants affect mechanical weathering through root action on the substrate on which they grow (e.g., root wedging). Mechanical weathering is facilitated by pre-existing weaknesses in the bedrock, such as joints.

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Chemical weathering Chemical reactions, often involving water, lead to formation of new minerals (clays, oxides, hydroxides), and removal or addition of chemical components. Chemical weathering is enhanced by presence of dissolved species that increase the acidity of infiltrating water.

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Products of chemical weathering Silicate minerals typically break down to form other minerals (clays, oxides, hydroxides). Other minerals, such as carbonates, completely dissolve in water leaving new pore space. marble (calcium carbonate) dissolution at Lincoln Memorial

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Chemical weathering Acid precipitation resulted in dissolution of calcite in tombstone over time. Intense chemical weathering of granite facilitated by exfoliation joints. Note the flaky, brittle appearance of the rock due to feldspar breaking down to clay minerals

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Weathering and soil Soil constituents: weathering products living organisms organic decay products

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Effect of climate on weathering Climate controls the rates of mechanical and chemical weathering. In cool, humid climates, mechanical weathering, in particular frost wedging, is important. Chemical weathering is favored by: 1. high humidity (more water as weathering agent) ; 2. high temperature (increases the rates at which chemical reactions proceed).

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Factors controlling weathering

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Soil profile Soils typically show a zonation from the surface to the unweathered basement including some or all of: a layer consisting primarily of organic matter (O horizon), a zone of leaching rich in organic matter (A horizon), a zone of leaching poor in organic matter (E horizon), and zone of accumulation (B horizon), and a zone of partially weathered basement (C-horizon).

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The nature of a soil is characterized by a number of properties (color, particle size and shape, mineral, organic, and chemical composition, structure, etc.). These properties depend on the starting materials and on the relative importance of the different weathering processes. Pedocal (rich in calcium-carbonates; typical of dry climate) Pedalfer (rich in Al and Fe, and relatively poor in calcium; typical of humid, temperate climate) Types of soil

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Types of soil: laterites In hot, humid regions, intense chemical weathering leads to leaching of all but the most insoluble components in the soil (aluminum and iron). This produces thick, clay-rich soils rich in Al and Fe oxide and hydroxide minerals, but generally poor in other elements (calcium, potassium, magnesium, etc.) necessary for plant growth. The soils are called laterites. Tropical rainforest soils are typically laterites. So how is the vegetable luxuriance possible??

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Native dryland plants are well suited to the dry conditions. They play an important role in stabilizing soil and storing moisture. They also serve as wind breakers. Removal of native vegetation to grow other plant species can lead to loss of soil if the new species do not thrive, or if soil is not covered year-round. Roots continually rework soil. If plants are removed, soil porosity may decrease, resulting in decreased water infiltration, and increased run-off and erosion. Soil degradation: Devegetation/cultivation

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Millet farm in sub-Saharan Africa left bare in off-season. Note lack of vegetation and how sand has blown over. Excessive working of soil in cultivated areas leads to enhanced soil erosion. Complete removal of plant cover after growing season exposes the soil to erosion and sand migration. Soil degradation: Cultivation/tilling practices

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Dust storm in Kansas, 1937 (Dust Bowl years).

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Devegetation and overgrazing Excessive use of plants for human and animal consumption can lead to permanent loss of vegetation. This problem of overgrazing is compounded during dry spells because animals consume plants not only for nutrients but as a source of moisture. At the same time, potential for soil erosion is increased because soil is dryer.

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Human impact The problem of loss of arable land by desertification is particularly acute in regions whose land-based subsistence is already precarious. Poor land-use practices in these areas can have disastrous consequence.

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Mitigation Mitigating desertification requires stabilization of soil and reduction of wind-driven sand (dune) migration. Revegetation stabilizes soil due to root action, increases soil moisture, and guards soil against water and wind erosion. Contained guaranteed grazing protects against roaming in and overgrazing of sensitive areas. Dune fixation limits sand migration.

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Mitigation Conservative tilling and planting practices can greatly reduce soil erosion and degradation. Contour plowing reduces down-slope runoff and sediment transport. Reduced tilling minimize soil exposure to wind. Residue from previous crops provide protection as well as soil nutrients for new crop.

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Mitigation: landmines?? In Kuwait, in the 11 years since the Gulf war, land mines have greatly reduced off-road riding by hunters. This has allowed natural revegetation and restoration of large expenses of degraded areas. Off-road vehicle use can lead to extensive soil degradation.

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The current global warming trend, spurred by anthropogenic release of greenhouse gases is speeding up the process of desertification worldwide. What should we do? Long-term outlook

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