Geoscience Reference
In-Depth Information
Desert soils
David L. Dunkerley
7.1
Introduction: the nature and
significance of desert soils
and surfaces upon which desert soil development takes
place, and on the particular properties of desert soils that
make their hydrologic response and erosional behaviour
distinctive. In interesting ways, the development of desert
soils is intimately connected with landscape development
and with the history of environmental change in the arid
and semi-arid regions, especially through the Quaternary
Period (see Chapter 3). Despite the lack of protective veg-
etation, some of the features of desert soils may contribute
to markedly lower rates of erosion than are seen in wetter
climates. Desert stone pavements in the hyper-arid Negev,
for example, have been shown to armour the surface to
an extent that yields exceptionally stable and long-lived
landforms (Matmon
et al.
, 2009).
As a background to the detailed study of desert soils,
we can note that the importance of these soils extends be-
yond the margins of the global drylands, and extends their
significance even to global environments. Dust is often
entrained from deserts and carried downwind to wetter
environments where vegetation taps dust, or rainout car-
ries it to the land surface. The result is soils that contain
exotic silts and clays, and whose depth and texture have
been modified by the accessions of desert dust. Grazing
pressure and other forms of land use in the dryland source
areas accelerate the deflation of dusts. The dusts have ad-
ditional effects so that, for instance, if the dust settles on
snowfields, it reduces the albedo and promotes melting.
Painter
et al.
(2007) showed that the duration of snow
cover in the seasonally blanketed San Juan Mountains
of Colorado was reduced by up to
In contrast to wetter areas, where soil mantles of pedo-
genically altered decomposed bedrock cover most or all of
the landscape, the surfaces of deserts may be only patchily
covered with soil. The surface over many desert uplands
is formed of outcropping bedrock, and in lower-lying ar-
eas there may be a cover of aeolian or fluvial sediments
so little modified that it hardly amounts to 'soil' by any
common definition (e.g. many fall into the
entisol
order
of the US system of taxonomy). In semi-arid areas, the ef-
fects of pedogenesis become more apparent, and a richer
array of soil types has been described from these envi-
ronments. Factors that contribute to climatic gradients in
soil properties include various climatic factors such as
rainfall and temperature, and also increasing biomass and
larger accessions of organic detritus in humid environ-
ments. Rainfall in drylands tends to decline with distance
from the ocean. Mean rates of rainfall decline are of-
ten around 1 mm/km, but the trend is really exponential
in form and regressions show that the rainfall declines
by 50% over distances of about 400 km (Makarieva and
Gorshkov, 2007; Sheil and Murdiyarso, 2009). In addition
to driving a decrease in plant cover, this pattern of dimin-
ishing rainfall results in less intense leaching of dryland
soils, and consequently quite steep regional gradients in
dryland soil properties occur. In areas of extensive forest,
rainfall decline with distance inland is much less evident
owing to the biotic recycling of precipitation, and biomass
and soil properties vary less steeply.
In the overview presented in this chapter, we will not
concentrate on the nutrient status or cropping potential of
desert soils. Rather, our goal is to review those aspects
that relate to the geomorphic evolution of the materials
1 month by the en-
hanced absorption of solar radiation caused by desert dust.
They speculated that the shortened snow season may have
been established in the 1800s following the introduction
of settlement and pastoralism in the western USA, with
its associated soil disturbance. Such changes in snowmelt
∼
