Geoscience Reference
In-Depth Information
ice and intense solar radiation and heating. Various
consequences of exposure to beating rain have been
described, but a critical one is the physical breakdown
of soil aggregates caused by the repeated raindrop im-
pacts and the production of a 'seal' or 'crust' at the
surface, which may be of very low permeability. This
will be discussed in detail later.
seeking to understand desert soils and their moisture
regime. It is worth noting that other resources may
move with flows of water, including organic litter par-
ticles, seeds and nutrients. The contrasting locations
in the landscape, either generating or absorbing over-
land flow, create an important patch structure that can
greatly modify the hillslope or larger-scale responses of
the landscape to rain events. Because of this structure,
overland flow may only move small distances across
the landscape before being reabsorbed.
2. The open 'interspaces', the gaps between the canopies
of vascular plants, provide an environment where there
is sufficient light and moisture for other specialised or-
ganisms such as algae and lichens. Some of these can
draw moisture from the air, or derive it from dew in
sufficient amounts, and can build extensive communi-
ties that are collectively known as biological soil crusts
(BSCs). These biological communities may also occur
beneath plant canopies that allow sufficient light and
moisture to reach the soil surface.
In later sections, we will consider the first two factors
in greater depth, beginning with the key effects of rain-
drop impact on bare soils and then turning to the nature
of the communities of nonvascular plants forming biolog-
ical soil crusts. First, we begin our examination of desert
soils with a short examination of their classification and
nomenclature.
Consequently, dryland soil surfaces can have properties
quite different from the deeper subsurface where raindrop
energy has no effect, and where light does not penetrate.
Over vast areas of the drylands, it is the uppermost few
millimetres of the soil that determine infiltrability and
resistance to erosion (Patrick, 2002). As noted earlier, it is
not uncommon for the infiltrability of dryland interspaces
to be quite low. Thus, even in rain events of moderate
rain rate, water is partitioned into overland flow and little
may infiltrate. This perpetuates the dry conditions that
exclude vascular plants from most of the landscape, and
is an example of pedologic aridity. On the other hand,
the surface flow can move downslope where, if it can
be absorbed as run-on water at a more permeable site, it
can supplement the scant rainfall and allow more plant
growth than would otherwise be possible. This leads to
a third characteristic of many dryland landscapes, that is
intimately connected with soil properties.
7.2
Taxonomy of desert soils
Desert soils are not widely documented. Those classifi-
cation schemes for soils developed primarily to support
food and fibre production in wetter areas reflect the impor-
tance of drainage, nutrient status, horizon characteristics
and other aspects that relate to the capacity of the soil to
support functions such as cultivation, irrigation and crop
growth. They do not in general lend themselves well to
the classification of desert soils. Desert soils may show
very little horizon differentiation, a key taxonomic tool
in humid areas, but often show features such as accu-
mulations of soluble salts, which are rare in soils used for
agriculture. Furthermore, many of the most important fea-
tures from the hydrologic and geomorphic perspectives,
such as desert stone mantles, surface seals due to beating
rain, microbiotic crusts and vesicularity in near-surface
layers, are developed with little dependence on pedologic
characteristics, such as the nutrient status, mineralogy or
horizonation of the deeper regolith. Rather, they occur in
soils of diverse depth and mineralogy. Thus, though of
extreme importance in landscape behaviour, because
of their effect on rainfall partitioning and the production
of overland flow, these features too are of relatively little
use taxonomically, and instead depend on factors includ-
ing the distance from the nearest vascular plant, position
in the landscape and the evolutionary history of the site.
Table 7.1 lists the major soil groups recognised in the
US classification system. A fuller examination of the prop-
erties of these soil groups (e.g. see Nettleton and Peter-
son, 1983; US Department of Agriculture, 1988; Watson,
3. Soil hydraulic properties and soil water balance can
vary greatly over short distances (metres to tens of me-
tres) and soil properties, spatially, are in fact often very
patchy. The soils present a mosaic pattern of areas hav-
ing contrasting properties, some areas exhibiting high
infiltrability while adjacent patches show much lower
infiltrability. There can be considerable contrasts in soil
properties between subcanopy sites, which are shaded
and receive inputs of organic material, and more open
interspace sites. The production of overland flow on
one patch or set of patches can drive quite important
transfers of the key ecosystem resource, water, to other
patches downslope where it can be absorbed. In other
