Geoscience Reference
In-Depth Information
et al.
, 1995; Berkeley, Thomas and Dougill, 2005; Gob-
erna
et al.
, 2007; Housman
et al.
, 2007). The structure of
nutrient pools, the distribution of organic matter and other
effects arising from these organisms may have their own
influence on soil hydraulic properties. The rhizosphere
(the zone of microbial activity associated with the root
systems of plants) forms another important component
of dryland soils that has significance for nutrient pools
and soil processes. Some discussion of the microorgan-
isms of the rhizosphere (as well as BSCs) can be found in
Bhatnagar and Bhatnagar (2005).
The identification and taxonomy of biological crust
organisms is a specialised pursuit. Reference should be
made to works such as Rosentreter, Bowker and Belnap
(2007) on the biological soil crusts of the western USA,
which contains a helpful glossary and collection of URLs
for internet sources of additional information. This and
other reports can be downloaded from www.soilcrust.org.
For the Australian drylands, Eldridge and Tozer (1997)
is a well-illustrated guidebook that includes dichotomous
keys to aid the identification of lichens, mosses and liver-
worts. Works on the biology of key organism groups in-
clude the extensive review of soil algae by Metting (1981),
the overview of crusts and their ecological roles by Evans
and Johansen (1999) and the major treatment of the bi-
ology and ecology of the cyanobacteria by Whitton and
Potts (2000). Belnap and Lange (2001) provided a com-
prehensive treatment of BSCs, with global coverage.
(a)
(b)
Figure 7.3
Biological crusts, arid western NSW, Australia: (a)
rugose crust of lichens and bryophytes on soil, where coin used
for scale is 23 mm in diameter; (b) lichen crust on calcareous
bedrock outcrop.
7.5.6
The habitats or niches exploited by
microphytic plants in drylands
Organisms exploit a range of habitats in drylands, grow-
ing on or within soil or rock, and also beneath small stones
(Figure 7.4). In the most hyper-arid drylands, conditions
are too severe for autotrophic photosynthetic organisms
to survive. Lichens and other crust organisms are largely
poikilohydric (lacking the ability to control desiccation)
and are thus vulnerable to weather and climate condi-
tions, especially if hot conditions occur while the organ-
isms are moist. For example, the central Atacama Desert
in Chile lacks any photosynthetic plants or cyanobacteria,
and indeed bacteria are only found with any abundance
within the soils at depths greater than about 20 cm (Moser,
2008). However, beyond such intensely dry locations, mi-
crophytic plants are found to be very widespread in lo-
cations where sufficient light reaches the ground. In the
hypolithic niche, algae and cyanobacteria are often found
growing beneath translucent quartz pebbles, as evidenced
by patches of green coloration on the undersides of stones.
Ocampo-Paus, 1967). Through a set of mechanisms some-
what different from those just discussed in relation to
inorganic seals, BSCs affect infiltration, overland flow
and erosion. We will now consider these biological crusts
and their place in dryland ecohydrological and erosional
processes. Biological soil crusts have considerable sig-
nificance in the sustainable management of drylands that
are used for pastoralism (e.g. Harper and Marble, 1988;
West, 1990; Bowker
et al.
, 2008) but the impact of graz-
ing pressure on BSCs is not covered here. In addition to
the surface and near-surface crust organisms, microbial
communities, including for instance heterotrophic bacte-
ria together with bacterivorous nematodes, rotifers and so
on, occur with depth in dryland soils, sometimes prefer-
entially in the sheltered 'resource islands' located beneath
and near the canopies of vascular plants but elsewhere dis-
