Geoscience Reference
In-Depth Information
explanations for this behaviour in terms of the processes
active on the surface were considered to involve the ef-
fects of the crusts on surface roughness and on aggregate
stability. Additional surface roughness may have resulted
in ephemeral ponding of water at some locations that was
sufficiently deep for the mechanical effects of raindrop im-
pact on the surface to be cushioned. The structural binding
effect of the crust organisms is also likely to diminish the
breakdown of soil aggregates into smaller components
that can be more readily moved by airsplash.
BSCs also modify the nature and rate of wind erosion
processes on dryland soils (Williams et al. , 1995). Using
a portable wind tunnel at field sites in Utah, Belnap and
Gillette (1997) showed that intact moss and lichen crusts,
with considerable surface roughness, were more than 500
times as resistant to wind erosion than was bare sand and
more than 60 times the resistance of flat, cyanobacterial
crusts. Additional data of this kind were presented by
Belnap and Gillette (1998), who showed that stones lying
on the soil and inorganic seals also conferred wind erosion
resistance. In both of these studies, the enhanced resis-
tance to wind erosion arising from biological crusts was
shown to be diminished quite readily by surface trampling
or disturbance. At dryland sites in southeastern Australia,
Eldridge and Leys (2003) used a portable wind tunnel
to explore the wind resistance of crusted soils that had
been mechanically disturbed by hand (using raking and
rotary cultivation). They showed that as the cover frac-
tion of BSC diminished, the flux of eroded soil increased
logarithmically. Very similar trends were shown for both
loamy and sandy soil materials, though the fluxes of wind-
eroded soil were greater for the sandy soils (Figure 7.11).
The various experiments with BSCs and wind erosion
cannot fully represent natural conditions owing to their
very small spatial and temporal scales. The very short-
term wind erosion experiments need to be supplemented
by longer-term studies of the cover of biological crusts
in drylands exposed to the range of stresses from people,
grazing stock, wind and occasional rain events.
Damage to biological crusts by vortex wind systems
(willy willies or dust devils), which are common over
many drylands in summer (e.g. Oke, Dunkerley and Tap-
per, 2007; Oke, Tapper and Dunkerley, 2007), have not
been explored. Often, wind systems of this kind carry sand
particles that can be very abrasive. In a similar way, winds
arriving with a considerable upslope fetch (not represented
in small wind tunnel trials) can be loaded with erosive sand
grains. In the face of such conditions, BSCs, especially the
thinner cyanobacterial crusts, which may be curled at the
edges and so allow the entry of wind and eroding particles,
can readily be fragmented and carried away (Figure 7.7).
In collisions with saltating sand grains, the mechanical
strength characteristics of the crusts are especially sig-
nificant. Measurements of relevant properties, such as
compressive and tensile strength, have been made in the
laboratory. For example, Wang, Zhou and Zheng (2006)
measured the tensile strength of a crust from Shapotou
in China, and reported rupture strengths of 30-42.8 kPa.
These values are comparable to those reported for natural
aggregates from soils of southern Australia (Dexter and
Chan, 1991), suggesting that the additional strength aris-
ing from the microorganisms in the crusted sand can be
similar to that arising from the cohesion generated in soils
by silts and clays. The elastic behaviour that some BSCs
exhibit allows surface deformation to reduce the impact
stress caused by saltating grains, but nevertheless sus-
tained impacts progressively cause a loss of structural in-
tegrity (McKenna Neuman, Maxwell and Boulton, 1996;
McKenna Neuman and Maxwell, 2002). In the drylands of
western NSW, Australia, the rainfall climate is strongly
modulated by multiyear quasi-cycles of above and be-
low average rainfall, related to the ENSO phenomenon.
In long and severe droughts connected with this climatic
system, abrasion of cyanobacterial crusts has been ob-
served during several extended droughts, becoming pro-
gressively more complete through the course of several
months. Clearly aligned, wind abrasion and scour fea-
tures result on exposed surfaces and wind scour lifts away
the crust, which fragments further during transportation
across the soil surface. As in the case of wind abrasion
of inorganic seals discussed earlier, critical factors in the
wind erosion of BSCs include the location and availabil-
ity of upwind sources of entrainable sand particles and
wind erosion data of Elderidge & Leys (2003)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
10
20 30 40
biological crust cover (%)
50
60
70
Figure 7.11 Plots of data from Eldridge and Leys (2003)
showing the way in which the wind erosion rate of loamy
and sandy soils declines with increasing stabilisation of the
soil surface by biological soil crusts. The soils had been artifi-
cially disturbed by raking prior to the experiments, in order to
 
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