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during ponding, but it is not clear whether such results
primarily reflect the effects of the crust removal and soil
disturbance and not of the crust itself. Presumably, in the
absence of BSCs, soil surfaces might develop inorganic
seals, and then the infiltrability would again be modified.
(a)
7.5.10
Effects of biological crusts on soil
stability and erosion resistance
The situation is somewhat better understood in relation to
local effects of crusts on erosional processes. In essence,
many crusts provide structures that link and bind soil par-
ticles into a more coherent structural framework able to
offer greater resistance to dislodgment by raindrop splash,
overland flow and wind. Indeed, the surface roughness of
mosses is known to promote the trapping of aeolian dust,
increasing accessions of exogenous materials to some dry-
land soils (Danin and Ganor, 1991). As with soils gener-
ally, the increased contribution of organic matter is likely
also to produce a better aggregated and more stable soil
structure. For example, laboratory splash cup experiments
with soils inoculated with cyanobacteria or autoclaved to
prevent their growth suggested that splash losses were re-
duced by >90% on both fine and coarse textured substrates
by cyanobacterial colonies (Hill, Nagarkar and Jayawar-
dena, 2002). This result cannot be extrapolated directly to
field conditions, of course, owing to the lack of strict corre-
spondence of conditions (lack of overland flow and so on).
In a multiyear study in semi-arid badlands in southeast-
ern Spain, it was found that bare sites eroded at moderate
rates but that slopes with lichen crusts were essentially
stable (Lazaro
et al.
, 2008). This does not establish that
the lichens directly reduce erosion, since these organisms
only become established on stable sites (Eldridge and
Greene, 1994), so that the pattern of crust types might
be affected by spatially varying erosion rates, as well as
partly contributing to that pattern.
Studies have often employed simulated rain of a single
intensity in order to explore the effects of BSCs on ero-
sion. For example, Kinnell, Chartres and Watson (1990)
exposed soil monoliths to overland flow and the impact
of simulated rain at 64 mm/h. They found that sediment
concentrations on uncrusted (but sealed) soils were 3-4
times higher, at 2-4 g/L, than from samples having BSCs
(approximately 0.2-0.8 g/L). Results on splash loss of
soil monoliths under simulated rain at 45 mm/h showed
that soil loss declined exponentially as the fraction of
the soil surface covered by BSC increased (Eldridge and
Greene, 1994). In tests lasting 20 minutes, soil loss fell
from
(b)
Figure 7.10
Closer views of bryophyte crusts, dominated by
Riccia limbata
, western NSW, Australia. Note the extensive cov-
erage of the soil surface in both (a) and (b), providing protec-
tion from striking raindrops: (a) in active state following rain;
(b) appearance following a long rainless period.
a moss-dominated crust yielded on average 3.2 times more
runoff than south-facing plots dominated by cyanobacte-
ria (Kidron, Barzilay and Sachs, 2000). Despite this, the
crust biomass was greater on the north-facing slopes, the
explanation apparently being that the cooler aspect al-
lowed soil wetness to persist for much longer following
rain, so permitting greater time for crust growth. Signif-
icant effects of slope aspect were also found in a study
of lichen crusts in the Tabernas Desert in Spain, where
morning sun on east-facing slopes shortened the time for
which dew was present and able to support photosynthesis
(Del Prado and Sancho, 2007).
Experimental studies have often involved the 'scalping'
or careful removal of crusts, with infiltration tests then
made on the exposed substrate (e.g. Graetz and Tongway,
1986; Williams, Dobrowolski and West, 1995; Eldridge,
Zaady and Shachak, 2000). Results have suggested that
300 g/m
2
with no BSC cover, to
50 g/m
2
at 75%
