Geoscience Reference
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while taxonomically the soils might be similar, in terms
of their function in the landscape they most certainly are
not. In many areas, it has been found that key nutrients
(notably nitrogen) are concentrated beneath and around
woody shrubs, to form 'resource islands' (Gutierrez et al. ,
1993; Cross and Schlesinger, 1999; Ewing et al. , 2007).
Very often the spatial variability of desert soils can be
regarded as involving repeated patterns of two distinct
regolith forms, forming what is termed a binary mosaic
or two-phase mosaic . One component of the mosaic con-
sists of soils supporting relatively dense vascular plant
cover (grasses, shrubs, small trees) while the other is rel-
atively devoid of plants. For example, Tongway and Lud-
wig (1990) have described binary mosaics in semi-arid
mulga woodland (dominant tree species Acacia aneura )
where groves of trees alternate downslope with treeless
intergroves. In association with this pattern are changes
in the abundance of soil fauna, notably termites, which
preferentially inhabit the wooded subareas (e.g. see Whit-
ford, Ludwig and Noble, 1992). Thus, the mosaic actually
involves a spatial patterning in biological activity, nutrient
cycling, organic matter decomposition and related aspects
of soil development.
Spatial mosaics of soil like this have been reported
widely from arid and semi-arid areas (Dunkerley and
Brown, 1995). Their functioning is important to the con-
tinued existence of the plant communities that grow in
these spatial mosaics. The key to this lies in the hydro-
logic response of the soil surfaces. The intergroves have
bare, sealed surfaces often underlain by vesicular hori-
zons. They are quite impermeable and absorb very little
of the rain that falls on them. Rather, the rainwater is
efficiently shed as runoff, and this trickles downslope to
infiltrate in the more protected, more organically rich and
more porous soil of the vegetated phase of the mosaic. Lit-
tle or no water passes through a typical grove on to the next
intergrove downslope. Thus, with a 50/50 mosaic pattern,
the groves receive water equivalent to almost twice the cli-
matological rainfall, while the intergroves are about twice
as arid as the rainfall would suggest. It has been argued
recently that binary mosaics may be an ancient feature of
the desert landscape, developing from changes triggered
by the transition from last-glacial to Holocene climatic
environments (Dunkerley and Brown, 1995). For the ge-
netic taxonomy of soils, this behaviour is problematic,
because it results in soils having quite different leach-
ing characteristics, salinity and moisture contents within
metres of each other, and developed under a uniform ex-
ternal climatic environment. For example, Monta na et al.
(1988), in a study of patchy vegetation in the Mexican
Chihuahuan Desert, found mean organic matter levels of
2.6% within vegetation patches but only 0.8% in the in-
tervening bare spaces. These differences occurred over
spatial scales of 10-100 m. In western Australia, Mabbutt
and Fanning (1987) reported that, beneath mulga bands
10-20 m in width, a siliceous hardpan was typically lo-
cated more deeply within the soil beneath groves than be-
neath intergroves. In fact, they described the depression
of the hardpan as forming a 'trench' beneath the groves,
which might act as a trap for water infiltrating there. Apart
from this, the main differences in the grove and intergrove
soils were restricted to the upper few cm.
The hydrologic efficiency of the runoff/runon process
in two-phase mosaics was demonstrated by Tongway and
Ludwig (1990) using artificial rainfall. In their mulga in-
tergroves, runoff began after only 7 minutes of rain at
29 mm/h, equivalent to 3.4 mm rain depth. In contrast,
mulga grove soils showed no runoff from this rain appli-
cation. Some of the other clear differences between the
mulga grove soils and those of the intergrove runoff areas
are summarised in Table 7.4. It is evident that the contrasts
are especially marked in the top few cm of the soil.
Other forms of spatial heterogeneity have been reported
from desert sites. For example, Rostagno, del Valle and
Table 7.4 Summary of grove and intergrove soil properties, semi-arid mulga woodland, NSW, Australia (after Tongway and
Ludwig, 1990).
Electrical
Exchangeable
Cation exchange
conductivity ( ยต S/cm)
calcium (meq/100 g)
capacity (meq/100 g)
Organic carbon (%)
Depth (cm)
Intergrove
Grove
Intergrove
Grove
Intergrove
Grove
Intergrove
Grove
0-1
22.0
57.9
2.69
4.78
7.38
10.50
0.71
1.97
1-3
16.5
36.3
2.46
3.99
6.63
9.37
0.43
0.95
3-5
22.1
29.4
3.68
5.05
8.40
9.21
0.40
0.71
25-50
44.2
17.9
5.05
3.56
9.21
7.87
0.23
0.30
 
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