Soil Nutrients in Drylands
It is well established that soil and water resources are not evenly spaced in dryland
environments. The spatial variability of rainfall and water results in patchy vegetation
cover, which in turn leads to resource islands of elevated productivity characterised
by increased organic matter, nutrients and microbial activity (Bennett and Adams,
1999). The work of Noy-Meir (1973, 1981, 1985) demonstrated that by concentrating
resources in patches the productivity of a landscape is greater than if resources were
evenly spread. This phenomena has been documented in a variety of dryland
environments, including the tiger bush landscapes of West Africa where the
vegetation is arranged in regular stripes ( ) or more commonly as irregular clumps of
vegetation in other dryland regions (see for example Tongway and Ludwig, 1994;
Bennett and Adams, 1999; Puigdefabregas et al., 1999).
Schlesinger et al. (1990) associated the development of spatial heterogeneity in soil
and water resources in the southwest United States with land degradation. Intensive
grazing reduced grass cover and resulted in an invasion of woody shrub species.
Once established, the bush encroachment was difficult to reverse because of the
development of `islands of fertility’ under the canopies. However, Ludwig et al.
(1996) found that if the patchiness of the resources was lost then there was a decrease
in the capacity of the landscape to capture rainfall as soil water by 25%, resulting in a
decrease in net primary productivity of 40%. They concluded that patchiness was a
vital component for landscape functioning and biodiversity in savannas.
The processes that lead to the concentration of soil and water resources in drylands
are just starting to be understood. Work in Honda, southern Spain has shown the
exchange of sediment and water occurs at two scales, i) at the patch scale where
vegetation is important in creating spatial heterogeneity and ii) at the hillslope scale
where sources and sinks are connected through first order channels and spatial
distribution controlled by topography and lithological factors (Puigdefabregas et al.,
1999).
Tongway and Ludwig (1994) suggest the regulation of resources heterogeneity occurs
through the density, morphology and spatial distribution of perennial plants as they
obstruct wind and water flows resulting in the retention of materials. Working in the
chenopod shrublands of Western Australia they found dunes developed around shrubs
and resulted in the localised concentration of nutrients and increased infiltration rates.
Dean et al. (1999) found that the large tree species of Acacia erioloba and A.
haematoxylon in the Nossob River valley of the Kalahari increase the nutrient content
of underlying soil because they attract animal activity. Nitrogen and potassium were
twice, and phosphorus 2.5 times that of adjacent areas.
Both Tongway and Ludwig (1994) and Schlesinger et al. (1996) found grasses also
led to small-localised increases in nutrients. In grasslands litter bridges between
adjacent plants slow overland flow, increase infiltration, reduce erosion, and help
retain water and nutrients. In degraded areas, where the grass cover is reduced, there
are fewer bridges and therefore higher volumes and velocities of overland flow. The
density of grasses is, therefore, vital in controlling erosion and as such these systems
are very sensitive to grazing pressures.
Landscapes, where plant regulation mechanisms predominate are generally regarded
as more vulnerable to degradation than areas where topographic factors predominate
(Tongway and Ludwig, 1994; Schlesinger et al., 1996; Puigdefabregas et al., 1999).
Another feature of dryland soil nutrients is that they are preferentially concentrated in
the surface layers (Tongway and Ludwig, 1994; Dougill et al., 1998; Bennett and
Adams, 1999). In part, this is due to the widespread occurrence of biological soil
crusts (Eldridge and Tozer, 1996; Rosentreter, 1997; Belnap et al., 2003), which are
able to fix atmospheric nitrogen (Belnap 1994, 1995, 2003; Evans and Lange, 2003)
and sequester carbon (Beymer and Klopatek, 1991; Evans and Lange, 2003).
Nitrogen in one of the main limiting factors to plant growth in drylands and, therefore,
the distribution of biological soil crusts is therefore vital in understanding soil
fertility. Vascular plant cover has an important influence on biological crust cover at
smaller scales. It is commonly reported that there is an inverse relationship between
biological crust cover and vascular plant cover because they are in direct competition
for light and moisture. Certain plants also have an allelopathic effect on the
microorganisms forming crusts and prevent their development (Rychert et al., 1978;
Skujins, 1984). Light is an essential factor to growth of the microorganisms forming
biological crusts and areas of the soil in constant shade, such as directly around a
shrub canopy base, are not ideal habitats for crust formation (Friedman and Galun,
1974). However, bush canopies can provide protection from disturbance, create shade
which controls the heat and light reaching the soil surface all of which can be
beneficial to growth (Belnap et al., 2001). The fine root systems of many plants can
also encourage cyanobacteria to colonise soils (Scott, 1982).
Recent work in the Kalahari has shown the relationship between crust cover and
vegetation cover is not simple (Thomas et al., 2002). Around all bush species, there
is a concentric pattern of unconsolidated soil and crust; the size of each zone varies
with bush species. The understorey of Acacia mellifera was shown to be ideal for
crust formation due to a combination of the grazing deterrent effect of the dense
double thorns and the small leaves, which allow light to reach the soil surface. The
bush, therefore, provides ideal conditions for crust formation with minimal
disturbance and high light levels reaching the surface soil. Grewia flava, in
comparison, has no thorns and large leaves producing a thick cover of litter on the
soil. Consequently, despite the similar canopy dimensions, crust development was
greatly reduced under this species.
This complex relationship between vegetation type and biological crust formation and
its relationship to the development of `islands of fertility’ has not previously been
studied. To fully understand the spatial and temporal dynamics of soil fertility and
degradation processes in dryland environments this additional layer of complexity
needs to be fully investigated.