SPATIAL DISTRIBUTION AND CHARACTERISTICS OF BIOLOGICAL SOIL
CRUSTS IN THE MOLOPO BASIN, SOUTHERN AFRICA
1
Thomas, A.D.* and 2Dougill, A. J.
– Department of Environmental and Geographical Sciences, John Dalton Building, Chester Street,
Manchester, M1 5GD, U.K; Fax: + 161 247 1568; a.d.thomas@mmu.ac.uk
1
– School of the Environment, University of Leeds, Leeds, LS2 9JT, UK; Fax: + 113 343 6716;
adougill@env.leeds.ac.uk
2
* - Correspondance to A.D. Thomas
Keywords: Biological soil crusts; Dryland Soils; Kalahari; Land Degradation
assessment
1
Introduction
Dryland soils are typically coarse and deficient in organic matter and nutrients, reflecting
the lack of moisture for vegetation growth and nutrient mineralisation. As a result they
have been widely reported as fragile and easily degraded with intensified agricultural use
(e.g. Oldeman et al., 1990; Pimentel et al. 1995; UNEP, 1997). Despite this, there is a
growing abundance of literature (e.g. Thomas and Middleton, 1994; Stocking, 1996;
Mortimore, 1998; Warren et al., 2001) that questions these conventional assessments.
This represents a paradigm shift in theories on dryland soils, emphasising the resilience
of their hydrochemical characteristics, rather than their fragility. To better understand
dryland soil resilience requires research into nutrient cycling and water retention
properties and processes.
Biological soil crusts, made up of communities of
cyanobacteria, algae, lichens and mosses, typify most dryland soils (Belnap and Lange,
2003). Despite their prevalence, they have been largely ignored in previous studies of
African dryland soils (see Ullmann and Büdel, 2003 for a recent review). This paper
aims to provide an analysis of the distribution of biological soil crusts in the Molopo
Basin, Southern Africa and to assess their potential role in affecting nutrient
characteristics, and thus the resilience of Kalahari soils.
There has been growing global recognition of the importance of biological soil crusts
(Eldridge and Greene, 1994; Evans, and Johansen, 1999; Belnap and Lange, 2003) and
they have been reported in numerous environments (see for example, Eldridge and Tozer,
1996; Karnieli et al., 1996; Rosentreter, 1997; Malam Issa et al., 1999). They have many
important functions, including; retaining soil moisture, discouraging weed growth,
reducing erosion by wind (Belnap and Gillette, 1997) and water (Alexander and Calvo,
1990), fixing atmospheric nitrogen (Rychert and Skujins, 1974) and sequestering carbon
(Beymer and Klopatek, 1991).
A variety of environmental factors influence the
distribution of crusts at a range of scales (Eldridge, 1999).
At a continental scale
temperature and rainfall are the greatest influences (Rogers, 1972). At the regional scale,
substrate is the predominant control (Johansen, 1993), with several studies showing that
biological crusts are less likely to develop on sandy soils due to their surface mobility
(e.g. Skujins, 1984; Belnap and Gillette, 1997). At a localised scale, vascular plant cover
2
has an important influence on biological crust cover. It is commonly reported (e.g.
Malam Issa et al., 1999) that there is a broadly 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 under a shrub canopy base, are not ideal
habitats for crust formation (Friedman and Galun, 1974). However, bush canopies can
provide protection from disturbance and create limited shade, which controls the heat and
light reaching the soil surface all of which can be beneficial to microbiological growth
(Belnap et al., 2003a).
The fine root systems of many plants can also encourage
cyanobacteria to colonise soils (Scott, 1982).
Consequently, the nature of crust -
vegetation relationships are complex and site-specific.
Crusts are also sensitive to physical disturbance. Belnap (1996) estimates they can take
250 years to recover after trampling by animals or humans. She argues that soils, which
are frequently disturbed, can only support large filamentous cyanobacteria as later
successional species are not able to form (Belnap and Eldridge, 2003), thus reducing the
ecological diversity and altering crust functioning. Marble and Harper (1989) found
biological crusts to be particularly susceptible to disturbance through mechanical damage
when dry and thus trampling by livestock to be one of the major inhibitors of dryland
crust development.
It is clear there are numerous factors influencing the development and distribution of
dryland biological soil crusts, notably substrate characteristics, vegetation cover and
disturbance levels. It is, however, difficult to isolate each causal factor because of the
complex interactions at a variety of spatial and temporal scales.
The extensive
heterogeneity of soil and climate conditions in drylands and the large number of species
forming biological crusts mean that there is considerable variation in their range (Skujins,
1984). It is surprising, therefore, that despite the extent and wide-ranging influence of
biological crusts there remains a dearth of evidence from the extensive Kalahari sandveld
3
with only one report of the presence of biological crusts in the western Kalahari of
Botswana (Skarpe and Henriksson, 1987).
This paper contains the first morphology-based classification of biological soil crusts in
the Molopo Basin on the south-eastern edge of the Kalahari sandveld of Southern Africa.
The aims are threefold:
1. To identify different biological crust types and determine their physical and chemical
characteristics.
2. To determine the controls on the spatial distribution of the different crust types,
particularly substrate, disturbance levels and vegetation cover. And,
3. To establish whether there is a significant difference in the nutrient content of the
different crust types and what affect a surface crust cover has on the soil nutrient content.
The implications of the findings for degradation processes and its assessment are then
discussed.
Study Area
The Kalahari is a large basin of wind-blown, nutrient deficient sediments (Thomas and
Shaw, 1990) characterised by sandy soils and an extensive vascular plant cover. Soils are
deep, structureless fine sands with limited organic matter, with primary productivity
restricted by water availability and to a lesser extent soil nitrogen and phosphorus
(Dougill et al., 1998). There is a high degree of livelihood dependency on traditional
communal grazing systems (Sporton and Thomas, 2002) leading to frequent and
extensive soil disturbance. The Molopo Basin (Figure 1) lies at the southern edge of the
Kalahari basin in the North West Province, South Africa and Southern District,
Botswana.
It is a semi-arid region, with a mean annual rainfall of c. 450 mm
concentrated in the summer-wet season (October – March). Rangeland fertility is vital
for success of smallholder farmers due to their dual reliance on livestock grazing and
4
manure inputs for arable production (Dougill et al., 2002). The population density and
thus intensity of agricultural land use is higher than elsewhere in the Kalahari because of
the relocation of outside populations by Apartheid policies and intensive agricultural
development projects in both countries.
Recent assessments by UNEP (1997) and
Hoffman and Ashwell (2001) have concluded the Molopo Basin is experiencing land
degradation through a variety of processes including both water and wind erosion.
The study sites were approximately 100 km west of Mafikeng in South Africa, between
the villages of Loporung and Tsidilamalomo, a site with a series of low parallel ridges of
calcrete and ironstone cutting across the Kalahari sand deposits (Figure 2). The soils and
consequently the vegetation developed on the ridges vary within a relatively short
distance. This enabled the investigation of a range of different soil, vegetation and
disturbance characteristics on crust development in a small area.
Research Design and Methods
Six locations along a 1500 m transect running perpendicular to the ridges were selected
with soil crust and vegetation characteristics determined within 50 m2 or 30 m2 grids
(depending on vegetation density).
The grids were located on Kalahari sands, an
ironstone ridge, colluvium derived from the ironstone, alluvium adjacent to an ephemeral
channel, colluvium derived from the calcrete and finally on the calcrete ridge (see Table
1 for site details). Within the grids, a dual-sampling framework of line transects and
quadrats was used. Soil crust cover (% by morphological type) and vegetation cover (%
by species) was assessed along three parallel line transects (of 30 or 50 m) at each
location. In addition, a metre square quadrat was placed every 5 m or 1.5 m (for 50 m
and 30 m transects respectively) and the following characteristics determined: total
biological crust cover; crust morphology and extent; vegetation species composition and
cover; crust depth and crust hardness (using a penetrometer). A relative level of
disturbance was also determined at each location using cattle track frequency (across line
transects) and dung density parameters (for all quadrats), using the methods of Perkins
and Thomas (1993). Samples of the different crust types and unconsolidated soils were
collected at this stage for subsequent chemical analyses.
5
To minimise post-sampling changes in available nutrients a portable field
spectrophotometer was used to determine extractable NH4+-N and PO43--P concentrations
according to the methods of Anderson and Ingram (1993). Salinity and pH of samples
were also determined in the field using portable probes. In addition, samples of all crust
types and unconsolidated soil were air-dried prior to laboratory determination of grain
size, organic matter, total-N and -P, total chlorophyll and chlorophyll a. Grain-size
distributions were determined on dispersed samples sieved at half-phi intervals in the
range - 1.0 to + 4.0 phi (2 mm to 0.063 mm) after removal of organic matter using H2O2.
Silt and clay were determined on the less than 0.063mm fraction using the sedimentation
method outlined in Rowell (1994). Organic matter was determined using loss-on-ignition
(Rowell, 1994). Total-N and total-P concentrations were assessed following a Kjedahl
digestion using the method of Anderson and Ingram (1993). Total chlorophyll and
chlorophyll a were determined colorimetrically after extraction with 85 % v/v acetone
according to the method of Allen (1989). This analysis was repeated after wetting
samples to investigate the microbiological response to moisture.
Preliminary light
microscopy analysis of the different crust types was also conducted to identify the main
microbiological constituents.
Results and Analysis
The characteristics of each crust type are presented and used to derive a classification
scheme for the different crust morphologies. Analyses of the findings are then provided
in relation to the aims outlined earlier, prior to subsequent discussion of the wider
significance to Kalahari sand soils and their resilience.
i. Crust Classification and Characteristics
There are many problems associated with the field identification and classification of
biological soil crusts due to the small size of the crust components and the difficulties
with identification of microbes to a species level (Eldridge and Rosentrenter, 1999).
Most classification schemes are therefore based on the surface form and morphology of
crusts (e.g. Danin et al., 1998), as there is a strong relationship between crust morphology
6
and their ecological function. Therefore, the classification developed and used in this
study (Figure 3) uses the form and morphology of the different crust types observed to
provide an objective and reliable classification of soil surface conditions. Subsequent
testing of each crust phase has shown each to have significantly different ecological,
physical and chemical properties, justifying the initial use of a morphology based
classification.
Crusted sites away from the alluvial channel and with no visible bacterial sheath material
were classified as physical crusts, forming through processes of raindrop impact and
relocation of fine sediments (Casenave and Valentin, 1992).
Thereafter, crusts of
increasing surface discolouration and microtopography are assumed to have an increasing
biological component and to represent different stages in crustal succession. This is
reflected in increasing chlorophyll content (Table 2). The chlorophyll content of the
biological crust samples was remeasured after wetting. In all cases this led to an increase
in both total chlorophyll and chlorophyll a, signifying that the initial determination was
measuring cyanobacteria rather than leaf litter or similar organic material (Table 2). With
the exception of the physical crusts (which are formed under very different conditions)
the hardness of surface soil crusts also increases with crust succession (Table 2). This
has implications for soil surface erodibility, which will be greatly reduced, where crust
development is most advanced and the surface soil is more strongly bound.
The
considerable hardness of the crusts is despite modest depths, typically of no more than 4
mm.
First stage biological crusts are weakly consolidated and have no surface discolouration,
but bacterial sheath material is visible below the crust when removed from the surface.
These appear equivalent to the class 1 crusts described for US sites by Belnap and
Gillette (1997) as `flat crusts, no visible lichen cover, low cyanobacteria biomass,
disturbed within 1 year’. The sheath material is indicative of the presence of species of
the genus Microcoleus, a filamentous cyanobacteria occurring in bundles, which
commonly initiates early stages of biological soil crust development. Microscope
7
analyses confirm the presence of extensive networks of Microcoleus species in the first
stage crusts.
Second stage biological crusts have a black or brown speckled surface that is more
consolidated than stage 1 crusts and typically 4 mm deep. Bacterial sheath material can
again be seen below the crust surface and in microscopic analyses, indicating the
presence of Microcoleus.
Third stage biological crusts have a bumpy (surface topography of up to 2 cm) and
intensely coloured black/brown surface. These appear equivalent to class 2 crusts under
the Belnap and Gillette (1997) system, which are described as, `moderately bumpy crust,
no lichen or moss, moderate cyanobacteria levels, disturbed 5-10 years previously’. The
surface topography is believed to originate from frost heaving prevalent in the Kalahari
climate where night-time winter temperatures frequently fall below 0 oC.
ii. Spatial Distribution of Crust Types
There are clear differences between the study locations in terms of both total and type of
crust cover (Table 3). The ironstone soils have the greatest biological crust cover (> 50
%), followed by the calcrete soils (c. 30 %). Crusts are least likely to develop on the
Kalahari sands. The cover of biological crust stage 3 crusts reflects the pattern of total
crust cover, such that soils with the greatest total cover also have the highest proportion
of this higher successional stage. Parent material is, therefore, of paramount importance
in regulating the nature and extent of crust cover.
Each site has a unique combination of vegetation, soil and disturbance characteristics, all
of which are likely to affect the development of surface crusts. Figure 4 shows the
relationship between the level of disturbance at each site and the cover of each crust type.
From this it is clear that the least developed crust types are resilient to relatively high
levels of disturbance, because they are able to rapidly reform. However, the betterdeveloped stage 3 crusts are only found in areas of lower disturbance. These data suggest
that disturbance, although reducing the diversity of crust types and the cover of more
8
established crust morphologies has little effect on total crust cover because of the rapid
formation of the Microcoleus-dominated crusts.
The relationship between vascular plant cover and biological crusts is complex and there
is no linear inverse relationship between crust and vegetation cover (r2 < 0.01). The
relationship is best described on an individual plant basis, with a concentric pattern of
unconsolidated soil and crust found around all bush species. The size of each of the
zones varies with bush species and the morphology of the canopy and leaves (Table 4).
Acacia mellifera is a bush with a dense canopy covered in double thorns that is highly
effective at reducing grazing under the canopy. The leaves of the Acacia spp. are small
and produce an incomplete cover on the soil surface. The bush, therefore, provides ideal
conditions for crust formation with minimal livestock disturbance and high light levels
reaching the surface.
Grewia flava, in comparison, has no thorns and large leaves
producing a thick surface cover of litter. Consequently, despite the similar canopy
dimensions, crust development is greatly reduced under this species. It is reasonable to
expect that such patterns will be repeated across wide areas of the Kalahari where the
substrate and vegetation cover are similar.
iii. Crust Chemical Characteristics
Salinity, pH, total-N and -P, extractable ammonium-N (NH4+-N) and phosphate-P (PO43-P) were determined for each crust type and at the soil surface where there was no crusting
(Table 5). All surfaces were neutral to slightly alkaline. There is a small decline in pH
with each biological crust stage. Salinity is consistently low in all surfaces. The total
nitrogen and phosphorus contents of the better-developed biological crust types 2 and 3
are both significantly higher than that of the less well developed type 1 crust (p < 0.01).
There is, however, no significant difference between the total-N and -P content of the
type 2 and 3 crusts (p > 0.05). Extractable NH4+-N is highest in the physical crusts,
which probably reflects the smaller grain-size and cation-exchange sites of the silty
material. Extractable NH4+-N concentrations in the first stage of biological crust
development are significantly higher than surface soils where there is no crusting.
Thereafter NH4+-N declines significantly with the biological crust stage. The trend is not
9
repeated, however, with phosphate. As such, although crust development appears to
enhance total-nutrient content, this is not inherently associated with increases in plantavailable inorganic forms.
iv. Soil Chemical Characteristics under Crusts
In addition to differences in characteristics between crust types it is important to establish
the effect the crusts have on the surrounding soil. The extractable NH4+-N concentration,
pH and salinity of each crust type and the soil immediately below the surface were
strongly correlated (Figure 5). However, there are some important differences. NH4+-N
concentrations were greater in the biological soil crusts than in the soil immediately
below, whereas in the absence of a biological crust the highest concentrations were found
below the soil surface. This suggests that crusts are reducing the movement of inorganic
nutrients to depth in soil profiles, due to the enhanced adsorption and retention of
nutrients within the biological soil crusts. Similarly, all biological crusts had equal or
reduced pH compared to the soil below the surface. This was especially apparent for the
stage 2 and 3 crusts. At sites without biological crusts the pH was highest at the surface.
The salinity of all types of biological crust was significantly lower than soils without
crust and the salinity is highest at the surface.
Discussion
Classification
This paper details the widespread occurrence of biological soil crusts in the Molopo
Basin of the Southern Kalahari and the factors affecting their spatial distribution in a
small area of differing substrates.
Classification of crust types using a visual
identification of morphology was shown to be applicable by verification with laboratory
analyses (Tables 2 and 5). Micro-geomorphological features of biological crusts have
been used previously to classify different stages of microbiotic succession on a series of
alluvial terraces in Israel (Danin et al., 1998). The three biological crust types found in
the Kalahari are shown to represent development of the crust biological components and
each had distinct properties. Hardness, and thus resistance to wind erosion, in particular,
increased with each stage of crust development.
10
Spatial Distribution
Substrate, vegetation and disturbance levels were investigated to determine their effect on
crust distribution and development. Of these, substrate was shown to have a dominant
influence. Kalahari Sand is the most common substrate in the southern Kalahari and
across the study area had an average biological crust cover of 25 %. At ironstone sites
over 50 % of the ground was covered in biological crust. Alluvium and calcrete sites had
an average crust cover of 40 % and 30 % respectively. Although these areas are less
representative of the wider Kalahari they form a significant part of the southern Kalahari
landscape (Thomas and Shaw, 1990). The limited crust cover on Kalahari Sand is
probably a function of the sandy surface that limits the opportunity for anything other
than the mobile and larger Microcoleus species of cyanobacteria to colonise (Belnap et
al., 2003b). The slightly finer, more consolidated soils developed on alluvium, calcrete
and ironstone are, therefore, better suited to crust development.
The level of disturbance (both human and from grazing animals) is a well-established
factor determining crust cover (Belnap, 1995; Belnap and Eldridge, 2003). In this study,
stocking levels of cattle and goats were high and all sites were frequently disturbed.
However, using a combination of dung and cattle-track density, it was possible to
differentiate the relative levels of disturbance at each site. Comparison with crust cover
showed disturbance restricts the development of biological crust succession (Figure 4).
Similar findings are reported in Belnap and Eldridge (2003) where disturbance is shown
to lead to a simplified community dominated by a few species of cyanobacteria.
However, the occurrence of high levels of stage 1 crusts in the most disturbed locations
suggests that Microcoleus spp. are able to quickly re-establish after disturbance. This has
important implications for both erodibility and soil fertility.
Stage 1 crusts are
significantly harder than sites without crusts (Table 2) and have the highest NH4+-N
content of all crusts (Table 5). Thus, although the poorly developed stage 1 crusts form
the majority of the crust types found on highly disturbed sites and across all Kalahari
Sand sites, they still have an impact on soil fertility and erodibility.
11
Clear patterns were also found around vegetation. The characteristics of Acacia mellifera
with its broad thorny canopies and small leaves encourage crust development by
providing an ideal combination of protection from grazers as well light penetration under
the canopy. Grewia flava, however, does not favour crust development as the bush
affords little protection from grazers and the large leaves restrict light levels reaching the
ground surface. Acacia mellifera is one of the main woody species responsible for bush
encroachment in the Kalahari (Thomas et al., 2000; Reed and Dougill, 2002) and the
preferential development of biological crusts under the canopy may well re-enforce this
pattern if the bush is able to utilise the nitrogen fixed by the crust. The shallow rooting
system of A. mellifera (Skarpe, 1990) suggests it may well be able to secure many of
these nutrients. Whether this bush encroachment represents degradation depends upon
the degree to which the sub-canopy niche can also support more nutritious grass species.
Findings elsewhere (Dougill and Thomas, in press) suggest that there are small increases
in the cover of the most nutritious grass species under bush canopies that could offset
reductions in ecological fodder diversity associated with A. mellifera encroachment.
Nutrients
Nutrients in dryland soils generally (Tongway and Ludwig, 1994; Bennett and Adams,
1999), and Kalahari soils specifically (Dougill et al., 1998), are preferentially
concentrated in the surface layers suggesting the role of crusts in affecting nutrient
availability is important. Total-N in all crusts was greater than at sites where there was
no crust (Table 5) and the better-developed type 2 and 3 crusts contained more total N
than the less developed type 1 crust implying that they may have an important N fixation
role. In contrast, there was no significant difference in available N (as extractable NH4+N) in biological crusts and soils without crusts. However, NH4+-N declines with crust
development and at crusted sites was preferentially concentrated at the surface whereas at
sites without crusts it was greater below the surface. This is a critical finding given the
slow mineralisation rates reported for Kalahari sand soils (Dougill et al., 1998) which
implies that the reduced inorganic-N pool is more important than increases in total-N for
the establishment of vascular plants. It also helps to explain the surface concentration of
available nutrients, and low rates of leaching of N in Kalahari soils observed in previous
12
process-based studies (Dougill et al., 1998), with crusts able to retain surface nutrients
and thus sufarce concentrations even with grazing-induced vegetation removal.
Available N, pH and salinity of the surface crust all correlated strongly with the
underlying soil (Figure 5) demonstrating the importance of crust nutrients for soil
fertility.
Implications for degradation
In the flat sandy savanna expanses of the Kalahari, the main threats to agricultural
sustainability are the ecological changes associated with the transition of the grassdominated rangelands into more uniform bush-encroached ecological communities (see
Dougill, 2002 for review). Similar problems are documented for many other semi-arid
rangeland environments and a better understanding of the environmental processes
regulating these ecological changes is therefore essential. Much recent discussion (e.g.
Schlesinger et al. 1990; Bennett and Adams, 1999) has focused on the links between the
spatial heterogeneity of soil and vegetation communities. It is in this regard that our
studies extend understanding of the processes regulating rangeland degradation, and
conversely the observed resilience of Kalahari grazing systems.
Previous studies have shown that the spatial variability of rainfall and water in dryland
regions results in patchy vegetation cover, which in turn leads to resource islands of
elevated soil fertility and ecological productivity characterised by increased organic
matter, nutrients and microbial activity (Bennett and Adams, 1999). By concentrating
resources in patches the productivity of a landscape is greater than if resources were
evenly spread (see for example Tongway and Ludwig, 1994; Bennett and Adams, 1999;
Puigdefabregas et al., 1999). Ludwig et al. (1999) 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, resulting in a decrease in net primary productivity. They concluded
that patchiness was a vital component for landscape functioning and biodiversity in
savannas. As resource heterogeneity is inextricably related to the distribution of both
vegetation and biological crusts they will have vitally important consequences for
degradation resilience.
13
The links between spatial resource heterogeneity and degradation processes and / or
ecosystem resilience remain unclear.
Schlesinger et al. (1990) associated the
development of spatial heterogeneity in soil and water resources in the southwest United
States with land degradation. They argued that intensive grazing reduced grass cover and
resulted in an invasion of woody shrub species and that once established, the bush
encroachment was difficult to reverse because of the development of `islands of fertility’
under bush canopies. The processes that lead to the concentration of soil and water
resources in drylands are just starting to be understood. Tongway and Ludwig (1994)
suggest the regulation of resource heterogeneity occurs through the density, morphology
and spatial distribution of perennial plants as they obstruct wind and water flows
resulting in sediment retention.
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. Similarly, Dean et al. (1999)
found that Acacia erioloba and Acacia haematoxylon in the Nossob valley of the
Kalahari increase the nutrient content of underlying soil because they attract large nesting
birds and mammals searching for shade. Our studies show that a further process leading
to enhanced nutrient concentrations around key bush species in the Kalahari is the
increased prevalence of well developed biological soil crusts in these sub-canopy niches.
The 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. Our findings suggest that crust distribution is related to the morphology of
bush canopies and that where biological soil crusts are formed nitrogen retention in the
surface layer is increased. This helps to explain the findings of process-based studies that
found no significant increases in nutrient leaching to depth in Kalahari soils, even
following vegetation removal and increased surface nutrient inputs (Dougill et al., 1998).
The implications of this to degradation assessment depend on the relative ability of bush
and grass species to both withstand grazing and to access the surface nutrients. It is the
dual ability of A. mellifera to withstand grazing, but also to take up surface layer nutrients
14
(due to its shallow rooting system) that make it the main encroaching species throughout
the Kalahari. The limited forage value of this species implies that very real degradation
concerns exist. Nutrient adsorption and retention characteristics of biological soil crusts
provide a possible explanation of the widely cited, but hitherto unexplained, resilience of
dryland soils. However, this soil resilience does not imply ecological resilience due to
the ability of A. mellifera to increase its spatial coverage independent of significant
changes in the soil hydrochemical properties (Dougill et al., 1999). Indeed, as crusts of
higher successional stages are formed preferentially under A. mellifera canopies it is
plausible that these crusts add to the competitive advantage this bush species is gaining in
grazed Kalahari rangelands.
The exact biochemical processes within crusts that fix, store and release nutrients require
further study before their ability to fix atmospheric nitrogen, sequester carbon and control
mineralisation is understood. Given the extensive cover of Kalahari soils, these processes
will also have an important, but largely unquantified influence over regional atmospheric
fluxes of carbon and nitrogen that are starting to be investigated (e.g. Scholes and
Scholes, 1998; Swap et al., 2003). Further studies of the biogenic emissions of NOx from
soil crusts and
15
N isotope studies of N fixation will clarify many of the key nutrient
cycling processes within Southern African soils.
In addition to the resilience to change in soil nutrient properties imparted by biological
soils crusts, they also have an important role to play in influencing erodibility. The soils
and ultimately the ecology of this dryland system are strongly affected by wind blown
sediment movements (Dougill and Thomas, 2002). Experimental studies on sandy
Australian soils by Eldridge and Leys (2003) suggest that a crust cover of over 20 % will
maintain low wind erosion losses due to the surface aggregation. Our findings show a
crust cover above this 20 % threshold at all sites (Table 3), including on Kalahari sand
soils, suggesting that crusts will contribute significantly to the limited wind erosion losses
observed within the Kalahari sandveld.
Conclusions
15
This paper has provided the first account of biological soil crusts in the Molopo Basin of
Southern Africa. It suggests a morphological-based classification of crust types and
relates it to succession of the microbiotic components. The importance of substrate,
disturbance and vegetation cover on crust distribution is reported and the nutrient content
of the crust types and the impact on surrounding soil chemical properties examined. A
better understanding of surface biological soil crusts is essential to improve
understanding of dryland soil functioning and assessments of degradation and resilience.
Our findings show that that the development, occurrence and functions of surface soil
crusts are vital in imparting the characteristic resilience of dryland soils. In particular, we
demonstrate that they can retain available N at the surface; even after the removal of
grass cover through grazing would be expected to enhance nutrient leaching to depth in
soil profiles. They also provide surface stability and aggregation that reduces topsoil loss
through wind erosion. It is however noted that this soil resilience does not intrinsically
imply ecological resilience due to the ability of the main encroaching bush species (A.
mellifera) to access nutrients from this surface layer.
Conversely, as nutrients
concentrate in the more highly developed biological soil crusts, that are preferentially
formed under Acacia mellifera canopies, there is the potential for a positive feedback
mechanism that can help to explain the rapid spread of bush encroachment in Kalahari
rangelands.
The wider environmental significance of biological soil crusts in the Kalahari remains in
urgent need of further research. In particular, we suggest that studies are needed to
examine their impact on nitrogen fixation and adsorption characteristics, carbon
sequestration potential and their control on nutrient mineralisation rates that control the
extent and nature of ecological growth following rainfall.
Acknowledgements
The authors are grateful for the financial support provided by the Manchester
Geographical Society and the Universities of Salford and Leeds. We also thank Kate
Berry and Jennifer Byrne who helped with the field data collection.
16
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21
Table 1 Grain size, disturbance and vegetation cover at each site
Location
Grain Size %
Vegetation Cover (%)
Disturbance
Coarse
Sand
Fine
Sand
Silt
Clay
Acacia
Spp.
Grewia
flava
Brach.
Rot.
Grass
Spp.
6
Cattle
Tracks
(/ 30 m)
2.5
Dung
Density
(/ 25 m2)
0.9
Kalahari
Sands
22.3
69.3
3.1
5.3
11
1
0
Ironstone
Ridge
26.9
61.8
4.0
7.3
15
4
10
13
2.4
0.2
Ironstone
Colluvium
22.5
65.2
7.7
4.6
25
3
12
5
3.3
0.4
Valley
Alluvium
25.8
67.0
0.5
6.7
5
29
0
26
6.0
2.3
Calcrete
Colluvium
23.8
60.2
11.2
4.8
20
13
13
10
4.3
1.4
Calcrete
Ridge
21.1
64.9
8.5
5.5
16
13
16
9
4.3
1.5
Table 2. Crust characteristics at all locations - means and (standard deviation)
Surface type
n
Depth mm
Hardness
kg/ cm2
Total
chlorophyll %
Chlorophyll a
(%)
Alluvial crust
10
9.82 (4.7)
4.05 (1.4)
-
-
Increase in
chlorophyll
a after
wetting (%)
-
Physical crust
22
3.48 (3.1)
1.37 (0.73)
-
-
-
Bio stage 1 crust
50
4.22 (1.98)
2.91 (1.67)
0.034 (0.012)
0.012 (0.004)
15.8
Bio stage 2 crust
72
4.04 (1.74)
3.44 (1.74)
0.043 (0.032)
0.020 (0.012)
45.6
Bio stage 3 crust
104
3.73 (2.02)
4.57 (1.71)
0.066 (0.029)
0.029 (0.021)
18.2
Table 3. Crust type characteristics at each site (% ground cover)
Location
Physical
Crust
Bio 1 crust
Bio 2 crust
Bio 3 crust
Alluvial
crust
Total
Crust
Kalahari Sands
8.0
16.0
1.3
-
-
25.3
Ironstone Ridge
-
2.7
12.0
36.4
-
51.1
Ironstone Colluvium
-
11.4
11.7
32.7
-
55.8
Valley Alluvium
-
5.1
4.6
14.6
15.1
39.4
0.8
2.7
13.7
14.8
-
32.0
-
1.1
7.6
20.7
-
29.4
Calcrete Colluvium
Calcrete Ridge
22
Asks for Figure Captions page, followed by the Figures
Table 5. Crust chemical characteristics
Surface Type
pH*
Salinity*
No crust
7.4  0.08
0.07  0.01
Physical crust
7.2  0.16
0.06  0.03
-
-
91.8  21.1
-
Bio 1 crust
7.5  0.06
0.05  0.02
0.05  0.005
1.7  0.25
83.7  29.7
5.2  0.8
Bio 2 crust
7.3  0.08
0.06  0.02
0.09  0.009
4.5  0.49
60.5  19.6
13.9  1.50
Bio 3 crust
7.1  0.04
0.06  0.01
0.08  0.006
2.4  0.44
48.6  18.6
7.2  1.36
*
Means with standard error
Total Nutrient
Concentration*
(%)
N
P
0.02  0.007
Extractable Inorganic
Nutrient Concentration*
(mgkg-1)
+
NH4 -N
PO43--P
65.7  28.9
- no data
23
Figure 4. Crust types and chemical characteristics at and below the soil surface
NH4-N (mg-kg)
100
80
60
Surface
40
Below
20
0
No crust Physical
crust
B1
B2
B3
7.5
7.4
pH
7.3
Surface
7.2
Below
7.1
7.0
6.9
No crust
Physical
crust
B1
B2
B3
0.10
Salinity
0.08
0.06
Surface
0.04
Below
0.02
0.00
No crust Physical
crust
B1
B2
B3
24