SPATIAL DISTRIBUTION AND CHARACTERISTICS OF CYANOBACTERIAL
SOIL CRUSTS IN THE MOLOPO BASIN, SOUTHERN AFRICA
1Thomas,
A. D. and 2Dougill, A. J.
– Department of Environmental and Geographical Sciences, Manchester Metropolitan University, John
Dalton Building, Chester Street, Manchester, M1 5GD, U.K
1
2
– School of the Environment, University of Leeds, Leeds, LS2 9JT, UK
Abstract
Dryland soils are typically covered in a biological soil crust consisting of cyanobacteria, lichens
and mosses. These living crusts can reduce erodibility, fix atmospheric nitrogen and sequester
carbon. Despite this, there are few studies on the occurrence and impact of biological crusts in
Southern Africa. This paper provides a morphological-based classification of crust types in the
Molopo Basin of Southern Africa and examines the importance of substrate, disturbance and
vegetation cover on their spatial distribution.
Three biological crust types were found with distinct morphologies and properties. Species of the
cyanobacteria Microcoleus were, however, dominant in all crusts. Hardness, chlorophyll, and
total nitrogen increased with crust development.
Where crusts were present NH4+-N
concentrations were greater at the surface, suggesting crusts are vital in retaining plant-available
nutrients in the root zone. Crusts were widespread at all sites (25 - 56 % of ground cover) despite
high levels of disturbance, but were most prevalent on soils developed on ironstone and calcrete.
Disturbance reduced the diversity of crust types by restricting the growth of the better-developed
crusts. Vegetation plays an important role in the spatial distribution of crusts with clear patterns
around bushes. Soil under the sub-canopy of Acacia mellifera is particularly well suited to crust
development with a combination of optimal light levels and protection from disturbance. As
total nutrient concentrations are enhanced in the cyanobacterial soil crusts, that are preferentially
formed under A. mellifera canopies, there is potential for a positive feedback mechanism that can
help to explain the spread of bush encroachment in Kalahari rangelands.
Keywords: Biological soil crusts; Dryland Soils; Kalahari; Land Degradation;
Cyanobacteria
1
Corresponding author. Fax: + 161 247 1568
Email addresses: a.d.thomas@mmu.ac.uk (A.D. Thomas), adougill@env.leeds.ac.uk (A.J. Dougill)
2
Fax: + 113 343 6716;
1
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. Previous soil hydrochemical process-based research in the
Kalahari of Southern Africa (Dougill et al., 1998) has shown that in the typically sandy
soils nutrient retention and cycling remain focused in the topsoil even following intensive
grazing, and associated ecological changes.
The factors enabling topsoil nutrient
retention, and thus resilience to degradation (Dougill et al., 1999) remain poorly
understood and require analysis of soil surface characteristics as provided here.
Biological soil crusts, made up of communities of cyanobacteria, algae, lichens and
mosses, typify many dryland soils (Belnap and Lange, 2003). There has been growing
global recognition of the environmental significance of biological soil crusts 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). Despite their
2
prevalence and influence on fertility they have been largely ignored in previous studies of
African dryland soils (see Ullmann and Büdel, 2003 for a recent review). They have
many important functions, including; retaining soil moisture, discouraging weed growth,
reducing erosion by wind and water, fixing atmospheric nitrogen and sequestering
carbon. A variety of environmental factors influence the distribution of crusts at a range
of scales (Eldridge, 2003). 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 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
(Skujins, 1984). 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 scale- and
site-specific.
Crusts are sensitive to physical disturbance. Belnap (1996) estimates that they can take
250 years to recover after trampling by animals or humans. She argues that soils, which
3
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.
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 heterogeneity of soil and
climatic conditions and the large number of species forming biological crusts mean that
there is considerable variation in their range. 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 with only one report of the presence of biological
crusts in the western Kalahari of Botswana (Skarpe and Henriksson, 1987).
This paper aims to provide an analysis of the distribution of biological soil crusts in the
Molopo Basin, Southern Africa and to assess their role in affecting nutrient
characteristics, and thus the resilience of Kalahari soils. It contains the first morphologybased classification of biological soil crusts in the Molopo Basin on the south-eastern
edge of the Kalahari. The objectives are threefold:
4
1. To identify different biological crust types and determine their physical and chemical
characteristics;
2. To determine the factors influencing 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 the impact a surface crust cover has on the soil nutrient content.
2. 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). Livelihoods are highly dependent 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 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.
Rangeland fertility is vital for the success of smallholder farmers due to their dual
reliance on livestock grazing and manure inputs for arable production (Dougill et al.,
2002). The population density and thus intensity of agricultural land use is higher than
5
elsewhere in the Kalahari because of the relocation of outside populations by Apartheid
policies and intensive agricultural development projects. Recent assessments by UNEP
(1997) and Hoffman and Ashwell (2001) have concluded that the Molopo Basin is
experiencing land degradation through a variety of processes including both water and
wind erosion.
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 on the ridges varies across small spatial scales. This enabled
the investigation of a range of different soil, vegetation and disturbance characteristics on
crust development.
3. Research Design and Methods
To investigate the variability in crust characteristics between sites of different substrate,
and within sites, a nested sampling framework was developed and used. This entailed
demarcation of a 50 metre by 50 metre grid (though a 30 m by 30 m grid was used on
ironstone site due to the dense thorny bush cover) in sites typical of the vegetation
community on each substrate (see Table 1 for site details). Within the demarcated grid a
dual-sampling framework of 5 m line transects and 1 m2 quadrats was used (as shown in
Figure 3). Three parallel 50 m transects were split into ten 5 m line transects along which
the following variables were quantified: soil surface morphology (using crust
6
classification scheme detailed below), vegetation cover (% by species), the number of
cattle tracks crossing the transect and the number of dung pats within a 2 m wide swathe
of the transect. The latter two grazing disturbance variables were used to provide a
livestock disturbance index based on the method of Perkins and Thomas (1993) that has
been used in other Kalahari soil and ecological studies (Dougill and Thomas, 2004).
Every 5 m a more detailed analysis of soil surface characteristics was conducted in 1 m 2
quadrats. This included estimates of surface morphology classification, including
differentiation of cover in sub bush canopy sites and open sites, and also involved
measurements of crust depth (assessed after breaking surface and measuring depth at 5
places in quadrat) and crust hardness (measured using a hand held soil penetrometer).
Samples of the different crust morphologies found in a quadrat were also collected by
carefully removing samples of intact crust (typically from 0 – 5 mm depth), with a sub
crust soil sample also being collected from 10 mm depth where crusts were sampled.
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 as there is a strong relationship between crust morphology and their ecological
function.
Therefore, the classification developed (Figure 4) uses the form and
morphology of different crust types to provide an objective classification of soil surface
conditions. Subsequent testing of each crust phase has shown each to have significantly
different ecological, physical and chemical properties (Dougill and Thomas, 2004),
7
justifying the use of a morphology-based classification. Crusts of increasing surface
discolouration and microtopography are assumed to have an increasing biological
component and to represent different stages in crustal succession.
Available nutrient concentrations in crust and soil samples taken were measured within
two days of sampling using a portable field spectrophotometer.
This 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 after extraction with distilled water at a 1 g: 5 ml ratio.
Samples of all crust types and unconsolidated soil were then 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 Kjeldahl 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.
8
Statistical analysis of the differences between chemical and microbiological
characteristics of the assigned crust types was performed using appropriate parametric
analytical methods. Significant differences in mean characteristics were compared using
t-tests and are only stated when p < 0.05.
4. Results and Analysis
Findings are presented in relation to the three research objectives; namely the
identification of crust types and their physical characteristics; an assessment of the spatial
distribution of crust types; and, investigation of the nutrient characteristics of each crust
type.
4.1. Crust Classification and Characteristics
First stage biological crusts are weakly consolidated and have no surface discolouration,
but bacterial sheath material is visible below the crust. 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 analyses confirm the presence of extensive networks
of Microcoleus species in the first stage crusts.
9
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 cyanobacteria as the predominant species.
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
where night-time winter temperatures frequently fall below 0 oC. Need to say something
about their spp make up.
4.2. Spatial Distribution of Crust Types
There are significant 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 (c. 25 %). 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.
10
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 5 shows the
relationship between the level of disturbance at each site and the cover of each crust type.
This indicates that the least developed crust types are resilient to relatively high levels of
disturbance, as they are able to rapidly reform. However, the better-developed stage 3
crusts are only found in areas of lower disturbance.
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. Typically, a zone of crust
formation is found under bush canopies. The size of each of these crust 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. Brachylaena rotundata is another
common bush species and the intermediate level of crust development under its canopy
reflects the larger leaves and consequently reduced light at the soil surface compared to
A. mellifera. 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.
11
4.3. 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 and salinity was consistently low.
The total -N and -P 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 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 repeated, however, with phosphate. As such, although crust development
appears to enhance total-nutrient content, this is not inherently associated with increases
in plant-available inorganic forms.
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, due to the enhanced adsorption and retention of nutrients within the
12
biological soil crusts. Similarly, all biological crusts had equal or reduced pH compared
to the soil below the surface.
5. Discussion
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). 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.
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 lower crust cover on Kalahari Sand is
probably a function of the greater component of fine sand (Table 1) that restricts
13
cyanobacterial colonisation to the large and mobile Microcoleus species (Belnap et al.,
2003b). The 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).
Stock
densities of cattle and goats are high in the Molopo 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
cyanobacteria species. A predominance 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 the biological crusts (Table 4). 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.
Clear patterns were also found around vegetation (Table 4). The characteristics of A.
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
14
under the canopy. G. 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. A. 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 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 a 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
15
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
process-based studies (Dougill et al., 1998), with crusts able to retain surface nutrients
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.
In the Kalahari, the main threats to agricultural sustainability are the ecological changes
associated with the transition of the grass-dominated rangelands into more uniform bushencroached 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.
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 (Noy-Meir 1973, 1985; Bennett and Adams,
16
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 by 25 %, resulting in a decrease in net primary
productivity of 40 %.
Patchiness, therefore, is 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.
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.
They found dunes forming around shrubs in the
chenopod shrublands of Western Australia resulted in the localised concentration of
nutrients and increased infiltration rates. Similarly, Dean et al. (1999) found that Acacia
17
erioloba and Acacia haematoxylon in the Nossob valley of the Kalahari increased 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
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 for degradation depend on the relative ability of bush and grass species
to both withstand grazing and to access surface nutrients. It is the dual ability of A.
mellifera to withstand grazing, but also to take up surface layer nutrients (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 a very real degradation
concerns exist.
18
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
19
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.
6. Conclusions
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.
They also provide surface stability and aggregation that reduces topsoil loss through wind
erosion. Soil resilience, however, 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. As total nutrient concentrations are enhanced in the more developed
20
biological soil crusts, that are preferentially formed under A. 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.
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25
List of Tables and Figures
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Grain size, disturbance and vegetation cover at each site
Crust characteristics at all sites - means and (standard deviation)
Crust type characteristics at each site (% ground cover)
Crust cover and bush species (mean values with s.d. in parentheses)
Crust chemical characteristics
Study Region
Profile of Sample Sites
Soil Crust Classification System
Crust types and chemical characteristics at and below the soil surface
Crust cover and type and disturbance at all sites
26
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
27
Table 2. Crust characteristics at all sites - 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
28
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
29
Table 4. Crust cover and bush species (mean values with s.d. in parentheses)
Bush Species
Canopy
Height (m)
Canopy
Width (m)
Area of Crust
under bush (m2)
Crust:Canopy
Height Ratio
Crust:Canopy
Width Ratio
Acacia
mellifera
1.50
(1.27)
1.87
(1.19)
2.13
(1.52)
0.42
(0.18)
0.33
(0.18)
Grewia flava
1.26
(0.29)
1.41
(0.47)
1.19
(0.71)
0.29
(0.12)
0.28
(0.15)
Brachylaena
rotundata
1.57
(0.45)
1.43
(0.55)
1.64
(2.18)
0.29
(0.21)
0.31
(0.19)
30
Table 5. Crust chemical characteristics
Surface Type
pH*
Salinity*
No crust
7.4  0.08
0.07  0.01
Alluvial 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
31