DISTRIBUTION AND CHARACTERISTICS OF CYANOBACTERIAL SOIL
CRUSTS IN THE MOLOPO BASIN, SOUTH 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 Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
Abstract
This paper provides an analysis of the physical and chemical characteristics of
cyanobacterial soil crusts in the Molopo Basin, South Africa. It details the influence that
soil type, livestock disturbance and shrubs have on the spatial distribution of crusts. Four
morphologically-distinct cyanobacterial crusts were identified and crust cover ranged
from 24 % to 55 %. Crust cover was significantly higher and characterised by darkened
type 3 and 4 crusts on Ironstone soils compared to Kalahari Sand. More frequently
disturbed sites had the least crust cover and had predominantly type 1 and 2 crusts. Type
3 and 4 crusts are more common on the less disturbed sites and under the canopies of
Acacia mellifera where soils are protected from livestock disturbance. Total nitrogen
concentrations were significantly higher in crusts compared to unconsolidated soil. There
is also a strong correlation between the pH and NH4+-N concentrations in crusts and the
soil immediately below the crust, suggesting that crusts have an influence on some of the
properties of the underlying soil. If the A. mellifera can utilise additional nitrogen from
crusts it may provide a competitive advantage to their establishment in formerly grassdominated grazing lands.
Keywords: Cyanobacterial soil crusts; Kalahari; Soil nutrients; Shrub encroachment;
Acacia mellifera
1
Corresponding author. Fax: + 44 161 247 1568
Email addresses: a.d.thomas@mmu.ac.uk (A.D. Thomas), adougill@env.leeds.ac.uk (A.J. Dougill)
2
Fax: + 44 113 343 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 reported as fragile and easily degraded (e.g. Oldeman et al., 1990; Pimentel et
al. 1995; UNEP, 1997). Despite this, there is a growing abundance of literature (e.g.
Stocking, 1996; Thomas and Middleton, 1994; Warren et al., 2001) that emphasises the
resilience (a return to functional stable equilibrium) and resistance (the ability to maintain
functional stability despite disturbances) of dryland soils. One important factor in this
regard is biological soil crusts that typify many dryland soils (see Ullman and Büdel,
2003 for review) but which have only been described at a few locations in the Kalahari
(Aranibar et al., 2003; Dougill and Thomas, 2004; Skarpe and Henriksson, 1987).
Biological crusts form from the association of soil particles and organic matter with
varying proportions of cyanobacteria, algae, lichens and mosses (Belnap et al., 2003a).
They have many important functions, including; soil moisture retention, inhibition of
weed growth, reduction of wind and water erosion, atmospheric nitrogen fixation and
carbon sequestration.
A variety of environmental factors influence crust form and distribution at a range of
scales. At a continental scale, temperature and rainfall are the greatest influences
(Rogers, 1972). At a regional scale, soil type (especially texture) is the predominant
control with biological crusts less likely to develop on sandy soils due to their surface
mobility (Belnap and Gillette, 1997). At a localised scale, there appears to be an inverse
2
relationship between biological crust cover and plant cover because they are in direct
competition for light and moisture (Malam Issa et al., 1999). Shrub canopies can,
however, provide protection from disturbance and create shade which can enhance
microbiological growth (Belnap et al., 2003b). Consequently, crust - vegetation
relationships are complex and scale- and site-specific. Crusts are also sensitive to
physical disturbance. Belnap and Eldridge (2003) argue that frequently disturbed soils
only support large filamentous cyanobacteria as later successional species are not able to
develop, thus limiting microbial diversity and altering crust functioning. Marble and
Harper (1989) also found biological crusts to be particularly susceptible to disturbance
through mechanical damage when dry and thus livestock trampling to be one of the major
inhibitors of crust development.
Consequently there are numerous inter-dependent factors, notably soil type, shrub cover
and disturbance influencing the development and distribution of biological soil crusts.
This paper aims to provide an analysis of the distribution of biological soil crusts in the
Molopo Basin, South Africa and to assess their role in affecting nutrient characteristics.
The objectives are to:
1. Document the spatial extent, morphology and taxonomy of the different crust types;
2. Determine the influence of soil type, disturbance and shrubs on the amount and type
of crust cover; and,
3. Determine whether crusts contribute to the retention and availability of nutrients at
the soil surface.
3
2. Study Area
The Kalahari is a large basin of wind-blown, nutrient-deficient fine sands covering over 2
million km2 of Southern Africa (Thomas and Shaw, 1990). There is an extensive surface
vegetation cover of open shrub savanna (FAO, 1991), with primary productivity limited
by water availability and soil nitrogen (Dougill et al., 1998). The Molopo Basin lies on
the southeastern edge of the Kalahari Sands, forming the border between South Africa
and Botswana. It is a semi-arid region, with a mean annual rainfall of c. 450 mm
concentrated in the summer-wet season. 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. The fertility of rangeland soils is important
for the livelihoods of communal pastoralists and mixed smallholder farmers due to their
reliance on livestock grazing and manure inputs for arable production (Dougill et al.,
2002). Recent assessments by UNEP (1997) and Hoffman and Ashwell (2001), however,
have concluded that the Molopo Basin is experiencing extensive land degradation.
The study area, located in the Molopo Basin, is approximately 100 km west of Mafikeng
in North West Province, South Africa (25o49’52’’S; 24o40’58’’E) between the villages of
Loporung and Tsidilamalomo, and is characterised by a series of low parallel ridges of
calcrete and ironstone cutting across the Kalahari Sands (Figure 1). Soil characteristics
and vegetation vary across the ridges, enabling the investigation of soil type, vegetation
and disturbance on crust development.
4
3. Research Design and Methods
3.1. Morphological and taxonomic characterisation of crust types
Because there are problems associated with field classification of biological soil crusts
due to the small size of the crust components and difficulties with taxonomic
identification of cyanobacteria (Eldridge and Rosentreter, 1999), most classification
schemes are based on surface crust morphology. There is a strong relationship between
crust morphology and ecological function (Belnap and Lange, 2003). The classification
used in this study is based on a descriptive in-field assessment of crust microtopography,
colouration and visible cyanobacterial sheath material to provide an objective
classification of soil surface conditions (full details are given in Dougill and Thomas,
2004). Four replicates of each crust type were subsequently analysed using light
microscopy to provide an indication of the cyanobacteria species in wetted duplicate
samples at a magnification of 200 times using the method of Alef and Nannipieri (1995).
Samples were assessed for the presence of the three main cyanobacteria species
(Microcoleus, Scytonema, Nostoc) identified by Skarpe and Henriksson (1987) in the
only previous study of Kalahari soil crust species.
3.2. Influence of soil type, site disturbance and vegetative cover on crust cover
5
Three parallel 50 metre line transects set 15 metres apart were surveyed at six study sites
(Figure 1). Sites were located on Kalahari Sands (site 1), an Ironstone ridge (site 2) and
slope colluvium (site 3), on alluvium adjacent to an ephemeral channel (site 4), a calcrete
ridge (site 5) and slope colluvium (site 6) (see Table 1 for site details). Along each
transect a series of ten 5 m line transects (n = 30 for each site) were used to quantify soil
crust area (% by type) and vegetation cover (% by species) by visualising cover using a
0.25 m2 quadrat along the length of the 5 m transect (as per line transect method of Reid
and Thompson, 1996). The number of cattle tracks and dung pats (within a 2 m wide
swathe of the transect) were quantified and used to provide a livestock disturbance index
based on the method of Perkins and Thomas (1993). Every 5 m along the transects the %
surface area covered by each crust type, crust depth (the mean at 5 locations) and
hardness (a soil penetrometer was used to measure resistance to compressive force) were
quantified within a 1 m2 quadrat. Intact samples of all crust types and samples of the soil
immediately below the crust from a depth of 10 mm were also collected. This provided
between 22 and 96 samples of each crust type for nutrient analysis. All sampling was
conducted in July and August during the winter dry season.
Patterns of crust cover around the three most common bush species (Acacia mellifera,
Grewia flava and Brachylaena rotundata) were also assessed. The canopy dimensions of
ten bushes of each species within the 50 m x 50 m quadrat were measured (less when
species numbers were lower across a site). Crust cover estimates under every canopy
were taken using 0.5 m x 0.5 m quadrats, adjacent to one another along a line extending
from the bowl to the canopy edge in both a northerly and southerly direction. The radial
6
distance (r) of the crusted surface area on each transect away from the bole was measured
and used to provide an estimate of the area of crust cover under each canopy (using πr2
based on assumed circularity of crust zones under canopies – a view subsequently
supported in analysis by Berkeley et al., 2005).
3.3. Chemical and nutrient characterisation of the crusts and underlying soil
Plant available nutrient concentrations in crust and soil samples were measured within 24
hours of sampling using a portable spectrophotometer. Available NH4+-N and PO43--P
concentrations were determined according to the methods of Anderson and Ingram
(1993). Salinity and pH were determined using portable probes after mixing with
distilled water at a soil to water ratio of 1 g to 5 ml. Samples of all crust types and
unconsolidated soil were air-dried prior to laboratory determination of grain size, organic
matter, total-N and total chlorophyll. Grain-size distributions were determined on
dispersed samples sieved at half-phi intervals from -1.0 to +4.0 (2 mm to 0.063 mm) after
removal of organic matter using H2O2. Silt and clay were determined using a
sedimentation method (Rowell, 1994). Organic matter was determined using loss-onignition (Rowell, 1994). Total-N concentrations were determined following a Kjeldahl
digestion (Anderson and Ingram, 1993). Total chlorophyll was determined
colorimetrically after extraction with 85 % v/v acetone according to the method of Allen
(1989).
7
Statistical analysis of differences between the different crust types in the means and
distributions of all the measured chemical and nutrient variables was conducted using
single factor ANOVA in SPSS ™. A post-hoc Sheffe’s test, based solely on the F ratio
statistic, was used to test differences between multiple data sets (Quinn and Keogh,
2002). Where applicable, regression analyses were also conducted in SPSS ™. For all
tests, statistically significant differences were assigned to p values of < 0.05.
4. Results and Analysis
4.1. Crust Morphology and Taxonomy
Five distinct crust types were identified on the basis of microtopography, colouration and
the presence of filamentous cyanobacterial sheaths. In three crust types, cyanobacterial
sheath material was clearly visible on the underside of the crust. A fourth, weakly
consolidated crust, had no obvious surface colouration and no visible sheath material but
cyanobacteria were identified using light microscopy. The fifth crust type was only
found on alluvium and was considered to form through desiccation of the finer sediment
independent of bacterial activity and is not considered further in this paper. Physical and
taxonomic characteristics of the four main crust types and unconsolidated soils at all sites
are summarised in Table 2.
Unconsolidated soils had Microcoleus present in all samples with Scytonema in two of
four samples. Type 1 crusts are very weakly consolidated with a mean compressive
strength of 1.38 kgcm-2 (significantly lower than all other crust types; p < 0.001). They
8
have no surface colouration or visible cyanobacterial sheath material, though light
microscopy identified Microcoleus in all samples and Scytonema in 1 of the 4 samples.
Type 2 crusts are better-consolidated with a mean compressive strength of 3.17 kgcm-2,
significantly greater than type 1 crusts (p < 0.001). There is no surface colouration, but
cyanobacterial sheath material is visible below the crust and total chlorophyll content is
0.034 % ± 0.01. Microcoleus was identified in all samples of type 2 crusts, as well as
Scytonema in 2 of the 4 samples. These appear equivalent to the class 1 crusts described
for US sites by Belnap and Gillette (1997; p. 356) as “flat crusts, with no visible frost
heaving or lichen cover, low cyanobacteria biomass, indicating disturbance within 1 year
of observation”. Type 3 crusts have a similar compressive strength (3.44 kgcm-2) to type
2 crusts (p = 0.840) but are characterised by a black or brown speckled surface.
Cyanobacterial sheath material is visible below the crust surface. Mean total chlorophyll
content is 0.0043 % ± 0.02 and Microcoleus and Scytonema were identified in all
samples. Type 4 crusts have a bumpy surface topography of up to 2 cm and an intensely
coloured black/brown surface. These appear equivalent to class 2 crusts under the Belnap
and Gillette (1997; p. 356) system, which are described as, “moderately bumpy biological
crusts with no lichen or moss development and moderate cyanobacterial biomass levels,
indicating disturbance 5 – 10 years prior to observation”. The compressive strength of
the type 4 crust is significantly greater than the rest (p < 0.001 for types 1 and 2; p = 0.03
for type 3). All samples contained Microcoleus and Scytonema and the mean total
chlorophyll content is 0.0066 % ± 0.02.
4.2. The influence of soil type, disturbance and vegetation on crust distribution
9
There are significant differences between the crust cover on different soil types (Table 3).
Crust cover is significantly higher on the soils of the Ironstone ridge (52.8 %) and the
Ironstone colluvium (54.8 %) than on the Kalahari Sand (25.3 %) (p = 0.03 in both
cases). Soils on the Ironstone also have the highest cover of type 4 crusts.
An index based on the number of cattle tracks and dung density was used to differentiate
sites on the basis of disturbance (Table 1). Due to the close proximity of villages
livestock frequently disturb all sites and differences in disturbance reflect the relative
palatability of vegetation growing on each soil type. The relative level of disturbance
affects the total cyanobacterial crust cover at each site, with more disturbed locations
having the lowest crust cover (Figure 2). In areas of relatively high disturbance only
crust types 1 and 2 are found, whereas crust types 3 and 4 are only found in areas of
lower disturbance and under shrub canopies.
The relationship between vegetation and crusts is complex and there were no significant
relationships between crust cover and total vegetation cover, shrub cover or grass cover.
The relationship between vegetation and crust development is best described on an
individual shrub basis as a zone of crust formation is found under canopies, where soils
are protected from disturbance by grazing animals. The size of these crust zones varies
with shrub species and the morphology of the canopy and leaf structure (Table 4).
Relative to the size of the canopy, crust areas are greatest under Acacia mellifera and
10
least under Grewia flava. There is a positive correlation (r2 = 0.51; p = 0.001) between
the canopy size of A. mellifera and the area of sub-canopy crust cover (Figure 3).
4.3. Crusts and nutrients
Salinity, pH, total-N, available ammonium-N (NH4+-N) and phosphate-P (PO43--P) were
determined for each crust type and for unconsolidated soils (Table 5). All samples are
neutral to slightly alkaline with low salinity. Total-N concentrations in all samples are
very low (< 0.1 % as per classification of Landon, 1991). Available PO43--P
concentrations are also low, reflecting the inherent infertility of Kalahari soils.
Total-N content is significantly higher in all crust types compared to the levels in
unconsolidated soil (p < 0.001 for all crust types). There are also significant differences
between crust types and total-N concentration. The total-N contents of type 3 and 4
crusts are significantly higher than for type 2 crusts (p = 0.008 and p = 0.039
respectively). Available NH4+-N concentrations, however, are not significantly different
between the crust types, or between crusts and unconsolidated soils (p > 0.05). PO43--P is
significantly greater in type 3 crusts compared to both type 2 and 4 crusts (p = 0.001 for
both), but there is no difference between type 2 and 4 crusts (p = 0.913).
The relationship between nutrients in crusts and the subsoil was also explored. There are
strong significant positive correlations between subsoil and crust pH and NH4+-N (r2 =
11
0.89, p < 0.001 for pH; r2 = 0.73, p < 0.001 for NH4+-N) suggesting that crusts have an
influence on some of the properties of the surrounding subsoil (Figure 4).
5. Discussion
This paper details the widespread occurrence of cyanobacterial soil crusts in a regularly
disturbed area of differing soil types in the Molopo Basin, South Africa and the factors
affecting their distribution. Four cyanobacterial crust types are identified with total
average crust cover ranging from 24 % to 55 % (Table 3), demonstrating that despite
frequent livestock disturbance, a significant crust cover remains. Two species of
cyanobacteria (Microcoleus and Scytonema) are found in all crust types. The presence of
both species in unconsolidated soils suggests they are able to lie dormant in soils before
rapidly forming crusts given favourable conditions. Microcoleus commonly occurs in
bundles surrounded by a polysaccharide sheath, which forms a web of sticky material in
the upper soil layer (Belnap et al., 2003a). Individual filaments become active upon
wetting, moving out of the sheaths and towards the soil surface in a phototactic reaction.
On drying, the exposed filaments secrete new sheaths that bind and aggregate soil
particles leading to crusting of the soil surface. Microcoleus and Scytonema are both able
to impart stability to the soil surface, thus decreasing surface sediment mobility.
Disturbance restricts the amount of crust cover and the occurrence of type 3 and 4 crusts
(Tables 1, 3 and Figure 2). Similar findings have been reported from the United States by
Belnap and Eldridge (2003) where disturbance leads to a simplified community
12
dominated by a few cyanobacteria species. A predominance of type 1 and 2 crusts in the
most disturbed locations across the study sites suggests that they are able to re-establish
quickly after disturbance. Crust recovery is therefore a key attribute of Kalahari Sand
soils and an important factor in their resilience to disturbance.
Soil type also affects the spatial extent and type of crust cover. Kalahari Sand is the most
common substrate in the Kalahari and across the study area an average of 25 % was
covered in cyanobacterial crust. This is similar to the cover of 19 to 40 % reported for
grazed rangelands on Kalahari Sand soils in Botswana and Namibia (Dougill and
Thomas, 2004). In contrast, over 50 % of the soils developed on Ironstone were covered
in cyanobacterial crust. Alluvium and calcrete sites had an average crust cover of 40 %
and 30 % respectively. The reduced crust cover on Kalahari Sand reflects the coarser
grain size that inhibits crust formation. The lower shrub cover on Kalahari Sand (Table
1) also reduces the area of protected sub-canopies better-suited to crust formation. Crusts
on Kalahari Sands are also subject to periodic burial under mobile wind-blown material
(Dougill and Thomas, 2002) and development is limited by a lack of light, which can
restrict cyanobacterial colonisation to the large and mobile Microcoleus species (Belnap
et al., 2003a). The more consolidated soils on calcrete and Ironstone are less erodible and
the shrub cover is denser making these soils better-suited to crust formation.
Shrub cover also affects crust distribution (Table 4). There is no relationship between the
amount and type of crust cover and shrub cover; however, there is a positive relationship
between the area of sub-canopy crust cover and A. mellifera canopy area (Figure 3). This
13
bush has a dense canopy covered in double thorns that is highly effective at reducing
disturbance under its canopy. The leaves are small and leaf litter rarely completely
covers the soil under the canopy. The sub-canopy of A. mellifera, therefore, provides
ideal conditions for crust formation with minimal livestock disturbance and adequate
light levels reaching the surface. G. flava, in comparison, has no thorns and large leaves
producing a thicker leaf litter cover. Consequently, despite similar canopy dimensions to
A. mellifera, crust development is less under G. flava canopies. Brachylaena rotundata
has an intermediate level of crust area under its canopy, reflecting the larger leaves and
reduced light at the surface compared to A. mellifera.
Total-N and available nutrient content of all soils was very low, reflecting the inherently
poor fertility of Kalahari soils. Crust cover did, however, enhance total-N concentrations
at the surface (p < 0.001). Furthermore, type 3 and 4 crusts have significantly higher
total-N than type 2 crust (p = 0.008 and p = 0.039). Previous research has shown that in
Kalahari Sand soils nutrients remain concentrated in a topsoil layer, even where intensive
grazing has removed the perennial grass cover (Dougill et al., 1998). Given their spatial
association with A. mellifera canopies, crusts could also explain observed patterns of
total-N enrichment under A. mellifera (Hagos and Smit, 2005). Cyanobacterial crust may
contribute to surface nutrient enrichment given their ability to fix and retain nitrogen.
Scytonema has heterocysts (cells devoid of oxygen) that allow the enzyme nitrogenase to
fix atmospheric dinitrogen (N2) into ammonium (NH4+-N) (Belnap et al., 2003b). In
addition, although non-heterocystic, Microcoleus can fix atmospheric nitrogen (Belnap,
1996) through association with other bacteria and because various processes exclude
14
oxygen from micro-sites on the bacteria. Given that recent research suggests that most
Kalahari shrub species (including Acacia species) are not able to fix atmospheric nitrogen
(Aranibar et al., 2004), the widespread cover of cyanobacterial crusts mean nitrogen
inputs from crusts could be important for the nutrition of both grass and shrub species.
The implications of these findings for soil fertility and shrub encroachment processes
remain uncertain. Samples were collected during the dry season when bacteria are
inactive and most nitrogen remains organically bound (Table 5). However, the
significantly higher total-N in crust types 3 and 4 (Table 5) can explain the enhanced
total-N pool under A. mellifera that after mineralisation could provide available nitrogen
to vascular plant roots in the vicinity of these crusts. The positive correlation between
crust and subsoil NH4+-N concentrations (Figure 4a) suggests crusts can improve the
fertility of surrounding soils.
In the Kalahari, the main threat to agricultural sustainability is the ecological transition of
grass-dominated rangelands into more uniform shrub-encroached ecological communities
(Dougill et al., 1999). A. mellifera is the most prevalent woody species responsible for
shrub encroachment in the Kalahari (Moleele et al., 2002). The shallow rooting system
of A. mellifera (Skarpe, 1990; Hipondoka et al., 2003) suggests it will be able to secure
surface available nutrients, especially when grass competition is diminished through
intensive grazing. Aranibar et al. (2004) suggest that A. mellifera do not fix atmospheric
nitrogen but obtain N from other mechanisms in addition to mineralization of soil organic
matter. Further work is needed to establish whether the cyanobacterial crusts under A.
15
mellifera canopies (Figure 3) are the source of this additional N and if the shrub is
afforded a competitive advantage in its invasion and domination of Kalahari grasslands.
6. Conclusions
This paper provides an account of the widespread occurrence of cyanobacterial soil crusts
in the Molopo Basin, South Africa. The relative importance of soil type, disturbance and
shrub cover on crust distribution is determined and the impact on nutrients examined.
Four cyanobacterial crust types were identified using surface microtopography,
colouration and visible sheath material. Soil type, shrub cover and livestock disturbance
levels influence spatial patterns of crust cover. Crusts are more extensive on ironstone
soils compared to Kalahari sands reflecting the soil texture, surface mobile layer and
shrub types found on Kalahari Sands. Disturbance reduces crust cover and the
occurrence of type 3 and 4 crusts. In disturbed locations, crusts are found preferentially
under A. mellifera shrub canopies where they are protected from livestock trampling. As
total-N concentrations are significantly greater in crusts compared to uncrusted sands,
especially in type 3 and 4 crusts, there is the potential for crusts to provide a competitive
advantage to A. mellifera shrubs once they have become established in formerly grassdominated grazing lands.
Acknowledgements
The authors are grateful for the financial support provided by the Manchester
Geographical Society and the Universities of Salford and Leeds. We are also grateful to
Dr David Eldridge who provided detailed and valuable comments on an earlier draft of
16
this paper and to the useful input of two anonymous reviewers. We thank Kate Berry and
Jennifer Byrne who helped with field data collection.
17
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