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 Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
Abstract
Recent research has suggested that Southern African soils are resilient to degradation
pressures associated with soil erosion and/or soil nutrient depletion. The reasons for this
resilience remain unclear, however, one potentially important factor are cyanobacterial
soil crusts that typify the soil surface. This paper provides an analysis of the physical,
microbiological and chemical characteristics of four morphologically distinct
cyanobacterial crust types found in the Molopo Basin, South Africa. It details the
influence that soil type, livestock disturbance and vegetation cover provide in controlling
the spatial distribution of crusts. Our findings show that as cyanobacterial crusts develop
they contribute to retention of total-N at the soil surface. Crusts are more widespread and
better developed on ironstone compared to Kalahari Sands. At a more localised scale,
crusts are better-developed underneath the canopies of Acacia mellifera where soils are
protected from livestock disturbance. This offers a potential contributing factor to explain
the increase in A. mellifera cover across Kalahari rangelands if the bushes can utilise the
nitrogen fixed by the crusts.
Keywords: Cyanobacterial soil crusts; Kalahari; Soil nutrients; Bush 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.
Thomas and Middleton, 1994; Stocking, 1996; Mortimore, 1998; 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
factor that may be important in this regard is biological soil crusts, that typify many
dryland soils, but which have only infrequently been described in previous research on
Kalahari soils (Skarpe and Henriksson, 1987; Aranibar et al., 2003, Dougill and Thomas,
2004). Biological soil 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; retaining soil moisture,
discouraging weed growth, reducing wind and water erosion, fixing atmospheric nitrogen
and sequestering carbon.
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, and several studies show 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
2
localised scale, there appears to be an inverse relationship between biological crust cover
and plant cover because they are in direct competition for light and moisture (e.g. 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
soils which are frequently disturbed 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) 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, vegetation
cover and disturbance, influencing the development and distribution of biological soil
crusts. It is, however, difficult to isolate each causal factor because of their complex
interactions across 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. Despite the extent and wide-ranging
influence of biological crusts there are only a limited number of studies on biological soil
crusts in Southern Africa (see Ullman 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, South Africa and to assess their role in affecting nutrient characteristics. The
objectives are to:
3
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.
2. Study Area
The Kalahari is a large basin of wind-blown, nutrient-deficient fine sands (Thomas and
Shaw, 1990). There is an extensive surface vegetation cover of open shrub savanna
(FAO, 1991), although primary productivity is limited by water availability and to a
lesser extent soil nitrogen and phosphorus (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 and subsequent intensive agricultural
development projects. 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.
4
Study sites were 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;
a site with 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.
3. Research Design and Methods
Three parallel 50 metre transects set 15 metres apart were demarcated at six study sites
(Figure 1). Sites were located on an ironstone ridge and slope colluvium, the Kalahari
Sands, a calcrete ridge and slope colluvium and on alluvium adjacent to an ephemeral
channel (see Table 1 for site details). Along the transects, crust area (% by
morphological type), vegetation cover (% by species), number of cattle tracks and dung
pats (within a 2 m wide swathe) were quantified. The latter two grazing disturbance
variables were 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 (using a soil
penetrometer to measure resistance to compressive force) were quantified within a 1 m2
quadrat. Intact samples of all crust types found in each quadrat and samples of the soil
immediately below the crust to 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 during July and August during the winter dry season.
5
There are many 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
therefore based on surface crust morphology as there is a strong relationship between
crust morphology and ecological function (reference?). The classification in this study
uses crust microtopography, colouration and visible cyanobacterial sheath material to
provide an objective classification of soil surface conditions.
Plant available nutrient concentrations in all crust and soil samples were measured within
24 hours of sampling using a portable spectrophotometer. Availaible 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, total chlorophyll and chlorophyll a. Grain-size distributions were
determined on dispersed samples sieved at half-phi intervals from -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 0.063 mm fraction using a sedimentation method (Rowell, 1994).
Organic matter was determined using loss-on-ignition (Rowell, 1994). Total-N
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
6
(1989). Light microscopy was used to provide a preliminary assessment of the presence
of cyanobacteria species in wetted duplicate samples at a magnification of 200 times
according to the method of Alef and Nannipieri (1995). Samples were assessed for the
presence of the three main cyanobacteria species identified by Skarpe and Henriksson
(1987) in the only previous study of Kalahari soils where crust species have been
identified.
Statistical analysis of differences in means and distributions between the different crust
types was conducted using single factor ANOVA in SPSS ™ after checking the data
were normally distributed. A post-hoc Sheffe’s test, which assumes equal variance, was
used to…? 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, it was possible
to see cyanobacterial sheath material on the underside of the crust with the naked eye. A
fourth, weakly consolidated crust, had no obvious surface colouration and no visible
sheath material but cyanobacteria were identified using light microscopy.
Unconsolidated soils had Microcoleus present in all samples with Scytonema in two of
four samples. The fifth crust type was only found on alluvium and was considered to
7
form through desiccation of the finer sediment and independently of bacterial activity and
therefore is not considered further in this paper.
Type 1 crusts are very weakly consolidated with a mean compressive strength of
1.38kgcm-2 (significantly lower than all other crust types; p < 0.001). They 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 (Table 2).
Type 2 crusts are better-consolidated with a mean compressive strength of 3.17kgcm-2,
significantly greater than type 1 crusts (p < 0.001). There is no surface colouration, but
bacterial sheath material is visible below the crust. Microscope analyses confirm the
presence of extensive networks of Microcoleus species in type 2 crusts, as well as
Scytonema in 2 of the 4 samples (Table 2). 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
(3.44kgcm-2) to type 2 crusts (p = 0.840) but are characterised by a black or brown
speckled surface. Bacterial sheath material is visible to the naked eye below the crust
surface and light microscopy confirms the presence of Microcoleus and Scytonema
(Table 2). 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
8
this crust type is significantly greater than all other crust types (p < 0.001 for types 1 and
2; p = 0.03 for type 3). All samples of the type 4 crusts contained Microcoleus and
Scytonema (Table 2).
4.2. The influence of soil type, disturbance and vegetation on crust distribution
There are significant differences between the crust cover on different soil types (p <
0.001) (Table 3). Crust cover is significantly higher on the soils of the ironstone ridge
and colluvium (54.8 %) than on the Kalahari Sand (25.3 %) (p = 0.03). Soils on the
ironstone also have the highest cover of type 3 and 4 crusts.
An index based on the number of cattle tracks and dung density was used to differentiate
sites on the basis of disturbance (Tables 1 and 3). 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
correlates with the amount of cyanobacterial crust cover at each site (r2 = 0.51; p = 0.???),
with more disturbed locations having the lowest crust cover (Figure 2). Only crust types
1 and 2 are found in areas of relatively high disturbance, 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
9
individual shrub basis as a zone of crust formation is typically 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 least under Grewia flava, such that at a site scale there is a positive
correlation (r2 = 0.47; p = 0.???) between A. mellifera cover and crust cover (Figure 3).
4.3. The influence of crusts on 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 crusts were
neutral to slightly alkaline with consistently low salinity. Total-N concentrations in all
samples were classified as very low (< 0.1 % as per classification of Landon, 1991).
Available PO43--P concentrations were also low, reflecting the inherent infertility of
Kalahari soils.
There are some significant differences between crust types and total-N concentration.
The total-N contents of crust types 3 and 4 are both significantly higher than for the type
2 crust (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 was significantly greater in type 3 crusts
compared to both type 2 and 4 crusts (p = 0.001 for both), but there was no difference
between type 2 and 4 crusts (p = 0.913).
10
The relationship between nutrients in crusts and the subsoil under each sample was also
explored. There are no significant differences in salinity, pH, NH4+-N and PO43--P in
surface crusts compared with the soil immediately below the crust (p > 0.05 in all cases).
Indeed, subsoil and crust pH, NH4+-N and PO43--P are correlated (see Figure 4 for r2 and
p values). Subsoil and crust pH and NH4+-N are positively correlated suggesting a close
relationship between crust properties and the underlying soil. Conversely, PO43--P is
negatively correlated indicative of its insolubility and the long-lived surface
concentration of P in Kalahari soils.
5. Discussion
This paper details the widespread occurrence of cyanobacterial soil crusts in the Molopo
Basin and the factors affecting their distribution in a small, regularly disturbed area of
differing soil types. Four cyanobacterial crust types were identified and total average
crust cover ranged from 24 % to 55 % (Table 3), demonstrating that despite frequent
livestock disturbance, a significant crust cover remains. Two species of cyanobacteria
(Microcoleus and Scytonema) were found in all crust types. The presence of both species
in unconsolidated soils suggests an ability to lie dormant and to rapidly reform crusts
given favourable conditions. Microcoleus and Scytonema are both filamentous
cyanobacteria and impart stability to the upper soil surface, thus decreasing surface
sediment mobility. Microcoleus commonly occurs in bundles surrounded by a
polysaccharide sheath, which forms a web of sticky material in the upper soil layers
11
(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 (Belnap et al., 2003a)
leading to the crusting of the soil surface.
Scytonema has heterocysts (cells devoid of oxygen) which allow the enzyme nitrogenase
to fix atmospheric dinitrogen (N2) into ammonium (NH4+-N) (Belnap et al., 2003b). In
addition, although non-heterocystic, Microcoleus has been shown to fix atmospheric
nitrogen (Belnap, 1996) through association with other bacteria and because various
processes exclude oxygen from micro-sites on the bacteria (Rogers and Gallon, 1988).
Given that recent research suggests that most shrub species (including Acacia species) are
not able to fix atmospheric nitrogen in the Kalahari (Aranibar et al., 2004), the
widespread cover of cyanobacterial crusts mean nitrogen inputs from crusts could be
important for the nutrition of many grass and shrub species.
Disturbance restricts the amount of crust cover and the occurrence of type 3 and 4 crusts
(Table 4, Figure 2). Similar findings have been reported from the United States by
Belnap and Eldridge (2003) where disturbance leads to a simplified community
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 these crust types are able to
re-establish quickly after disturbance. Crust recovery is seen as a key attribute of
Kalahari Sands imparting a level of resilience to regular livestock disturbance.
12
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 and an average of 25 % was
covered in cyanobacterial crust. 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 and the surface mobile surface layer which both inhibit the formation
of crusts (Dougill and Thomas, 2002). The lower shrub cover on Kalahari Sand (Table 1)
also reduces the area of protected sub-canopies better-suited to crust formation.
Developing crusts are subject to periodic burial under wind-blown material and may die
from lack of light, restricting 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 vegetation cover is denser and these soils are therefore
better suited to crust development.
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 amount of crust cover and A. mellifera (Figure 3). This species 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 surface
under the canopy. 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
13
greatly reduced under this species. Brachylaena rotundata has an intermediate level of
crust development under its canopy, reflecting the larger leaves and consequently reduced
light at the surface compared to A. mellifera.
The total-N and available nutrient content of all soils was very low, reflecting the
inherently poor fertility of Kalahari soils. Crust cover did, however, affect total-N
concentrations at the surface. Type 3 and 4 crusts have significantly higher total-N than
type 2 crust (p = 0.008 and p = 0.039).
Previous research in Southern Africa (Dougill et
al., 1998) has shown that in sandy Kalahari soils nutrients remain concentrated in a
topsoil layer (0 – 20 cm depth), even where intensive grazing has removed the perennial
grass cover. Cyanobacterial crust may be a factor contributing to nutrient retention at the
surface.
The pH and NH4+-N of the soil below crusts are also correlated with that in the overlying
crusts (Figure 4), indicating an impact of crusts on the soils immediately below them.
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) will provide an enhanced
organic-N pool for mineralisation after rainfall, capable of providing available nitrogen to
vascular plant roots in the vicinity of these crust types.
14
The situation for phosphorus is different. The insolubility of PO43--P in Kalahari soils
results in a longevity of PO43--P surface enrichment that override the shorter-term impacts
of biological crusts on P mineralisation and retention. Patterns in P availability are
therefore determined by a number of small-scale soil type, vegetation or historical land
use differences that control P cycling in dryland soils (Blackmore et al., 1990; Tiessen,
1995).
In the Kalahari, the main threat to agricultural sustainability is the ecological change
associated with the transition of grass-dominated rangelands into more uniform shrubencroached 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 works is needed to establish
whether the enhanced cyanobacterial crusts under A. mellifera canopies (Figure 3) are the
source of this additional N and if the shrub is afforded a competitive advantage that could
lead to the stability of the woody plant encroached ecosystem.
6. Conclusions
15
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 is examined.
Our findings have identified four cyanobacterial crust types using surface topography,
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 enhanced in type 3 and 4 crusts, there the potential for a
positive feedback mechanism that can help to explain the rapid spread of A. mellifera
encroachment in Kalahari rangelands requires further investigation.
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 useful comments on an earlier draft of this
paper and to the useful input of two anonymous reviewers. We thank Kate Berry and
Jennifer Byrne who helped with field data collection.
16
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