DISTRIBUTION AND CHARACTERISTICS OF CYANOBACTERIAL SOIL CRUSTS IN THE MOLOPO BASIN, SOUTH AFRICA
[1]Thomas, A. D. and [2]Dougill, A. J.
1 – Department of Environmental and Geographical Sciences, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester, M1 5GD, U.K
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 grass-dominated grazing lands.
Keywords: Cyanobacterial soil crusts; Kalahari; Soil nutrients; Shrub encroachment; Acacia mellifera
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 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;
- Determine the influence of soil type, disturbance and shrubs on the amount and type of crust cover; and,
- 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 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.
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
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 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). [a1]
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. AvailableNH4+-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-on-ignition (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).
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 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
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 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 = 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 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.