SPATIAL DISTRIBUTION AND CHARACTERISTICS OF CYANOBACTERIAL SOIL CRUSTS IN THE MOLOPO BASIN, SOUTHERN 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 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. 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 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 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 morphology-based classification of biological soil crusts in the Molopo Basin on the south-eastern edge of the Kalahari. The objectives are threefold:
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 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 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 soil and ecological studies across the Kalahari (Dougill and Thomas, 2004). Every 5 m a more detailed analysis of soil surface characteristics was conducted in 1 m2 quadrats. This included further 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 at this stage 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 and used in this study (Figure 4) uses the form and morphology of the 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), 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. Measuring the chlorophyll content of the crust samples and remeasuremenet after wetting in the laboratory was designed to test this assertion.
Available nutrient concentrations in all 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: 5ml 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.
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.