Download Nutrients in Dryland Soils

Kalahari Sand Soils: Spatial Heterogeneity, Biological Soil Crusts
and Land Degradation
Dougill, A.J.1 and Thomas, A.D2
1
- School of the Environment, University of Leeds, Leeds, LS2 9JT, UK;
[email protected]; tel - (+ 44) 113 343 6782; fax - (+44) 113 343 6715
2
- Department of Environmental and Geographical Sciences, Manchester Metropolitan
University, Manchester, M1 5GD, UK; [email protected]
Abstract
This paper identifies spatial associations between surface nutrients, biological soil crusts and
vegetation in the Kalahari. Four locations, with different land use and substrate
characteristics were used to determine the extent of biological crust cover and the factors
affecting the spatial heterogeneity in soil nutrients and ecology. Despite the sandy texture of
Kalahari soils and regular surface disturbances, there is a significant biological soil crust
cover (19 – 40 %) at all locations. This is due to a combination of resistance to trampling,
sub-canopy niches protected from disturbance and the prevalence of Microcoleus vaginatus
cyanobacteria which are able to rapidly reform crusts. Crust cover and diversity is enhanced
on ironstone and calcrete soils. The spatial variability of soil nutrients is low but is increased
by grazing-induced bush encroachment. The preferential development of nitrogen-fixing
biological soil crusts under bushes could enhance the competitive advantage of Acacia
mellifera favouring further bush encroachment. Whether this constitutes land degradation is
dependent upon the extent to which sub-bush canopy niches retain palatable grass species.
Keywords:
Kalahari soils; Biological soil crusts; Soil Degradation; Bush
Encroachment; Spatial heterogeneity.
1
Nutrients in Dryland Soils
The spatial and temporal variability of rainfall in drylands results in an incomplete vegetation
cover and a complex association between vegetation and soils, notably organic matter,
nutrients and microbial activity (Bennett and Adams, 1999). Spatial heterogeneity is a vital
component for landscape functioning and biodiversity. The work of Noy-Meir (1973, 1985)
demonstrated that the concentration of soil nutrients in patches increases the ecological
productivity compared to where resources are evenly spread. It can take the form of the
regular banding of tiger bush landscape (e.g. Valentin et al., 1999) or as irregular vegetation
clumps (e.g. Tongway and Ludwig, 1994; Bennett and Adams, 1999; Puigdefabregas et al.,
1999).
Existing research suggests the processes leading to elevated nutrients and organic matter
under bush canopies can be grouped into two categories: i) vegetation related, where subcanopy soil nutrients are elevated by organic matter additions and faunal activity (e.g. Dean et
al. 1999); and ii) sediment transport related, associated with deposition of enriched eroded
material (e.g. Tongway and Ludwig, 1994). The implications of this spatial heterogeneity
are, however, contested and may be site specific. Some authors have questioned the link
between enhanced spatial heterogeneity and increased ecological productivity. 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 that once established,
was difficult to reverse due to the development of 'islands of fertility' under bushes.
The two-fold causation model for sub-canopy nutrient enrichment may be oversimplistic in
that it fails to address internal processes within dryland soils. Dryland soil nutrients are
preferentially concentrated at, or near, the soil surface (Tongway and Ludwig, 1994; Dougill
et al., 1998; Bennett and Adams, 1999). In part, this is due to the widespread occurrence of
biological soil crusts consisting of cyanobacteria, algae, microfungi, lichens and bryophtes
which live within the top few millimetres of soil (Belnap et al., 2003). Crusts are able to fix
atmospheric nitrogen (Belnap 1994, 1995, 2003; Evans and Lange, 2003) and sequester
carbon (Beymer and Klopatek, 1991; Evans and Lange, 2003). They typify all dryland soils,
but their composition and distribution varies between locations with substrate, vegetation
cover and disturbance (Belnap and Lange, 2003). The relationship between crusts and
vegetation is particularly complex. Bush canopies can deter crust development as they reduce
the amount of light reaching the surface. However, they also provide protection from
disturbance and trap fine sediment, both of which can be beneficial to crust development.
To fully understand the spatial and temporal dynamics of soil fertility and degradation
processes in any dryland environment this additional layer of complexity needs to be
investigated. This study aims to provide new analyses of the spatial associations between soil
nutrients, biological soil crusts and vegetation canopies for the Kalahari sand soils of
Southern Africa.
Kalahari Sand Soils, Ecosystems and Degradation
The Kalahari is "the extensive elevated, flat, sand-covered plain that occupies a substantial
part of the southern Africa interior" (Thomas, 2002; p.21). The geological unit of the Kalahari
Sands covers 2.5 million km2 (Thomas and Shaw, 1991). Recent intensification of pastoral
land use in the semi-arid core of the Kalahari (comprising much of Botswana, eastern
Namibia and northern South Africa) based on the drilling of deep boreholes, has led to
concerns of widespread land degradation (Government of Botswana, 1997; SADC, 1997).
Kalahari sand soils typically consist of over 95 % fine sand-sized, aeolian-deposited sediment
(Thomas and Shaw, 1991) and are, predominantly deep, structureless and lacking in N, P and
organic matter (Skarpe and Bergström, 1986; Dougill et al., 1998). The natural flora of the
Kalahari is well adapted to low water and nutrient availability and therefore the vegetation is
2
sensitive to changes in soil properties (Skarpe, 1990). For example, extensive bush
encroachment associated with grazing intensification has been linked to increased leaching of
water and nutrients (Walker and Noy-Meir, 1982; Perkins and Thomas, 1993). However,
research has failed to identify any association between soil water and nutrient availability and
ecological changes (Dougill et al., 1999) implying that Kalahari sand soils are resilient to
chemical degradation. The processes leading to this resilience remain unclear, and the spatial
association with the main ecological degradation concern of bush encroachment unknown.
Research Objectives
This paper aims to identify spatial associations between surface nutrients, biological soil
crusts and vegetation in the Kalahari. Four locations across the Kalahari are used to examine
the processes retaining nutrients at the surface. The objectives of the paper are:

To determine the extent of biological crust cover in contrasting Kalahari environments.

To determine the factors affecting the spatial heterogeneity in soil nutrients.

To examine the association between the spatial heterogeneity in soil biochemical
characteristics and the ecological communities found at different locations.
In addressing these issues a clearer assessment of the factors contributing to the resilience of
Kalahari soils and ecosystems to degradation will be obtained.
Research Design and Methods
Site Selection
Four sites across the Kalahari were selected to incorporate a range of land uses and climatic
conditions (Figure 1). They are the commercially owned Makoba Ranch Block, in the
Eastern Kalahari; the mixed farming area of the Molopo Basin in the South East Kalahari;
communal rangelands of Omaheke District, in the Western Kalahari; and in southern
Kgalagadi District, South Western Kalahari. This provides a diverse range of substrates,
grazing intensities and ecological community characteristics.
Field Measurements
Ecological and soil-based studies at the sites have been ongoing for the last decade. The
research focus and approaches have evolved through time and thus the exact nature of the
methods at each site differs.
i.
Makoba Ranch Block.
Integrated soil nutrient and ecological analyses were conducted at plot- (4 m2), site- (30 m x
30 m) and ranch- (8 km x 8km) scales (see Dougill et al., 1999). Topsoils (n = 23) were taken
from random points across 30 m x 30 m sites in both an intensively-grazed bush-dominant
site (800 m from Uwe Aboo borehole) and a less intensively-grazed grass-dominant site (4
km from Uwe Aboo borehole). Inorganic nutrients (NO3--N, NH4+-N and PO43--P) were
determined using methods outlined by Anderson and Ingram (1993).
ii.
Molopo Basin, South Africa and Botswana.
This site was approximately 100 km west of Mafikeng in an area with a series a low parallel
ridges of calcrete and ironstone cutting across Kalahari sand deposits. Six sites along a 1500
m transect perpendicular to the ridges were selected and soil crust characteristics determined
within 30 m by 30 m grids. Topsoils from Kalahari sands, ironstone, alluvium and calcrete
were analysed. A disturbance index was calculated for each site based on the number of
cattle tracks, surface dung density and grass height. At all sites the percentage crust cover
and type, vegetation species and cover was determined in 1 m2 quadrats every 1.5 m along
three parallel 30 m line transects. The crust type was assigned using a classification scheme
based on crust form and morphology (Figure 2). Soil nutrients and grass biomass were also
3
determined underneath the three main bush species (Acacia mellifera, Grewia flava and
Brachylaena rotundata) at each of the sites and compared to adjacent open sites.
iii.
Omaheke District, Namibia.
The extent of biological crust cover and the nutrient content of the surface soil was
determined at four sites in Omaheke District, Eastern Namibia. Studies were undertaken at
three communally-managed farms (Netso in Tsjaka Fram Block; Corridor No.4 Farm and
Okonyoka in Aminuis Reserve) and a commercially owned ranch (Hogusrus). At all
locations, sampling sites were designated at 0, 100, 200, 400, 800, 1600, 3200 and 4000 m
from a waterpoint along a 'piosphere' of declining grazing intensity (Perkins and Thomas,
1993). At each site, soil crust and vegetation cover was determined within sixteen
systematically located 1 m2 quadrats and crust samples collected for total-N, total-P and
organic matter analysis (Anderson and Ingram, 1993).
iv.
Southern Kgalagadi District, Botswana.
Soil crust cover, nutrients and bush and grass cover were determined on a communal
rangeland adjacent to Berry Bush farm, 10 km north east of Tshabong in Kgalagadi District,
Botswana using the methodological framework established at the Omaheke District site.
Results and Analysis
i.
Distribution of Biological Soil Crusts
Despite the fine-sand texture of Kalahari soils and regular surface disturbances, there is a
significant biological soil crust cover (19 – 40 %) at all locations (Table 1). This is a result of
a combination of resistance to trampling, shown by the high compressive strengths of
between 2.9 (mean for crust type 1) and 4.6 kgcm-2 (mean for crust type 3); sub-canopy niches
protected from livestock disturbance; and the prevalence of Microcoleus vaginatus
cyanobacteria which rapidly reform crusts. At the Molopo sites, the extent of all but the most
well developed crusts appeared unaffected by disturbance (Figure 3), reflecting the
dominance of cyanobacterial crusts. Similarly, significant crust cover is retained throughout
ranches in the Namibian and Kgalagadi rangelands (Figure 4), except around the small
'sacrifice zone' surrounding waterpoints.
The Molopo Basin sites enabled an assessment of the impact of different parent material on
crust formation. Table 2 summarises the cover of the different crust types for soils on the
four substrates. This shows that crust formation is least prevalent, and includes fewer betterdeveloped type 2 and 3 crusts, on Kalahari sand soils. In contrast, on ironstone soils there is
over 50 % crust cover and a high proportion of type 2 and 3 crusts. This implies that any
landscape-scale spatial variations in parent material will add to crust extent and development.
ii.
Spatial Heterogeneity in Soil Biochemical Characteristics
Previous studies on Kalahari soils have assessed the impacts of grazing regimes on mean soil
nutrient concentrations and ecology (e.g. Skarpe, 1990; de Queiroz, 1993; Perkins and
Thomas, 1993; Dougill et al., 1999). These studies have found no significant differences in
soil chemical characteristics between intensively grazed bush-encroached sites and the less
intensively grazed grass-dominant sites. This, however, could be a consequence of examining
differences in mean nutrient concentrations where there is high spatial variability and low
concentrations. In addition, some studies (e.g. Perkins and Thomas, 1993) integrate nutrient
concentrations from a range of depths, rather than just the surface layer where nutrients are
focused.
In this study nutrients were determined for 23 randomly located topsoils from 30 m by 30 m
plots at both intensively grazed bush-dominant, and less intensively grazed grass-dominant
sites at the Makoba site. Table 3 shows the mean nutrient concentrations, and the coeffiecient
of variation, used as an index of the spatial variation in soil properties. The spatial variability
within a plot was low (11 to 28 %) compared to shrublands in the southwestern United States
4
(Schlesinger et al., 1996) where the coefficient of variation in N and P ranged from 59 to 103
%. The significant difference in variance of phosphate availability between bush and grassdominated plots indicates that a shift to more bush-dominated cover with intensive grazing
will increase the spatial heterogeneity of nutrients (Table 3).
Analysis of the impact of bushes on the spatial heterogeneity in soil fertility at the Molopo
site supports this assumption. Table 4 shows that sub-bush canopy sites have enhanced
ammonium and phosphate concentrations, organic matter and grass biomass. The lower
nitrate concentration under bush canopies results from the efficient plant uptake of this
nutrient, due to the synchrony of nitrification after rains and plant uptake. This has been
shown to leave significantly lower nitrate than ammonium concentrations where a plant cover
exists (Dougill et al., 1999). Efficient N uptake remains prevalent in bush encroached sites
due to the extensive surface layer rooting system of encroaching Acacia species (Skarpe,
1990) and the grass cover remaining under canopies (Table 4). Previous work in the Molopo
Basin has shown that wind transported sediments deposited under bushes are not significantly
enriched in nutrients (Dougill and Thomas, 2002) and that elevated nutrient concentrations
under bushes are due to autogenic processes. This differs from the findings for other drylands
such as the SW United States (e.g. Reynolds et al., 2001) and Sahelian Africa (e.g. Sterk,
2003) where enhanced nutrient concentrations in aeolian dust are recorded. The small
increases in organic matter under bushes are not sufficient to explain all the nutrient
enrichment. Thus we also investigated the impacts of biological soil crusts in contributing to
the enhanced nutrient concentrations under bushes.
Around all bushes, there was a concentric pattern of unconsolidated soil and crust with the
size of each zone varying with bush species (Table 5). The understorey of Acacia mellifera is
better suited to crust formation due to the grazing deterrent effect of its double thorns and the
small leaves, which allow light to reach the soil surface. Grewia flava, in comparison, has no
thorns and larger leaves producing a thick cover of litter on the soil. Consequently, despite
the similar canopy dimensions, crust development was greatly reduced under Grewia flava.
This has important implications for persistence of Acacia mellifera in the landscape as once
established the crust will supply additional nutrients to the plant.
iii.
Spatial Variability in Vegetation Characteristics
The ecological implications of soil crust formation under bushes depend on the associated
herbaceous grass cover in these niches. At the Molopo study site, there was increased grass
biomass under bushes (Table 4), as well as differences in the herbaceous species between subcanopy and open locations. Grass species cover was grouped (Table 6) according to 'Grazing
Value' classes, as assigned by van Oudtshoorn (1999). Grazing values are based on the
species palatability, production, nutritional value, growth vigour, digestibility and habitat
preference. There is evidence that more nutritious grasses are protected from grazing under
bush canopies (Table 6). Enhanced grass biomass, and prevalence of nutritious grass species,
under bushes was also found at the Makoba study sites (Perkins and Thomas, 1993; Dougill
and Trodd, 1999). Although the dry season cover of these grasses is low, the enhanced
nutritious grass cover under bushes is important in retaining a seed resource for future
germination and maintaining ecological fodder diversity for livestock.
Further investigations of the complex inter-relationships between bush canopies, soil
biochemical characteristics and herbaceous vegetation assemblages therefore remains a key
area for future research to better understand ecological change and potential processes of
degradation in Kalahari rangelands.
Discussion and Conclusions
Findings from across the Kalahari are used to provide an insight into the factors controlling
soil surface nutrient dynamics. Dryland soils are characterised by low fertility, with a surface
concentration of nutrients (Bennett and Adams, 1999) and in 'islands of fertility' (Schlesinger
5
et al., 1990). There is an ongoing debate on the implications of nutrient spatial heterogeneity
for land degradation (e.g. Noy-Meir, 1985; Schlesinger et al., 1990; Ludwig et al., 1999). Our
findings suggest that in the Kalahari, the spatial heterogeneity of soil nutrients is relatively
low. Nutrient enrichment of wind-eroded material is limited (Dougill and Thomas, 2002),
and therefore not the principal cause of nutrient heterogeneity. Some nutrient enrichment
under bush canopies results from organic inputs from vegetation canopies. However, further
processes are required to explain the nutrient heterogeneity with biological soil crust
formation the most likely process.
Our research shows that biological soil crusts are widespread across the Kalahari. They exist
in a variety of settings and are able to persist in landscapes where there is a high level of
disturbance from livestock. This is a result of the predominance of Microcoleus spp. which
are able to rapidly reform after disturbance and their colonisation of protected sites under
bushes. High levels of disturbance restrict the formation of better-developed crusts with
mixed bacterial and lichen compositions.
The implications of the widespread occurrence of simple Microcoleus crusts in the Kalahari
are uncertain. They will enhance soil surface stability and, therefore, reduce erosion. In
addition, they are able to fix nitrogen (Evans and Lange, 2003) thus increase soil fertility.
Nitrogen fixation usually occurs in specialised cells or heterocysts within the bacteria.
Microcoleus, however, is non-heterocystous but can still fix N2 using a variety of strategies
which exclude O2 from microzones or through association with other bacteria (Belnap, 2003).
Our findings show that crusts develop preferentially under Acacia mellifera, which is the
major bush encroaching species in the Kalahari (Skarpe, 1990; Thomas, 2002). As such,
nutrient enrichment is not evenly distributed across landscapes, but is concentrated under
encroaching bush species and may encourage the persistence of A. mellifera at the expense of
other species. The shallow rooting system of A. mellifera (Skarpe, 1990) secures many of
these nutrients for the bush. Whether this bush encroachment represents degradation depends
upon the degree to which the sub-canopy niche can also support more nutritious grass species.
The evidence presented here suggests that for our study sites there are small increases in the
the most nutritious grass species under bushes that could offset reductions in ecological
fodder diversity associated with A. mellifera encroachment. The net effect of these processes
in the Molopo and Makoba sites is to increase the bush cover, but also to retain a grass cover
and seed resource, that enables continued livestock utilisation. However, degradation
concerns remain due to evidence elsewhere in the Kalahari (e.g. Skarpe, 1990; Adams, 1996;
Twyman et al., 2001) that when A. mellifera occurs in dense thickets the palatable grasses
may no longer be accessible to livestock. It is these ecological changes that reduce the overall
fodder diversity that represent a process of land degradation.
Acknowledgements
Research in Botswana was conducted with the Republic of Botswana Research Permit No.
OP46/1XCVI(87). Funding was provided by the Royal Geographical Society (HSBC
Holdings Grant), Royal Society (Dudley Stamp Memorial Fund), Manchester Geographical
Society and the Universities of Leeds, Sheffield and Salford.
Field assistance provided by Kate Berry, Jenny Byrne, George Davies, Liz Metcalfe, Ed
Sherratt, Nigel Trodd and Eleanor van der Linde is gratefully acknowledged.
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Figure Captions:
Figure 1. Locations of study sites and Kalahari sand soils.
9
Figure 2. Soil crust classifications based on crust form and morphology.
10
Figure 3. Soil crust cover and type in relation to disturbance index in Molopo Basin
Crust Cover %
40
30
20
10
0
0
0.5
1
1.5
2
2.5
Disturbance Index
Bio 1 Crust
Bio 2 Crust
Bio 3 Crust
11
Figure 4. Crust cover with distance from waterpoints on Namibian and Kgalagadi ranches.
80
Crust cover (%)
70
60
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
Distance (m)
Netso
Hogusrus
C4b
Okonyoka
Berry Bush
12
Table 1. Soil physical characteristics, crust cover and environmental conditions at each
location (see Figure 1).
Site
Land Use
Climate mean
annual
rainfall (c.
mm p.a.)
Soil
Texture
Analysis
Sand
(%)
Silt (%)
Clay
(%)
Soil Crust
Cover
(mean %)
Vegetation
Status
Makoba
Ranch
Block
Commercial
ranch
380
97.6
1.8
0.6
40
Mixed bush:grass
savanna extensive bush
encroachment
Molopo
Communal
rangelands
450
95.9
1.9
2.2
39
Mixed bush:grass
savanna extensive bush
encroachment
Omaheke
Communal
rangelands
300
95.6
2.9
1.5
39
Grass:bush
savanna extensive bush
encroachment
Omaheke
Commercial
ranch
300
95.2
2.8
2.0
19
Grass: bush
savanna remains
due to bush
clearance
Berry
Bush
Commercial
ranch
320
97.9
1.5
0.6
26
Grass:bush
savanna extensive bush
encroachment
Sources: Particle size data summarised from Dougill et al. (1998, 2002), Metcalfe
(unpublished) and van der Linde (unpublished).
13
Table 2. Soil crust type (classification as detailed in Figure 2) at sites of different substrate
(% ground cover) at Molopo Basin site.
Location
Biological soil
crust 1
Biological
soil crust 2
Biological
soil crust 3
Alluvial
crust
Total
Kalahari Sands
24
1
-
-
25
Ironstone
7
12
35
-
54
Valley Alluvium
5
5
15
15
40
Calcrete
2
11
18
Crust
31
14
Table 3. Surface soil nutrient concentrations and variability within bush-encroached and
grass-dominant sites on Uwe Aboo Ranch, Makoba Ranch Block.
Soil Chemical
Characteristic
Bushdominant
site
Coefficient of
variation (%)
[(s.d. ÷ mean)
x 100]
Grassdominant
site
Coefficient of
variation (%)
[(s.d. ÷ mean)
x 100]
Statistical
implication
(t-test on
means)
Statistical
implication
(F-test on
variance)
NO3--N
(mgN100g-1)
4.7 ± 2.5
n = 23
28
6.3 ± 2.0
n = 23
16
x
x
NH4+-N
(mgN100g-1)
5.1 ± 1.3
n = 23
13
5.1 ± 1.3
n = 23
13
x
x
PO43--P
(mgP100g-1)
1.1 ± 0.4
n = 23
20
1.2 ± 0.3
n = 23
11
x
*
x No significant difference; * significant difference at 95 % confidence interval
15
Table 4. Mean nutrient concentrations (and standard deviations) and grass biomass for subbush canopy and neighbouring open sites at Molopo Basin.
Environmental Parameter
Sub-bush canopy sites
Open sites
NO3--N concentration
(mgN100g-1)
54.2
(40.9)
102.0
(92.4)
NH4+-N concentration
(mgN100g-1)
283.9
(204.4)
45.8
(22.4)
PO43--P concentration
(mgP100g-1)
115.5
(97.0)
87.5
(63.2)
Organic Matter Content (%)
2.8
(1.2)
1.9
(0.6)
Grass biomass cover (gm-2 dry
weight)
17.2
(5.2)
0.9
(0.6)
Source: Nutrient data summarised from Dougill and Thomas (2002).
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Table 5. Bush canopy characteristics and extent of undisturbed crust cover for the main bush
species at the Molopo Basin site (means with standard deviations in parentheses)
Bush Species
Acacia mellifera
Grewia flava
Brachylaena rotundata
Canopy Height (m)
Canopy Width
(m)
Area of Undisturbed Crust
under bush (m2)
1.5
1.9
2.1
(1.3)
(1.2)
(1.5)
1.3
1.4
1.2
(0.3)
(0.5)
(0.7)
1.6
1.4
1.6
(0.5)
(0.6)
(2.2)
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Table 6. Mean herbaceous species cover (and standard deviations), by grazing value class for
sub-bush canopy and open sites in the Molopo Basin (species list by class given below).
Herbaceous Species Class (by
grazing value - van
Oudstshoorn, 1999)
Mean cover (%) in sub-bush
canopy sites
(n = 144)
Mean cover (%) in open sites
(n = 125)
High Grazing Value *
1.1
(1.8)
0.3
(0.8)
Average Grazing Value +
2.2
(2.1)
2.3
(1.8)
Low Grazing Value ^
5.5
(7.0)
3.6
(4.4)
*
- species in this class were Anthepora pubescens, Digitaria eriantha and Schmidtia
pappophoroides, Themeda triandra.
+
- species in this class were Enneapogon cenchroides, Eragrostis echinochloidea, Eragrostis
trichophora, Fingerhuthia africana and Stipagrostis uniplumis.
^
- species in this class were Aristida canescens, Aristida diffusa, Aristida stipitata, Eragrostis
biflora, Eragrostis pallens, Pogonarthria squarrosa and Perotis patens.
18