Transcript No Slide Title

Kalahari Sand Soils: Resilience
Mechanisms and yet Degradation
Processes
Dr Andrew Dougill
School of the Environment,
University of Leeds
My Research Aim:
“To characterise the processes that regulate interactions
between ecological change, soil properties and grazing for
Kalahari rangelands”
Key Research Questions:
• Do changes in soil water and nutrient cycling occur with,
&/or cause, ecological change (notably bush encroachment)?
• What mechanisms / processes can explain the limited soil
degradation in Kalahari farming systems?
• What are the main areas requiring further research?
Research Studies
•4 main study sites from
across Kalahari sandveld
•Range of climate, land
use and sand properties
My Research Context
4 main soil-ecology projects that contribute to literature on
Kalahari ecosystem functioning & soil resilience
1. “Soil hydrochemical characteristics and rangeland environmental change in
the Kalahari, Botswana” PhD (1992 -1995)
Dougill et al., 1998 - Hydr. Proc.; Dougill et al., 1999 - Annals of Am. Assoc. Geogr.
2. “Spatial patterns of rangeland ecological change & the use of EO data for
dryland degradation monitoring” (1995 - 98)
Trodd & Dougill, 1998 - Appl. Geog; Dougill & Trodd, 99 - Global Ecology and Biogeog.
3. “Soil nutrient fluxes in mixed farming systems” (1999 - 2002)
Dougill & Thomas, 2002 - J Arid Envs; Dougill et al., 2002 - The Geographical Journal
4. “Dryland Soil Crusts, Resilience and Biogeochemical Cycling” (2001 - )
Dougill & Thomas, in press - Land Deg & Dev; Thomas & Dougill, submitted - Geoderma
Soil - Ecological Change Links in Grazed Rangelands
Extended piosphere studies (e.g. Tolsma et al., 1987; Perkins &
Thomas, 1993) by examining depth profiles of soil water and
nutrients along a grazing gradient
•No sig difference between water or
nutrient profiles between bush and
grass-dominant sites
Soil Nutrient Cycling Process Studies
Tested two-layer model (Walker & Noy-Meir, 1982) that grazing
induced grass removal, & surface dung/urine inputs, will increase
nutrient leaching to subsoil
“both the magnitude of soil water & concentration of soil nutrients
leached into the subsoil is largely unaffected by the ecological and
biochemical effects of increased cattle use”
Dougill et al., 1998
p.443.
•Nutrient adsorption in surface layer noted as key mechanism that
explains the resilience of Kalahari soils
•Low mineralisation rates enable synchrony between nutrient
availability and uptake by surface roots of any vegetation
•Soils changes cannot explain bush encroachment processes &
alternative models of ecological change needed
Alternative Model of Ecological Change
State and transition model without soils-based component due to
resilience to ecological change that Kalahari soils provide
Ecological Mapping of Bush Encroachment Processes
•Problems caused by the difficulty in using EO data to
characterise vegetation structure due to darkening effects of any
vegetation on the bright soils (Trodd & Dougill, 98)
•Spatial patterns (Dougill & Trodd, 1999) show that bush
encroachment acts through expansion and aggregation of bush
clumps as ‘protected’, nutrient-enriched, sites for germination and
establishment
•Sub bush canopy niches can also support nutritious grass
species that provide seed source that retains ecological resilience
for grass regrowth following drought
•Enhanced grazing levels during drought
events most likely cause of removal of
grass cover & permanent ecological change
Nutrient fluxes in Kalahari rangelands
•Ranch-scale nutrient flow from grazing lands to borehole & kraal
could decrease range productivity. Not picked up by existing
studies due to spatial heterogeneity
•Local-scale redistribution of surface sediment & nutrients by wind
erosion. However, nebkha dune & fenceline studies (Dougill &
Thomas, 2002) show only a small aeolian nutrient redistribution,
with plant canopy inputs & impacts more important in causing
nutrient enrichment & increasing spatial heterogeneity
Nutrient fluxes in mixed farming systems - Molopo Basin
•Field-scale nutrient budget analysis (Dougill et al., 2002) highlight
that nutrient inputs exceed outputs when farmers add compound
fertiliser inputs - excess nutrients cause soil acidification
•Manure inputs alone lead to soil nutrient depletion
•Perception of need for no fertiliser
addition to groundnuts leading to
soil nutrient depletion
•Integrated nutrient management
used by 2 of 15 study farmers
capable of avoiding most soil
degradation processes
Surface Controls on Nutrient Retention & Heterogeneity
•New biochemical analyses to examine surface nutrient retention,
soil resilience & spatial patterns of nutrient availability shown that
biological soil crusts are extensive, and vitally important, factor
Surface Controls on Nutrient Retention & Heterogeneity
•Biological soil crusts cause surface stabilisation & nutrient fixation
and adsorption retaining a near surface concentration of nutrients
•Weak crusts form rapidly due to Microcoleus spp.
•Succession to higher crusts (black staining & microtopography)
with lichens limited by disturbance, such that these crusts found
mainly under certain bush canopies adding to ‘island of fertility’
effects - supported by Aranibar et al., 2003
Crust Cover %
40
30
20
10
0
0
0.5
1
1.5
2
Disturbance Index
Physical Crust
Bio 1 Crust
Bio 2 Crust
Bio 3 Crust
2.5
Ecological Implications of Surface Crusts
•Crusts provide an ideal surface habitat for germination & their
spatial distribution regulates many processes of ecological change
•Details will be site specific dependent on disturbance, rainfall
& fire histories, as well as seed bank resources
•Soil resilience imparted by surface crusts does not imply
ecological resilience due to the ability of encroaching bush
species (notably Acacia mellifera) to access surface nutrients
•Rooting studies (Hipondoka et al., 2003) support this
supposition implying a +ve feedback could operate due to
‘islands of fertility’ under bush canopies
Summary - Soil Resilience but Ecological Degradation
•Surface crusts provide mechanisms preventing soil erosion &
chemical degradation =>
Soil resilience
•Crusts retain surface nutrient concentrations even with grazing
such that all vegetation needs efficient surface rooting to retain /
enhance competitive dominance
•As crusts, and nutrients, are concentrated under bush canopies
there is a potential positive feedback mechanism that can help to
explain the extensive nature of bush encroachment (notably of
Acacia mellifera) across Kalahari =>
Ecological degradation ?
Key Future Research Questions
•What livelihood adaptations can enable secure rural livelihoods
despite bush encroachment?
•Shift to smallstock keeping?
•Charcoal production?
•Use of social networks to move cattle across regions?
•Dependence on Government ‘safety nets’
•What factors control the retention of nutritious grass cover in subcanopy habitats?
•Interactions of grazing levels, rainfall and bush stand morphology
•What is the regional / global significance of the biological soil
crusts that typify Kalahari soils?
•Studies proposed of N fixation, adsorption & mineralisation, C
sequestration and of the soil microbiology / biodiversity