Dr Andrew Dougill

School of the Environment, University of Leeds

Kalahari Sand Soils: Resilience Mechanisms and yet Degradation

Processes

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 fromacross 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 ornutrient 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 fertiliseraddition to groundnuts leading tosoil nutrient depletion•Integrated nutrient management used by 2 of 15 study farmerscapable 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

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 sub-canopy 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