Dr Andrew Dougill School of the Environment, University of Leeds

1. Kalahari Sand Soils: Resilience Mechanisms and yet Degradation Processes Dr Andrew Dougill School of the Environment, University of Leeds

2. 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?

3. Research Studies • 4 main study sites fromacross Kalahari sandveld • Range of climate, land use and sand properties

4. 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

5. 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

6. 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

7. Alternative Model of Ecological Change State and transition model without soils-based component due to resilience to ecological change that Kalahari soils provide

8. 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

9. 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

10. 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

11. 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

12. Surface Controls on Nutrient Retention & Heterogeneity • Biological soil crustscause 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

13. 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

14. 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 ?

15. 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