Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands

1. Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas

2. Kalahari Degradation Perspectives Main degradation problems viewed as - • Bush encroachment • Declines in palatable grass cover • Soil erosion on arable lands Permanence of ecological changes dependent on - • Soil hydrochemical characteristics • Fire and rainfall regimes

3. Resilience of Kalahari Sand Soils - Evidence & Implications • Rangeland ecological changes occur without soil profile changes => livestock management practices lead directly to changes, and could potentially reverse trends • Nutrient-enriched sub-canopies retain nutritious grass cover to enable rehabilitation of ‘degraded’ rangelands • Soil erosion minimal with low associated nutrient loss

4. Soil Studies on Kalahari Sand Soils in Mixed Farming Systems • Is the classification of mixed farming regions on Kalahari sands, as moderately degraded by wind erosion, applicable ? • Limited aeolian sediment transport losses imply soil erosion not a major problem, with chemical soil degradation the main concern

5. Resilience of Kalahari Sand Soils - Evidence & Causes • Nutrient adsorption at surface due to biological crusts • Sub-canopy enrichment due to sediment & organic inputs, and maintenance of undisturbed crusts • Erosion losses limited by rapid formation of surface aggregates and soil crusts

6. Dryland Soil Crusts - Form and Function • Biological soil crusts reported for drylands globally, consisting of various assemblages of cyanobacteria, bacteria, fungi, lichens, algae, mosses & liverworts • Important functions include - • Moisture retention • Erosion prevention • Nitrogen fixation • Carbon sequestration • Nutrient adsorption • No previous studies in Southern Africa => aim to classify and explain distribution and degradation implications

7. Kalahari Soil Crusts - Research Design & Methods • Distribution mapping in SW Botswana, E Namibia and Molopo Basin (SA/Bots) shown extensive regional cover • Molopo Basin used to investigate impact of substrate, disturbance and bush canopies on crust formation

8. Kalahari Crusts - Classification Scheme • Classification scheme developed uses crust form and morphology to provide rapid, objective assessment

9. Crust Characteristics • Hardness of crusts ’s with stage showing  erodibility • Chlorophyll content ’s with stage indicating  cyanobacteria and crust succession • C sequestration and N speciation to be investigated

10. Crust Cover by Substrate • High biological crust cover on all soil types, though higher successional stages less on Kalahari sands

11. Crust Cover & Disturbance • Crusts resilient to disturbance, except with higher successional stages

12. Crusts and Vegetation • Typically, crusts found in protected sub-canopy sites

13. Kalahari Soil Crusts - Formation and Implications • Crust formation and succession leads to increased soil heterogeneity. Sub-canopy sites become fertile patches responsible for much of the nutrient cycling and also nutritious grass cover •  soil heterogeneity occurs without sediment movement •  soil heterogeneity  degradation (as per US models)

14. Kalahari Soil Crusts - Chemical Resilience & Further Studies • Rapid formation of surface crusts after rainfall will limit leaching losses and erosion • Nitrogen fixation and mineralisation likely to be increased - to be investigated by process-based studies • Role in C sequestration largely unknown