soil crusts, nutrient heterogeneity & rangeland degradation assessments in kalahari rangelands...
Post on 21-Dec-2015
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![Page 1: Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas](https://reader030.vdocuments.site/reader030/viewer/2022032522/56649d6c5503460f94a4b65b/html5/thumbnails/1.jpg)
Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation
Assessments in Kalahari rangelands
Andrew Dougill & Andrew Thomas
![Page 2: Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas](https://reader030.vdocuments.site/reader030/viewer/2022032522/56649d6c5503460f94a4b65b/html5/thumbnails/2.jpg)
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
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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
![Page 4: Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas](https://reader030.vdocuments.site/reader030/viewer/2022032522/56649d6c5503460f94a4b65b/html5/thumbnails/4.jpg)
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
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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
![Page 6: Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas](https://reader030.vdocuments.site/reader030/viewer/2022032522/56649d6c5503460f94a4b65b/html5/thumbnails/6.jpg)
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
![Page 7: Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation Assessments in Kalahari rangelands Andrew Dougill & Andrew Thomas](https://reader030.vdocuments.site/reader030/viewer/2022032522/56649d6c5503460f94a4b65b/html5/thumbnails/7.jpg)
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
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Kalahari Crusts - Classification Scheme
• Classification scheme developed uses crust form and morphology to provide rapid, objective assessment
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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
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Crust Cover by Substrate
• High biological crust cover on all soil types, though higher successional stages less on Kalahari sands
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Crust Cover & Disturbance
• Crusts resilient to disturbance, except with higher successional stages
0
10
20
30
40
0 0.5 1 1.5 2 2.5
Disturbance Index
Cru
st C
over
%
Physical Crust Bio 1 Crust Bio 2 Crust Bio 3 Crust
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Crusts and Vegetation
• Typically, crusts found in protected sub-canopy sites
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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)
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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