Soil%20Crusts,%20Nutrient%20Heterogeneity%20

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Soil Crusts, Nutrient Heterogeneity Rangeland
Degradation Assessments in Kalahari rangelands

• Andrew Dougill Andrew Thomas

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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 gt 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

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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

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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 gt aim to
  classify and explain distribution and degradation
  implications

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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

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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