Soil Crusts, Nutrient Heterogeneity & Rangeland Degradation

Assessments in Kalahari rangelands

Andrew Dougill & Andrew Thomas

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

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

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

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

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

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

Kalahari Crusts - Classification Scheme

• Classification scheme developed uses crust form and morphology to provide rapid, objective assessment

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

Crust Cover by Substrate

• High biological crust cover on all soil types, though higher successional stages less on Kalahari sands

Crust Cover & Disturbance

• Crusts resilient to disturbance, except with higher successional stages

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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

Crusts and Vegetation

• Typically, crusts found in protected sub-canopy sites

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)

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