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This paper provides an analysis of the physical and chemical characteristics of cyanobacterial soil crusts in the Molopo Basin, South Africa. It details the influence that soil type, livestock disturbance and shrubs have on the spatial distribution of crusts. Four morphologically distinct cyanobacterial crusts were identified and crust cover ranged from 24% to 55%. Crust cover was significantly higher and characterized by darkened type 3 and 4 crusts on Ironstone soils compared to Kalahari Sand. More frequently disturbed sites had the least crust cover and had predominantly type 1 and 2 crusts. Type 3 and 4 crusts are more common on the less disturbed sites and under the canopies of Acacia mellifera where soils are protected from livestock disturbance. Total nitrogen concentrations were significantly higher in crusts compared to unconsolidated soil. There is also a strong correlation between the pH and NH4+-N concentrations in crusts and the soil immediately below the crust, suggesting that crusts have an influence on some of the properties of the underlying soil. If the A. mellifera can utilize additional nitrogen from crusts it may provide a competitive advantage to their establishment in formerly grass-dominated grazing lands. (c) 2005 Elsevier Ltd. All rights reserved.