Chapter 19 - Physical and Biological Surface Crusts and Seals

Abstract

Soil crusts are widespread in arid and semiarid regions, but they also commonly occur in cultivated soils in other climates. Two morphological types of crusts are detailed in this chapter - physical crusts and biological crusts. Crusts of all types have been shown to influence soil hydrologic processes and plant germination, however, the micromorphological features of crusts strongly drive their impact on soil function. Physical crusts are subdivided into structural, vesicular and sedimentary crusts. Structural crusts are formed through in situ morphological alteration of the soil surface via raindrop impact and wetting and drying cycles. Structural crusts may be further classified into four types: disruptional crusts, coalescing crusts, erosional crusts, and sieving crusts. Vesicular crusts most commonly occur in dry areas and are characterised by a more or less parallel distribution of spherical voids and subhorizontal planar voids. Sedimentary crusts, which may cover disruptional and vesicular crusts, form as a result of lateral water transport and deposition of eroded material. Biological soil crusts are colonized by communities of microorganisms (e.g., cyanobacteria, algae, fungi, lichens, mosses) that grow in and around soil particles, producing a matrix composed of biological structures and mineral grains. These crusts are classified according to their component organisms and surface morphological features. Biological soil crusts play an important ecological role by protecting soils against erosion and by contributing to dust capture and nutrient accumulation.