Two-Dimensional Porosity of Crusted Silty Soils: Indicators of Soil Quality in Semiarid Rangelands?

Little is known about the morphological characteristics of pores in soil crusts. The objective was to characterize the two-dimensional (2D) porosity (amount, shape, size, and area of pores) of soil crusts to ascertain their potential as indicators of soil quality for natural crusted soils. The 2D-porosity was described in thin sections and measured by image analysis of polished resin-impregnated soil blocks. Physical soil crust and incipient biological soil crusts appear to be the lowest-quality soils in terms of number of pores (average of 131-133 cm(-1)) and area occupied by pores or meso-macroporosity (3.5-4.2%). Their most abundant pore types were small unconnected rounded pores. Soil crust infiltration coefficients (65-72% annual) were among the lowest and their high erosion rates (81-204 g m(-2) yr(-1)) were not only due to their lower total porosity, but also to their pore shapes and sizes. Biological soil crusts appear on higher-quality soil, where the higher the organic C content, the more evolved the soil crust is (with lichens and cyanobacteria). Such soil crusts have better developed pore-systems with specific meso-and macropore morphologies, for example, large, interconnected elongated and irregular pores. Biological soil crusts (BSCs) dominated by lichens have the largest meso-macroporosity (up to 23.65%) due to the predominance of elongated pores. In many cases, infiltration is low (46-57%) because the biological crusts are somewhat detached from the soil underneath, but the armouring effect decreases erosion rates (7-23 g m(-2) yr(-1)). Soil crust pore numbers, size, and shape were useful indicators of soil quality.