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

Abstract

Little is known about the morphological characteristics of pores in soil crusts. The objective was to characterize the 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. 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 to 133 cm-1) and area occupied by pores or meso-macroporosity (3.5 to 4.2%). Their most abundant pore types were small unconnected rounded pores. Soil crust infiltration coefficients (65-75% annual) were among the lowest and their high erosion rates (81 to 204 g m-2 y-1) and low infiltrability (47 to 61%) 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 poresystems with specific meso- and macropore morphologies, e.g., large, interconnected elongated and irregular pores. Biological soil crusts 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 to 57%) because the biological crusts are somewhat detached from the soil underneath, but the armouring effect decreases erosion rates (7 to 23 g m-2 y-1).Soil crust pore numbers, size and shape were useful indicators of soil quality.