Morphology, lacunarity and entropy of intra-aggregate pores: Aggregate size and soil management effects

Abstract

In a hierarchical model of soil structure, small size aggregates are less porous and contain smaller pores than larger aggregates. Fractal dimensions obtained from aggregate mass-ratio data seem to confirm a hierarchical organization of intra-aggregate pores that may be altered by tillage. The objective of this study was to use image analysis to directly characterize the morphology and spatial arrangement of intra-aggregate pores as a function of aggregate size and soil management. The surfaces of wooded and cultivated sites of a Typic Hapludults, an Aquic Hapludults and an Ultic Hapludalfs were sampled. Hand-picked aggregates from each site were separated in three size groups having masses of 5–6 g ( M 5), 2 g ( M 2) and 0.5 g ( M 0.5) and 6 aggregates per size group were resin-impregnated and imaged. Measurements on the 108 images included pore shape (planar, irregular and rounded), the geometric mean of the Feret (longest) pore diameter (GMFD) for each shape group and pores greater than 300 μm 2, and properties of the spatial arrangement of pores characterized with a normalized entropy and a lacunarity function. Distributions of GMFD by shape were quantified with the Kullback–Leibler Divergence and a chi-square test. Planar pores had greater GMFD values than irregular and rounded pores and their distributions were significantly different ( P < 0.05) across aggregate sizes. Parameters from the normalized entropy and lacunarity functions were significantly ( P < 0.05): 1) different in cultivated than in wooded sites and in the M 0.5 than in the M 5 and M 2 size groups (entropy function only) and 2) correlated to the porosity, pore number and to GMFD (entropy function only) values of each pore shape group. The lacunarity function revealed a fractal or multifractal structure of pores within aggregates, while the normalized entropy function detected significantly lower entropy in aggregates of the M 0.5 size group. The GMFD of planar pores showed well defined trends with fractal parameters obtained from data (reported elsewhere) on aggregate mass-ratio. These results are evidence of differences in the organization and distribution of pores within aggregates of different sizes that may be of a hierarchical (fractal) type.