FRACTAL DIMENSION TO DESCRIBE SOIL MACROPORE STRUCTURE USING X-RAY COMPUTED-TOMOGRAPHY

The size, shape, and arrangement of structured voids 1-10 mm in size p lay an important role in the transport of water and solutes through so il. However, these characteristics are complex and difficult to quanti fy. Improved methods are needed to quantify the characteristics of the se voids to better understand and predict the behavior of water and so lutes passing through them. This study applied fractal analysis to soi l bulk density data measured by X ray computed tomography (CT), a rela tively new tool for nondestructively measuring macropore-scale density in soil cores. Studies were conducted using undisturbed soil cores (7 .6 cm ID) from forested and cultivated sites in the A horizon of a Men fro silt loam soil containing macropores and using two groups of soil cores which were uniformly packed with Menfro aggregates from 1-2 mm i n diameter for one group and < 1 mm in diameter for the other group. S amples were probed using CT to produce a 512 by 512 digital matrix of CT pixel values corresponding to bulk density. Pixels above a specifie d CT ''cutoff'' value were designated as occupied. A box-counting meth od was used to find the fractal dimension of the perimeters between oc cupied and unoccupied pixels and of the areas formed by the unoccupied pixels. For length scales from 1 to 10 mm, perimeters and areas of th ese regions appeared to be fractal systems. Single degree of freedom o rthogonal contrast tests determined from analysis of variance showed s ignificant differences between the fractal dimension for (1) forest an d cultivated cores versus uniformly packed cores, (2) two groups of un iformly packed cores made of different aggregate sizes, and (3) forest versus cultivated cores.