Parameterizing temporal changes in aggregation in a sludge amended soil

Statistical comparison of the changes in aggregate size distributions (ASD) with time among treatments in a given soil or among soils is difficult. The objective of this study was to compare the use of fractal dimension (D), geometric mean diameter (GMD), and alpha (alpha) parameters as indices to characterize the temporal changes in ASD of a soil subsequent to the incorporation of an oily waste sludge. Soil samples for dry and wet aggregate size distribution (DASD and WASD, respectively) measurements were collected from a loamy sand during a 4-yr period following sludge Incorporation. Sludge incorporation produced a continuous oil content, i.e., 0, 1.0, 1.5, 3.0, and 7.0%, variable in the soil (by mass). The wind erodible fraction (WEF; <0.84 mm) in the sludge treated plots decreased from 80 to 38% but increased with time to 78% 4 yr later. The reduction in WEF was reflected as gains in larger size fractions which decreased with time. The 4.76- to 8-mm water stable aggregates in the sludge treated plots increased from 10 to 80% but decreased with time to 40% 4 yr later. An opposite trend was observed for smaller size aggregates. After sludge incorporation, the D for DASD and WASD decreased with increasing oil content and increased with time. An opposite trend was observed for GMD and a. Initial oil content and time of sampling accounted for 91% of the variability in DASD characterized by GMD, 89% by D, and 60% by a. Initial oil content and time accounted for 78% of the variability in WASD characterized by D and 67% by GMD. Our results indicate that the temporal changes in DASD can be characterized equally well by GMD and D and the changes in WASD by D.