Fractal mass–size scaling of wetting soil aggregates

Abstract

Structure is an important factor of soil functioning in ecosystems. Soil aggregate size distributions are commonly used to characterize soil structure. Relationships between density of dry soil aggregates and aggregate size present a different way to use aggregate-related information in soil structure characterization. Those relationships have been simulated assuming soils to be mass fractals. Aggregates in field soil are not air-dry. The relationships between mass and size differ between dry aggregates and wet aggregates because aggregates shrink as water content decreases. Our objective was to find out whether the mass fractal model can be applied to wet aggregates. Aggregates from the plow layer of Greyzem soil were brought to four different levels of water contents, and the kerosene method was used to measure volume of aggregates within diameter ranges of 3–5, 5–7, and 7–10 mm. It appeared that the wetter aggregates were less prone to loosening as the water increased. The mass fractal model was applicable to wet aggregates under the assumption of linear dependence of the fractal dimension D m and the unit size aggregate mass a on the gravimetric water content w (g g −1). Dependencies D m=2.925+0.284 w and a=0.808−0.123 w resulted in R 2=0.9999 for the regression line of simulated versus measured aggregate mass. Fractal modeling of mass–size scaling in wet soil aggregates presents a set of aggregate-based parameters for soil structure that may reflect soil properties and can be explored as an index of soil ability to support functions of ecosystems.