Investigating Fractal Distribution of Mass From the Millimeter- to Decimeter-Scale in Two Kansas Soils.

Fractal models that describe the distribution of aggregate mass and the hierarchical organization of soil structure at scales relevant to hydrological process have been tested only over a small range of aggregate sizes. The objectives of this work were to examine the ability of a fractal model to describe the mass-volume relationship from the millimeter to decimeter scale and to assess the variability of fractal parameters obtained from individual clods sampled within the same horizon. Soils at a native prairie (NP) and a restored prairie (RP) in northeastern Kansas were studied. Six large clods sampled from the A or Ap and Btss1 horizons from the NP and RP sites were sequentially broken and volumes measured. Volumes were paired with their respective masses and fit with a power law expression to obtain D_m (fractal dimension of mass) and k_m (the mass of an aggregate of unit diameter). Most fits demonstrated two domains separated at a breakpoint, d_b, with values of ~1 cm. We found a strong relationship between d_b and the combination of organic carbon and silt+clay content suggesting that these properties interact to control aggregation in aggregates with diameters smaller than d_b. D_m-values for the NP Btss1 and A horizons were not fractal for small aggregates and were fractal for large aggregates. We found the opposite trend in the RP soil likely due to high concentrations of roots and organic carbon observed in this horizon. Variation of D_m and k_m within any given horizon was large and comparable to the variation of similar values obtained from water retention from a variety of soils of contrasting textures found in other studies, suggesting that a more thorough understanding of the horizon-scale variability of these parameters is needed in order to appropriately apply fractal models of water retention.