Ultrasonic Aggregate Stability Assessment of Soils on Drastically Disturbed Hillslopes: Constituent Particle-size Distribution and Associated Organic Matter in Water-stable Macroaggregates.

Mechanisms of water-stable aggregation in drastically disturbed hillslope soils are not well-understood, and standard practices for promoting aggregation in these areas have demonstrated limited long-term viability. This study investigated physical and chemical properties water-stable macroaggregates associated with incidences of long-term plant recolonization on highway road cuts in California, and compared these results to those obtained from nearby unvegetated soil. Ultrasonic processing and laser granulometry were employed to assess aggregate stability and analyze the size-distribution of constituent particles. The distribution of organic matter in studied aggregates was assessed by measuring changes in total C and N of selected size fractions over various levels of ultrasonic agitation. For studied roadcut slopes, vegetated samples exhibited significantly (p<0.05) greater volume of macroaggregates (1000-2000 μm) than unvegegated samples. Both 1000-2000 μm and 250 – 1000 μm macroaggregates in vegetated samples were significantly (p<0.05) more resistant to ultrasonic agitation than those of unvegetated samples. Macroaggregates in vegetated samples were also composed of coarser distribution of particles sizes than unvegetated samples (0.04 – 250 μm and 0.04 – 20 μm, respectively).