Hydropedology of Vesicular Horizons: Response to Changes In Structure and Porosity.

Vesicular horizons are common surface horizons in arid and semi-arid environments.  They occur predominantly in the interspace between shrub islands.  They restrict infiltration rates, promoting runoff and a patchy distribution of soil water storage.  This pattern favors desert shrub communities that are adapted to a patchy distribution of resources.  The mechanism by which vesicular horizons restrict infiltration rates is unclear, but may be related to the predominance of non-interconnected vesicular pores.  Vesicular pores, formed by air bubbles trapped during wetting of dry soil, are a dynamic soil property, capable of formation and alteration on short time scales.  This study was conducted to determine to what extent the infiltration rates of vesicular horizons is controlled by vesicular porosity, as well as characteristic structure associated with the horizons (e.g., prismatic and platy structure).   We considered: 1) the correlation between infiltration rates and structure and porosity of vesicular horizons and 2) changes in infiltration rates when structure and porosity is disturbed and allowed to reform in the field.  Vesicular horizons were analyzed at 15 sites occurring across three ecoregions of the Basin and Range province (Sonoran, Mojave, and Great Basin Deserts).   At each site, infiltration rates and morphological descriptions of the vesicular horizons were made in shallow (approx. 10 cm) excavations.  The excavations were refilled with crushed soil material and left for one year to allow vesicular pores to reform under natural wetting and drying cycles.  Infiltration measurements and morphological descriptions were repeated in the following year to detect hydraulic changes in response to morphological alteration of the vesicular horizons.  Infiltration rates of the undisturbed vesicular horizon were strongly regulated by cracks between prismatic units and were altered in the reformed vesicular horizons.  Vesicular horizons and their associated dyanamic morphologic properties are critical regulators of infiltration in desert ecosystems.