Hydrologic Characterization of Desert Soils with Varying Degrees of Pedogenesis: 1. Field Experiments Evaluating Plant-Relevant Soil Water Behavior

To assess the effect of pedogenesis on the soil moisture dynamics influencing the character and quality of ecological habitat, we conducted infiltration and redistribution experiments on three alluvial deposits in the Mojave National Preserve: (i) recently deposited active wash sediments, (ii) a soil of early Holocene age, and (iii) a highly developed soil of late Pleistocene age. At each, we ponded water in a 1-m-diameter infiltration ring for 2.3 h and monitored soil water content and matric pressure during and after infiltration, using probes and electrical resistivity imaging (ERI). Infiltration and downward flow rates were greater in younger material, favoring deep-rooted species. Deep-rooted species tend to colonize the margins of washes, where they are unaffected by sediment transport that inhibits colonization. The ERI results support important generalizations, for example that shallower than 0.5 m, infiltrated water persists longer in highly developed soil, favoring shallow-rooted species. Soil moisture data for the two youngest soils suggested that saturation overshoot, which may have significant but unexplored hydroecologic and pedogenic effects,occurred at the horizontally advancing wetting front. Spatial heterogeneity of soil properties generally increased with pedogenic development. Evidence suggested that some early-stage developmental processes may promote uniformity; the inter-mediate-age soil appeared to have the least heterogeneity in terms of textural variation with depth, and also the least anisotropy. Lateral heterogeneity was pronounced in older soil, having a multitude of effects on the distribution and retention of soil water, and may facilitate certain water-conserving strategies of plants over what would be possible in a laterally homogeneous soil.