William A. Hobson's research while affiliated with University of California, Davis and other places

Vernal pool soils in California's Mediterranean climate experience extremes in pedogenesis driven by prolonged saturation to extended desiccation. Four northern California vernal pool soil catenas (summit, rim, and basin) were assessed to determine how soil properties and hydric soil indicators vary in response to duration of standing water and landscape position. Each catena had differences in parent material or degree of soil development. Soil properties differed subtly across each microtopographic sequence. In the well-developed soils, the geochemical signature of horizons overlying the duripans changed sharply compared with horizons below the restrictive layers, suggesting polygenic origins of the soil profiles. The presence and abundance of redoximorphic features (RMFs) in profiles corresponded poorly with the duration of standing water at the four sites. Instead, the abundance of RMFs coincided better with the thickness of the soil above the restrictive horizons in all settings with duripans. Hydric soils were identified in the basin positions of each catena. Most rim positions contained hydric soils and most summit positions had soils that were not hydric. Indicators F8 (redox depressions) and TF2 (test indicator for red parent materials) were most commonly applied. None of the vernal pool catena soils met F9 (vernal pools hydric soil indicator), thus the hydric soil criteria for vernal pools may need to be revised.

Vernal pool soils in California's Mediterranean climate experience extremes in pedogenesis driven by prolonged saturation to extended desiccation. Four northern California vernal pool soil catenas (summit, rim, and basin) were assessed to determine how soil properties and hydric soil indicators vary in response to duration of standing water and landscape position. Each catena had differences in parent material or degree of soil development. Soil properties differed subtly across each microtopographic sequence. In the well-developed soils, the geochemical signature of horizons overlying the duripans changed sharply compared with horizons below the restrictive layers, suggesting polygenic origins of the soil profiles. The presence and abundance of redoximorphic features (RMFs) in profiles corresponded poorly with the duration of standing water at the four sites. Instead, the abundance of RMFs coincided better with the thickness of the soil above the restrictive horizons in all settings with duripans. Hydric soils were identified in the basin positions of each catena. Most rim positions contained hydric soils and most summit positions had soils that were not hydric. Indicators F8 (redox depressions) and TF2 (test indicator for red parent materials) were most commonly applied. None of the vernal pool catena soils met F9 (vernal pools hydric soil indicator), thus the hydric soil criteria for vernal pools may need to be revised.

Vernal pools and swales represent small yet complete ecosystems whose existence is threatened due to urban expansion and agricultural development. This study was conducted to evaluate and understand the unique biogeochemical processes occurring in these endangered ecosystems, in order to enhance preservation and mitigation efforts. Soils in the summit, rim, and basin geomorphic positions of four pools and one swale were investigated to correlate soil morphological properties to soil chemical and mineralogical properties. Processes were determined by solid-phase characterization, soil solution analysis, and in situ measurement of redox potential. The dominant soil processes are ferrolysis, organic matter accumulation, clay formation and translocation, duripan formation, and calcium carbonate accumulation. These seasonally episaturated wetlands exhibit cyclic oxidation and reduction of Fe and Mn (ferrolysis) resulting in low pH and accelerated weathering above the duripan. By-products of ferrolysis are bases and metal cations in solution which move downward and accumulate above the duripan as the pools dry. Oxidation of Fe and Mn creates the redoximorphic features of high chroma Fe mottles, and neutral Mn stains, concentrations, and masses. Organic matter accumulation was highest in the surface horizons (0-8 cm), ranging from 9.3 to 76.2 g kg -1 . Clay formation and translocation occurred in all geomorphic positions with thicker soil profiles in the summit position than either the rim or basin positions. Clay accumulation produced subsur-face horizons enriched in clay (37-56 %). Duripan formation occurs as illuvial weathering products, dominantly silica with accessory iron oxides and calcium carbonate, cement a subsurface horizon to varying degrees.