Hydropedological model of vertisol formation along the Gulf Coast Prairie land resource area of Texas

摘要

Vertisols are clayey soils containing slickensides and wedge-shapedaggregates formed by shrink-swell processes in seasonally wet climates. Thedynamic distribution of macro- and microvoids as a by-product of this uniquepedoturbation process, accompanied by microtopographic lows and highs(gilgai), mitigate our ability to make accurate and precise interpretationsof aquic and hydric conditions in these problem soils. We studied Vertisolsacross a subhumid to humid climosequence to assess the formation ofredoximorphic features on shallow, linear (nondepressional) landscapepositions in response to varying levels of rainfall. Approximately 1000 mmof mean annual precipitation (MAP) is required to form soft iron masses thatthen increase in abundance, and to shallower depths, with increasingrainfall. Soft iron masses with diffuse boundaries become more abundant withhigher rainfall in microlows, whereas masses with nondiffuse boundariesbecome more common in microhighs. Most soft iron masses form in oxygenatedped interiors as water first saturates and then reduces void walls whereiron depletions form. In contrast, at least 1276 mm of MAP is needed to formiron pore linings in both microlow and microhigh topographic positions. Irondepletions do not correlate with rainfall in terms of abundance or depth ofoccurrence. The quantity of crayfish burrows co-varies with rainfall andfirst appears coincidentally with soft iron masses in microlows near 1000 mmof MAP; they do not appear until nearly 1400 mm of MAP in microhighs.Dithionite-citrate extractable and ammonium-oxalate extractable iron oxidesincrease systematically with rainfall indicating more frequent episodes ofiron reduction and precipitation into pedogenic segregations. The sum of ourdata suggests that Vertisols forming in the Coast Prairie of Texas with MAPgreater than 1276 mm should be classified as aquerts because of the presenceof aquic conditions. These same soils may also meet the definition of hydricas one criterion for the identification of Federally-protected wetlands.However, there is a considerable disjunct between protracted periods ofsaturation and limited periods of reduction in these soils. Based on thedistribution of redoximorphic features in the study area, regional watertable data, and recent electrical resistivity data from a nearby uplandVertisol, non-Darcian bypass flow is the principle mechanism governing theflux of water through deep cracks where water first accumulates and thenpersists in microlow bowls at depths of 1 to 2 m.