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-shaped aggregates formed by shrink-swell pro-cesses in seasonally wet climates. The dynamic distribution of macro- and microvoids as a by-product of this unique pe-doturbation process, accompanied by microtopographic lows and highs (gilgai), mitigate our ability to make accurate and precise interpretations of aquic and hydric conditions in these problem soils. We studied Vertisols across a subhumid to hu-mid climosequence to assess the formation of redoximorphic features on shallow, linear (nondepressional) landscape posi-tions in response to varying levels of rainfall. Approximately 1000 mm of mean annual precipitation (MAP) is required to form soft iron masses that then increase in abundance, and to shallower depths, with increasing rainfall. Soft iron masses with diffuse boundaries become more abundant with higher rainfall in microlows, whereas masses with nondif-fuse boundaries become more common in microhighs. Most soft iron masses form in oxygenated ped interiors as water first saturates and then reduces void walls where iron deple-tions form. In contrast, at least 1276 mm of MAP is needed to form iron pore linings in both microlow and microhigh topographic positions. Iron depletions do not correlate with rainfall in terms of abundance or depth of occurrence. The quantity of crayfish burrows co-varies with rainfall and first appears coincidentally with soft iron masses in microlows near 1000 mm of MAP; they do not appear until nearly 1400 mm of MAP in microhighs. Dithionite-citrate extractable and ammonium-oxalate extractable iron oxides increase sys-tematically with rainfall indicating more frequent episodes of iron reduction and precipitation into pedogenic segrega-tions. The sum of our data suggests that Vertisols form-ing in the Coast Prairie of Texas with MAP greater than 1276 mm should be classified as aquerts because of the pres-ence of aquic conditions. These same soils may also meet the definition of hydric as one criterion for the identification of Federally-protected wetlands. However, there is a con-siderable disjunct between protracted periods of saturation and limited periods of reduction in these soils. Based on the distribution of redoximorphic features in the study area, re-gional water fable data, and recent electrical resistivity data from a nearby upland Vertisol, non-Darcian bypass flow is the principle mechanism governing the flux of water through deep cracks where water first accumulates and then persists in microlow bowls atidepths of 1 to 2 m.