Soils, surficial geology, and geomicrobiology of saline-sodic wetlands, North Platte River Valley, Nebraska, USA

Abstract

Saline-sodic wetlands along a 200-km stretch of the North Platte River Valley in western Nebraska, USA lie within an important agricultural region, but their processes, salt mineralogy, and geomicrobiology have not previously been investigated. Putative anthropogenic salinization has long been a concern, yet early historical accounts of widespread surface salts in the area have never been applied as comparative standards. Surface salts in the area originate from soil capillarity and surface evaporation. Thenardite (Na 2SO 4) and/or mirabilite (NaSO 4·10 H 2O) dominate, depending on ambient conditions. Bloedite (Na 2Mg[SO 4] 2·4[H 2O]), halite (NaCl), burkeite (Na 6CO 3[SO 4] 2), and calcite (CaCO 3) are minor constituents. Historical accounts indicate that salts accumulated naturally long before Euramerican settlement, apparently as a result of rock–water interaction in nearby volcaniclastic sediments of the Brule Formation. Ephemeral to permanent water-holding basins in the wetlands contain Na +-rich waters that vary widely in electrical conductivity (as high as 159 mS/cm) and in ionic composition, but local spring waters are extremely dilute. Basin floors exhibit a unique type of microrelief, which appears to form by the filling of microlows with water and the dispersal of soil material therein by Na +, followed by dewatering and collapse of the soil with drying. Illite dominates basin surface soils, but smectite dominates at depth; high soil pH, available K +, and frequent wetting–drying cycles in the wetlands suggest that in-situ illitization may have occurred. Soil crusts and vesicular surface horizons are common as are upward increases in electrical conductivity. The activity of sulfate-reducing microbes forms prominent near-surface horizons of sulfate reduction in saturated soils, which retract or disappear entirely during dry episodes. Saline-sodic wetland soils in the study area change on daily to seasonal scales. Cycles of surface salt development, microbial activity, and microrelief genesis are all controlled by regular wetting–drying cycles and the interaction of ponded surface waters and shallow groundwaters. Relatively unique aspects of microbial ecology and surface processes make the soils important as “geomicrobial reactors” wherein important parts of hydrological and geochemical cycles occur.