L.P. Wilding

Characteristics of aeolian dusts in Niger, West Africa

Dust-laden Harmattan winds are a recognized phenomenon of the Sahel region of Africa. Atmospheric dust inputs have been monitored at two sites in Niger, West Africa since July 1985. Dust traps of the open bucket type were placed 2.5 and 5 m above the soil surface. Dust was collected and analyzed quarterly for total infall, particle size, mineralogy, and water-soluble and exchangeable cations. Surface soil samples adjacent to the dust traps were also analyzed for the same properties. Of the ∼2000 kg ha−1 yr−1 of dust infall, over half came during April to July, after the Harmattan wind season. Atmospheric dust inputs varied by season as did particle size, being coarser during periods of minimal infall. The dust contributes ∼ 4.8, 2.8, 1.1 and 0.6 kg ha−1 yr−1 of exchangeable plus watersoluble Ca, K, Mg and Na, respectively. The nutrient status of the dust is substantially greater than the native soil and may serve as a nutrient renewal vector.

Quantifying pedogenic carbonate accumulations using stable carbon isotopes

Abstract Four pedons from a late Quaternary chronosequence developed in calcareous alluvium in central Texas were investigated to assess the ability of the stable C isotope method to partition and quantify pedogenic carbonate accumulations. To quantify pedogenic carbonate accumulations with this method, δ 13 C values of bulk, pedogenic, and parent carbonate must be known. For each pedon, δ 13 C values of bulk carbonate were measured on a horizon by horizon basis. The parent carbonate end-member for all pedons was approximated by averaging the bulk δ 13 C values for all horizons from the weakly developed floodplain soil. The diffusion model of Cerling (1984) and Quade et al. (1989) was used to estimate the pedogenic carbonate end-member. Quantification of pedogenic carbonate accumulations by the isotopic method was compared to quantitative estimates conducted by field morphology, binocular light microscope point counts, and thin-section point counts. Results suggest that the isotopic method is superior to other methods of quantifying pedogenic carbonate accumulations. Whole-soil pedogenic carbonate accumulations as calculated by the isotopic method for the chronosequence were: (1) 1 to 4 vol.% by 2000 years; (2) 4 to 15 vol.% by 5000 years; and (3) 1 to 12 vol.% after 15,000 years. These results reveal net soil carbonate loss through time for central Texas climates. This corroborates similar conclusions drawn for the same chronosequence using mass balance analysis to calculate the flux of carbonate. Net carbonate loss through time also indicates that these soils do not serve as long-term carbonate C sequesters.

Differentiation of pedogenic and lithogenic carbonate forms in Texas

Abstract Sixteen pedons, representing soils developed from limestone and fluvial/deltaic sediments, were sampled in central and western Texas (300–800 mm annual precipitation) to differentiate pedogenic (secondary) and lithogenic (inherited) carbonate forms by field and laboratory techniques. Lithogenic carbonate forms identified in the field included indurated limestone bedrock and coarse limestone fragments (lithorelicts). Field identified pedogenic forms included petrocalcic horizons, thin laminar cappings on indurated limestone, carbonate joint seams, pendants on pebbles, and carbonate films and threads on ped faces. Small (

Recharge and discharge of groundwater in aquic conditions illustrated with flownet analysis

Abstract We applied a flownet analysis to several landscapes that we had examined in the past. From a flownet analysis it can be seen that water flow in wetlands acts as a recharge, throughflow, or discharge of groundwater. The long-term direction of the flow alters the type of soil that forms in these conditions. Recharge waters leach and remove soluble materials. Discharge waters tend to add materials by chemical precipitation. In flowthrough conditions, the lateral water flow changes the direction of most soil processes. Flow direction affects areas next to wetlands (the edge effect). Downward flow results in dissolution, and removal of chemical material while upward flow contributes chemical precipitates at the edge.

Micromorphic record and interpretations of carbonate forms in the Rolling Plains of Texas

Abstract The Rolling Plains of Texas comprises about 10 million hectares of soils developed in Permianage deposits. Most soils occur on erosional landscape positions. Soils have ustic moisture regimes and thermic temperature regimes. These soils have developed in a wide variety of sediments ranging from red bed shales to sandstones. Most of the soils have carbonate-enriched Bk horizons while parent materials are slightly- to non-calcareous. Pedogenic calcite usually occurs as nodules, calcans, neocalcans and crystallaria. Calcite nodules are the dominant form in upper Bk horizons, while the other forms occur primarily in lower Bk/Cr horizons. Additional forms of calcite include pseudomorphs after gypsum and equant anhedral calcite which fill fracture or joint seams, and sand-calcite crystals (poikilotopic). These are considered pedogenic although formed under an earlier climatic regime. Dolomite, a post-depositional lithogenic mineral, occurs as nodules, intercalary crystals and clustered to dispersed microsparite crystals. Micromorphic forms of calcite indicate that the soils have been through several major and minor fluctuations in the soil chemical environment. On stable landscape positions, carbonate minerals preserve the long-term pedogenic record of soil development.

Clay minerals of four soils formed in eolian and tephra materials in Iceland

Abstract Clay minerals in a Vitricryand and three Haplocryands derived from eolian and tephra materials in different parts of Iceland and in glacial till samples representative of underlying strata were studied by a combination of methods. The soils are primarily used as rangelands and are highly susceptible to wind erosion. Allophanes, imogolite and poorly-crystalline ferrihydrite are abundant (> 700 g/kg of the clay) throughout the soils, including A horizons. This clay mineralogy would give rise to stable, low-density aggregates that influence the susceptibility of the soils to wind erosion. It also affects chemical properties such as high phosphate retention of the soils, even of the Cryands with relatively low clay contents (160–275 g/kg). Imogolite was found in some but not all horizons of all soils. No particular relationship of imogolite to measured Si/Al molar ratios of the oxalate-oxalic acid soluble fractions of the clays (0.51 to 1.13) was evident. Layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities. Plagioclase, augite and/or their weathering residues were found in the clay and silt separates. Dark-colored glass is predominant in the fine sand throughout the soils and the tills. One till sample had a clay mineral composition similar to the eolian-andic soils, while the other contained smectite and possibly a spherical halloysite-smectite intermediate, but not allophane and imogolite, as major weathering products. The clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions.

Micromorphology, submicroscopy and microprobe study of carbonate pedofeatures in a Vertisol gilgai soil complex, South Russia

Abstract Carbonate nodules and soft masses were studied in a gilgai soil complex in the North Caucasus, South Russia. Microrelief with an amplitude about 30 cm resulted in a wetter environment with stronger leaching in the microlow and a drier pedoenvironment with carbonate accumulation in the microhigh. Various macroforms of carbonate nodules and soft masses were identified in soil pits and sampled for micromorphology, scanning electron microscopy (SEM), and microprobe analyses to better characterize them and elucidate their pedogenesis. Common and unique attributes were described for carbonate pedofeatures depending on loci in gilgai soil complex. The most probable hypothesis for their formation is as follows. Nodules represent early pedogenic products that were initiated before gilgai formation. Modern hydrology resulted in variability of dissolution/recrystallization of the nodules along the gilgai microtopography. The variability in degree of impregnation, aggregation into pellets, and presence of hard nodular cores reflects several generations of soft masses.

Microspatial variability and sampling concepts in soil porosity studies of vertisols

Abstract Image analysis techniques and analysis of variance components were used to help design sampling schemes for micromorphometric porosity studies. The objective was to estimate how many Kubiena samples, thin sections from each Kubiena tin, and microscopic fields from each section, should be analyzed to accurately and efficiently characterize soil porosity. The analysis of variance components allowed estimation of variability at different levels of the nested sampling design. Image analysis facilitated the inclusion of a large number of samples in the experiment. Percentage of macroporosity, pore orientation, and two shape factors were quantified for a Vertisol in Texas. Micro-spatial variability of these pore-space characteristics is property-specific: variability at the Kubiena level is large for some attributes while small for some others. Consequently, when addressing several properties simultaneously, the sampling scheme will be a compromise between the required statistical accuracy and the relative costs of collecting Kubiena samples, preparing thin sections, and analyzing microscopic frames.

Steepland resources: characteristics, stability and micromorphology

Abstract Steeplands of Central America are a major land resource, but due to population pressures they are rapidly being deforested leading to landscape instability. Microwatersheds of Southern Honduras were examined to correlate soil type, landform, slope gradient and slump potential. Soils most susceptible to slumping were moderately deep (0.5–1 m) Haplustalfs and Haplustepts. These soils occupy about 25% of the landscape, and have slope gradients of 45–90%. Soils are developed on plagioclase-rich andesitic parent material, are loam to clay loam in texture and have high base status. However, due to weathering of parent material, sand and silt fractions are dominated by quartz with smaller amounts of plagioclase, vermiculite and kaolinite. The A and B horizons have an open porphyric related distribution with plagioclase as the coarse fraction in a fine-grained groundmass. The andesitic parent rock contains abundant hydrothermally altered plagioclase phenocrysts in a single-spaced porphyric-related (c/f5μm ratio of 4:1) distribution. Some feldspars are partially to completely altered to clay pseudomorphs, probably vermiculite. Microfabric analysis of soil and pararock did not show any striated b-fabric indicative of shear failure. However, microfabrics do confirm translocated clay into the subsoil as typic pore coatings indicating long-term landscape stability prior to deforestation.

Micromorphology of calcium carbonate in bauxite processing waste

Abstract Secondary calcium carbonate accumulations were observed in bauxite processing waste, a waste product of aluminum production. The waste material is saline-sodic, calcareous, and highly alkaline (pH 11.8). The waste was packed as a 50% solids slurry into cylindrical acrylic columns and subjected to treatment combinations of differing drainage regimes, and gypsum and sulfuric acid additions. After eight wetting and drying cycles, samples were taken from the columns for analysis of carbonate distribution and thin-section analysis. Micromorphic forms of carbonate found in the waste included infillings, calcitic coatings and hypocoatings of voids, acicular calcite found in vughs, and calcium carbonate replacing gypsum. These results suggest that secondary calcite forms commonly found in soils may form very rapidly under certain conditions.