| dc.description.abstract | Although soils developed from ultramafic parent materials have significance to
agriculture, ecology and health, their bio-geochemistry is poorly understood. The
mineralogical and bio-geochemistry of soils formed from the ultramafic parent materials
of the Great Dyke, Zimbabwe and Gillespie County, Texas was investigated. The
objectives were to determine the mineralogical and bio-geochemical properties of the
soils in order to assess the potential impact and challenges to agriculture, and
environmental quality. Soil samples were taken from the crest, shoulder, footslope and
the toeslope. Chemical analyses were performed by nuclear and spectroscopic
techniques. Mineral characterization was conducted by x-ray diffraction (XRD) and
spectroscopic techniques. Microbial whole-community structure was determined by the
fatty acid methyl esters (FAME) technique. The results indicate wide chemical and
mineralogical compositions among the studied sites. The soils contain relatively high
concentrations of heavy metals (some sites contain Cr(VI)), but low levels of K and Ca.
The highest concentrations of trace metal were associated with chromite, Fe oxides and
serpentinite. The concentrations of Mg were higher than those of Ca and varied between
Zimbabwe and Texas soils largely due to the parent materials.
Unique to these soils is the occurrence of talc, serpentine, chlorite, Fe-rich smectite,
amphiboles, pyroxenes, Fe and Cr oxides in relatively large amounts. These soils also
lack micas and have neglible amounts of kaolinite and feldspars. Palygorskite and
serpentine occurred in specific soil horizons and at specific landscape positions.
FAME profiles indicate that the soil microbial community structure is predominantly
bacteria and fungi (including arbuscular mycorrhiza fungi) at each landscape position
across the transect. Biomarkers for actinomycetes were undetectable. The proportions of
Gram-positive bacteria were higher than those of the Gram-negative bacteria.
Very low levels of nutrients (Ca and K), higher Mg/Ca molar ratios, and the relatively
high concentrations of heavy metals in these soils impact agricultural productivity. High
concentrations of heavy metals, the presence of the Cr(VI) as well as its great potential
to form in these soils might impact microbial activity and environmental quality. The
occurrence of fibrous minerals (e.g serpentine and amphiboles) in these soils will likely
impact human health. | en |
