Comparing the weathering of soil and sedimentary palygorskite in the rhizosphere zone

Rhizosphere and microorganisms can play a central role in the weathering of clay minerals. Rhizospheric and bacterial studies to understand the weathering of clay minerals, however, are still lacking. Most previous studies have been carried out on industrial or geologic minerals. Therefore, the main objective of the present study was to compare the transformation of palygorskite found in sedimentary rocks with palygorskite from soils. Mineral transformations were induced by silicate solubilizing bacteria and the sorghum rhizosphere. The plant extractable Mg as a criterion of the ability of plants to extract Mg from these minerals was also considered. A pot experiment was conducted. The growth medium was a mixture of quartz sand as filling material, sedimentary or soil palygorskite and silicate solubilizing bacteria. After 100 days from cultivation, plants were harvested and their Mg uptake was measured by an inductively coupled plasma atomic emission spectroscopy instrument following dry ash extraction. The X-ray diffraction analyses illustrated that sedimentary palygorskite weathered almost completely in plant and/or silicate solubilizing bacteria treatments. Soil palygorskite however, did not weather to a detectable extent. The Mg content extracted by the sorghum was significantly greater for sedimentary palygorskite than the soil derived mineral. Root and silicate solubilizing bacterial activities appear to have increased rhizosphere acidity which, in turn, resulted in the release of structural Mg from mineral lattice. Higher Mg release and uptake and complete weathering of sedimentary palygorskite, could indicate a higher weatherability of this source. The higher disequilibrium between the geologic and pedogenic environments increases the weatherability of geological minerals. Studies under the soil conditions could therefore be helpful to understand the weathering and management of soils with dominant palygorskite.