Chao Shang, Virginia Tech, Dept of Crop & Soil Env. Sciences, Blacksburg, VA 24061
The association of soil organic matter (SOM) with mineral components was examined in the soil samples from Brazil and Mexico using physical fractionation including particle-size, density, and magnetic gradient fractionation, chemical selective dissolution and x-ray diffraction. Particle fractionation is useful for locating SOM in soil matrix (i.e., at various levels of aggregation), density fractionation can be used for isolating light-fraction SOM, SOM- and nutrients-rich aggregates in silt size or larger and the mineral components of these fractions, and magnetic gradient separation is effective in concentrating clay-sized Fe oxides and associated mineral phases, which is important for studying the interaction between SOM as humus and mineral phases. In the acidic Brazilian soils, amorphous form of Al and Fe (particularly Al) shows a significant correlation with organic C level in light fractions, whereas free oxides of Al and Fe give a negative correlation with organic C abundance. In clay-sized mineral-dominant fractions, the association of extracted metals with SOM, however, was not evident, indicating a complicated role played by mineral phases. Low-activity minerals including mica, quartz and feldspars can be easily separated from organic components, whereas goethite and kaolinite are more tightly associated with each other and with SOM. In tropical calcareous soils, carbonates-coating on coarse SOM is an important mechanism for slowing decomposition, and carbonates are clearly responsible for formation of the silt-size microaggregates that stabilize SOM.
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