SYNOPSIS OF DOCTORAL THESIS: Ultramicroscopy of a Brazilian Oxisol

Gordon Vrdoljak

Environmental Geochemistry Group

University of California at Berkeley

Soils of tropical regions (Oxisols) are under increasing demands for agricultural use due to expanding populations. Current methods of slash and burn agriculture allow these soils to remain productive for only a few years, thus contributing significant amounts of greenhouse gases to the atmosphere, while destroying more than 10 million hectares of tropical forest each year. After cultivation, reestablishment of forest or other groundcover is not always successful. The loss in productivity and the difficulty in vegetation reestablishment are thought to be from a change in the Oxisol soil structure.

Little is known about the microstructure of these fragile soils in comparison to temperate-zone soils. Contradicting theories have been proposed in the literature to explain their structure and behaviour. To resolve this conundrum and to aid in the development of sustainable agricultural practices, the structure of a benchmark Brazilian Oxisol under tropical rainforest was studied intensively. The results were compared directly to those for an identical soil placed under continuous agricultural use for 20 years.

Light, scanning electron, transmission electron, scanning transmission electron, and atomic force microscopy were used to resolve the microstructural elements of the soil. Individual minerals, amorphous materials, biota, and organic materials were imaged and characterized by the microscopic techniques. Such a multiple microscopic method was needed to observe the scale-dependent structure of the soil, and to visualize interactions between its components. Electron diffraction and energy dispersive X-ray spectroscopy were used to identify and characterize chemically the mineral and amorphous phases present.

The microaggregates of the Oxisol were formed by processes of aggregation and microstructuration. Interactions between kaolinite clay, nanosize goethite particles, and organic materials was found to control the process of soil structure formation. Clay particles form the bulk of the soil and are intimately associated with goethite, while coating other soil components. Organic matter within the forest soil was found to act as a strong binding agent, bridging parts of aggregates together. In the soil under cultivation, a loss in organic matter was observed, which destroyed much of the aggregate internal strength and nutrient content, and allowed the dispersion of clay to occur. With clay dispersion followed by reflocculation, the cultivated soil formed much larger aggregates (clods), less productive for vegetation.

With a better understanding of the microaggregate structure of Oxisols comes a more adequate model for the formation of soils in tropical regions. This model can be of use in the development of sustainable agricultural management practices to reduce the damaging effects of slash and burn agriculture.