Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 11/10/2008
Publication Date: 2/13/2009
Citation: Laird, D.A., Thompson, M. 2009. The Ultrastructure of Clay-Humic Complexes in an Iowa Mollisol. In: Laird, D.A., Cervini-Silva, J., editors. Carbon Stabilization by Clays in the Environment: Process and Characterization Methods, CMS Workshop Lectures, Volume 16. Chantilly, VA: Clay Minerals Society. p. 95-118. Interpretive Summary: Interactions between clay minerals and humic materials in soils have a large influence on soil quality and the ability of soils to sequester organic carbon. In this book chapter we used scanning and transmission electron microscopy to show that two different types of humic materials are present in soils and interact differently with soil clays. One type of humic material exists as loose material that tends to surround and cover clay particles while the other type of humic material appears to be discrete particles of charcoal. This research will help scientists to better understand the interactions between clay minerals and soil organic matter. This research also provides definitive evidence that clay size charcoal particles can be physically separated from soil materials and is therefore likely to stimulate new research to characterize the physical and chemical changes that occur during aging of charcoal in soil environments.
Technical Abstract: The chemical and physical activity of clay minerals in soils, particularly in surface horizons, is significantly mediated by interactions with organic components. The reactivity of soil organic matter, including its resistance to decomposition, is regulated by interactions with clay minerals. This marriage of organic and inorganic soil components has profound implications on our ability to quantitatively predict the consequences of alternative soil management practices that could improve food and energy production, protect water and air quality, or mitigate the impacts of greenhouse gases on global climate. In this paper, we discuss observations of clay-humic complexes made by electron microscopy and other complementary techniques. Data reported in this paper come from various analyses of a smectite-rich Mollisol in Iowa. Scanning and transmission electron micrographs of both minimally disturbed soil clay-humic complexes and microaggregates combined with scanning electron microscope (SEM) and transmission electron microscopy (TEM) images of physically extracted soil clay subfractions reveal two distinct phases of soil organic matter, diffuse humic materials and discrete particles of black carbon. The humic materials are observed to enmesh and apparently stabilize soil smectite quasicrystals and microaggregates. The black carbon, believed to be aged charcoal from prairie fires, exist as discrete particles some of which have a solid almost "glassy" appearance while others are highly vesicular. The discrete charcoal particles are admixed with the coarse clay fraction but can be separated from it by combined particle size and density fractionation procedures.