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DIVISION S-2 - SOIL CHEMISTRY

Nature of Clay–Humic Complexes in an Agricultural Soil

II. Scanning Electron Microscopy Analysis

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Drive, Ames, IA 50011

* Corresponding author (laird{at}nstl.gov)

The most stable forms of organic matter in soils are the clay–humic complexes. An understanding of mechanisms and processes influencing the formation of clay–humic complexes may facilitate development of agricultural management systems that increase the long-term sequestration of C in soils and improve soil quality. The specific objective of this study was to visualize associations between humic substances and clay minerals separated from a typical agricultural soil. The soil sample used in this study was from the Ap horizon of a Webster (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) soil located near Waseca, MN. The whole soil clay fraction (<2 µm particle-size fraction) was separated by a relatively mild sedimentation technique, and coarse, medium, and fine clay fractions (0.2–2.0, 0.02–0.2, and <0.02 µm size fractions, respectively) were separated from a portion of the whole clay sample by an aggressive sonication-centrifugation technique. All samples were Ca-saturated, dialyzed, and freeze-dried. The samples were analyzed by scanning electron microscopy to obtain images of clay–humic complexes and energy dispersive x-ray analysis to obtain maps of elemental distributions. Two distinct types of clay-associated humic substances were identified. The first type exists as diffuse filamentous films that cover basal surfaces of 2:1 phyllosilicates in the medium and fine clay fractions. The second type exists as discrete particles of high-density metal–humic substance complexes in the coarse clay fraction.

Abbreviations: MAS-NMR, magic angle spinning-nuclear magnetic resonance spectroscopy • SEM, scanning electron microscopy • XRD, x-ray diffraction

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