Nature of clay-humic complexes in an agricultural soil: II. Scanning electron microscopy analysis

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 mum 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 lm 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.