Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2001
Publication Date: N/A
Interpretive Summary: Increasing the amount of carbon stored in soil as soil organic matter both improves the quality of the soil and reduces levels of greenhouse gases in the atmosphere. Clay-organic matter complexes are the most stable form of carbon in soils. When added to soil, fresh plant reside decomposes in a few weeks, but clay-organic matter complexes remain in soils for 500 to 3000 years. We discovered two different types of clay-organic matter complexes. One type exists as films that coat the surfaces of clay particles. The other type exists as discrete particles that are primarily made of carbon and iron. Scientist are the immediate beneficiaries of this research because the new knowledge will help them to understand how soil organic matter forms in soils. In the long-term, the American people will benefit because the new knowledge will help scientists develop new management systems for agricultural soils that both improve soil quality and reduce the threat of global warming.
Technical Abstract: 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, mesic Typic Haplaquoll) soil located near Waseca, Minnesota. The whole soil clay fraction (less than 2 um particle size fraction) was separated by a noninvasive sedimentation technique, and coarse, medium, and fine clay fractions (0.2-2.0, 0.02-0.2, and less than 0.02 um size fractions, respectively) were separated from a portion of the whole clay sample by an invasive sonication-cetrifugation 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 or types of clay-associated humic substances were identified. The first type exists as a diffuse filamentous film that covers the basal surfaces of 2:1 phyllosilicates the medium and fine clay fractions. The second type exists as discrete particles of high-density Fe-humic substance complexes in the coarse clay fraction.