Skip to main content
ARS Home » Research » Publications at this Location » Publication #105816
Title: NATURE OF CLAY-HUMIC COMPLEXES IN AN AGRICULTURAL SOIL: (I) CHEMICAL, BIOCHEMICAL, AND SPECTROSCOPIC ANALYSES

Author
item Laird, David
item Martens, Dean
item KINGERY, WILLIAM - MISSISSIPPI STATE UNIVER.

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Storage of carbon as soil organic matter both improves soil quality and reduces levels of greenhouse gases in the atmosphere, thereby reducing the threat of global warming. Adding more crop residue to soils will not necessarily increase soil organic matter because crop residue decomposes rapidly in soils (within a few months) releasing carbon dioxide back to the atmosphere. The most stable forms of soil organic matter are clay-humic complexes. Clay-humic complexes are stable for 500 to 3000 years in soils. Thus, there is a need to understand how clay-humic complexes are formed. We discovered that the chemistry of humic substances varies with the type of clay minerals in a soil. This suggests that clay mineralogy has a big influence on the formation of clay-humic complexes. This new knowledge will help scientists to better understand how soil organic matter is formed and stabilized and ultimately design better management systems that optimize formation of soil organic matter. The American people will benefit in the long-term from enhanced soil quality and reduced levels of greenhouse gas emissions.

Technical Abstract: Soil management systems that encourage the formation and stabilization of recalcitrant clay-humic complexes will have the greatest long-term impact on C sequestration and soil quality. The objective of this study was to determine the relationship between the chemical, biochemical, and spectroscopic characteristics of humic substances and clay mineralogy. Clay-humic complexes were separated from the Ap horizon of a Webster soil (Typic Haplaquoll) by an invasive sonication-centrifugation technique. The samples were analyzed from mineralogy by XRD, chemical composition by ICP-AES, C and N by thermal combustion, C chemistry by solid state **13C MAS-NMR and both GC and HPLC analyses of extractable organic compounds. The coarse, medium, and fine clay fractions are dominated by quartz, a low-charged interstratified phase, and smectite, respectively. Extractable organic compounds, 30 to 52% of the total C, are dominated by basic amino acids and polyunsaturated fatty acids. Less than 3% of the extractable C is monosaccharides and amino sugars and only trace levels of phenolic acids were found. The C:N ratios of humic substances associated with the coarse, medium and fine clay fractions are 17, 10, and 10, respectively. The coarse clay fraction has stronger carboxyl and O-alkyl **13C-NMR peaks and lower levels of extractable amino acids, fatty acids, monosaccharides, and amino sugars than humics associated with the fine clay fraction. The results indicate that the biochemistry of the clay-humic complexes differs substantially from that of whole soils and that soil clay mineralogy strongly influences humification.