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United States Department of Agriculture

Agricultural Research Service

Title: Analysis of Humic Substances by C-13 and N-15 Methodology

Authors
item Hayes, Michael - UNIV OF BIRMINGHAM
item Clapp, Charles
item Malcolm, Ronald - USGS, DENVER, CO
item Layese, Milegua - UNIVERSITY OF MINNESOTA

Submitted to: International Soil Science Society Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: August 15, 1995
Publication Date: N/A

Interpretive Summary: Humic and fulvic acids are the major and vital macromolecules of the organic matter fractions in soils, sediments, waters, peats and coals. These are highly mixed substances arising from the transformations of plant and animal tissues through a variety of chemical and biological processes. Plants that follow a 3-carbon pathway for sugar synthesis do not incorporate the natural abundance of the carbon-13 isotope as readily as do plants that use a 4-carbon synthesis for metabolism. Therefore, the 4-carbon plants such as corn, some grasses, and tropical plants have carbon-13 isotope ratio values of approximately -12. The 3-carbon plants, such as soybean, wheat, cool season grasses and forest trees have carbon- 13 isotope ratio values of the order of -27. From analyses of the isotope ratio values of humic fractions isolated from their natural environments, some clues can be obtained as to the nature of the plant materials contributing to these humic substances. Isolated humic and fulvic acids from world-wide water sources ranged from -27 to -31. Values from peat were -24 to -27; from soils, -17 to -26; and from coal, -22 to -28. A better awareness of the types of biological molecules which make up humic substances will aid our interpretations of analytical data, and of the possible origins in the humic macromolecules of products identified in chemical and biochemical reactions

Technical Abstract: The formation of humic substances can be traced to the type of originating plant residue by use of delta 13C analyses. C4 (Hatch-Slack cycle) plants such as corn, some grasses and tropical plants have delta 13C values of approximately -12/mil, whereas C3 (Calvin cycle) plants such as small grains, soybean, trees, and most temperate and cold climate plants have delta 13C values of about -27/mil. Total C contents were typical of humic and fulvic acids. Isolated humic substances from water in the U.S., U.K., Ireland, and Norway gave delta 13C values ranging from -27 to -31/mil. Other ranges in values were: for peats from Minnesota, New York, and England (-24 to -27/mil), for soils from Minnesota, Alaska, Canada, and Israel (-17 to -26), for coals from England (-22 to -24/mil), and for several IHSS standard and reference samples (-22 to -28/mil). A relationship appears to exist between the nature of the humic substances and the original plants and/or the organic matter in the soils.

Last Modified: 10/1/2014