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United States Department of Agriculture

Agricultural Research Service

Research Project: DRYLAND CROPPING SYSTEMS MANAGEMENT FOR THE CENTRAL GREAT PLAINS

Location: Central Plains Resources Management Research

Title: Mid-Infrared Spectroscopic Properties of Humic Acid and Fulvic Acid-Soil Mixtures.

Authors
item Calderon, Francisco
item Reeves Iii, James
item Vigil, Merle

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 19, 2010
Publication Date: July 19, 2010
Citation: Calderon, F.J., Reeves III, J.B., Vigil, M.F. 2010. Mid-Infrared Spectroscopic Properties of Humic Acid and Fulvic Acid-Soil Mixtures.. Meeting Proceedings. 15th Annual Meeting of the International Humic Substances Society, June 27-July 2, 2010. Tenerife, Spain.

Interpretive Summary: In this work, we added different amounts of humic substances to soils in order to show how infrared spectroscopy can be used to measure the quality of humic substances in environmental samples. This is important because humic substances are an important part of soil organic matter, and they have important implications for soil quality and carbon sequestration. Soils that are exceptionally fertile, such as Midwest Corn Belt soils, are rich in organic matter and humic substances. For this reason it is important to go beyond total carbon in soil and measure humic substances. The technology that we are developing will help to measure small changes in soil organic matter, which in turn will permit us to carry out more detailed field studies about how agricultural practices can be used to improve our soils.

Technical Abstract: The detection of humic materials in soils is essential in order to determine organic matter (SOM) stability and C sequestration on agricultural land. Mid-Infrared (MidIR) spectroscopy has been used to characterize SOM quality [1], study extracted soil humic acids [2], develop calibrations for quantifying SOM [3], and to study decomposition of organic matter in soil [4]. However, infrared spectra from soils are the result of a multitude of combined absorbances from organic and mineral bands, and some of the spectral signatures of humic acids can be lost or confounded in soil matrices. In this study, we added different amounts of authentic humic and fulvic acid standards to ashed soil in order to identify reliable spectral MidIR bands for marking the presence and amount of stable organic matter in soil. Several regions of the MidIR spectrum decrease in soils upon ashing, and at the same time are highly absorbed by humic and/or fulvic acids. These regions include 3500-2000, 1830-1520, and 1260-990 cm-1 and should thus be considered organic matter bands in soils. Other MidIR regions outside these bands are predominantly of mineral absorption or from a mixture of organic-mineral absorption. The 2870-2950 cm-1 is one of the few regions of the MidIR soil spectrum where absorbance is almost exclusively from organics, with little absorbance due to mineral sources. Our spectral subtraction approach suggests that 3400-2800 and 1730-1630 cm-1 are regions that can be attributed to stable organic compounds in soil spectra.

Last Modified: 9/2/2014