NMR Imaging of Soil Organic Carbon

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By William L. Kingery, Professor, Mississippi State University, and Michael E. Lilly, Mississippi State Soil Scientist, Natural Resources Conservation Service.

Typical soil chemistry studies have used methodologies whereby components of soils are isolated, e.g., extraction using solvents, or physicochemical procedures, such as the isolation of clay-sized particles. More recent technologies allow for chemical observations to be made using intact soil samples. An example is nuclear magnetic resonance spectroscopy (NMR). The NMR spectrometer is similar to the magnetic resonance imaging (MRI) equipment found in hospitals. Both rely on advanced technology that uses magnetic fields and radio waves to acquire detailed information. MRI looks at the human body. NMR works on a much smaller scale, looking at the compounds found in organic matter. The NMR can be used to examine soil, leaves, air particles—really anything found in nature. NMR basically produces a molecular map.

In a cooperative research project involving Billy Kingery, soil scientist at the Mississippi Agricultural and Forestry Experiment Station; Mike Lilly, State Soil Scientist, Mississippi NRCS; Delaney Johnson, soil scientist, Mississippi NRCS; and the husband-and-wife team of Myrna and Andre Simpson at the University of Toronto at Scarborough; NMR microimaging technology is being used to study water, organic matter, and pollutants in soil ecosystems. In addition to Kingery, Mississippi State University personnel working on the project include graduate assistant Rachel Stout and research technician Grady Jackson. Stout and Jackson are in the Department of Plant and Soil Sciences.

In this project, NMR microimaging allows for a holistic look at the biochemistry and structures of soil organic matter. From an agricultural standpoint, the study will provide information about water and the fate of pesticides in soils.

NMR microimaging was applied to examine the distribution of water in soils cores and the transport of pollutants. In order to reflect a range of characteristics of organic matter, four soil series were selected for sampling. The soils in the study were Memphis (Alfisol), Bowdre (Mollisol), Johnston (Inceptisol), and Croatan (Histosol). Microimaging samples were prepared by gently forcing a glass-walled core (3 cm in diameter and 15 cm in length) into the soil surface. The samples were shipped to the Bruker Biospin Applications Laboratory in Rheinstetten, Germany, for microimaging by personnel of Simpsons and Bruker.

Soil water was imaged using protons as the target nuclei. Protons in water molecules have resonance characteristics that allow them to be distinguished from protons in other molecular structures in soils. Hexafluorobenze was used to representing a pesticide contaminant. It was imaged using the fluorides contained in its structure as the target for imaging. Images and videos of the results are at the bottom of the page.

NMR microimaging is a powerful tool for the study of water and contaminant distribution in soil. The images indicate that the NMR microimaging techniques are sensitive enough to distinguish between adsorbed and free water in complex soil systems. Further, while the contaminant studies show the potential for imaging in pollutant transport research, sensitivity remains a challenge to simulations of actual environmental concentrations.

In addition to tracing what happens to crop inputs, such as fertilizer and pesticides, the NMR process can also be used to track carbon. The release of carbon in the form of CO₂ can influence climate. A significant portion of the earth’s carbon is tied up in the soil, but it is unclear how long the movement of soil carbon into a different part of the carbon cycle takes. NMR technologies offer the potential for improvements in the accounting of global carbon.

Videos and Graphics of NMR Microimaging

Video 1 (AVI; 464 KB): 3-D microimage build from the resonance signals of protons in benzene molecules following the application of benzene to a Croatan soil. The benzene protons can be distinguished from protons in other structures, such as water.

Video 2 (AVI; 579 KB): 3-D microimage of protons in the water of a Croatan soil sample as it was collected from the field in Jackson County, Mississippi (see article above for more details).

Video 3 (AVI; 1.9 MB): 3-D microimage of the protons attached to water molecules in a Croatan soil as the signals build up over time to give a final image.

Figure 1.—Proton (water) images of different types of soils that vary in content of organic matter: Bowdre (image A), Croatan (image B), Johnston (image C), and Memphis (image D).

Figure 2.—A series of images taken at different times showing the process of  hexafluorobenzene penetration into the Croatan soil. The brighter the region, the higher the concentration of hexafluorobenzene. The four images were taken at 0 hours (A), 1.5 hours (B), 8 hours (C), and 16 hours (D).

To learn more about NMR in environmental sciences, visit http://www.utsc.utoronto.ca/~asimpson/.

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