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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOGEOCHEMICAL PROCESSES INFLUENCING FORMATION AND STABILIZATION OF SOIL ORGANIC MATTER AND SOIL STRUCTURE Title: Chemical structure of soil organic matter in slickspots as investigated by advanced solid-state NMR

Authors
item Mao, J -
item Palazzo, D -
item Olk, Daniel
item Clapp, Charles
item Senesi, N -
item Bushore, T -
item Cao, X -

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 21, 2010
Publication Date: July 2, 2010
Citation: Mao, J., Palazzo, D.C., Olk, D.C., Clapp, C.E., Senesi, N., Bushore, T.I., Cao, X. 2010. Chemical structure of soil organic matter in slickspots as investigated by advanced solid-state NMR. Soil Science. 175:329-338.

Interpretive Summary: In Idaho, some soils in the mountainous regions have high salt concentrations and do not support much vegetation. The reason for the lack of vegetation growth is unknown. One possible explanation is that the salt in the soil changes the type or quality of the organic nutrients in the soil so that they become less available to plants. But little is known regarding the organic nutrients of salt-affected soils. In this study we used new laboratory methods to measure the organic compounds that were found in two salt-affected soils and compared them with two nearby soils that were not affected by salt. We found one difference in the types of organic compounds between soils that were salt-affected and the soils that were not salt-affected. This information will serve as useful background information to other researchers who are studying the growth of young plants in these salt-affected soils. Our results will also benefit other researchers of salt-affected soils and researchers who study the organic chemistry of soils, especially those who use our laboratory methods.

Technical Abstract: Slickspot soils are saline, and knowledge of their humic chemistry would contribute to our limited understanding how salinity affects soil C and N stocks. We characterized humic acids (HAs) from slickspot soils with solid-state 13C nuclear magnetic resonance (NMR). Expanding on previous use of cross polarization/magic angle spinning (CP/MAS) NMR, we used direct polarization (DP) and yet more advanced spectral editing techniques to identify specific functional groups, detect the connectivities of different functional groups, and selectively observe fused ring carbons. A series of soil HAs was extracted from soil layers having different physical properties: silt texture, vesicular structure, and clay texture. They were compared to HAs from corresponding depths in soils adjacent to the slickspots. All HAs consisted of five main structural components: aliphatic chains; peptides; sugar rings; lignin residues, and aromatics/olefinics. For all soils except one outside slickspots, the HAs from the vesicular and clay layers contained less nonpolar alkyls and more aromatics than those from surface silt layers, but their spectral proportions differed when the 13C NMR was performed using CP/MAS instead of DP. HAs from the surface layers inside the slickspots had lower aromaticity than those from outside the slickspots. Advanced spectral editing techniques allowed for the selection of nonprotonated carbons and mobile groups, alkyls, CH, and CH2 groups which would otherwise be buried in the heavily-overlapped spectrum. They provided more structural information than was obtained by routine 13C CP/MAS or DP.

Last Modified: 11/25/2014
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