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DIVISION S-2-SOIL CHEMISTRY

Nitrite Fixation by Humic Substances

Nitrogen-15 Nuclear Magnetic Resonance Evidence for Potential Intermediates in Chemodenitrification

^a U.S. Geological Survey, Box 25046 M.S. 408, Denver Federal Center, Denver, CO 80225 USA
^b Dep. of Chemistry, California State University, Bakersfield, CA 93311 USA

kathorn{at}usgs.gov

Studies have suggested that NO^-₂, produced during nitrification and denitrification, can become incorporated into soil organic matter and, in one of the processes associated with chemodenitrification, react with organic matter to form trace N gases, including N₂O. To gain an understanding of the nitrosation chemistry on a molecular level, soil and aquatic humic substances were reacted with ¹⁵N-labeled NaNO₂, and analyzed by liquid phase ¹⁵N and ¹³C nuclear magnetic resonance (NMR). The International Humic Substances Society (IHSS) Pahokee peat and peat humic acid were also reacted with Na¹⁵NO₂ and analyzed by solid-state ¹⁵N NMR. In Suwannee River, Armadale, and Laurentian fulvic acids, phenolic rings and activated methylene groups underwent nitrosation to form nitrosophenols (quinone monoximes) and ketoximes, respectively. The oximes underwent Beckmann rearrangements to 2° amides, and Beckmann fragmentations to nitriles. The nitriles in turn underwent hydrolysis to 1° amides. Peaks tentatively identified as imine, indophenol, or azoxybenzene nitrogens were clearly present in spectra of samples nitrosated at pH 6 but diminished at pH 3. The ¹⁵N NMR spectrum of the peat humic acid exhibited peaks corresponding with N-nitroso groups in addition to nitrosophenols, ketoximes, and secondary Beckmann reaction products. Formation of N-nitroso groups was more significant in the whole peat compared with the peat humic acid. Carbon-13 NMR analyses also indicated the occurrence of nitrosative demethoxylation in peat and soil humic acids. Reaction of ¹⁵N-NH₃ fixated fulvic acid with unlabeled NO^-₂ resulted in nitrosative deamination of aminohydroquinone N, suggesting a previously unrecognized pathway for production of N₂ gas in soils fertilized with NH₃.

Abbreviations: IHSS, International Humic Substances Society • LFA, Laurentian soil fulvic acid • NMR, nuclear magnetic resonance • NOE, nuclear Overhauser enhancement • SRFA, Suwannee River fulvic acid