Nitrite Fixation by Humic Substances Nitrogen-15 Nuclear Magnetic Resonance Evidence for Potential Intermediates in Chemodenitrification
2000
Thorn, K. A. | Mikita, M. A.
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
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