Nitrite fixation by humic substances: Nitrogen-15 nuclear magnetic resonance evidence for potential intermediates in chemodenitrification

Studies have Suggested that NO,, produced during nitrification and denitrif ication, can become incorporated into soil organic matter and, in one of th e processes associated with chemodenitrification, react with organic matter to form trace N gases, including N2O. To gain an understanding of the nitr osation chemistry on a molecular level, soil and aquatic humic substances w ere reacted with N-15-labeled NaNO2, and analyzed by liquid phase N-15 and UC nuclear magnetic resonance (NMR). The International Humic Substances Soc iety (IHSS) Pahokee peat and peat humic acid were also reacted with (NaNO2) -N-15 and analyzed by solid-state N-15 NMR. In Suwannee River, Armadale, an d Laurentian fulvic acids, phenolic rings and activated methylene groups un derwent nitrosation to form nitrosophenols (quinone monoximes) and ketoxime s, respectively. The oximes underwent Beckmann rearrangements to 2 degrees amides, and Beckmann Fragmentations to nitriles. The nitriles in turn under went hydrolysis to 1 degrees amides. Peaks tentatively identified as imine, indophenol, or azoxybenzene nitrogens were clearly present in spectra of s amples nitrosated at pH 6 but diminished at pH 3. The N-15 NMR spectrum of the peat humic acid exhibited peaks corresponding with N-nitroso groups in addition to nitrosophenols, ketoximes, and secondary Beckmann reaction prod ucts. Formation of N-nitroso groups was more significant in the whole peat compared with the peat humic acid. Carbon-13 NMR analyses also indicated th e occurrence of nitrosative demethoxylation in peat and soil humic acids. R eaction of N-15-NH3 fixated fulvic acid with unlabeled NO2- resulted in nit rosative deamination of aminohydroquinone N, suggesting a previously unreco gnized pathway for production of N-2 gas in soils fertilized with NH3.