Fig. 3
From: Fire-derived organic matter retains ammonia through covalent bond formation
Nitrogen K-edge NEXAFS, FTIR, and NMR spectra of oxidized PyOM samples. Nitrogen (N) K-edge near-edge X-ray absorption fine structure (NEXAFS) (a), Fourier transform infrared (FTIR) (b), and nuclear magnetic resonance (NMR) (c) spectra collected from oxidized PyOM, oxidized PyOM following exposure to ammonium (NH4+), and oxidized pyrogenic organic matter (PyOM) following exposure to ammonia (NH3). a Shaded bands represent the range of peak centers consistent with selected spectral features: 397.88–399.20 eV for C=N bonds in 1N and 2N aromatic six-membered rings (orange), 400.00 for nitrile bonds (dark blue), 399.76–400.27 for C=N bonds in 2N five-membered rings (light blue), 401.15 for C–N bonds in non-aromatic six-membered rings (red), 401.20–402.40 for C–N bonds in 1N and 2N aromatic five-membered rings (light blue), 403.00–403.75 for aliphatic N bonded to aromatic rings (yellow), and 405.00–406.58 for aliphatic amines and N–H bonds (green). Model chemical structures are shown at the top of the figure. b FTIR spectra of oxidized PyOM, oxidized PyOM following exposure to NH4+, and oxidized PyOM following exposure to NH3. c 15N-NMR spin echo direct polarization magic angle spinning (SEDPMAS) spectrum of oxidized PyOM following exposure to [15N]-NH3, and 15N-NMR cross-polarization magic angle spinning (CPMAS) spectra of oxidized PyOM following exposure to [15N]-NH3 and [15N]-NH4+. The spectra suggest that NH3–N retention mechanisms could include NH4+ adsorption (represented by the peak at 20 ppm), and the formation of covalent C–N bonds such as amines (20 ppm), amides (~107 ppm), and aromatic five-membered heterocycles (chemical shifts between ~130–165 ppm)16,33,39,40
