Fig. 4 Liquid-phase ACOUSTIC ¹⁵N nuclear magnetic resonance (NMR) spectra (nuclear Overhauser enhancement retained) of Suwannee River fulvic acid (SRFA) (a) reacted with ¹⁵NH₂OH, (b) reacted with Na¹⁵NO₂ at pH 3, and (c) reacted with Na¹⁵NO₂ at pH 6