Fig. 5 Liquid phase ACOUSTIC ¹⁵N nuclear magnetic resonance (NMR) spectra (nuclear Overhauser enhancement retained) of soil fulvic acids (FA) reacted with Na¹⁵NO₂. (a) Armadale, pH 3; (b) Laurentian, pH 3; (c) Laurentian, pH 6; (d) Laurentian, dilute, pH 4