Fig. 4. Cross-polarization, magic angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectra of the humin fractions from Site 1 soil (Oa1 and Oa2 horizons), Site 2 soil (Oe and O/A horizons), and Site 3 soil (Cf horizon). 150917 and 61612 scans for the Site 1 soil Oa1 and Oa2 horizons, respectively; 18825 and 80194 scans for the Site 2 Soil Oe and O/A horizons, respectively; and 199912 scans for the Site 3 Cf horizon