Fig. 3. Solid-state nuclear magnetic resonance spectra of ¹³C bonded to ¹⁴N in the (a) mobile humic acid fraction and (b) whole soil organic matter for the surface layer of a triple-cropped rice soil in the Philippines. The N–C=O signal is indicative of amides, the N–C_arom signal represents the aromatic N form anilide, and the N–CH signal is an aliphatic resonance of peptides. Figure 3a is reprinted from Schmidt-Rohr et al. (2004) with permission from the National Academy of Sciences of the USA. Figure 3b is from K. Schmidt-Rohr (2005, unpublished data).