### Spectral Analysis  
\({}^{13}\)C CP-MAS NMR experiments on (vermi)compost or digestate samples are usually performed to investigate and compare OM composition in the feedstock (biomasses or wastes) and throughout (or only at the end of) the decomposition process. The spectra are typically analyzed by distinguishing carbons from different functional groups on the basis of the chemical shift, which is directly related to the chemical shielding. Given the complex and heterogeneous chemical nature of OM, spectra span a chemical shift range of over 200 ppm, with a strong overlap of signals (Figure 1). Therefore, spectral regions, rather than individual signals, summarized in Table 1, are identified and assigned to specific organic species: (i) the 0-45 ppm spectral region is ascribed to alkyl carbons, with short chain aliphatic carbons from volatile fatty acids and steroid-like molecules resonating between 0 and 28 ppm [47] and long chain aliphatic carbons from plant aliphatic biopolymers (suberin, cutin, waxes, fatty acids) [48] and proteins [49] between 28 and 45 ppm; (ii) O-alkyl and N-alkyl carbon signals, ascribable to lignin, carbohydrates (i.e., cellulose and hemicellulose), and proteins [50], are observed between 45 and 110 ppm; (iii) the 110-160 spectral region is assigned to aromatic carbons present, for instance, in lignin and polyphenols [51]; (iv) signals of carboxyl carbons in esters, acids, and amides (160-190 ppm) and carbonyl carbons in aldehydes and ketones (190-220 ppm) are observed between 160 and 220 ppm. It must be noted that slightly different spectral region ranges are indicated in the literature by different authors. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Shift Range** & **Assignment** & **Conventional Region Name** \\ \hline
0–28 ppm & CH\({}_{3}\) and CH\({}_{2}\) in short chain and simple aliphatics & Alkyl C \\
28-45 ppm & CH\({}_{2}\) and CH in long aliphatic chains & Alkyl C \\ \hline
45–60 ppm & O-CH\({}_{3}\); CH-N; aliphatic quaternary C & \\
60–95 ppm 1  & C2-C6 in cellulose and hemicellulose; alcohols; ethers & O-Alkyl C \\
95–110 ppm 2  & C1 of cellulose and hemicellulose; anomeric carbon of polysaccharides & \\ \hline
110–145 ppm 3  & unsubstituted or alkyl-substituted aromatic C & Aromatic C \\
145–160 ppm 4  & O-N-substituted aromatic & Carboxyl/carbonyl C \\ \hline
160–190 ppm & Carboxylic acids; esters; amides & \\
190–220 ppm & ketones; aldehydes & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spectral regions used in the analysis of \({}^{13}\)C MAS NMR spectra of OM and assignment to carbons in different functional groups. Figure 1: Typical \({}^{13}\)C MAS NMR spectrum of OM with division in spectral regions assigned to functional groups (see Table 1) and assignment to specific molecular structures. Reproduced with permission from Ref. [43]. Some signals are also assigned to functional groups belonging to specific biomacromolecules present in biomass samples [52; 53]. The peak at about 56 ppm has been assigned to methoxyl groups in lignin. This signal may overlap a contribution from CH-N groups in proteins. Lignin also shows peaks between 60 and 100 ppm, with resonances at 60, 72, and 84 ppm from ether and secondary alcohol groups, as well as minor contributions from fully saturated carbons not linked to oxygen between 15 and 45 ppm. Peaks at about 154 and 148 ppm arise from C-3 and C-5 carbon atoms in syringl and guiaacyl units of lignin. The spectral region between 60 and 110 ppm is dominated by peaks that are characteristic of polysaccharides and, to a minor extent, proteins. In particular, the peak at 63-65 ppm has been ascribed to C6 and C5 in crystalline hexose and pentose, respectively. Signals around 72-75 ppm may be ascribed to C2, C3, and C5 from cellulose, as well as carbons from xylans. Peaks at 83 and 88 ppm may be due to C4 of non-crystalline and crystalline components of cellulose, and the peak at 105 ppm to dioxygenated anomeric C1. This last peak can overlap the peak of C1 carbons from xylans at 103 ppm.