## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion
Compost of digestate solid fraction (DSF) is sufficiently stable for nuisance-free storage and handling and mature enough to be safely used in agriculture. However, the beneficial effects of digestate composing on OM stabilization are still under debate. To better understand this point, \({}^{13}\)C CP-MAS NMR spectroscopy was applied to \begin{table}
\begin{tabular}{c c c c c} \hline
**Ref. 1**  & **Feedstock 2**  & **Sample 3**  & **\(A\)/\(OA\)** & \(ARM\) \\ \hline \multirow{10}{*}{[132]} & Starch/creals/forage/com silage & FS & 0.1–0.3 & \\  & D & 0.2–0.3 & \\  & & Post-D & 0.3 & \\  & Cereals/sugar beets/crop/corn silage & FS & 0.1 & \\  & D & 0.3–0.5 & \\  & Post-D & 0.4–0.6 & \\  & Pig slurry/ChM/HM/deep litter/creals & FS & 0.3–0.6 & \\  & & D & 0.4 & \\  & MFW/SW/food industry W & FS & 0.4 & \\  & D & 1.1 & \\  & & Post-D & 1.1 & \\  & MFW/vegetables/food package/SW/food & FS & 0.9–1.1 & \\  & industry W/SM/GSS & D & 0.7–0.9 & \\  & & Post-D & 0.7–0.8 & \\  & Municipal food waste & FS & 0.4 & \\  & & D & 0.9–1.1 & \\  & Post-D & 1.1–1.2 & \\  & Municipal food waste & FS & 0.3 & \\  & & D & 0.5 & \\ \hline \multirow{10}{*}{[133]} & & FS & & 8.12 \\  & & C & & 14.07 \\  & & D & & 15.17 \\  & & FS & & 12.11 \\  & Green waste/food waste & C & & 12.46 \\  & & D & & 15.34 \\  & & FS & & 14.71 \\  & & C & & 15.54 \\  & & D & & 17.12 \\ \hline \multirow{10}{*}{[9]} & & C 4 d & & 15.8 \\  & & C 18 d & & 15.4 \\  & & C 31 d & & 14.5 \\  & & C 40 d & & 15.5 \\  & & C 57 d & & 16.9 \\  & & C 67 d & & 15.6 \\  & & C 84 d & & 17.6 \\  & & C 101 d & & 17.3 \\  & & C 114 d & & 18.1 \\  & & C 128 d & & 16.2 \\  & & C 146 d & & 17.6 \\ \hline \multirow{10}{*}{[124]} & & FS & & 6.2 \\  & CM (+straw) & D & & 9.5 \\  & & C & & 13.0 \\  & & Mixed & & 15.2 \\  & & FS & & 6.3 \\  & PM (+straw) & D & & 13.8 \\  & & Mixed & & 6.9 \\ \hline \multirow{10}{*}{[130]} & & FS & 1.45 & \\  & & D & 1.93 & \\  & & FS & 0.53 & \\  & & C & 1.08 & \\  & & C & 0.39 & \\  & & FS & 0.75 & \\  & & C & 1.05 & \\  & Municipal solid waste & FS & 0.37 & \\  & & C & 0.68 & \\  & & FS & 0.37 & \\  & & C & 0.85 & \\  & & AF & 1.53 & \\  & & FS & 0.37 & \\  & & Alt & 0.57 & \\  & & Fixed & 0.89 & \\ \hline \end{tabular}
\end{table} 
Table 7: \(A\)/\(OA\) and \(ARM\) indices determined from \({}^{13}\)C CP-MAS NMR spectra for digestate samples and corresponding feedstocks reported in the literature. investigate the composition of DSF separated from digestates by a screw press separator and their composts [135]. Digestates were obtained from mixtures of pig slurry, energy crops, and agro-industrial residues in different ratios, while the composting process was applied to mixtures of DSF and lignocellulosic bulking agent in different proportions. It was found that AD itself was able to transform OM to obtain digestate with high biological stability. As a consequence, composting led to only a slight modification of digestate DSF, except for ammonia content, which significantly decreased because of its volatilization. \({}^{13}\)C CP-MAS NMR spectra of composts were dominated by signals from carbohydrate molecules and also showed signals from lignin. These components had higher concentration than in non-composted digestates, indicating that lignocellulosic materials used as bulking agents were only partially degraded and strongly affected compost composition. In a study by [PERSON] et al. [9], the transformation of OM during composting (from 4 to 146 days) of digested municipal sewage sludge mixed with green waste and pine bark was investigated. A detailed analysis of the \({}^{13}\)C CP-MAS NMR spectra recorded on 4 replicates of samples collected at 11 different times, including a deconvolution procedure and a statistical analysis by PCA, revealed an increase in the relative content of aromatic structures with composting duration and a concomitant decrease in alkyl structures arising from lipids, proteins and hemicellulose, correlated with an increased humification of organic matter during composting.