## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion  
\({}^{13}\)C CP-MAS NMR experiments have been used for monitoring the transformation of OM in anaerobic digestion (AD) of different feedstocks with the aim of evaluating digestate stability in view of its possible applications as soil amendment or fertilizer. Amendment properties are related to the presence in digestate of organic fractions that can contribute to soil organic matter turnover, as this process influences biological, chemical, and physical soil characteristics. On the other hand, fertilizer properties come from the content of nutrient elements in the digestate organic matter and their chemical forms. Figure 6: Relative areas of the \({}^{13}\)C NMR spectral regions recorded for HAs extracted from feedstocks (CM, CM+sugar cane bagasse (SB), CM+sunflower cake (SFC), and CM+SB+SFC) and vermicomposts matured for 120 days (Data taken from Ref. [115]). During AD, starting organic matrices are transformed, with a high degradation of polysaccharide components and preservation of recalcitrant molecules (lignin-like molecules and complex lipids and steroids), which are concentrated. These molecules were found to be precursors of humus in the soil [123]; therefore, macromolecular composition of OM in digestate determined by \({}^{13}\)C CP-MAS NMR spectra is particularly useful for evaluating amendment properties. The relative integral intensities over the spectral regions defined in Table 1, reported in the reviewed studies, are listed in Table 6, while values of the _A/OA_ and _ARM_ indices are given in Table 7. These data are used in the following to describe the conversion of OM during AD and OM composition in digestate in relation to application properties. [PERSON] et al. [124] studied the AD of CM and poultry manure (PM), also containing straw as residual bedding material, to obtain digestate to be used as a soil amendment. AD was performed as a solid-state process in a pilot plant (3 L) under mesophilic conditions (34 \({}^{\circ}\)C) in batch operation mode. The digestion process was concluded when the daily gas production was negligible (45 days for CM and 35 days for PM). For both feedstocks, digestion removed a considerable part of the polysaccharides, and an increase in the relative content of aromatic and carboxylic, amidic and ester structures was observed (Figure 7), the aromatic structures mostly arising from lignin. Stability was also evaluated by investigating the thermal behavior of feedstocks and digestates under inert atmosphere. A correlation could be drawn between the higher aromaticity degree (see _ARM_ values in Table 7) and the lower degree of volatilization in pyrolysis, indicating a higher stability of digestate. [PERSON] et al. [125; 126; 127] studied the OM composition of different biomasses (mixtures of energetic crops, cow slurry, agro-industrial waste and organic fraction of municipal solid waste; mixtures of pig slurry, energy crops and agro-industrial residues; mixtures of sorghum silage, beef cattle slurry and agro-industrial residues; mixtures of sorghum silage, olive mill wastewater, olive residues and agro-industrial residues; pig slurry) and of the corresponding digestates obtained by AD under mesophilic conditions in full-scale biogas production plants located in Italy (Lombardia [125; 126], Emilia Romagna [127], and Umbria [127] regions). The aim of their work was to investigate the quality and fertilizer  
Figure 7: \({}^{13}\)C CP-MAS NMR spectra of CM (**left**) and PM (**right**) before and after anaerobic digestion, compositing, and a mixed aerobic-anaerobic process. Adapted from Ref. [124]. value of digestates in view of their use in agriculture. The analysis of \({}^{13}\)C CP-MAS NMR spectra indicated that anaerobic digestion proceeds through preferential degradation of carbohydrates, such as cellulose and hemicellulose, and, as a consequence, concentration of more recalcitrant aliphatic molecules (non-hydrolysable lipids and steroid-like molecules) occurs.