## 3 \({}^{13}\)C Solid-State NMR Applications to Composing
By recording variable contact time \({}^{13}\)C CP-MAS NMR spectra of the film before and after composting, the authors detected changes in the \({}^{1}\)H \(T_{1\varrho}\) values that were ascribed to an alteration of the inter-molecular linkages among the organic components of the plastic, due to the progressive decomposition of its constituents. The effect of specific additives on different composts was also investigated. [PERSON] et al. [84] studied the decomposition of straw in the presence and absence of inorganic matrices, such as sand, kaolin, and loamy sand, and found that straw incubated in inorganic matrices decomposes faster than straw alone and that sand seems to preserve alkyl groups. Considering that biochar in ruminant diets is being assessed as a method for simultaneously improving animal production and reducing enteric CH\({}_{4}\) emissions, [PERSON] et al. [59]examined the effects of feeding a pine-based biochar to beef cattle on manure composting. \({}^{13}\)C CP-MAS NMR showed that biochar increases compost aromaticity, without altering the bulk C speciation of manure. [PERSON] et al. [62] investigated the effect of adding biochar to a swine manure compost, and found a promoting influence of biochar on the aromatization process. \({}^{13}\)C SSNMR has also been applied to freeze-dried extracts of compost to aid the understanding of the chemical structure of this complex material, as well as the changes occurring in OM during composting. Results on the contribution of the different functional groups to the \({}^{13}\)C CP-MAS NMR spectra of different types of extracts obtained from various composts and feedstocks are reported in Table 3.  
As evidenced by the data reported in the table, the chemical composition of compost extracts is strongly dependent on the feedstock. [PERSON] [85] applied \({}^{13}\)C CP-MAS NMR spectroscopy to characterize HAs extracted from three types of composted feedstocks: agricultural crop plants, date palm fronds, and animal waste. For all samples, the most intense spectral region was the alkyl C one, with HAs of compost from agricultural crop plants showing the highest contribution, counterbalanced by a lower amount of carbohydrate and aromatic carbons. On the other hand, HAs from animal waste compost had the lowest alkyl C content but the highest aromatic C content, while HAs of compost from date palms had the lowest amount of carboxyl carbons. None of the samples showed intensity in the carbonyl region. Similarly, HSs extracted from composts obtained from artichoke, pepper or coffee wastes showed significantly different \({}^{13}\)C CP-MAS NMR spectra, particularly regarding the alkyl C and O-alkyl C spectral regions [86]. Additionally, the data by [PERSON] et al. [87] on HSs extracted from composts of different origin (animal manure, olive mill and winery wastes, domestic wastes) showed significant differences among the spectral intensities of the different regions. Bulking agents were also found to affect the composition of compost extracts. The \({}^{13}\)C CP-MAS NMR data recorded on HSs extracted from compost obtained from artichoke residues mixed with corn straw and wood chips [86] and from the same residues mixed with maize straw [37] showed major differences, particularly in the aromatic spectral region. Even the use of additives was found to influence HA composition after composting. [PERSON] et al. [88] studied compost obtained from pig manure and sawdust with the addition of different amounts of sepiolite, a clay mineral, which is indeed expected to affect the composting process due to its high absorption capacity and cation exchange ability, possibly favoring the humification process. The \({}^{13}\)C CP-MAS NMR spectra of HAs extracted from compost obtained from samples with increasing amounts of sepiolite confirmed the increasing extent of humification, as highlighted by the higher relative amount of aromatic C and concomitant reduction in the contribution of alkyl and O-alkyl C. Sepiolite also induced the formation of carbonyl and carboxyl groups.