## 3 \({}^{13}\)C Solid-State NMR Applications to Composing
The authors suggested that the reduction in bioavailability of toxic metals, such as Cu and Zn, abundant in pig manure, in sepiolite-amended compost might be related to the higher amount of phenolic and oxygen-containing functional groups of the resulting HAs. The effect of biochar on compost HAs was also investigated. Biochar has been demonstrated to have positive effects on compost, for example improving compost porosity [89] and favoring carbon and nitrogen retention [90; 91]. [PERSON] et al. [62] studied the effect of adding biochar to swine manure and rice straw on the composition of HAs, as well as on compost, as previously mentioned. A comparison of \({}^{13}\)C CP-MAS NMR spectra and carbon species distribution of compost and HA samples during composting is shown in Figure 3. Three mixtures of manure, straw as bulking agent, and biochar in different proportions, i.e., 4:1:0, 8:1:0 and 8:1:1 by wet weight, were investigated. In Table 3 the data relative to the different NMR spectral regions for HAs obtained for the three different samples before and after composting are reported. The authors observed a positive effect of biochar on HA formation on the basis of the higher overall spectral intensity. In all cases, the alkyl C contribution sensibly decreased after 40 days of composting, while the aromatic and O-alkyl C contents increased. This was taken as an indication of the decomposition of macromolecules, such as lignin, hemicellulose and cellulose, contributing to the formation of HAs [92]. The amount of carboxyl C in the HA sample with the biochar added was higher with respect to the other samples before composting and also remained high after composting, with a value similar to that of the sample with a higher amount of bulking agent. A high proportion of both aromatic and carboxyl groups in compost HAs, as observed in the presence of biochar and with a higher amount of bulking agent, has been suggested to have positive agronomic effects [93]. \({}^{13}\)C SSNMR was more extensively applied to monitor the changes occurring in HAs during composting, although, again, the changes observed were strongly dependent on the feedstock. In fact, although it is generally stated that, during composting, carbohydrates  
Figure 3: \({}^{13}\)C CP-MAS NMR spectra of composts (**a**-**c**) and HA samples (**d**–**f**) and relative contributions of the different spectral regions. Feedstocks were constituted of mixtures of swine manure/rice straw/biochar in the ratios: 4:1:0 (M4S1), 8:1:0 (M8S1), and 8:1:1 (M8S1B1). Reproduced with permission from Ref. [62]. are decomposed and new aromatic groups are formed, no fixed trends were reported in the case of compost HAs. Data in agreement with the expected trend, were reported by [PERSON] and [PERSON] [94] on a mixture of domestic waste, plant trimmings and tobacco residues. After 2 months of composoting, the HA spectrum was dominated by alkyl and O-alkyl signals, the former probably deriving from lipids, waxes, plant polyesters, and peptidic compounds, the latter from sugars, but also from ether and epoxy groups of plant polyesters. The aromatic signals were mainly associated with H- or C-substituted carbons, with a low content of O-aryl carbons. Dipolar dephasing NMR experiments showed a low contribution from lignin. On the other hand, there was a large contribution from carboxyl groups in aliphatic acids and/or amino acid amide groups. After 5 months of composoting, the extracted HAs showed a significantly lower carbohydrate content and a higher proportion of alkyl C due to the incorporation of stable lipids. Observable, but minor increases were found for the aromatic and carboxyl C signal intensities. The results suggested a preferential preservation in the HA fraction of hydrophobic alkyl components, as well as the inclusion of labile peptidic compounds. [PERSON] et al. [95] studied the transformation of HAs during the composoting of MSW with the addition of agricultural wastes.