## 6 Conclusions and Perspectives  
\({}^{13}\)C SSNMR, widely used in the field of materials science, has revealed as a powerful tool for the investigation of organic natural matter of relevance in agriculture and environmental science. In fact, \({}^{13}\)C SSNMR gives access to information on the composition of complex organic materials that cannot be obtained otherwise, and which can be of great relevance for the optimization of waste recycling processes and the exploitation of waste materials that presently are directly disposed of in landfills. \({}^{13}\)C SSNMR has been applied to investigate the (vermi)composting or anaerobic digestion of materials of varied nature, from relatively homogeneous garden wastes to widely heterogenous municipal solid wastes. Most studies have shown that the compositional features of compost or digestate depend on the starting feedstock and this is important for the application of these materials, which are often intended to be used as fertilizers or amendments in agricultural soils. Nevertheless, \({}^{13}\)C SSNMR has highlighted the degradation of labile OM components (mainly carbohydrates) and the preservation of the recalcitrant ones (lipids, waxes, lignin). The relative amounts of the different components obtained from NMR data have been correlated with stability and maturity indices determined by analytical methods. In some cases, the results indicate that waste pretreatments may be required, as for example in the case of municipal solid waste where non-biodegradable plastic materials remain in the compost or digestate. Although not conclusive alone, \({}^{13}\)C SSNMR has contributed to the understanding of structure-bioactivity relationships of composts, highlighting the role of hydrophobic and polar chemical functionalities in determining the biostimulation properties of compost of interest in agriculture and the antibacterial, anti-inflammatory and antioxidant properties of extracts of relevance for their application in pharmacology. Most of the studies cited relied on spectral analysis based on the division of the \({}^{13}\)C MAS NMR spectra in regions related to specific functional groups. This is certainly use ful for understanding organic matter evolution during (vermi)composing or anaerobic digestion. However, the results of the different studies indicate that it is not always the standard decomposition pathway, in which carbohydrates are decomposed faster and recalcitrant lignin and lipids are concentrated, that is followed and no univocal indices of humification and maturity, as those typically used in soil science, can be defined. This is probably due to the variable and complex composition of waste materials, especially in the case of municipal solid waste. Nonetheless, comparison of \({}^{13}\)C MAS NMR spectra before and after the composting or digestion process yields detailed information on the transformations that have occurred, which is important for process optimization. To this end, molecular models can be of help, especially for the characterization and quality assessment of the final products and would certainly deserve more consideration and possibly further refinement. Moreover, given the complex nature of OM in waste transformation, a combined \({}^{13}\)C SSNMR investigation of the whole sample and different extracts is advisable for a better comprehension of the degradation processes and the chemical structure of the final products. As evidenced by the growing literature in the field and by the results therein reported, \({}^{13}\)C SSNMR is an important instrument for the investigation of organic waste transformation within a circular economy model.