## 4 Implications of the Findings for a Better Understanding of the Nature of Biochar  
The present analysis of the cellulose char produced at a 350 \({}^{\circ}\)C by 2D NMR spectroscopic techniques taught us that the common assignment of the aromatic C signal in the 1D \({}^{13}\)C NMR spectrum to PAH structures of which some of outer rim carbons are substituted by hydroxyl groups represents an oversimplified interpretation. The application of this technique unveiled new information about the nature of the aromatic network in pyrochar that is rarely accessible by other techniques but highly valuable for a better predication of the efficiency of biochar application to compost and soils. One major observation is that almost all-if not all-O-aryl Cs occur in furan and benzofuran-type structures contributing to a network with few domains of high molecular weight PAHs. Figure 6 shows a model of the cellulose derived char network including the main constituents that were identified in the present work. Indeed, furan and benzofuran-type structures seem to play a more important role for defining the overall structure and properties of the residual char than commonly assumed. In the present sample, their C\(\alpha\) comprises 30% of the total aromatic C, since phenols were identified as negligible entity. Rather than carboxyl groups, they explain the oxygen content of the sample. This finding will certainly have some practical implications and may improve our understanding of the behavior of low-temperature biochar in soils or as filtering system, since the nature of the oxygen functions within the aromatic network affects not only sorption of cations or organic molecules, wettability of the sample, but also the accessibility of aromatic domains for microbial degradation. Bearing in mind the high cellulose content of woody biomass, it can be concluded that these furan/benzofuran-type domains are indeed more relevant in defining the physical and chemical properties of a wood-derived biochar than generally considered. Further important information obtained from the present analysis is related to the nature of the alkyl residues. This fraction has been interpreted as extractable not yet condensed low molecular weight degradation products or residual carbohydrates [39]. They are supposed to be highly accessible to further thermal degradation and after the application of the biochar to soils and compost to microbial decomposition [40]. The results obtained in the present study show a different picture. There is strong evidence that rather than being separate entities, a considerable part of the alkyl and _O_-alkyl groups are directly connected to the aromatic domains. This is in line with observations by [52] suggesting that above 300 \({}^{\circ}\)C, a three-dimensional network was formed from oligosaccharides, aliphatic hydrocarbons and aromatics which are connected via ether linkages, although remaining saccharides must have been already transformed at the temperature used in our study. Carboxyl and carbonyl functional groups seem to be mainly incorporated into these alkyl chains. However, the question if the alkyl C and carbonyl C entities act preferentially as bridging units between the different aromatic domains or occur as side chains cannot be unbiasedly answered with the data obtained here. On the other hand, considering that those units contribute with 52% to the total organic C of the pyrochar, it is expected that they are not only determining the molecular structure and rigidity of its aromatic network but are also having an impact on its physical and chemical properties. Depending whether they occur as side chains or bridging unit, their influence on porosity, thus gas diffusion, accessibility of nutrients, water, or microorganisms will be different. Comparably, their contribution to interactions with the compost or soil components will vary.