###### Abstract  
The chemistry and nature of biochars are still far from being well understood. In the present work, solid-state 2D HETCOR 1H-13C NMR spectroscopy is introduced for an improved characterization of the aromatic network in biochars. To that end, a pyrochar obtained from the pyrolysis of cellulose at 350 \({}^{\circ}\)C for 1 h was used as an example. Variation of the contact time during cross polarization from 50 us, to 200 us and 1000 us gave information about the protonation degree of the different C groups and their interactions. We demonstrated that carbohydrates did not survive the used pyrolysis conditions. Therefore, O-alkyl C was assigned to ethers. Phenols were not identified to a higher extent suggesting that furan and benzofuran-type units determine the O-funnodality of the aromatic domains. The latter are directly connected to alkyl chains. Those features are expected to affect chemical but also physical properties of the biochar. Based on our results, we developed a new concept describing the nature of the aromatic network in the studied cellulose-based pyrochars. The latter contrasts common views about the chemical nature of biochar, possibly because pyrolysis temperatures > 350 \({}^{\circ}\)C are required for achieving advanced condensation of the aromatic domains. cellulose charring; role ofurans; benzofuran; pyrolysis; improved model for biochar structure; cross polarization technique +
Footnote †: journal: Applied Sciences  
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Footnote †: journal: Applied Sciences  
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Footnote 0: _Applied Sciences_  
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Footnote 0: _Applied Sciences_  
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_Applied Sciences_  
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With respect to composting, biochar addition is expected to improve the humification process, enhancing microbial diversity and activity as well as reducing the production of greenhouse gas emissions by their sequestration [8]. Further, it is thought to immobilize toxic metals [5] and organic pollutants [9] associated with the feedstock of the compost. Complementarily, biochar addition increases the water holding capacity (WHC) which is mainly explained by its high porosity. The increasing research related to biochar evidenced that depending on the chemistry of the feedstock and the production condition, the chemical composition and the physical properties of biochars vary strongly [10]. Thus, biochar application is not a \"one-size fit-all paradigm\" [11], but requires careful consideration of the product properties and how those contribute to the fulfillment of the application purpose. In spite of this, most reports about biochar characterization are still limited to basic descriptive data such as the content of pollutants, pH, WHC, nutrient content, and elemental composition. In particular, the latter is used to elucidate the carbonization degree by its atomic H/C and O/C ratios. The lower these ratios, the higher is the aromaticity which is commonly related to increasing porosity. This is paid with a lowering of the chemical reactivity and the decrease of acid functional groups such as carboxylic acids [12; 13], and as a possible consequence a reduction of the adsorption capacity in particular for metals or nutrients. However, often the sole description of basic parameters fails to explain why different feedstocks or pyrolysis conditions result in products with a large variation in porosity, adsorption, or biochemical recalcitrance. In some cases, they show unexpected behaviors during their application. The latter can be avoided if the scarce understanding of the chemical changes of biomass residues during pyrolysis and how these changes affect the chemical and physical properties of the final product is improved. Indeed, considerable literature reports on the analysis of volatile or soluble thermal degradation products of cellulose or lignin [14; 15], being the two major biopolymers of woody biochar precursors. However, a more detailed description of the charred solid residues which goes beyond the identification of condensed aromatic structures is still rare [16].