# Agricultural Waste-Based Biochar for Agronomic Applications
## 2 Physiological Characteristics of Biochar  
Physico-chemical characteristics of biochar are controlled by the type of feedstock and pyrolysis conditions [38; 39]. Surface area, porosity, density, water retention capacity, and hydrophobicity are the main physical properties of biochar [40]. Biochar is porous in nature and generally characterized by its low bulk density (less than 0.6 g/cm\({}^{3}\)) [26]. The pyrolysis temperature has a strong impact on biochar porosity; volatile substances are released at high temperatures, which supports the formation of pores [41]. [PERSON] et al. claimed that a higher porosity of wood-derived biochar is obtained, from 50 to 70%, when pyrolysis is performed at temperatures from 300 to 850 \({}^{\circ}\)C [42]. Biochar is mainly microporous, with more than 80% of its total pore volume being micropores [40]. The biochar surface area increases similarly to porosity with rising pyrolysis temperature. For most biomasses and under suitable pyrolysis conditions, the surface area can reach values between 100 and 800 m\({}^{2}\)/g, while for biochars derived from sewage sludge, the surface area shows values below 100 m\({}^{2}\)/g [39]. The biochar's water-holding capacity is also enhanced at higher pyrolysis temperatures. However, in some works it was observed that on the biochar surface the functional groups decrease at higher temperatures, which alters the biochar's affinity to water [40]. Surface functional groups are responsible for biochar reactivity and form the interface where different biological and chemical reactions occur. Among many functional groups found on biochar surface, such as aliphatic, aromatic, hydroxyl, epoxy, acyl, ether, amido, sulfonic, and azyl groups, phenolic-OH, carboxyl, carbonyl, and ester groups are the most abundant. The association/dissociation of these groups with protons results in biochar alkalis, hydrogen bond-induced sorption, hydrophilicity/hydrobicity, surface charge alterations and CEC [43]. The dissociation/associations of protons provided by biochar functional groups as a function of pH allow biochar-incorporated soils to resist the variation in pH and to increase their buffering capacity for longer periods [44]. The pH value of biochar is generally varying from 5.9 to 12.3, with an average of 8.9 [29]. The biochar pH value depends strongly on the pyrolysis temperature. With the increase in temperature (>350 \({}^{\circ}\)C), alkali salts start to separate from organic materials and basic cations increase in biochars, which increases the biochar pH [45]. In contrast, at low pyrolysis temperatures (200-300 \({}^{\circ}\)C), acidic biochar is produced due to the decomposition of cellulose and hemicelluloses, which yields organic acids [46]. In addition to pH and CEC, the elemental composition and the atomic ratios of H/C and O/C are the main chemical properties of biochar. Biochar is determined by its stable C existing in the inorganic phase in the form of carbonate and bicarbonate, and in the organic phase as aliphatic carbon, aromatic carbon, and functionalized carbon [32; 43]. The elemental composition of biochar is constituted by major elements (C, H, O, and N) and other elements (P, S, Si, Cu, Fe, Zn, and Mn) present in variable percentages depending on the nature of feedstock and the pyrolysis conditions [47; 48]. These elements make the surface of the biochar heterogeneous and reactive [49; 50]. Higher pyrolysis temperatures intensified polymerization reactions and reduced the atomic ratios of H/C and O/C [38; 48]. Biochars typically have a lower molar H/C ratio than that of feedstocks [51; 52], indicating a high content of aromatic structures and a potential recalcitrance of the biochar. The N in biomass is generally present in the form of proteins, amino acid, pyrrol, and pyridine. Peptide-N bonds transform during pyrolysis into N-heteroaromatic carbon compounds, and some amide-N reduces at elevated pyrolysis temperature.