## 3 Results and Discussion  
### Elemental Analysis  
Table 2 shows the elemental analysis of the feedstock (broom and gorse) and the corresponding biochars. Among the biochars derived from broom, the N content did not statistically differ from that of the feedstock. However, for gorse biochars, the N content increased with temperature and residence time, except for BG600-3H. The nitrogen content of the broom-derived biochars was between 2.62 \(\pm\) 0.2% and 3.12 \(\pm\) 0.2% and was higher than that reported in previous studies [69; 70]. In all cases, the C content increased with pyrolysis temperature, with BG600-3H being the sample with the highest value (72.32%). The C content of the broom-derived biochars was lower to that obtained in previous studies [69; 70] and was due to the low heating rates used in our treatments. However, these biochars could be suitable for soil amelioration and C fixation [71; 72]. In general, the H content of the biochars decreased with pyrolysis temperature and residence time as a result of the polymerization and condensation reactions leading to more aromatic structures and a decreased content of aliphatic groups. Related to these transformations, the O content of the biochars decreased with pyrolysis temperature and residence time for the biochars at 300 \({}^{\circ}\)C. No statistical differences were found in the O content of the biochars prepared at 600 \({}^{\circ}\)C between the two residence times. Similar results were previously obtained by different authors [50; 54]. The S contents of the biochars were below 0.05% for broom and gorse materials. The S contents of the broom-derived biochars were lower in biochars than in the broom feedstock, but, in the case of gorse-derived biochars, the S content did not present statistical differences between the biochars and the feedstock. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **\%N** & **\%C** & **\%H** & **\%S** & **\%O** & **(O + N)/C** \\ \hline BROOM & 2.93 \(\pm\) 0.07 a,b & 42.37 \(\pm\) 0.31 a & 5.76 \(\pm\) 0.17 e & 0.11 \(\pm\) 0.01 b & 45.67 \(\pm\) 0.92 d & 0.87 \(\pm\) 0.01 d \\ BB300-1H & 2.93 \(\pm\) 0.26 a,b & 59.4 \(\pm\) 2.24 b & 5.16 \(\pm\) 0.06 d & 0.03 \(\pm\) 0 a & 28.57 \(\pm\) 1.79 c & 0.40 \(\pm\) 0.03 c \\ BB300-3H & 3.12 \(\pm\) 0.2 b & 62.68 \(\pm\) 1.91 b & 4.81 \(\pm\) 0.14 c & 0.02 \(\pm\) 0 a & 25.04 \(\pm\) 1.39 b & 0.34 \(\pm\) 0.02 b \\ BB600-1H & 2.62 \(\pm\) 0.2 a & 68.66 \(\pm\) 1 c & 1.92 \(\pm\) 0.14 b & 0.03 \(\pm\) 0 a & 20.88 \(\pm\) 0.37 a & 0.26 \(\pm\) 0 a \\ BB600-3H & 2.63 \(\pm\) 0.16 a & 67.67 \(\pm\) 2.78 c & 1.54 \(\pm\) 0.15 a & 0.03 \(\pm\) 0 a & 20.54 \(\pm\) 1 a & 0.26 \(\pm\) 0.02 a \\ \hline \end{tabular}
\end{table} 
Table 2: Elemental analyses and (O + N)/C atomic ratio (dry basis) for broom, gorse, and the biochars. Means with the same letter are not significantly different (\(p>0.05\)) using [PERSON]’s test for broom (lower case letters) and gorse (upper case letters) independently. Finally, the (O + N)/C ratio decreased with pyrolysis temperature and, at 300 \({}^{\circ}\)C, with residence time, indicating the loss of nitrogenated and oxygenated functional groups during pyrolysis and the polycondensation reactions that take place during pyrolysis. Regarding the atomic H/C and O/C ratios, Figure 1 shows the Van Krevelen diagram of the prepared biochars compared to those of different coals. It can be noted that the H/C and O/C ratios of the biochars decreased with pyrolysis temperature. The H/C atomic ratio presented similar values to those obtained by [PERSON] et al. [53] for gorse-derived biochars. The Van Krevelen diagram shows some differences between the feedstock (black circle), biochars at 300 \({}^{\circ}\)C (discontinuous black circle), and biochars prepared at 600 \({}^{\circ}\)C (double-lined black circle). The biochars obtained by pyrolysis at 300 \({}^{\circ}\)C presented H/C and O/C ratios close to those of low grade sub-bituminous coals [61] and lignite coal [73].