## 3 Results and Discussion
### Initial Direct Effects of Biochar on Soil Characteristics
Different letters and * indicate significant difference (\(p<0.05\), Duncan test and one-way ANOVA, respectively). Figure 2: (**a**) Water-holding capacity (WHC) and (**b**) cation exchange capacity (CEC) of studied soil (S), with biochar (S + B), and with different treatments, CT: soil + 100% chemical fertilization, BT20 and BT50: soil + biochar + 20, or 50% of chemical fertilization, respectively, and BBfT: soil + biochar + bioferilization. Different letters and * indicate significant difference (\(p<0.05\), Duncan test and one-way ANOVA, respectively). Other positive changes in the soil fertility were significant increases (\(p<0.05\)) in the content of OM from very low (2%) to low content (3%); CEC improved from medium (16 cmol+k\(\cdot\)g\({}^{-1}\)) to high content (33 cmol+k\(\cdot\)g\({}^{-1}\)); N\({}_{\rm{inorg}}\) showed the greatest increase (from 0.5 to 7 mg\(\cdot\)kg\({}^{-1}\)), although the content is still very low. Likewise, P\({}_{\rm{av}}\) raised from 24 to 35 mg\(\cdot\)kg\({}^{-1}\), although the difference was not significant (\(p<0.05\)). Regarding nutrients, with the biochar addition, K and Fe contents increased significantly (\(p<0.05\)) while Ca, Cu, and Zn showed a small increment, and Mn decreased slightly. High CEC and functional groups of biochar explain other changes detected in the soil. For example, diminution in pH could be due to the oxidation of biochar surface, the process responsible for the formation of acidic organic functional groups, such as carboxylic and phenolic groups [67, 68]. The mineral ash content in biochar depends on feedstock compositions as well as the pyrolysis temperature. While Ca, Fe, Mg, Mn, and Si are usually retained in biochar because their release requires high temperatures (>1000 \({}^{\circ}\)C), P and K start to vaporize at 700-800 \({}^{\circ}\)C and N at 200 \({}^{\circ}\)C [69]. Our results indicate that bamboo biochar provides mainly N, P, K, Fe, and modest quantities of Ca, Cu, and Zn, but not Mg or Mn. It is important to remark that the aforementioned changes in soil quality were obtained by adding 8 Mg\(\cdot\)ha\({}^{-1}\) (0.2%) of biochar, a lower rate than ones commonly applied (10-100 Mg\(\cdot\)ha\({}^{-1}\) or \(\geq\)1%) [22, 28, 31, 32]. We consider that the biochar used and the application rate applied were correct because the study soil did not require pH correction since its initial value was slightly alkaline. A higher biochar dose could have drastically altered pH because the study soil had low contents of components responsible for buffer capacity (OM and clay). To say that coarse-textured soils are more responsive to biochar addition is a general assertion that is not entirely true. Although tropical soils and arid and semi-arid soils have a coarse texture in common, they respond very differently to the addition of biochar because they have characteristics that clearly differentiate them, such as pH. It is also true that there may be particular cases within the soil groups, such as the study soil, which, despite being a semi-arid soil, does not present the buffering capacity that confers a high concentration of carbonates to this type of soil, so it responded to a small application rate of biochar.