## 3 Results and Discussion
### Short-Term Effects of Biochar and Biofertilizer on Soil Characteristics
However, only with BBfT was the increment significant, between 24 and 41 times higher than the rest of the treatments. The _Opuntia_-based biofertilizer used is an important K source. In the case of Ca, it showed significant variations (\(p<0.05\)) between treatments and between cycles; however, the final concentrations were significantly higher than the initial ones (Table 3). On the contrary, Mg tended to decrease significantly (\(p<0.05\)) with all treatments in both cycles, except for BT50 (Table 3). Even with the decrease, the final Mg content is high, although it is necessary to add Mg to reestablish the ratios between Ca, Mg, and K in soil (Ca/Mg: 2-5, Ca/K: 5-25, Mg/K: 2.5-15, (Ca+Mg)/K: 10-40) and thus to avoid possible deficiencies in plants due to the antagonism between them. In this study, the increase in Ca and K caused a deficiency of Mg, which was accentuated with each crop cycle, and to a greater extent with BBfT. Sawdust and waste wood are two biochar feedstocks rich in Mg and locally available to supply this element. Micronutrients Cu, Fe, and Zn increased significantly (\(p<0.05\)) with all treatments with respect to the initial content and soil + biochar (Table 3). Cu and Zn augmented with each crop cycle, whereas Fe decreased between 36-51% in the second cycle with respect to the first one, except BT50, which increased by 26%. These nutrients were provided mainly by biofertilizer (in BBfT), biochar, and in small concentrations by chemical fertilizers, where they are found in trace concentrations. On the contrary, Mn diminished 10-20% with all treatments in the first crop cycle and increased in the second one; however, Mn concentrations for CT, BT20, and BT50 were 5% lower than the initial one. Pb, Cd, and Ni concentrations remained lower than the detection limit in all treatments (Table 3), which was beneficial in the case of Pb and Cd because of their toxicity for living organisms. The case of Ni is a bit more complex; on the one hand, although it is an essential element for higher plants [69], it is considered a toxic one because its functional role in humans has not been demonstrated, while in animals, it is not clear. On the other hand, the Ni toxic effects on human health are well documented [70]. The Ni content in the soil used in this study was too low (<0.0042 mg\(\cdot\)kg\({}^{-1}\)), but it was not added because it is a common trace element in fertilizers and biofertilizers. Practically any biomass is useful to produce biochar whenever it does not contain toxic elements. In this case, residual bamboo biomass used complies with this requirement. Biochar had a positive effect on soil CEC; initially it changed from 16 to 33 cmol+kg\({}^{-1}\), and although later this parameter diminished significantly with all treatments with respect to the soil + biochar, after the second cycle, it remained 33, 56, 31, and 12.5% higher for CT, BT20, BT50, and BBFT, respectively, relative to the initial value (Figure 2b). According to [PERSON] [6], when biochar had higher exchangeable Ca\({}^{2+}\) than the soil, CEC increases, as was the case in this study, where biochar increased exchangeable Ca\({}^{2+}\) and CEC in soil (Table 3, Figure 2b). [PERSON] et al. [36] confirmed the aforementioned; they found minimal increases in CEC adding 8 Mg\(\cdot\)ha\({}^{-1}\) of biochars produced at different temperatures to a soil rich in CaCO\({}_{3}\). However, contrary to expectations, BBft, the treatment with the highest pH and OM values showed the lowest increase in CEC. Despite its recalcitrant nature, biochar contributes organic C, P, and N to soil [7, 22]. Figure 3a shows that biochar treatments significantly increased (_p_ < 0.05) soil OM from the first crop cycle compared to CT. After the second cycle, OM was significantly higher in BBft than in the rest of the treatments due to the OM addition from the biofertilizer. Application of biochar and biofertilizer to the soil raised the OM content to a medium level (4.1-6.0%), while in CT, its OM content continued to be low (>4).