## 3 Results and Discussion
### Use of Biochars Mixed with Compost  
Total Nutrient Content of the TPS  
A requirement to use organic amendments in a sustainable way is to quantify the amount of nutrients for plants that could be taken up by a crop [59]. Table 6 illustrates some of the main characteristics of the human compost (TPS) used to prepare the soil amendments before the addition of the biochars. The nutrients correspond to the total content in the compost. These results show that the compost is almost neutral and slightly saline. Comparing the C:N of 31.5 reported by [PERSON] et al. [60] for municipal solid organic waste and fecal matter, the C:N ratio obtained is quite low. This compost made of dry composted feces and coffee skin was relatively high in nitrogen content and therefore, suitable as a terra preta-type soil amendment. Besides nitrogen, plants use five other nutrients in relatively large amounts: phosphorus, potassium, sulphur, calcium and magnesium [61]. In terms of phosphorus and potassium content, the results showed medium values ranging between cow manure and vermicompost, as used by [PERSON] and [PERSON] [62] in their investigation.  
Figure 4: Cation exchange capacity (CEC) and anion exchange capacity (AEC) of six biochar samples. Notice that BB and WB indicate bamboo and pinewood biochar, respectively. Different letters mean statistical significant differences using [PERSON]’s test (\(p<0.05\)).  
Figure 5 compares the experimental data of available water content (AWC), as the relative amount of water in grams retained per gram of soil amendment. The average AWC (\(n=4\)) of twelve TPS + biochar treatments (mixtures) and a TPS control treatment without biochar addition (i.e., TPS), were compared at a 95% of confidence. The control TPS showed a total lack of water retention capacity. The most remarkable result is that TPS + BB1--10% showed by far the highest AWC, equivalent to 1.35 g g\({}^{-1}\) (\(p<0.05\)). Interestingly, BB1 had as well the highest CEC, the lowest charring temperature and the lowest holding time above 200 \({}^{\circ}\)C, which was approximately 25 min. The BB1 biochar had as well the highest SSA value in its group. Notwithstanding, SSA values in the wood biochars were much higher than those of the bamboo biochars, but their AWC values were not significantly different from those of the two bamboo chars with low AWC values. A dependency of AWC on O:C ratio and hydrophobicity is hereby suggested, which requires direct testing. The results for the rest of the biochars used in the soil amendments are not very encouraging, since the differences between them and the TPS control were not significant. However, as can be seen from Figure 5, there was a trend suggesting that, overall, biochar raised AWC from 20% to 50%. This increment in AWC agreed with data from [PERSON] et al. (2018), who reported an increase of 45% in available soil water content compared to control soil using 44 ton ha\({}^{-1}\) of application rate. It is therefore likely that biochar can bring potential benefits in terms of AWC and CEC, whether applied directly in soils or included within a soil amendment. Our results suggest that by reducing the holding times and temperatures for bamboo when charring, biochars similar to BB1 could be obtained, which could significantly improve the CEC and AWC of a soil. However, establishing the relationship of AWC and CEC to charring conditions requires specifically designed experiments that could not be carried out in TLUD stoves, and was therefore not the purpose of this article.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Analysis** & **Results** \\ \hline Moisture \% & 67 \\ pH H\({}_{2}\)O (1:2) & 6.7 \\ EC (mS/cm) & 8.10 \\ C:N & 12.50 \\ C \% & 45.23 \\ N \% & 3.61 \\ P \% & 0.48 \\ K \% & 1.37 \\ S \% & 0.36 \\ Ca \% & 0.89 \\ Mg \% & 0.36 \\ Fe (mg/kg) & 1631 \\ Cu (mg/kg) & 34 \\ Zn (mg/kg) & 122 \\ Mn (mg/kg) & 159 \\ B (mg/kg) & 14 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Chemical analysis of the terra preta sanitation (TPS).