###### Abstract  
Biochar is a carbon-rich organic material, obtained by the thermochemical conversion of biomass in an oxygen-limited environment, used as a soil amendment to stimulate soil fertility and improve soil quality. There is a clear need in developing countries for access to low cost, low technology options for biochar production, for example, top-lit updraft (TLUD) stoves, which are popular and spread worldwide. However, TLUD biochars are inevitably very variable in their properties for a variety of reasons. We present laboratory triplicate tests carried out on TLUD biochars obtained from waste pinewood and a _Guadua_ bamboo. Analyzed properties include specific surface area (A-BET), porosity, skeletal density, hydrophobicity, proximal and elemental composition, cation exchange capacity (CEC), relative liming capacity and pH. SEM images of the bamboo and wood biochars are compared. The biochars were mixed with composted human excreta at 5% and 10% biochar content, and available water content (AWC) was analyzed. Operating temperatures in the TLUD were recorded, showing different behaviors among the feedstocks during the process. Differences in operating temperatures during charring of the bamboo samples seem to have led to differences in A-BET, hydrophobicity and CEC, following unprecedented trends. For the mixtures of the biochars with compost, at 5% biochar no significant differences were observed for AWC. However, in the 10% biochar mixtures, bamboo biochar showed an unexpectedly high AWC. Overall, variations of chemical and physical properties between bamboo biochars were greater, while pinewood biochars showed similar properties, consistent with more homogeneous charring temperatures. top-lit updraft; TLUD; charring temperature; cation exchange capacity; available water content; biochar +
Footnote †: journal: Applied Soft  
for pollutants and nutrients [3]. These contributions of biochar vary with its quality. Biochar quality for soil amendment relies primarily on the volume and size of its pores and on the functional groups on the surface [4]. These aspects are driven by key factors; known as the feedstock type [5,6], the biochar particle size [7] and the pyrolysis conditions, for instance, the charring temperature, the heating rate and the residence or holding time [8]. [PERSON] et al. [9] considered biochar to be a key factor of the terra preta, which owes its name to the Amazonian dark earths, combined in the soil with massive amounts of nutrients and microbial processes. Biochar enhances soil quality by improving its holding capacity for nutrients and water, increasing their availability, augmenting populations of beneficial soil microorganisms and optimizing the water and air permeability of the soil [3]. A directly derived concept is terra preta sanitation (TPS) [10], which merges biochar with bio-waste management and sanitation. The bio-wastes comprehend human excreta collected in urine-diverting toilets [11] and animal excrements [9]. To ensure a thorough sanitization during composting, TPS relies on a lactic acid fermentation pathway for the wastes [12]. Probably, the major contributions that added biochar can make to the agronomic value of soils are the entailing improvements in the soil's cation exchange capacity (CEC), the liming effect [6], and its water retention capacity [13]. CEC is a measure of the negative charges in a material, which allows it to retain positive exchangeable cations such as K\({}^{+}\), Ca\({}^{2+}\) and NH\({}_{4}\)\({}^{+}\), and is therefore a desirable quality in a soil. Specific studies for application of biochars in soil remediation [14] have shown that pyrolysis conditions affect the chemical functional groups formed at the surface, determining their CEC. In fact, [PERSON] et al. [15] found evidence that a higher CEC provides better resistance against soil acidification. According to [PERSON] et al. [16], CEC can be mainly attributed to carboxyl groups (-COO-) formed by oxidation.