# Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar
## 2 Materials and Methods
### Physicochemical Characterization of Biochars  
The sewage sludge biochar (SSB) specific surface areas and pore volumes were determined by N2 adsorption isotherms at -196.2 \({}^{\circ}\)C in a surface area analyzer, NOVA 2200. The sample was degassed under vacuum at 300 \({}^{\circ}\)C for three hours before the measurement. Total C, N, and H contents in SS and SSB were determined using a CHN Elemental analyzer (model PE 2400, series II CHNS/O, PerkinElmer, Norwalk, CT, USA). Organic carbon (OC) was determined following Mebius [29]. According to this method, the digestion mixture of potassium dichromate with sulfuric acid was submitted to external heating on an electric plate at 100 \({}^{\circ}\)C for 30 min under reflux. OC was also determined according to Walkley-Black procedure with no external heating. The pH was determined in a 0.01 M CaCl2 solution, using a 1:5 (_w/v_) biochar:solution ratio suspension. Electrical conductivity (EC) was measured in a 1:10  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Property** & **Value** \\ \hline pH (CaCl2) & 5.1 \(\pm\) 0.3 \\ OC \({}^{\circ}\) (g kg\({}^{-1}\)) & 19.4 \(\pm\) 1.0 \\ P (mg kg\({}^{-1}\)) & 0.7 \(\pm\) 0.1 \\ K\({}^{+}\) (mg kg\({}^{-1}\)) & 46.1 \(\pm\) 4.0 \\ Ca+2 (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 14 \(\pm\) 2 \\ Mg+2 (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 8 \(\pm\) 1 \\ SB \({}^{\circ}\) (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 34 \(\pm\) 5 \\ Al+3 (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 0 \(\pm\) 0 \\ H+Al (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 37 \(\pm\) 4 \\ CEC \({}^{\text{d}}\) (mmol\({}_{\text{c}}\) kg\({}^{-1}\)) & 71 \(\pm\) 9 \\ V \({}^{\circ}\) (\%) & 48.0 \(\pm\) 4.0 \\ Sand (g kg\({}^{-1}\)) & 140.1 \(\pm\) 5.0 \\ Silt (g kg\({}^{-1}\)) & 270.1 \(\pm\) 10.0 \\ Clay (g kg\({}^{-1}\)) & 580.8 \(\pm\) 25.0 \\ \hline \hline \end{tabular} \({}^{\text{a}}\) According to the methods described by [PERSON] et al. [27], mean values, \(n\) = 3;\({}^{\text{b}}\) organic carbon by the Walkley–Black procedure [28]; \({}^{\circ}\) sum of bases; \({}^{\text{d}}\) cation exchange capacity at pH 7; \({}^{\circ}\) base saturation.  
\end{table}
Table 1: Soil chemical and physical properties \({}^{\text{a}}\).  
(g mL\({}^{-1}\)) ratio using a conductivity meter. After drying, grounding, and sieving through a 0.50 mm mesh sieve, samples were subjected to acid digestion with concentrated HCl/HNO\({}_{3}\) according to USEPA 3050B [30], and the elements were quantified according to the following procedures: phosphorus was determined by the molybdenumadate phosphoric acid method, potassium by flame photometry. Calcium, magnesium, sulfur, and heavy metals were analyzed by ICP-OES (ICPE-9000, Shimadzu, Kyoto, Japan). Nitrate and ammonium were determined by the Kjeldhal method [31]. Moisture was determined by calculating the weight loss after heating samples at 105 \({}^{\circ}\)C in porcelain crucibles until constant weight. Ash was determined as the residual remaining after heating to 600 \({}^{\circ}\)C and held at this temperature until the sample weight stabilized. The yield was calculated by the ratio between the mass of biochar and the feedstock.