## 2 Materials and Methods
### Analysis of Odorous Compounds  
The odor mitigation effect of biochar was evaluated by measuring various odorous compounds. These include ammonia (NH\({}_{3}\)), sulfur compounds like hydrogen sulfide (H\({}_{2}\)S), methyl mercaptan (MM), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and volatile fatty acids (VFAs) such as acetic acid, propionic acid, isobutyric acid, butyric acid, isovaleric acid, and valeric acid. Additionally, phenol, \(p\)-cresol, and indole were measured. NH\({}_{3}\) was measured using a tunable diode laser spectrometry-based laser gas detector (LGD Compact-A NH\({}_{3}\)/H\({}_{2}\)O, Axteris AG, 6056 Kaegiswil, Switzerland). The sulfur compounds were analyzed using a gas chromatograph (CP-3800, Varian Inc., Palo Alto, CA, USA) equipped with a pulsed flame photometric detector, a capillary column (CP-Sil 5 CB, length: 60 m, inner diameter: 0.32 mm, film thickness: 5 um), and a thermal desorption instrument (UNITY series 1, Markes International Ltd., Bridgend, UK). Under these analytical conditions, with a sample preconcentration volume of 200 mL, the limit of detection (LOD) for sulfur compounds was approximately 0.625 \(\upmu\)L m\({}^{-3}\). VFAs and other organic compounds (phenol, \(p\)-cresol, and indole) were absorbed into Tenax TA-stainless steel tubes using an integrating pump (MP-\(\sum\)30 KNII, Sibata Scientific Technology Ltd., Saitama, Japan) and then analyzed using a thermal desorption instrument (TD100-xr, Markes International Ltd., Bridgend, UK) and a gas chromatograph (7890A, Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a flame ionization detector and a capillary column (DB-Wax Ultra Inert, length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 \(\upmu\)m). The column was heated from 40 \({}^{\circ}\)C to 150 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C min\({}^{-1}\) and then heated at a rate of 18 \({}^{\circ}\)C min\({}^{-1}\) up to 240 \({}^{\circ}\)C. For VFAs and other target organic compounds, when adsorbing 500 mL onto the Tenax TA tubes and analyzing with split ratios of 20:1, the estimated LOD was 2.5 ng. Samples for NH\({}_{3}\), sulfur compounds, VFAs, phenol, \(p\)-cresol, and indole were taken every one to five days. Odor unit samples were collected weekly. Every seven days, additional manure was mixed into the bedded pack, and all target gas samples for analysis were taken before and three hours after mixing to assess its impact on gas emissions. For each measurement, 8 L of gas was sampled from each reactor using a sampling pump (PCXR8, SKC Inc., Eighty Four, PA, USA). The 8 L of gas was collected in polyvinyl fluoride gas sampling bags (Tedlar bag, CEL Scientific Corp., Cerritos, CA, USA). The emissions of odorous gases were calculated based on the gas concentration, ventilation rate, and gas temperature during each measurement using the following Equation (1):  
\[ \begin{split}\text{Odorous gas emission}\Big{(}\text{gm}^{-2}\;\text{day}^{-1}\Big{)}\\ =C_{gas}\times V\times\frac{273.15}{273.15+T_{gas}}\times\frac{ \text{MW}}{224.14}\div BSA\end{split}  \tag{1}\] \begin{table}
\begin{tabular}{l c} \hline
**Property** & **Rice Husk Biochar** \\ \hline Moisture content (\% w.b.) \({}^{\ddagger}\) & 2.0 \(\pm\) 0.1 \\ Volatile solid (\% d.b.) \({}^{\ddagger}\) & 6.3 \(\pm\) 0.0 \\ Total nitrogen (\%, w.b.) & 0.8 \(\pm\) 0.2 \\ Total carbon (\%, w.b.) & 55.6 \(\pm\) 3.2 \\ pH & 10.4 \\ Specific surface area (m\({}^{2}\) g\({}^{-1}\)) & 31.9 \\ Micropore surface area (m\({}^{2}\) g\({}^{-1}\)) & 26.0 \\ Total pore volume (cm\({}^{3}\) g\({}^{-1}\)) & 0.02 \\ Micropore volume (cm\({}^{3}\) g\({}^{-1}\)) & 0.01 \\ Adsorption average pore diameter (nm) & 15.5 \\ \hline \end{tabular} \({}^{\ddagger}\) w.b. = Wet basis; \({}^{\ddagger}\) d.b. = Dry basis.  
\end{table} 
Table 2: Characteristics of rice husk biochar. (Mean \(\pm\) S.D., \(n\) = 3).