## 2 Materials and Methods  
### Experiment Design  
The lactating Holstein cow manure was collected from a bedded pack dairy barn four days before the start of the experiment. The dairy manure was stored at 4 \({}^{\circ}\)C and then incubated at 28 \({}^{\circ}\)C in a constant-temperature chamber for 48 h prior to the experiment. Each experimental unit consisted of 2.5 kg of dairy manure and 0.225 kg of sawdust. According to the housing standards set by the South Korean government, dairy cows housed in a bedded pack barn require a minimum of 16.5 m\({}^{2}\) of space per head [13]. Additionally, the government has set a standard daily manure production rate of 30.1 kg for a 450 kg dairy cow [14]. Given the simulated bedded pack area of 0.031 m\({}^{2}\), the average daily production of dairy cattle manure is 0.057 kg. Thus, 2.5 kg of dairy manure is equivalent to the amount accumulated over six weeks. The standard plan for livestock manure treatment facilities in South Korea recommends an optimal bedding material depth of 10 cm [15]. Based on the sawdust bulk density of 0.3 kg L\({}^{-1}\) and the simulated bedded pack area of 0.031 m\({}^{2}\), the appropriate amount of sawdust was calculated to be 0.94 kg (wet basis). However, to enhance odor emissions and better evaluate the odor mitigation effects of biochar, this study intentionally applied only 0.225 kg of sawdust, which is 24% (wet basis) of the recommended amount. This reduced bedding quantity was intended to simulate a highly loaded bedded pack condition, reflecting a scenario where the bedding has not been replaced for an extended period. Based on the calculated mixing ratio of 2.5 kg of manure to 0.225 kg of sawdust, this composition corresponds to approximately six months of manure accumulation following bedding replacement. Given that applying10 cm of sawdust bedding and rearing dairy cows for six months results in a manure-to-sawdust weight ratio of approximately 11:1, the same as the ratio used in this study, the experimental conditions can be interpreted as simulating the state of the bedding pack six months after replacement. According to the 2023 Livestock Environment Survey conducted by the Republic of Korea's Ministry of Agriculture, Food and Rural Affairs [16], the most common manure removal frequency in Korean dairy farms ranges from three to six months, reported by approximately 30% of surveyed operations. Therefore, this setup was designed to simulate a worst-case odor emission scenario under typical field conditions, thereby providing a robust basis for evaluating the mitigation performance of rice husk biochar. The control group consisted of a mixture of dairy manure and sawdust without biochar. Rice husk biochar was incorporated into the control at rates of 5% (35 g) and 10% (70 g) by dry weight, referred to as the 5% biochar treatment and 10% biochar treatment, respectively. Each mixture was placed into a 14.1 L lab-scale simulated bedded pack chamber, and each experimental group was evaluated in triplicate (Figure 1). The experiment was conducted over 21 days, during which all chambers were placed in a constant-temperature room maintained at 28 \({}^{\circ}\)C. This temperature reflects the average maximum summer temperature in the Republic of Korea during the 2011-2020 period (29.1 \(\pm\) 1.4 \({}^{\circ}\)C), aligning the experimental setting with conditions under which odor emissions from bedded pack systems are typically most pronounced [17]. To simulate the accumulation of manure in the barn, 0.4 kg of dairy manure was added to each experimental unit at 7-day intervals, for a total of two additions (on days 7 and 14) over the 21-day experimental period. Manure samples were taken pre- and post-manure addition. Sampling losses represent the sum of the collected sample mass and any residual material on the gloves. Each chamber had a perforated plate in the lower section to facilitate representative odorous gas and leachate sampling (Figure 1).