## 4 Conclusions  
This study investigated the potential of rice husk biochar to mitigate odor emissions from a simulated dairy barn bedded pack system, focusing on the impact of biochar application rates (5% and 10% by dry weight). Our findings demonstrate that biochar can significantly reduce ammonia (NH3) and dimethyl sulfide (DMS) emissions. This reduction is attributed to enhanced adsorption and improved air permeability, suppressing anaerobic bacterial activity. Biochar application led to NH3 emission reductions of 27% and 43% and DMS emission reductions of 53% and 75% at 5% and 10% addition levels, respectively, with the most significant reductions observed within the first two weeks. This improvement is likely due to biochar's microporous and mesoporous structures, which facilitate the adsorption of odorous compounds like NH3 and DMS while simultaneously enhancing air permeability to support aerobic microbial activity. Furthermore, biochar amendment improved total nitrogen retention within the bedding material. In addition, biochar in manure can enhance its value as a fertilizer, as its carbon-rich and porous nature helps improve soil structure, moisture retention, and microbial activity. These benefits aid odor control and promote crop growth and long-term soil health, offering both environmental and agronomic advantages. This is supported by the findings of [PERSON] et al. [88], who demonstrated that combining biochar with compost, digestate, or mineral fertilizers significantly increased maize yield--by up to 42% depending on the treatment--and improved nutrient uptake while reducing the accumulation of heavy metals in plants. [PERSON] et al. [89] also reported that the addition of biochar (up to 40% \(v/v\)) to poultry manure compost reduced nitrogen loss and increased potato tuber yield by 40%, highlighting its effectiveness in both nutrient conservation and crop productivity. [PERSON] et al. [90] found that applying biochar co-compost at an equivalent carbon rate (8 Mg C ha-1) significantly increased soil water holding capacity and plant biomass, reduced cumulative CO2 emissions, and lowered nutrient leaching compared to biochar or compost alone. Although the 10% biochar treatment achieved the highest reduction, the 5% treatment proved nearly as effective, attaining 63% of the NH3 reduction and 70% of the DMS reduction observed with 10% biochar. Considering its lower material cost and reduced manure management burden, the 5% biochar treatment appears to be the more practical and scalable option for odor control in dairy barns. However, a potential limitation of rice husk biochar is its high silica (SiO2) content. While SiO2 contributes to the structural integrity of biochar, it can also hinder pore formation and reduce adsorption efficiency. While biochar effectively mitigated NH3 and DMS emissions, no substantial reductions were observed for other odorous compounds. Odor unit reduction was observed only temporarily on day 7, suggesting limitations in sustained odor control. While the 21-day lab-scale experiment provided valuable insights into short-term odor mitigation, longer-duration, and field-scale studies are essential to fully capture the dynamics of biochar performance under realistic manure accumulation and environmental conditions. NH3 and DMS emission patterns may shift over time due to factors such as adsorption saturation, microbial adaptation, and changes in bedding composition, which could affect the sustainability of observed reductions. To address this and other remaining challenges, future research should focus on:  
* Developing chemical or physical modification techniques to overcome limitations posed by high silica (SiO2) content in rice husk biochar, thereby enhancing its surface area and functional adsorption sites. * Investigating the influence of silica content on pore structure formation and its correlation with odorant adsorption efficiency.