## 1 Introduction  
The livestock industry has long faced challenges with odor emissions from manure storage, treatment facilities, and livestock housing. Cattle are well-adapted to cold weather and are therefore typically housed in open or partially open barns to manage humidity and air quality [1]. The reliance on natural ventilation in these barns limits the effectiveness of collection-based odor control systems, such as biofilters and wet scrubbers. Consequently, in-barn approaches to reduce odors, such as applying microbial additives, oxidants, and adsorbents, have become a preferred method for directly targeting odor sources within barns [2].  
Biochar has gained significant attention among various odor mitigation agents due to its unique structural and chemical properties that facilitate the removal of odorous compounds [3]. [PERSON] et al. [4] reported that adding green waste biochar and -poultry litter biochar to a composting mixture of poultry litter and straw resulted in a 30% and 44% reduction in ammonia emissions, respectively, when each type of biochar comprised 10% of the total dry weight of the mixture. [PERSON] et al. [5] investigated the effect of adding cornstalk biochar treated with HNO\({}_{3}\) and/or H\({}_{2}\)O\({}_{2}\) on ammonia emissions during layer manure composting. They observed that incorporating 10% (by dry weight) of this treated biochar led to a 46% to 62% reduction in ammonia emissions. [PERSON] et al. [6] demonstrated that surficial treatment of corn stover or red oak biochar on swine manure significantly reduced hydrogen sulfide emissions by 62% to 67%. [PERSON] et al. [7] also reported that a pilot-scale odor removal system filled with pine biomass biochar effectively reduced volatile organic compounds emitted from a swine gestation barn to undetectable levels.  
While bedded pack barns offer advantages for livestock welfare, improper management of the bedding's moisture content can lead to an increased risk of mastitis and odor problems [8]. Given its large pore volume and high water-holding capacity, biochar presents a promising option for improving bedding conditions [9; 10; 11]. Biochar's ability to absorb moisture from manure and adsorb odorous compounds can contribute to optimal bedding moisture levels and reduce odor emissions. Despite these potential benefits, few studies have explored the effects of biochar-amended bedding on odor reduction and overall cattle housing environments.  
Determining the appropriate biochar addition rate is crucial for effective odor mitigation in cattle barns. Excessive biochar use can significantly increase bedding management costs and produce a substantial volume of manure requiring treatment [2]. [PERSON] et al. [12] reported a reduction in volatile organic compounds (VOCs) in a lab-scale experiment by applying 10% (by weight) of cattle manure biochar to raw cattle manure. However, this approach may pose practical challenges due to increased treatment costs and the need to handle larger manure volumes. Therefore, this study aims to evaluate the effectiveness of rice husk biochar in reducing odor emissions from a bedded pack dairy barn, with a particular focus on the impact of the biochar application rate.