## 1 Introduction  
Livestock production always plays a very important role in our daily life. This industry provides millions of people with jobs and food. However, along with all the benefits of livestock production, unwanted gas emissions, such as ammonia (NH\({}_{3}\)), greenhouse gases (GHG), volatile organic compounds (VOCs), and odor are raising world-wide concerns regarding their impact on the environment [1]. NH\({}_{3}\) emissions are a major nitrogen (N) pollutant and responsible for the formation of secondary particulate matter (PM\({}_{2.5}\)) aerosols. GHGs, such as carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), and nitrous oxide (N\({}_{2}\)O), are related to concerns about climate change [2]. VOCs, such as phenolics, fatty acids, sulfur-containing compounds, are major contributors to the odorous emissions from the swine barns [3]. Research has been conducted to mitigate gas emissions from livestock agriculture. The user-friendly Air Management Practices Assessment Tool (AMPAT) organized 12 different mitigation technologies targeting gaseous emissions from stored manure [4]. Manure additives (i.e., proprietary mixtures of bacteria or chemicals) are popular low-cost products used by some United States (US) farmers. Small doses of manure additives can be easily applied to manure without changes to the current waste management structures. However, in recent research, 12 commercial manure additives did not show significant impacts on the targeted gases in controlled side-by-side trials [5; 6]. Thus, there is still a need to develop and test manure additives that are adaptable to current animal farming systems. Research shows that some experimental manure additives can have a positive impact on the targeted emissions. Zeolite applications to laying hen manure showed significant effectiveness in controlling odorous VOCs; the average reduction in the total odor was reduced up to 67% [7]. The pilot-scale study of using soybean peroxidase with calcium peroxide to mitigate the emissions from stored swine manure showed significant reductions of NH\({}_{3}\) by up to 68%, dimethyl disulfide (DMDS) by up to 85%, p-cresol by up to 90%, and skatole by up to 93%; also, significant generations were observed in CH\({}_{4}\) by up to \(\sim\)200% and in CO\({}_{2}\) by up to \(\sim\)124% [8]. Furthermore, the farm-scale testing of soybean peroxidase and calcium peroxide in the deep-pit swine barns observed significant reductions in NH\({}_{3}\) by 22%, H\({}_{2}\)S by 80%, and some targeted VOCs by 30% to 40%; no significant changes to GHGs [9]. Surficial (one-time) application of biochar as a manure additive was proposed for the comprehensive mitigation of gas emissions from stored manure. Biochar (a carbonaceous material) can be made from various types of biomass and waste through pyrolysis or torrefraction. Different feedstock and process conditions will produce biochar with different physicochemical properties [10; 11; 12; 13; 14; 15; 16]. Thus, biochar with desired properties (e.g., pH, porosity, chemical moiety) could be explored and used to mitigate targeted gaseous emissions. Recent research using biochar as a manure additive to mitigate gas emissions from livestock manure focused on a one-time application. For example, [PERSON] et al. reported that a thin layer of biochar (pH: 7.28) resulted in a significant reduction in NH\({}_{3}\) and a significant generation of CH\({}_{4}\) as applied to stored swine manure in a pilot-scaled setup [9]. [PERSON] et al. showed that biochar (pH: 9.32) had a significant reduction in NH\({}_{3}\) up to 22% when applied to dairy manure [17]. [PERSON] et al. showed that the one-time surficial application of biochar to swine manure significantly reduced NH\({}_{3}\) and phenol [18]. However, we observed that the mitigation effect decreased with time during the 30-day trials [18]. In this research, we hypothesize that bi-weekly reapplication of biochar can improve the mitigation effect on a wider range of compounds and their gaseous emissions using larger scale and longer trials.