# Mitigation of Gaseous Emissions from Swine Manure with the Surficial Application of Biochars
## 1 Introduction  
Animal production has a significant impact on local and regional air quality because of emissions of volatile organic compounds (VOC), ammonia (NH\({}_{3}\)) and hydrogen sulfide (H\({}_{2}\)S), greenhouse gases (GHG), and odor. These emissions have many adverse effects on the environment [1]. Manure management accounts for about 15% of the total GHG emissions from the agriculture sector in the United States [2]. The median emission rates from swine facilities were 2.08 and 0.20 kg\(\cdot\)yr\({}^{-1}\) pig for NH\({}_{3}\) and H\({}_{2}\)S, respectively [3]. Iowa State University Extension and Outreach Air Management Practices Assessment Tool summarized 12 methods of gaseous emissions mitigation in livestock, manure storage, and handling [4]. Farmers use manure additives because of their ease in application and low-cost in comparison with other odor mitigation technologies that can be complex and require expensive equipment. The performance of manure additives is not well studied, especially on a farm-scale. It is estimated that only \(\sim\)25% of technologies were tested at the farm scale while the remaining mitigation methods and technologies have not emerged from the laboratory or pilot scale [5]. Microbe-based manure additives treatments are the most popular and are marketed and used by farmers for mitigation of emissions. However, their performance is difficult to assess objectively. Most research is concentrated on a particular type of gas that a proposed treatment reduces and sometimes ignore other emissions that can be negatively affected by the treatment [5]. Thus, comprehensive, practical, and low-cost treatments to gaseous emissions are still in demand. We have been advancing surficial treatments to livestock and poultry manure based on the physicochemical mode of action. These include various mineral sorbents (e.g., zeolites and bentonite), plant-based enzymes (e.g., soybean peroxidase), and urease inhibitors [6; 7; 8; 9; 10]. One of the manure additives that could potentially simultaneously reduce different types of gas emission is biochar. Biochar is the carbon-rich material (a.k.a. char, biocool) obtained from pyrolysis, torrefraction, or gasification of various types of biomass and biowaste [11; 12; 13; 14; 15; 16]. Most of the early applications of biochars are focused on soil amendment and nutrient management [17]. Biochar is obtained as the chemical structure of biomass changes when heated in the absence of oxygen (O), resulting in a loss of hydrogen (H), nitrogen (N), and O relative to carbon (C). The C atoms are firmly bound to one another, which is difficult for microorganisms to break down [18]. A considerable number of studies show how biochar positively affects soil parameters [19; 20]. The evidence of recent research on applications of biochar in crop and animal production agriculture is reviewed elsewhere [21]. To date, there are only a few research papers published that test biochar application on stored manure to mitigate gaseous emissions. A recent study by [PERSON] et al., (2017) [22] showed that biochar has the potential for being an economical and effective treatment for odorous gaseous emissions from swine deep-pit barn. In experiments conducted by [PERSON] et al. [22] and [PERSON] et al. [23], biochar was applied surficially to swine and dairy manure, respectively. [PERSON] et al. reported up to 23% reduction of NH\({}_{3}\) emissions from surficially-treated swine manure. Interestingly, CH\({}_{4}\) was generated (\(\sim\)25% increase in emissions) [22]. [PERSON] et al., (2017) [23] reported that biochar made of Douglas fir bark mixed with wood fiber showed up to 80% reduction of NH\({}_{3}\). A summary of current research on the uses of biochar as a manure additive and its effect on gaseous emissions is presented in Table S1 (Supplementary Materials). The mechanism of biochar in mitigation of gaseous emissions is complex, and while it can reduce some emissions, it can also increase emissions for other gases (e.g., GHGs in [22]).