## 4 Discussion
### Effect of Bi-Weekly Biochar Reapplication  
Overall, the bi-weekly biochar reapplication resulted in much higher percentage reductions and a greater number of odorous VOCs with statistically significant percentage reductions. The percentage reductions for NH\({}_{3}\) and all the targeted VOCs (phenol, p-cresol, indole, skatole, and 4-ethyl phenol) were statistically significant. In the case of skatole, the biochar reapplication increased the percentage reduction from 42% to 80%. The 2 kg/m\({}^{2}\) bi-weekly reapplication showed an 89% reduction of phenol, a 74% reduction of p-cresol, a 78% reduction of indole, an 80% reduction of skatole, and an 87% reduction of 4-ethyl phenol.  
Considering NH\({}_{3}\), the reapplication doubled the percentage reduction from 33% to 53% compared to the one-time 2 kg/m\({}^{2}\) treatment. As shown in Figure 1, NH\({}_{3}\) emissions were significantly reduced immediately after each addition of biochar. The biochar reapplication was clearly solving the main motivation for this research, i.e., addressing the decreasing effectiveness over time reported for the 30-day trials with one-time biochar application [2].  
The reapplication nearly doubled the percentage reduction of odor (from 11% to 22%), and an almost tripled generation of CH\({}_{4}\) (from \(-\)15% to \(-\)46%) was also observed, yet without statistical significance. HAP biochar had no significant impact on CO\({}_{2}\) and N\({}_{2}\)O emissions regardless of the treatment.  
Several recommendations could be made for future research directions. There is an opportunity to further explore short-term biochar application to stored manure immediately before agitation and pump out. We already showed significant percentage reductions of NH\({}_{3}\) and H\({}_{2}\)S emissions during swine manure agitation [19; 20]. The testing of biochar addition on mitigating odor and odorous VOCs emissions during agitation, pump out, and land application is warranted. Treatment of gaseous emissions from open sources,such as manure lagoons, open dirt feedlots, could also be explored. Biochars with different physicochemical properties should be explored to target the mitigation of specific gases. Since biochar is a very fine powder, the pelletization of biochar along with different application and reapplication methods should be explored before testing at the farm-scale. Enhanced generation of biogas (and its main component, CH\({}_{4}\)) from biochar-treated manure could provide additional options for bioreenwable energy production on a farm biogas plant. Furthermore, the biochar and manure mixture has shown the potential to be better fertilizers and helps stop leaching [21]. In addition, biochar treatment of manure from other types of livestock and poultry and other gaseous emissions sources, such as municipal and industrial wastewater and landfills, could be explored.  
Thus, biochars can still be considered for a comprehensive solution on the food-energy-water nexus that could improve the sustainability of animal and crop agriculture. Specifically, mitigating gaseous emissions with biochar, a byproduct of bioreenwable energy production, followed by the benefits stemming from the application of manure and biochar mixture on soil, minimizing risks to runoff and water quality, and enhancing crop production.