# Mitigation of Gaseous Emissions from Swine Manure with the Surficial Application of Biochars
## 3 Results  
### Observation of Biochar Layer Interaction with Swine Manure  
In trial 1, HAP biochar started sinking right after application. At the end of the trial, there was a thin layer of wet biochar floating near the surface; some biochar was in suspension, while most of it settled on the bottom. All RO was wet by day 4. RO had a thicker wet layer floating near the surface of the manure and settled fewer particles on the bottom in comparison with HAP. During trial 2, RO biochar was floating for the first 2 weeks on top of the manure, and then it started to incorporate into manure after day \(\sim\)15. On the last day of the trial, about half of RO biochar was wet and was floating near the top of the manure; another half settled on the bottom. HAP biochar absorbed liquid manure where it was interacting with it, then crusted and partially separated from manure. Thus, a headspace between the HAP layer and manure was created near the perimeter, and only \(\sim\)30% of HAP biochar was still in contact with manure near the center of the storage container (Figure S5). In trial 3, HAP biochar started to incorporate into manure on day 3 and completely sank 8 days after application. At the end of the trial, all of the HAP biochar settled on the bottom of the container. A layer of RO biochar applied on the manure started getting wet after 2 weeks, and at the end of the trial, half of the RO biochar became wet and was floating on the manure surface while another half settled on the bottom of the container (Figure S6). Our working hypothesis was that the biochar layer influences manure pH, which could inhibit gas transfer from liquid to air. However, as is shown in Table 3, because of the differences between pre-trial and other treatment manures as well as because there were no significant differences between control and biochar-treated manures over all three trials, no effect of biochar on the manure pH can be assumed. Biochar-treated manure gained solids and volatile solids in comparison with the control manure. The total N content dropped 1.5-2 times compared to control over all three trials, likely due to the high biochars porosity that might adsorb N (Table 3). The possibility of N adsorption and possible slow release to (e.g.,) soil after biochar-treated manure is applied to the land should be further explored as the mitigation of gaseous emissions could be followed by agronomic benefits to the soils. One of the reasons for different biochars behavior in the meaning of sinking or floating on the manure surface could be a difference in hydrophobicity of biochars. Higher hydrophobicity decreases liquid absorption, which can prevent biochar from sinking in manure [45]. Hydrophobicity of biochar is associated with residual alkyl functional groups, which can be decomposed by a higher temperature of charring. Thus, hydrophobicity depends on feedstock and temperature of charring. It is possible to compare the hydrophobicity of biochars by comparing the peak area in a range of 3000-2800 cm\({}^{-1}\) on FTIR spectra [46]. As it can be seen in Figure S7, RO biochar has a small peak on that range, while the graph for HAP biochar is flat, which supports the above-mentioned hypothesis. Another property of HAP biochar that made it sink in manure could be high ash content in comparison with RO biochar. In addition, the differences in pore size associated with HAP and RO biochars (visible in SEM images, [34]) could also affect the liquid penetration. Another parameter that could affect biochar floating on top of manure is the solid content in manure. Trial 1 showed that HAP and RO started to incorporate with manure immediately after application and on day 2, respectively. Then, most of the particles of biochar with the lower solid content completely sank in manure. In trial 2, where manure with higher solid content was used, RO biochar became wet only after \(\sim\)2 weeks, and there was a floating wet layer of biochar still near the manure surface.