## 3 Results and Discussion
### Concentration of Volatile Fatty Acid (VFA) in Pig Slurry  
Figure 4 depicts the changes in VFA concentration in the pig slurry over the storage period. Considering the temporal variations in TVFA concentration observed in the pig slurry of PFS and CTL, it can be concluded that the concentration decreased until day 28, increased until day 42, and then decreased again until day 56. This trend indicates a complex dynamic in the formation and degradation of VFA in the pig slurry. A negative correlation between VFA concentration and CH\({}_{4}\) emissions in pig slurry [20] implies that the period of TVFA accumulation (28-42 days) was related to a decrease in CH\({}_{4}\) emissions, whereas the periods of TVFA decrease (0-28 days, 42-56 days) were related to an increase in CH\({}_{4}\) emissions. During anaerobic digestion, VFA is sequentially generated through hydrolysis and acidogenesis. Methanogenic microorganisms then utilize this VFA to produce CH\({}_{4}\). As a result, the concentration of VFA tends to increase immediately before the increase in CH\({}_{4}\) emissions and then decrease shortly after the increase in CH\({}_{4}\) emissions. This pattern is because the production of VFA precedes the production of CH\({}_{4}\) in the anaerobic digestion process. In the first two weeks of storage (0-14 d), the concentration of TVFA and emission of CO\({}_{2}\) decreased, and CH\({}_{4}\) emission increased (Table 3, Figure 4). It is assumed that the methanogenesis activity increased due to the slurry's anaerobic decomposition of organic substances. In the third period (15-28 d), CO\({}_{2}\) increased, and TVFA concentration and CH\({}_{4}\) emission decreased. From the second week of pig slurry storage, the rate of methanogenesis was stagnant, while fermentation and aerobic respiration of pig slurry  
Figure 4: Changes in VFA concentration over time in PFS and CTL during the experimental period (56 d). TVFA includes acetic acid, propionic acid, butyric acid, isobutyric acid, isovaleric acid, n-valeric acid, isocaproic acid, n-caproic acid, and heptanoic acid (Supplementary Materials). The statistical differences (\(p\) < 0.05) between PFS and CTL are marked as ***. were presumed to be active [20]. In the fourth period (29-42 d), only TVFA concentration increased, while emissions of both CO\({}_{2}\) and CH\({}_{4}\) decreased. The proliferation of acidogenic fermentative bacteria in the pig slurry led to the buildup of VFA, which, in turn, suppressed the emissions of CO\({}_{2}\) and CH\({}_{4}\). During the fifth experiment period (43-56 d), a decrease in TVFA concentration and CO\({}_{2}\) emission was observed, accompanied by an increase in CH\({}_{4}\) emission. The results indicate that PFS tended to lower TVFA concentrations by 37 to 44% compared with CTL (Figure 4). However, statistical analyses revealed no significant differences between the PFS and CTL regarding TVFA, except propionic and butyric acid. Previous research has demonstrated that HS can elicit hydrolysis inhibition within a concentration range of 0.5-5.0 g\(\cdot\)L\({}^{-1}\)[28]. Two putative mechanisms underlying this inhibition have been posited: threshold-type inhibition, which involves the binding of humic acid to active sites of hydrolytic enzymes, thereby blocking substrate access and disrupting crucial cellular transport processes via binding to bacterial cell walls [26]. HS represent an important outcome of the aerobic composite process and aerobic digestion, such as autothermal thermophilic aerobic digestion (ATAD), and are a crucial indicator for evaluating compost maturity [44; 45]. As a core product of the process, they are vital for achieving a mature compost that can be used for soil amendments, which have been shown to enhance plant growth [46]. Therefore, aerobic digestion or composite systems are a better method for treating pig slurry with high levels of humic substances than anaerobic digestion. Previous studies have explored the increased VFA content in pig slurry resulting from fibrous feedstuffs [47; 48].