## Results
### Diversity of Bacterial Community in Yellow Water and Pit Mud  
The sequencing of the yellow water (nine samples) and pit mud (three samples) resulted in 477,218 qualified sequences with an average length of 418.69 bp for all samples, and samples contained 30,381-52,008 sequences (Table 1 and Table S3). Overall, 501 different OTUs (97% similarity) from all 12 samples were obtained.  
The Shannon index of yellow water gradually increased with the increase of fermentation time, from 1.31 \(\pm\) 0.05 at 30 days to 2.58 \(\pm\) 0.35 at 60 days, but this was less than the 2.86 \(\pm\) 0.24 for the mud. The yellow water Chao1 index also increased with time, from 362.81 \(\pm\) 23.10 to 418.72 \(\pm\) 35.66, while the mud Chao1 was 294.78 \(\pm\) 31.33 and lower in all yellow water samples (Table 1).  
According to the Shannon index, the bacterial diversity in yellow water increased with time, but the diversity of mud was the highest. The yellow water Chao1 index increased with time, and the index of pit mud was the lowest. This indicated that the number of OTUs increased gradually and the species richness increased, which was consistent with the description of the Shannon index.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Sequence Number** & **Shannon** & **Chao1** \\ \hline HS30 & 35918.33 \(\pm\) 4036.73 & 1.31 \(\pm\) 0.05 & 362.81 \(\pm\) 23.10 \\ HS45 & 35285.67 \(\pm\) 4621.46 & 1.87 \(\pm\) 0.72 & 371.99 \(\pm\) 37.31 \\ HS60 & 42233.67 \(\pm\) 8418.46 & 2.58 \(\pm\) 0.35 & 418.72 \(\pm\) 35.66 \\ JN & 45635 \(\pm\) 5843.77 & 2.86 \(\pm\) 0.24 & 294.78 \(\pm\) 31.33 \\ \hline \hline \end{tabular} HS30: yellow water at 30 th day, HS45: yellow water at 45 th day, HS60: yellow water at 60 th day, JN: pit mud.  
\end{table}
Table 1: Sequence number and diversity indices calculated based on a cutoff of 97% similarity.  
Figure 1: Percentage of aroma components at different fermentation times. AA: acetic acid, BA: butyric acid, CA: caproic acid, EA: ethyl acetate, EL: ethyl lactate, EB: ethyl butyrate, EC: ethyl caproate. HS30, yellow water at 30 th day, HS45, yellow water at 45 th day, HS60, yellow water at 60 th day.  
However, the Chao1 index in the mud was smaller than that in the yellow water, and the Shannon index indicated that the bacteria in the mud was richer than the yellow water, which indicated that the rare OTU in the yellow water was increased, and the rare OTU in the mud was decreased.  
Yellow water had an average of 153 genera at 30 days, 161 genera at 45 days, and 174 genera at 60 days. With the prolongation of fermentation time, the microbial population of yellow water increased and was greater than the pit mud (137 genera). There were 143 common genera between HS30, HS45 and HS60, which indicated that the microbial population of yellow water increased from 30 days to 60 days, but there was little increase in the number of increased microorganisms, and the common genera microorganisms made up the majority. When the yellow water was in contact with the mud for a long time, the microorganisms were definitely affected by the mud. The common genus number between the yellow water and pit mud was 98 (Figure 2A). With the prolonged contact time between the yellow water and the mud, there were more and more common bacteria in them. At 30 days, there were 103 genera between yellow water and mud, and the common number of microorganisms increased to 118 at 60 days.  
HS30 & HS45 & HS60 & JN on genus level Microbial community pieplot  
Figure 2: \(Cont.\)  
Among the bacteria shared by pit mud and yellow water, Lactobacillus accounted for the most, at about 56.96%. Other common bacteria were _Sedimentibacter_, 9.76%, _Syntrophacticus_, 7.14%, _Capriciproducers_, 3.60%, _Hydrogenispora_, 3.12%, _g_norank_f_Peptococcaeae_, 2.69%, _g_norank_f_Clostridiaceae_1, 1.75% (Figure 2B).