## 3 Results and Discussion
### The Impacts of Returning Biogas Slurry to the Field on the Yield of CO and Phosphorus Content
The Impacts of Returning Biogas Slurry to the Field on the Soil Microbial Phosphorus Metabolic Functions  
4.1 Distribution of Soil Microbial Phosphorus Metabolic Function Genes with Different Amounts of Biogas Slurry Returned to the Field  
Based on the phosphorus metabolic functions of microorganisms, a set of genes related to phosphorus metabolism was established (Figure 6), as seen in Table 6, and the absolute abundance of phosphorus metabolism genes was subjected to a variance analysis of the differences between the biogas slurry application groups (Figure 7). The results showed that there were extremely significant differences between the groups for the phosphorus metabolism genes _plmP_, _plmM_, _pstB_, and _appA_. Meanwhile, _ploR_, _ppkI_, _glpQ_, _phoB_, _ugpC_, _plmJ_, and _plmK_ showed significant differences between groups, while the remaining phosphorus metabolism genes did not exhibit significant intergroup differences. The _phoR_ (gene for phosphorus deficiency response regulation) and _glpQ_ (gene for phosphorus transport) genes showed an overall consistent pattern, with their abundance gradually decreasing as the concentration of biogas slurry application increased. The organic phosphorus mineralization genes _plmM_ and _plmJ_ also showed an overall consistent pattern, with their abundance gradually increasing as the concentration of biogas slurry application increased. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Function** & **Number** & **Gene** \\ \hline Organic P mineralization & 17 &
\begin{tabular}{c} _PHO_, _plmX_, _plmW_, _phoA_, _plmQ_, _appA_, _plmC_, _plmH_, _plmH_, _plmH_, _plmH_, _plmH_,  
The Pearson analysis results of phosphorus metabolism genes and the amount of biogas slurry returned to the field show that the application amount of biogas slurry was negatively correlated with _pstS_, _spoT_, _pstB_, _glpQ_, and _plmK_, while it was positively correlated with other phosphorus metabolism genes. However, none of the correlations between the application rate of biogas slurry and the phosphorus metabolism genes reached a significant level (Table 7). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & & _pstS_ & _spoT_ & _plmX_ & _pstI_ & _ppx-gppA_ & _gcd_ & _plmA_ & _phR_ & _ppkI_ \\ \hline \multirow{2}{*}{Treatments} & coefficient & \(-0.173\) & \(-0.096\) & \(0.066\) & \(0.351\) & \(0.202\) & \(0.251\) & \(0.281\) & \(0.102\) & \(0.351\) \\  & \(p\) & 0.537 & 0.735 & 0.816 & 0.199 & 0.470 & 0.367 & 0.311 & 0.717 & 0.199 \\ \hline \multirow{4}{*}{Treatments} & _pstB_ & _glpQ_ & _plbB_ & _plbB_ & _qqC_ & _plmM_ & _plbB_ & _qqpA_ & _plbK_ \\  & \(p\) & \(-0.164\) & \(-0.092\) & \(0.488\) & \(0.430\) & \(0.029\) & \(0.380\) & \(0.336\) & \(0.223\) & \(-0.144\) \\ \cline{1-1}  & \(p\) & 0.599 & 0.744 & 0.065 & 0.109 & 0.918 & 0.162 & 0.220 & 0.425 & 0.608 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Correlation analysis between soil phosphorus metabolism genes and biogas slurry use. Figure 6: Distribution of phosphorus metabolism functional genes in each group. Figure 7: Functional genes with significant differences among the treatment groups (* \(p\) \(<\) 0.05, **_p_ \(<\) 0.01, *** \(p\) \(<\) 0.001). (**a**) Genes with significant differences and abundance ranking in the top 7–11, (**b**) Genes with significant differences and abundance ranking in the top 1–6. _plmP_, _plmM_, _pstB_, and _appA_ showed highly significant group differences, and _ploR_, _ppkI_, _glpQ_, _phB_, _ugpc_, _plmJ_, and _plmK_ showed significant group differences.