## 3 Results and Discussion
### The Impacts of Returning Biogas Slurry to the Field on the Yield of CO and Phosphorus Content
#### 3.4.2 The Impact of Biogas Slurry Application on Phosphorus Metabolism Functional Genes  
Based on the migration and transformation characteristics of inorganic phosphorus and organic phosphorus in microbial phosphorus metabolism genes, as well as the transformation characteristics of phosphorus in living organisms, a phosphorus cycling process in the soil for CO planting was created, as detailed in Figure 8. The abundance of the _ppx-gppA_ gene in each biogas digestate application group increased compared to that in the CK group, while the relative abundance of the _pstS_ gene decreased. The application of biogas digestate increased the solubility of inorganic phosphorus in the soil through the microbial community action of the _ppx-gppA_ and _gcd_ genes. After dissociating inorganic phosphate (Pi), it is carried and transported by the phosphorus transport gene community (such as the _pstS_ gene community). Organic phosphorus is mineralized by the _phnA_ and _phnX_ gene communities, converted into Pi, and then absorbed by biological cells. With the increase in the amount of biogas digestate, the abundance of the _phnA_ gene significantly increased; the _phnX_ abundance in each biogas digestate application group significantly increased compared to the CK group, effectively promoting the conversion of soil organic phosphorus to inorganic phosphorus. The primary gene responsible for polyphosphate synthesis in the test soil was _ppkI_, and the abundance of _ppkI_ in each biogas digestate application group was higher than in that the CK group. The gene responsible for polyphosphate degradation was _spoT_, and the abundance of _spoT_ after biogas treatment increased slightly compared to that in the CK group. This study found that, compared to CK, the application of digested slurry increased the abundance of _ppx-gppA_ and _gcd_, indicating that the application of digested slurry can promote microbial participation in the dissociation of inorganic phosphorus from soil into Pi, thus increasing the solubility of inorganic phosphorus. The relative abundances of _PhnA_ and _phnX_ increased, suggesting that the application of digested slurry enhanced the decomposition and mineralization rate of organic phosphorus by microorganisms. Organic phosphorus, under the action of certain bacteria (such as Strep  
Figure 8: Schematic diagram of phosphorus cycling under the influence of soil microorganisms. Single gene abundance was compared with CK on the background of different biogas slurry application rates, and included inorganic phosphorus solubilization, organic phosphorus mineralization, phosphorus transfer, polyphosphate synthesis, and polyphosphate degradation. Bacillus, and Gram-negative bacteria) and a few fungi and actinomycetes, is primarily mineralized to inorganic phosphorus through phosphatase enzyme activity and the secretion of certain organic acids (such as citric acid, lactic acid, and gluconic acid) [42; 43]. Inorganic phosphorus is dissolved from soil phosphates by organic acids (citric acid, malic acid, etc.) and phosphatases secreted by phosphate-solubilizing and phosphorus-dissolving bacteria [44; 45], releasing soluble phosphorus. Simultaneously, some bacteria and fungi in the soil can also promote the dissolution of inorganic phosphorus by secreting acidic metabolic products to change the soil pH conditions. The analysis of soil _phmM_ and _phml_ gene abundance in this study found that with an increase in the concentration of digested slurry application, both the _phmM_ and _phml_ genes related to organic phosphorus mineralization showed a trend of increasing expression abundance. This suggests that with an increase in the amount of digested slurry applied, the organic fraction of organic phosphorus in the soil increases, leading to an increased demand for the mineralization of organic matter such as organic phosphorus, resulting in an increase in the expression of the microbial _phmM_ and _phml_ genes [46].