## 3 Results and Discussion
### The Impacts of Returning Biogas Slurry to the Field on the Yield of CO and Phosphorus Content
However, the linear increase in the amount of biogas slurry returned to the field did not exhibit a uniform pattern. Figure 3: Community composition of phosphorus-metabolizing microbial phyla. Soil phosphate-solubilizing microflora were dominated by _Actinobacteria_, _Proteobacteria_, _Chloroflexi_, and _Acidobacteria_, with no significant differences between these four microbial groups (_p_ < 0.05). _Germatimonadetes_, _Candidatus_Rokubacteria_, and _Candidatus_Eremiobacteraeota_ exhibited better variability. At various levels, _Chloroflexi_bacterium_, _Aicobacteria_bacterium_, _Actinomycteia_bacterium_, _Streptosporangiaceae_bacterium_, _Hyphomicrobiales_bacterium_, _Hyphomicrobiales_bacterium_, _Alphaproteobacteria_bacterium_, _Terrabacteria_group_bacterium_, _Candidatus_Doribacreia_bacterium_, _Actinobacteria_bacterium_, and _Gemmatimonadetes_bacterium_ were the dominant bacterial species (Figure 5). Compared to the CK group, the biogas digestate treatment group showed increases in the relative abundances of _Chloroflexi_bacterium_, _Actobacteria_bacterium_, _Hypomicrobiales_bacterium_, _Terrabacteria_group_bacterium_, _ANGPI_, and _Actinobacteria_bacterium_, whereas decreases in the relative abundances of _Actinomycteia_bacterium_ and _Streptosporangiaceae_bacterium_ were observed. No significant differences were observed in the remaining species. In the current study, the application of biogas slurry increased the contents of phosphates and organic matter in the soil, providing a rich source of nutrients for such bacteria, which was conducive to the growth and reproduction of nutrient-rich microorganisms such as _Chloroflexi_bacterium_ and _Acidobacteria_bacterium_ (Figure 5) [38; 39]. However, _Actinomycteia_bacterium_ and _Streptosporangiaceae_bacterium_, owing to their poor adaptability to the soil environmental changes caused by the application of biogas slurry, faced increased competition for living space and nutritional resources, resulting in a decrease in the relative abundances of these populations. Previous studies have shown that the role of microorganisms in the soil phosphorus cycle is influenced by various factors, such  
Figure 5: Community structure of phosphorus-metabolizing microorganisms at the species level. _Chloroflexi_bacterium_, _Acidobacteria_bacterium_, _Actinomycteia_bacterium_, _Hyphomicrobiales_bacterium_, _Alphaproteobacteria_bacterium_, _Termobacteria_group_bacterium_, _Candidatus_Dormibacreia_bacterium_, _Actinobacteria_bacterium_, _Gemmatimonadetes_bacterium_ were the dominant species. as the soil type, environmental conditions, and plant species [40]; therefore, their specific roles in the phosphorus cycle may vary with the environment. Currently, the specific roles and contributions of certain microbial phyla to the phosphorus cycle are not yet fully understood. For example, the role of one microbe may be specifically in promoting the decomposition and mineralization of organic phosphorus, whereas the role of other microbes may indirectly affect the phosphorus cycle through ecosystem functions. Some studies suggest that Actinobacteria can mineralize organic phosphorus and promote plant phosphorus absorption by producing phosphatases and forming mycorrhizal symbiosis with the roots of CO. _Chlorofexi_ primarily participates in the mineralization of organic phosphorus to inorganic phosphorus in the soil phosphorus cycle and can release phosphorus by decomposing organic matter. _Acidobacteria_ may be involved in the mineralization of organic phosphorus to inorganic phosphorus, and may play an important role in maintaining the availability of phosphorus in the acidic soil of the experimental area. Overall, the phosphorus metabolism of these functional microorganisms is mainly achieved through pathways such as releasing phosphorus by decomposing organic matter, using inorganic phosphates as phosphorus sources to promote phosphorus cycling, and decomposing organic phosphates into small-molecule inorganic phosphates [41].