## 3 Results and Discussion  
The Impacts of Returning Biogas Slurry to the Field on the Physical and Chemical Properties of CO Planting Soil and the Distribution of Phosphorus Forms  
The results of the indoor soil incubation experiment showed that the soil in CO plantations was dominated by inorganic phosphorus (accounting for approximately 80%), with Fe-P and O-P being the main forms of inorganic phosphorus. It can be observed that within a 30-day incubation period, the increase in the amount of slurry used could effectively raise the total phosphorus level in the CO planting soil, significantly increasing the contents of Al-P and Fe-P, while the impacts on O-P, Ca-P, and organic phosphorus in the soil were not significant. As the incubation time increased (30 days), the total phosphorus content in the soil decreased significantly, with the levels of the inorganic phosphorus forms Al-P and Fe-P also significantly decreasing, gradually transforming into O-P and Ca-P, and the organic phosphorus content also significantly decreasing (Table 2). After the biogas slurry is applied to the soil, the dissolved inorganic phosphorus in the slurry is rapidly bound by the abundant Fe\({}^{3+}\) and Al\({}^{3+}\) in the acidic soil of CO plantations, forming unstable Fe-P and Al-P, which are temporarily stored. As the application time extends, the organic phosphorus in the biogas slurry is transformed into inorganic phosphorus by soil microorganisms and oxidative environments, and Fe-P and Al-P are further oxidized, gradually encapsulated by iron oxides, and converted into the closed storage form O-P [30], with significantly reduced availability. It is commonly believed that phosphorus in the forms bound to aluminum, iron, and calcium in acidic soils can be slowly absorbed and utilized by plants [31]; although the absorption rate is slower than that of dissolved phosphorus (Pi), it can slowly release a phosphorus source over a longer period, while closed storage (O-P) phosphorus is extremely difficult to utilize [32]. This study monitored the soil physicochemical indicators and phosphorus forms during the harvest period (Table 3) and found that the application of the biogas slurry significantly improved the available phosphorus content in the soil, increased the content of Al-P, and significantly reduced the content of Ca-P. However, owing to the obvious spatial heterogeneity under field application, the effect of the variation in the amount of biogas slurry applied on the physicochemical properties of the soil for CO cultivation, soil phosphorus, and its forms is not clear. Research shows that biogas slurry contains a higher amount of phosphorus, mostly in the form of inorganic phosphates, phosphoric acid, and phosphate ions, and contains some organic phosphorus compounds, such as fatty acid phosphates, but usually at lower levels [33]. The application of biogas slurry can effectively supplement phosphorus in the soil and has a certain activation effect on the phosphorus fixed in the soil phosphorus pool [34; 35]. Al-P, which is a phosphorus form with good plant availability in acidic soils [36], also showed a significant increase, confirming that the return of biogas slurry to the field for CO cultivation can promote the absorption of soil phosphorus by plants. The significant reduction in the Ca-P content, combined with the view that Ca-P has the highest plant availability in acidic soils, suggests that the reason may be that during the shooting period, the growth and metabolic rate of CO is rapid, and the soil nutrients are quickly absorbed and assimilated by the plants. Therefore, the effect of applying biogas slurry on