## 2 Materials and Methods  
### Experimental Design and Sampling  
The field positioning experiment was conducted in Yuping County, Tongren City, Guizhou Province, China (27.3451\({}^{\circ}\) N, 108.9475\({}^{\circ}\) E). This area is located in the transitional zone from the Yunnan-Guizhou Plateau to the Xiangxi Hills, with an altitude of 530 m. It has a subtropical monsoon humid climate, with an annual average temperature of 16.4 \({}^{\circ}\)C, an average annual sunshine duration of 1227.8 h, and rainfall ranging from 1200 to 1600 mm [23].  
The soil type of the experimental site is yellow soil [24], with basic physicochemical properties as follows: pH value of 4.87 \(\pm\) 0.52, EC of 215.17 \(\pm\) 8.44 uS/cm, total nitrogen content of 1.59 \(\pm\) 0.26 g/kg, total phosphorus content of 0.79 \(\pm\) 0.17 g/kg, total potassium content of 14.30 \(\pm\) 0.51 g/kg, soil organic matter content of 30.38 \(\pm\) 6.4 g/kg, and available phosphorus content of 8.90 mg/kg. The biogas slurry used in the experiment was collected from the biogas slurry collection pond of the biogasification project of Guizhou Gengze Agricultural Technology Co., Ltd. The basic physicochemical indicators of the biogas slurry are as follows: pH 7.97, EC of 16.14 ms/cm, redox potential of \(-\)363 mv, chemical oxygen demand of 4550 mg/L, ammonia nitrogen content of 1268 mg/L, total phosphorus content of 29.17 mg/L, and suspended solid content of 2645 mg/L.  
The field experiment began in 2021 using the CO variety Xianglin 210, with tree ages ranging from 8 to 10 years. The planting density of CO was approximately 1665 trees per hectare, adopting a randomized block design, with each tree as an observation group (Figure 1). The treatments included: (1) no fertilizer treatment (CK); (2) low-concentration treatment ZY1 with a biogas slurry application rate of 13 kg/tree; (3) medium-concentration treatment ZY2 with a biogas slurry application rate of 27 kg/tree; (4) higher-concentration treatment ZY3 with a biogas slurry application rate of 40 kg/tree; and (5) high-concentration treatment ZY4 with a biogas slurry application rate of 53 kg/tree. Biogas slurry was applied to the field during the shooting period of CO in May 2022, with a single application. The specific design of the biogas slurry application rate is presented in Table 1. Corresponding indoor soil incubation experiments were carried out using soil collected from the field site. After air-drying and passing through a 20-mesh sieve, 500 g of soil was weighed and placed in plastic pots, with the same treatment groups and repetitions as those used in the field experiment for biogas slurry application.  
Soil samples were collected during the camellia oil harvest period in October 2022. For each treatment group (each camellia plant), three different points around it were selected to collect and mix the surface soil (0-40 cm). After removing impurities such as leaves and stones, the soil for metagenomic sequencing was stored at \(-\)80 \({}^{\circ}\)C for testing [25], and the soil for determining soil physicochemical properties was naturally air-dried and sieved (100 mesh) before testing. An indoor soil culture experiment was set up with the same treatment groups and repetitions, and then placed in an artificial climate chamber under conditions of 20 \({}^{\circ}\)C and 75% humidity to be cultured for 1 d and 30 days before undisturbed sampling was conducted. After culturing, the soil samples were naturally air-dried and sealed for testing.