Co-Adding Biochar and Zeolite to Chicken Manure Compost: Nutrient Retention, Humification, and Bacterial Community Responses

Compost instability and low nutrient content remain the primary constraints on the resource utilization of livestock manure compost. Although biochar and zeolite have individually demonstrated ameliorative effects, their combined application and the associated microbial mechanisms remain insufficiently understood. This study ran a 35-day composting trial with four treatments, which were a no-amendment control (CK); amended with 10% biochar (w/w, dry weight basis, T1); amended with 10% zeolite (w/w, dry weight basis, T2); and amended with 5% biochar plus 5% zeolite (w/w, dry weight basis, T3). The T3 group achieved an average thermophilic temperature of 58.37 °C, sustained a thermophilic phase for 10 days, and had the highest seed germination index (118.31%), exceeding those of the other groups. This group maintained higher observed total phosphorus and total potassium concentrations at compost maturity. It also presented a higher NH4+-N concentration (410.00 mg/kg DM). Humic acid climbed to 57.67 g/kg DM by day 35. Microbial analysis showed that Actinobacteriota made up 60.54% of the bacterial community in T3, almost twice the share in CK (32.97%). These observational results indicate that biochar–zeolite co-addition provided complementary benefits for improving compost maturity and nutrient concentrations, offering a practical strategy for sustainable chicken manure management.