Superphosphate-driven low-pH regime reduces greenhouse gas emissions during manure composting

Additives are widely used to mitigate greenhouse gas (GHG) emissions during manure composting, but whether additive combinations improve mitigation remains unclear. Here, we compared zeolite, ferrous sulfate, superphosphate, and their combinations in cattle manure composting. Superphosphate alone showed the strongest overall mitigation, reducing cumulative CH₄ and N₂O emissions by 87% and 38%, respectively, relative to the control. The best-performing combined treatment (FL) reduced cumulative CH₄ and N₂O emissions by 52% and 48%, respectively, indicating that FL performed better for N₂O, whereas superphosphate alone achieved much stronger CH₄ suppression and the best overall GHG mitigation. This effect was associated with a relatively lower-pH regime, higher NH₄⁺-N retention, lower NO₃^- accumulation, and clear shifts in bacterial, archaeal, and functional gene profiles. Stage-specific multivariate analyses further showed that pH was strongly associated with late-stage functional reorganization, while treatment identity retained an independent effect after accounting for measured environmental variables. Overall, effective compost mitigation depended more on establishing a favorable physicochemical regime than on increasing additive complexity, and superphosphate treatment represented the most effective low-emission strategy in this system.