A Meta analysis of N2O Emission Characteristics and Influencing Factors in Facultative Composting and an Estimation of Emissions in China.

Facultative composting, as a low-cost and operationally simple technology, treats over 75% of livestock and poultry manure in China. This study used meta-analysis techniques to systematically evaluate the dynamics of N2O emissions during facultative composting, with a focus on identifying key influencing factors. Results indicate that N2O emission peaks in facultative composting were mainly observed during the initial and maturation phases, with emissions persisting over a prolonged period. Compared to aerobic composting processes, the environmental conditions in facultative composting are more variable. Changes in raw material characteristics and composting parameters have a more significant impact on facultative composting. N2O emissions are high in farmers' composting methods that lack ventilation, turning, and plastic mulching. Individually, turning or covering the pile tends to increase N2O emissions; however, their combined application achieves substantial reduction. Reducing pH, lowering initial moisture content, and decreasing turning frequency all mitigate N2O emissions under facultative conditions. In addition, N2O emissions from reactor composting are significantly higher than those from windrow composting and static composting. Peak N2O emissions during reactor composting primarily occur during the thermophilic phase, whereas emissions from large-scale static composting mainly occur during the maturation phase. Based on China's annual livestock manure production of approximately 3.8 billion tons, we estimate that facultative composting emits 21.07 Mt CO2-eq annually. These findings challenge the direct use of IPCC emission factors derived from aerobic systems in facultative composting accounting and underscore the need for process-specific mitigation strategies to reduce the climate and health impacts of China's dominant manure management practice.