Particle-scale visualization of the evolution of methanogens and methanotrophs and its correlation with CH₄ emissions...

Methane (CH₄) emissions are a major environmental concern in composting facilities. Therefore, this study initially visualized the dynamic distribution and quantity of methanogens and methanotrophs in composting particles during manure aerobic composting using fluorescence in situ hybridization-confocal laser scanning microscopy (FISH-CLSM) and quantified their correlation with CH₄ emissions. The visualization results showed that methanogens existed inside the particles, while methanotrophs clustered in the outer layer; a facultative anaerobic zone existed in between. The quantification results of integral optical density of methanogens and methanotrophs per unit particle area (U_gen and U_oxi, respectively) indicated that, in the cooling phase, CH₄ generation and oxidation could still be high and could strike a balance if the initial organic matter content of composting materials is high, while both could be extremely low if the content is low. A strong linearity between U_gen obtained by FISH-CLSM and methyl-coenzyme M reductase copy number obtained by quantitative polymerase chain reaction analysis (R² = 0.88) was observed, which justified the effectiveness of the FISH-CLSM method and demonstrated that macro-scale CH₄ emissions were essentially an accumulation of particle-scale CH₄ emissions. CH₄ emissions were equal to 3.3297 × 10⁷U_gen - 3.1814 × 10⁶U_oxi - 3902.9900 (R² = 0.98). Overall, the results showed that methanogens exerted more influence on CH₄ emissions than methanotrophs. Combining these results with CH₄-generation and -oxidation kinetics may help illustrate CH₄-emission mechanisms, improve particle-scale CH₄-emission models, and thereby provide theoretical guidance for operation optimization and emission reduction in composting processes.