Organic matter components rather than microbial enzymes and genes predominate CO₂/CH₄ emissions during composting...

The composting process is accompanied by CO₂ and CH₄ emissions, leading to environmental pollution and lower fertilizer efficiency. Biochar has been widely applied to mitigate CO₂ and CH₄ emissions, while its effect when added at different composting stages is till unclear. Therefore, this study investigated the effects of biochar added on day 0, 10, or both days on CO₂ and CH₄ emissions during composting process, and explored the mechanisms from multiple aspects including physicochemcial properties of compost matrix, degradation of organic matter (OM) components, functional enzyme activities and genes abundances. The findings showed that biochar enhanced the activities of lignocellulolytic enzymes, thus facilitating the degradation of cellulose, hemicellulose and water-soluble organic carbon (WSOC). All biochar addition strategies resulted in higher CO₂ emissions and C cycling genes abundances, while addition of 5 % or 10 % biochar on day 10 effectively reduced CH₄ emissions by 17 % and 50 %, respectively. Mantel analysis and partial least squares path modeling revealed that OM components played the primary role in influencing CO₂ emissions, with physicochemical properties, such as temperature, C/N and pH, playing a secondary role by influencing the degradation of OM and WSOC, while no significant factors were found to directly affected CH₄ emissions. Moreover, the lignocellulolytic enzymes and C cycling genes did not show significant impacts on their emissions. This study provides important insights for improving the mitigation on CO₂ and CH₄ emissions by biochar.