Organic matter components rather than microbial enzymes and genes predominate CO2/CH4 emissions during composting amended with biochar at different stages.

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.