Evaluation of the synergistic effects of biochar and biogas residue on CO2 and CH4 emission, functional genes, and enzyme activity during straw composting

[Display omitted] •Biogas residue increased CO2 and CH4 emissions, while biochar decreased them.•CO2 release was significantly linked with BG and pmoA enzyme and mcrA gene.•Biochar improved ACS and BG activity and decreased methanotrophs pmoA activity.•Biochar played a vital role in mitigating carbon loss from biogas residue addition.•OM was the significant variable affecting CO2 and CH4 emissions. This study examined the effects of biochar, biogas residue, and their combined amendments on CO2 and CH4 emission, enzyme activity, and related functional genes during rice straw composting. Results showed that the biogas residue increased CO2 and CH4 emissions by 13.07 % and 74.65 %, while biochar had more obvious inhibition. Biogas residue addition enhanced functional gene abundance more than biochar. Biogas residue raised the methanogens mcrA gene by 2.5 times. Biochar improved the Acetyl-CoA synthase and β-glucosidase activities related to carbon fixation and decreased coenzyme activities related to methanogens. Biochar and biogas residue combined amendments enhanced the acsB gene abundance for CO2 assimilation process and decreased methyl-coenzyme M reductase α subunit activity. Pearson correlation analysis indicated that organic matter was the significant variable affecting CO2 and CH4 emissions (P