Biochar reinforced the populations of cbbL-containing autotrophic microbes and humic substance formation via sequestrating CO2 in composting process.

The quality of compost is drastically reduced due to the loss of carbon, which negatively impacts the environment. Carbon emission reduction and carbon dioxide (CO2) fixation have attracted much attention in composting research. In this study, the relationship between CO2 emission, humic substances (HS) formation and cbbL-containing autotrophic microbes (CCAM) was analyzed by adding biochar during cow manure composting. The results showed that biochar can facilitate the degradation of organic matter (OM) and formation of HS, as well as reinforce the diversity and abundance of CCAM community, thereby promoting CO2 fixation and reducing carbon loss during composting. High-throughput sequencing analysis revealed significant increase in Actinobacteriota and Proteobacteria abundance by 30.97 % and 10.48 %, respectively, thus increasing carbon fixation by 32.07 %. Additionally, Alpha diversity index increased significantly during thermophilic phase, while Shannon index increased by 143.12 % and Sobs index increased by 51.62 %. Redundancy analysis (RDA) indicated that CO2 was positively correlated with C/N, temperature, HS and dissolved organic matter (DOM), while the abundance of Paeniclostridium, Corynebacterium, Bifidobacterium, Clostridium, Turicibacter and Romboutsia were positively correlated with temperature, CO2, C/N and E2/E4 (p <  0.01).