Synergistic effects of malonic acid and biochar on carbon emission reduction, humification, and microbial regulation during green waste composting.

During green waste (GW) composting, substantial emissions of carbon dioxide (CO2) and methane (CH4) contribute to carbon loss, inhibit humification, and increase environment burdens. This study investigated the effects of malonic acid (MA; 0 %, 0.5 %, 1.0 %) and biochar (BC; 0 %, 10 %, 20 %) on carbon loss, humic substance (HS) formation, and the dynamics of cbbL-containing autotrophic microorganism (CCAM) during GW composting. Among all treatments, the co-application of 0.5 % MA and 20 % BC significantly enhanced composting performance by reducing CO2 and CH4 emissions by 34.3 % and 30.6 %, respectively, and increasing HS content by 35.5 % relative to the control. Moreover, MA and BC synergistically reshaped CCAM community, enriching dominant autotrophic taxa (Proteobacteria and Actinobacteria). Mantel Test analysis revealed strong correlations between CCAM community and key composting parameters. These findings provide new insights into microbial-mediated carbon regulation and offer an effective strategy to enhance compost quality while mitigating greenhouse gas emissions during GW management.